Heat Exchangers

A heat exchanger is a heat-transfer device that is used for transfer of internal thermal energy between two or more fluids available at different temperatures. In most heat exchangers, the fluids are separated by a heat-transfer surface, and ideally they do not mix.

Heat exchangers are used in the process, power, petroleum, transportation, air conditioning, refrigeration, cryogenic, heat recovery, alternate fuels, and other industries. Common examples of heat exchangers familiar to us in day-to-day use are automobile radiators, condensers, evaporators, air preheaters, and oil coolers.

Shell and tube heat exchangers represent the most widely used vehicle for heat transfer in process applications. They frequently are selected for duties such as:

Process liquid or gas cooling.

Process or refrigerant vapor or steam condensing.

Process liquid, steam or refrigerant evaporation.

Process heat removal and preheating of feedwater.

Thermal energy conservation efforts and heat recovery

Types of heat exchangers used in the plant

U-Tube Exchangers

In this type of construction, tube bundle as well as individual tubes are free to expand and the tube bundle is removable. A U-tube exchanger is shown in Fig.1. U-tube exchangers can be used for the following services:

1. Clean fluid on the tube side2. Extreme high pressure on one side

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3. Temperature conditions requiring thermal relief by expansion4. For H2 service in extreme pressures, utilizing an all welded construction with a non-removable bundle5. To allow the shell inlet nozzle to be located beyond the bundle

Figure 1 TEMA U-tube heat exchanger.

Shortcomings of U-Tube ExchangersSome of the demerits associated with U-tube exchangers are:

1. Mechanical cleaning from inside tubes is difficult, the chemical cleaning is possible.

2. Flow-induced vibration can also be a problem in the U-bend region for the tubes in the outermost row because of long unsupported span.

3. Individual tubes cannot be replaced.

Shell and Tube Heat Exchangers Design

The most commonly used heat exchanger is the shell and tube type. It is the “workhorse” of industrial process heat transfer. It has many applications in the power generation, chemical, and process industries. Other types of heat exchangers are used when economical. Though the application of other types of heat exchangers is increasing, the shell and tube heat exchanger will continue its popularity for a long time, largely because of its versatility

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CONSTRUCTION DETAILS FOR SHELL AND TUBE EXCHANGERS

The major components of a shell and tube exchanger are tubes, baffles, shell, front head, rear head, tube sheet(s), and nozzles. Expansion joint is an important component in the case of fixed tube-sheet exchanger for certain design conditions. The selection criteria for a proper combination of these components are dependent upon the operating pressures, temperatures, thermal stresses, corrosion characteristics of fluids, fouling, cleanability , and cost. Other components include nozzles and supports. Mechanical Design of Shell and Tube Heat Exchangers. A large number of geometrical variables are associated with each component and they are discussed in detail in this chapter. Major components of shell and tube heat exchangers are shown in Fig. 2.

TUBES

Tubes of circular cross section are exclusively used in exchangers. Since the desired heat transfer in the exchanger takes place across the tube surface, the selection of tube geometrical variables is important from the performance point of view. Important tube geometrical variables include tube outside diameter, tube wall thickness, tube pitch, and tube layout patterns (Fig. 2). Tubes should be able to withstand the following1. Operating temperature and pressure on both sides2. Thermal stresses due to the differential thermal expansion between the shell and the tube bundle

3. Corrosive nature of both the shell-side and the tube-side fluidsThere are two types of tubes: straight tubes and U-tubes. The tubes are further classified as

1. Plain tubes

2. Finned tubes3. Duplex or bimetallic tubes

4. Enhanced surface tubes

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Figure 2 Shell and tube heat exchanger

1. tube bundle 2. shell 3. tube 4. baffle 5. vent nozzle 6. inlet tubeside 7. Tubesheet 8. drain nozzle 9. shellside

BAFFLES

Baffles must generally be employed on the shell-side to support the tubes, to maintain the tube spacing, and to direct the shell-side fluid across or along the tube bundle in a specified manner.

There are a number of different types of baffles and these may be installed in different ways to provide the flow pattern required for a given application.

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TUBESHEET

A tube sheet is an important component of a heat exchanger. It is the principal barrier between the shell-side and tube-side fluids. Proper design of a tube sheet is important for safety and reliability of the heat exchanger. Tube sheets are mostly circular with uniform pattern of drilled holes. Tube sheets of surface condensers are rectangular shape.

TUBE BUNDLEA tube bundle is an assembly of tubes, baffles, tube sheets, spacers and tie rods, and longitudinal baffles, if any. Spacers and tie rods are required for maintaining the space between baffles. Refer to TEMA for details on spacers and tie rods.

Bundle WeightThe maximum bundle weight that can conveniently be pulled should be specified and should allow for the buildup of fouling and scaling deposits. Offshore applications are particularly sensitive to weight.

Spacers, Tie Rods, and Sealing DevicesThe tube bundle is held together and the baffles located in their correct positions by a number of tie rods and spacers. The tie rods are screwed into the stationary tube sheet and extend the length of the bundle up to the last baffle, where they are secured by lock nuts. Between baffles, tie rods have spacers fitted over them. Tie rods and spacers may also be used as a sealing device to block bypass paths due to pass partition lanes or the clearance between the shell and the tube bundle.

Outer Tube LimitThe outer tube limit (OTL) is the diameter of the largest circle, drawn around the tube-sheet center, beyond which no tube may encroach.

SHELLSHeat exchanger shells are manufactured in a large range of standard sizes, materials, and thickness. Smaller sizes are usually fabricated from standard size pipes. Larger sizes are fabricated from plate by rolling. The cost of the shell is much more than the

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cost of the tubes; hence a designer tries to accommodate the required heat-transfer surface in one shell.

OTHER COMPONENTSSince the shell-side fluid is at a different temperature than the tube-side fluid, there will be a corresponding difference in the expansion of shell and tube. If the temperature difference is high, the differential thermal expansion will be excessive, and hence the thermal stresses induced in the shell and the tube bundle will be high. This is particularly true for fixed tubesheet exchangers. In fixed tube-sheet exchangers, the differential thermal expansion problem is overcome by incorporating an expansion joint into the shell. For U-tube exchangers and floating head exchangers, this is taken care of by the inherent design.

Drains and Vents

All exchangers need to be drained and vented; therefore, care should be taken to properly locate and size drains and vents. Additional openings may be required for instruments such as pressure gages and thermocouples.

Nozzles and Impingement Protection

Nozzles are used to convey fluids into and out of the exchanger. These nozzles are pipes of constant cross section welded to the shell and the channels. The nozzles must be sized with the understanding that the tube bundle will partially block the opening. Whenever a high velocity fluid is entering the shell some type of impingement protection is required to avoid tube erosion and vibration. Some forms of impingement protection include (1) impingement plate (Fig.3) (2) impingement rods, and (3) annular distributors.

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Figure 3 Impingement plate protection

Minimum Nozzle Size

A methodology for determining the minimum inside diameter of a nozzle for various types of fluid entering or leaving the unit is presented. The actual size of nozzle used will depend on the pressure, material, corrosion allowance, and pipe schedule.

FLUID PROPERTIES AND ALLOCATIONTo determine which fluid should be routed through the shell side and which fluid on the tube side, consider the following factors.

Corrosion

Fewer corrosion resistant alloys or clad components are needed if the corrosive fluid is placed on the tube side.

Fouling

This can be minimized by placing the fouling fluid in the tubes to allow better velocity control; increased velocities tend to reduce fouling.

Cleanability

The shell side is difficult to clean; chemical cleaning is usually not effective on the shell side because of bypassing, and requires the cleaner fluid. Straight tubes can be physically cleaned without

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removing the tube bundle; chemical cleaning can usually be done better on the tube side.

Temperature

For high-temperature services requiring expensive alloy materials, fewer alloy components are needed when the hot fluid is placed on the tube side.

Pressure

Placing a high-pressure fluid in the tubes will require fewer costly high-pressure components and the shell thickness will be less.

Pressure drop

If the pressure drop of one fluid is critical and must be accurately predicted, then that fluid should generally be placed on the tube side.

Viscosity

Higher heat-transfer rates are generally obtained by placing a viscous fluid on the shell side. The critical Reynolds number for turbulent flow in the shell is about 2000 hence, when the flow in the tubes is laminar, it may be turbulent if the same fluid is placed on the shell side. However, if the flow is still laminar when in the shell, it is better to place the viscous fluid only on the tube side since it is somewhat easier to predict both heat transfer and flow distribution.

Toxic and lethal fluid

Generally, the toxic fluid should be placed on the tubeside, using a double tube sheet to minimize the possibility of leakage. Construction code requirements for lethal service must be followed.

Flow rate

Placing the fluid with the lower flow rate on the shell side usually results in a more economical design and a design safe from flow-induced vibration, Turbulence exists on the shell side at much lower velocities than on the tube side.

PLATE HEAT EXCHANGER CONSTRUCTION-GENERAL

A plate heat exchanger (PHE), as shown in Fig. 4, is usually comprised of a stack of corrugated or embossed metal plates in mutual contact, each plate having four apertures serving as inlet

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and outlet ports, and seals designed so as to direct the fluids in alternate flow passages. The flow passages are formed by adjacent plates so that the two streams exchange heat while passing through alternate channels. When assembled, the spacing between adjacent plates ranges from 1.3 to 6.4 mm. The number and size of the plates are determined by the flow rate, physical properties of the fluids, pressure drop, and temperature program. The plate corrugations promote fluid turbulence and support the plates against differential pressure. The stack of plates is held together in a frame by a pressure arrangement. The periphery of each plate is grooved to house a molded gasket, each open to the atmosphere. Gaskets are generally cemented in, but snap-on gaskets are available that do not require cement. Gasket failure cannot result in fluid intermixing but merely in leakage to the atmosphere. Proper selection of gasket material and operating conditions will eliminate the leakage risk.

Frame assembly tightly holds the plates pack. It ensures optimum compression and leak tightness. The elements of the frame are a fixed plate, compression plate, pressing equipment, and connecting ports. Every plate is notched at the center of its top and bottom edges so that it may be suspended from the top carrier bar and guided by the bottom guide bar, and the plates are free to slide along the bars. The movable head plate is similarly notched and free to slide along both bars. Both the upper carrying bar and the lower guiding bar are fixed to the support columns. The plate pack is tightened by means of either a mechanical or hydraulic tightening device. Connections are located in the frame cover or, if either or both fluids make more than a single pass within the unit, the frame and pressure plates. By including intermediate separating konnecting plates, three or more separate fluid streams can be handled. Frames are usually free standing; for smaller units, they are attached to structural steel work.

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Figure 4

Standard Performance Limits

Standard performance limits for Alpha-Lava1 PHE are:Maximum operating pressure 25 bar (360 psi)With special construction 30 bar (435 psi)Maximum temperature 160°C (320°F)With special gaskets 200°C (390°F)Maximum flow rate 3600 m'/h (950,000 USG/min)Heat-transfer coefficient 3500-7500 W/m'-"C(600-1300) BTU/ft? hr*"F)Heat-transfer area 0.1-2200 m' (1-24,000 ft')Maximum connection size 450 mm (1 8 in)

Thermo-hydraulic Data

Temperature approach as low as 1°CHeat recovery as high as 93%Heat-transfer coefficient 3000-7000 W/m'*"C(water-water duties with normal fouling resistance)NTU 0.3-4.0Pressure drop 30 kPa per NTU

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High turbulence, absence of stagnant areas, uniform fluid flow, and the smooth plate surface reduce fouling and the need for frequent cleaning.

BENEFITS OFFERED BY PLATE HEAT EXCHANGERS

Cross-Contamination Eliminated

In PHE, each medium is individually gasketed. The space between gasket is vented to atmosphere, eliminating the possibility of any cross-contamination of fluids (Fig. 5).

Figure 5 Elimination of cross contamination by venting of gasket space.

True Counteflow

In PHE, fluids can be made to flow in opposite directions, resulting in greater effective temperature difference.

Close Approach Temperature In PHE, very close approach temperatures of 1-2°F are possible because of the true counterflow, advantageous flow rate characteristics, and high heattransfer efficiency of the plates.

Multiple Duties With a Single Unit. It is possible to heat or cool two or more fluids within the same unit by simply installing intermediate divider sections between the heat transfer plates (Fig. 6).

Expandable

Due to modular construction, true flexibility is unique to the PHE both in initial design and after installation.

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Figure 6 Handling of multiple duties.

Easy to Inspect and Clean, and Less Maintenance A PHE can be easily opened for inspection, cleaning, and gasket replacement. By simply removing the compression bolts and sliding away the movable end frame, one can visually inspect the entire heat-transfer surface. Easy cleanability and thin layer of product to ensure a good bacteriological effect during pasteurization are advantages in the dairy industry, where PHE are cleaned once a day.

Lightweight

The PHE unit is lighter in total weight than other types of heat exchangers because of reduced liquid volume space and less surface area for a given application.

High- Viscosity Applications, Because the PHE induces turbulence at low fluid velocities, it has practical application for high-viscosity fluids. Note that a fluid sufficiently viscous to produce laminar flow in a shell and tube heat exchanger may well be in turbulent flow in PHE.

Saves Space and Servicing Time

The PHE fits into an area one-fifth to one-half that required for a comparable shell and tube heat exchanger. The plate heat exchanger can be opened for inspection, maintenance, or rebuilding all within the length of the frame, while the shell and tube heat exchanger with removable tube bundle requires double its length to remove the tube bundle.

Less Operational Problems

In plate heat exchangers, flow-induced vibration, noise, and erosion- corrosion due to impingement attack are not present as in shell and tube heat exchangers.

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Lower Cost

Plate heat exchangers are generally more economical than other types of equivalent duty heat exchangers, due to the higher thermal efficiency and lower manufacturing costs of plate heat exchangers.

Quick Process Control

Owing to the thin channels created between the two adjacent plates, the volume of fluid contained in PHE is small; it quickly reacts to the new process condition and is thereby easier to control.

PLATE HEAT EXCHANGER-DETAILED CONSTRUCTION FEATURESWith a brief discussion of PHE construction, additional construction features and materials of construction for components such as plates, gaskets, frame, and connectors are discussed next.

PlatePlate Pattern

Plates are available in a variety of corrugated or embossed patterns. The basic objective of providing corrugation to the plates is to impart high turbulence to the fluids, which results in a very high heat-transfer coefficient compared to those obtainable in a shell and tube heat exchanger for similar duties. These embossing patterns also result in increased effective surface areas and provide additional strength to the plates by means of many contact points over the plates to withstand differential pressure that exists between the adjacent plates. Plate thickness as low as 0.6 mm (0.024 in) can therefore be used for working pressures as high as 230 psig, particularly when using the cross-corrugated, herringbone pattern.

Types of Plate Corrugation

Over 60 different plate patterns have been developed worldwide. The pattern and geometry are proprietary. The most widely used corrugation types are the intermating troughs or wash bourd and the chevron or herringbone pattern. Figure 6 shows both types. The construction features of these two patterns are discussed next.

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Intermating Troughs Pattern

In the intermating troughs or washboard pattern, the corrugations are usually pressed deeper than the plate spacing. The plates nestle into one another when the plate pack is assembled (Fig. 7a). The plate spacing is maintained by dimples, which are pressed into the crests and troughs and contact one another on adjacent plates. Since the washboard type has fewer contact points, and due to its greater corrugation depth than the herringbone type, it operates at lower pressures. The maximum channel gap, b (shown in Fig. 7a), varies from 3 to 5 mm and the minimum channel gap varies from 1.5 to 3 mm. Typical liquid velocity range in turbulent flow is from 0.2 to 3 m/s, depending upon the required pressure drop.

Figure 7 Two most widely used corrugation types. (a) Intermating troughs or washboard pattern; and

(b) chevron or herringbone pattern.

Chevron or Herringbone Trough Pattern

In the herringbone pattern, the corrugations are pressed to the same depth as the plate spacing.The chevron angle is reversed on adjacent plates so that when the plates are clamped together the corrugations cross one another to provide numerous contract points. The herringbone type therefore

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has greater strength than the washboard type, which enables it to withstand higher pressures with smaller plate thickness. It is the most common type in use today. The contact arrangements of the plates are shown in Fig. 7b. The corrugation depth generally varies from 3 to 5 mm. Typical liquid velocities (in turbulent flow) range from 0.1 to 1 m/s.

Plate MaterialsMaterials that are suitable for cold pressing and corrosion resistant are the standard materials of construction. Table 1 gives a list of most common materials used for fabrication of plates. Carbon steel is rarely used due to its poor corrosion resistance.

Figure 8 Cross-section of two neighboring plates (contact). (a) Intermating troughs; and (b) and (c)

chevron troughs.

Table 1 Common PHE Materials

Gasket Selection

When selecting the gasket material, the important requirements to be met are chemical and temperature resistance coupled with

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good sealing properties and shape over an acceptable period of life. Much work has been done to develop elastomer formulations that increase the temperature range and chemical resistance of gaskets. Typical gasket materials and their maximum operating temperature are given in Table 2.

FramesFrames are classified as (1) B frame, suspension-type frame used with larger PHEs, (2) C frame, compact cantilever-type frame used with smaller PHEs, and (3) F frame, an intermediate size suspension-type frame. The frame is usually constructed in carbon steel and painted for corrosion resistance. Where stringent cleanliness requirements apply, such as in pharmaceuticals and in dairy, food, and soft drinks industries, the frame may be supplied in stainless steel. Stainless-steel-clad frames are available for highly corrosive environments. The units are normally floor mounted, but small units may be wall mounted.

Table 2 Gasket Materials and Maximum Operating Temperature

Nozzles

Nozzles are located in one or both end plates. In single-pass arrangement, both the inlet and outlet ports for both fluids are located in the fixed head end, and hence, the unit may be opened up without disturbing the external piping. But with multipass arrangements the ports must always be located on both heads. This means that the unit cannot be opened up without disturbing the external piping at the movable head end. To be corrosion resistant, nozzles are usually same as plate material. Typical nozzle materials include stainless steel type 316, rubber clad, titanium, Incoloy 825, and Hastelloy C-276.

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Tie BoltsThe tie bolts are usually made of 1% Cr-0.5% MO low-alloy steel. The packing of large units may be compressed by hydraulic, pneumatic, or electric tightening devices.

Connector PlatesIt is possible to process three or more fluids in a single gasketed PHE. This is being achieved by means of connector plates. This practice is widely used in food processing and permits heating, cooling, and heat recovery of the fluids in a single unit

Air cooled Heat Exchangers Design

An Air Cooled Heat Exchanger is a heat transfer device for rejecting heat from a hot fluid directly to fan-blowing ambient air.

Air cooled heat exchangers are used for two primary reasons:

1. They increase plant efficiency 2. They are a "green" solution as compared to cooling towers

and shell and tube heat exchangers because they do not require an auxiliary water supply (water lost due to drift and evaporation, plus no water treatment chemicals are required).

Fields of application of air-cooled heat exchangers

Air coolers are installed throughout the world in the following applications:

• Forced and induced draft air cooled heat exchangers • Recirculation and shoe-box air cooled heat exchangers • Hydrocarbon process and steam condensers • Large engine radiators • Turbine lube oil coolers • Turbine intercoolers • Natural gas and vapor coolers • Combustion pre-heaters • Flue gas re-heaters • Lethal service • Unique customizations

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Figure 9 Typical components of air cooled heat exchangers

Air-Cooled Heat Exchangers

Air-cooled exchangers become especially attractive in locations where water is scarce or expensive to treat. Although an air-cooled unit requires a higher initial investment than its water-cooled counterpart, maintenance and operating costs are usually less.

The air-cooled units have axial-flow fans that force or induce a flow of ambient air across a bank of externally Finned tubes. A typical exchanger has a bank of finned tubing, a steel supporting structure with plenum chambers and fan rings, axial-flow fans, drive assemblies and miscellaneous accessories such as louvers, fan guards, fencing, hail screens and vibration switches.

Typical air-cooling systems are illustrated for forced-draft fans (Fig. 10) and induced-draft fans (Fig.11) In these systems, the air cooler consists of one or more rectangular bundles of finned tubes arranged in staggered rows and suitably supported on a steel structure. Both ends of the tubes are fixed in tubesheets in channels that have holes opposite the tubes, or removable covers, for tube rolling and cleaning. The channels are baffled to provide the desired number of tube passes. In some instances of high

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pressure, the channels may consist of drilled steel billets into which the tubes are rolled.

A fan below the tubes (Fig. 9, 10) forces air up through the bundle, or a fan located above (Fig. 10,11) draws the air through. These are axial propeller fans varying from 4 to 12 ft diameter. and having four to six blades, which may be of aluminum, plastic or, in the case of corrosive atmospheres, stainless steel. The drive can be an electric motor with gears or V-belts, or may be a steam turbine

The tubes are usually 1in. diameter, with wrapped on aluminum fins spaced from 8 to 16 per in. and varying from 3/8 in. to 5/8 in. high, and from 0.012 to 0.02 in. thick. These tubes are arranged in standard bundles ranging from 4 to 40 ft long and from 4 to 201 Wide.

The temperature of a process fluid passing through these coolers may be controlled by: 1) Louvers or shutters either manual or automatic; 2) variable speed steam turbine drives; 3) variable pitch fans (for accurate control); or 4) bypass control of the process fluid.

Three important design criteria affecting these systems are air data, type of draft, and type of fin.

Figure 10 Forced draft air-fin cooler

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Figure 11 Induced draft air fin cooler

The advantages and disadvantages of these types

In many cases the plate heat exchanger has replaced the shell and tube exchanger (STHE) for services within the latter’s operational limits. For identical duties (liquid-liquid service) and in those cases when the working limits of the gaskets are not exceeded, the merits of the PHE over the STHE are given in Table 3.

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Table 3 Comparison between PHE and Shell and Tube Heat Exchanger

Kettle Type Re-boiler (figure11)

Kettle reboilers consist of a bundle of tubes in an oversize shell. Submergence of the tubes is assured by an overflow weir, typically 5-15cm higher than the topmost tubes. An open tube bundle is preferred, with pitch to diameter ratios in the range of 1.5-2.

Temperature in the kettle is substantially uniform. Residence time is high so that kettles are not favored for thermally sensitive materials. The large shell diameters make kettles uneconomic for high pressure operation. Deentraining mesh pads often are incorporated. Tube bundles installed directly in the tower bottom are inexpensive but the amount of surface that can be installed is limited.

The K shell is used for partially vaporizing the shell fluid. It is used as a kettle reboiler in the process industry. Usually, it consists of a horizontal bundle of heated U tubes or floating head placed in an oversized shell. The tube bundle is free to move and it is removable. Its diameter is about 50-70% of the shell diameter.

The large empty space above the tube bundle acts as a vapor disengage space. The liquid to be vaporized enters at the bottom,

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near the tube sheet, and covers the tube bundle; the vapor occupies the upper space in the shell, and the dry vapor exits from the top nozzle(s), while a weir helps to maintain the liquid level over the tube bundle. The bottom nozzle in this space is used to drain the excess liquid.

Figure 11 Kettle Reboiler

CONDENSER TYPES

There are two primary types of condensers that can be used in a power plant:

1. Direct Contact2. Surface

Direct contact condensers condense the turbine exhaust steam by mixing it directly with cooling water. The older type Barometric and Jet-Type condensers operate on similar principles.

Surface Condenser

Steam surface condensers are the most commonly used condensers in modern power plants. The exhaust steam from the turbine flows on the shellside (under vacuum) of the condenser, while the plant’s circulating water flows in the tubeside. The source of the circulating water can be either a closed-loop (i.e. cooling tower, spray pond, etc.) or once through (i.e. from a lake, ocean, or river). The condensed steam from the turbine, called condensate,

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is collected in the bottom of the condenser, which is called a hotwell. The condensate is then pumped back to the steam generator to repeat the cycle.

Surface condenser is the commonly used term for a water cooled shell and tube heat exchanger installed on the exhaust steam from a steam turbine in thermal power stations. These condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric pressure. Where cooling water is in short supply, an air-cooled condenser is often used. An air-cooled condenser is however significantly more expensive and cannot achieve as low a steam turbine exhaust pressure as a surface condenser.

Surface condensers are also used in applications and industries other than the condensing of steam turbine exhaust in power plants

In thermal power plants, the primary purpose of a surface condenser is to condense the exhaust steam from a steam turbine to obtain maximum efficiency and also to convert the turbine exhaust steam into pure water (referred to as steam condensate) so that it may be reused in the steam generator or boiler as boiler feed water.

Why is it required?

The steam turbine itself is a device to convert the heat in steam to mechanical power. The difference between the heat of steam per unit weight at the inlet to the turbine and the heat of steam per unit weight at the outlet to the turbine represents the heat which is converted to mechanical power. Therefore, the more the conversion of heat per pound or kilogram of steam to mechanical power in the turbine, the better is its efficiency. By condensing the exhaust steam of a turbine at a pressure below atmospheric pressure, the steam pressure drop between the inlet and exhaust of the turbine is increased, which increases the amount heat available for conversion to mechanical power. Most of the heat liberated due to condensation of the exhaust steam is carried away by the cooling medium (water or air) used by the surface condenser.

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Diagram of water-cooled surface condenser

Figure (12) Diagram of a typical water-cooled surface condenser

The adjacent diagram depicts a typical water-cooled surface condenser as used in power stations to condense the exhaust steam from a steam turbine driving an electrical generator as well in other applications. There are many fabrication design variations depending on the manufacturer, the size of the steam turbine, and other site-specific conditions.

Shell

The shell is the condenser's outermost body and contains the heat exchanger tubes. The shell is fabricated from carbon steel plates and is stiffened as needed to provide rigidity for the shell. When required by the selected design, intermediate plates are installed to serve as baffle plates that provide the desired flow path of the condensing steam. The plates also provide support that help prevent sagging of long tube lengths.

At the bottom of the shell, where the condensate collects, an outlet is installed. In some designs, a sump (often referred to as the hotwell) is provided. Condensate is pumped from the outlet or the hotwell for reuse as boiler feedwater.

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For most water-cooled surface condensers, the shell is under vacuum during normal operating conditions.

Vacuum system

Figure (13) Diagram of a typical modern injector or ejector. For a steam ejector, the motive fluid is steam.

For water-cooled surface condensers, the shell's internal vacuum is most commonly supplied by and maintained by an external steam jet ejector system. Such an ejector system uses steam as the motive fluid to remove any non-condensible gases that may be present in the surface condenser. The Venturi effect, which is a particular case of Bernoulli's principle, applies to the operation of steam jet ejectors.

Motor driven mechanical vacuum pumps, such as liquid ring type vacuum pumps, are also popular for this service.

Tube sheets

At each end of the shell, a sheet of sufficient thickness usually made of stainless steel is provided, with holes for the tubes to be inserted and rolled. The inlet end of each tube is also bellmouthed for streamlined entry of water. This is to avoid eddies at the inlet of each tube giving rise to erosion, and to reduce flow friction. Some makers also recommend plastic inserts at the entry of tubes to avoid eddies eroding the inlet end. In smaller units some

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manufacturers use ferrules to seal the tube ends instead of rolling. To take care of length wise expansion of tubes some designs have expansion joint between the shell and the tube sheet allowing the latter to move longitudinally. In smaller units some sag is given to the tubes to take care of tube expansion with both end water boxes fixed rigidly to the shell.

Tubes

Generally the tubes are made of stainless steel, copper alloys such as brass or bronze, cupro nickel, or titanium depending on several selection criteria. The use of copper bearing alloys such as brass or cupro nickel is rare in new plants, due to environmental concerns of toxic copper alloys. Also depending on the steam cycle water treatment for the boiler, it may be desirable to avoid tube materials containing copper. Titanium condenser tubes are usually the best technical choice, however the use of titanium condenser tubes has been virtually eliminated by the sharp increases in the costs for this material. The tube lengths range to about 55 ft (17 m) for modern power plants, depending on the size of the condenser. The size chosen is based on transportability from the manufacturers’ site and ease of erection at the installation site. The outer diameter of condenser tubes typically ranges from 3/4 inch to 1-1/4 inch, based on condenser cooling water friction considerations and overall condenser size.

Waterboxes

The tube sheet at each end with tube ends rolled, for each end of the condenser is closed by a fabricated box cover known as a waterbox, with flanged connection to the tube sheet or condenser shell. The waterbox is usually provided with man holes on hinged covers to allow inspection and cleaning.

These waterboxes on inlet side will also have flanged connections for cooling water inlet butterfly valves, small vent pipe with hand valve for air venting at higher level, and hand operated drain valve at bottom to drain the waterbox for maintenance. Similarly on the outlet waterbox the cooling water connection will have large flanges, butterfly valves, vent connection also at higher level and drain connections at lower level. Similarly thermometer pockets are located at inlet and outlet pipes for local measurements of cooling water temperature.

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In smaller units, some manufacturers make the condenser shell as well as waterboxes of cast iron.

Figure (14)

STEAM SURFACE CONDENSER OPERATION

The main heat transfer mechanisms in a surface condenser are the condensing of saturated steam on the outside of the tubes and the heating of the circulating water inside the tubes. Thus for a given circulating water flow rate, the water inlet temperature to the condenser determines the operating pressure of the condenser. As this temperature is decreased, the condenser pressure will also decrease. As described above, this decrease in the pressure will increase the plant output and efficiency.

Due to the fact that a surface condenser operates under vacuum, non-condensable gases will migrate towards the condenser. The non-condensable gases consist of mostly air that has leaked into the cycle from components that are operating below atmospheric pressure (like the condenser). These gases can also result from caused by the decomposition of water into oxygen and hydrogen

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by thermal or chemical reactions. These gases must be vented from the condenser for the following reasons:

• The gases will increase the operating pressure of the condenser. Since the total pressure of the condenser will be the sum of partial pressures of the steam and the gases, as more gas is leaked into the system, the condenser pressure will rise. This rise in pressure will decrease the turbine output and efficiency.

• The gases will blanket the outer surface of the tubes. This will severely decrease the heat transfer of the steam to the circulating water. Again, the pressure in the condenser will increase.

• The corrosiveness of the condensate in the condenser increases as the oxygen content increases. Oxygen causes corrosion, mostly in the steam generator.

Thus, these gases must be removed in order to extend the life of cycle components.

STEAM SURFACE CONDENSER AIR REMOVAL

The two main devices that are used to vent the noncondensable gases are Steam Jet Air Ejectors and Liquid Ring Vacuum Pumps. Steam Jet Air Ejectors (SJAE) use high-pressure motive steam to evacuate the noncondensables from the condenser (Jet Pump). Liquid Ring Vacuum Pumps use a liquid compressant to compress the evacuated noncondensables and then discharges them to the atmosphere.

To aid in the removal of the noncondensable gases, condensers are equipped with an Air-Cooler section. The Air- Cooler section of the condenser consists of a quantity of tubes that are baffled to collect the non-condensables.

Cooling of the non-condensables reduces their volume and the required size of the air removal equipment.

Air removal equipment must operate in two modes: hogging and holding. Prior to admitting exhaust steam to a condenser, all the non-condensables must be vented from the condenser. In hogging

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mode, large volumes of air are quickly removed from the condenser in order to reduce the condenser pressure from atmospheric to a predetermined level. Once the desired pressure is achieved, the air removal system can be operated in holding mode to remove all non-condensable gases

Two Stage Steam EjectorsAir and water vapor are removed from the main steam condenser, enter the 1st stage ejector and are compressed to the interstage pressure by means of the high pressure motive steam. The load and motive steam are discharged to the inter condenser and a portion of the water vapor load and motive steam are condensed by means of cooling water or condensate from the main condenser. Non-condensibles and associated water vapor are removed from the inter condenser by the 2nd stage ejector, compressed to atmospheric pressure and are discharged through the after condenser. (Fig. 15)

Figure (15) Two stage ejector system

Two stage condensing ejector systems can be designed to operate at any condenser pressure and designs are not limited by the available cooling water temperature to the intercondenser (condensate cooled systems are common). These systems have no moving parts, are the most reliable, require the least maintenance of all venting systems, and are the least expensive in initial cost. Two stage ejector systems require a reliable steam source, generally 100-150 psig (690-1034 kPa) steam is used. Once equipment is built for a given motive steam pressure that

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pressure must be maintained or the ejector will become unstable and lose vacuum.

TWO STAGE LIQUID RING VACUUM PUMPS

The working parts of the liquid ring vacuum pump consist of a multi-bladed impeller mounted eccentrically in a round casing which is partly filled with the seal liquid, usually water. (Fig. 16) As the impeller rotates, the liquid is thrown by centrifugal force to form a liquid ring which is concentric with the periphery of the casing.

Figure 16 Vacuum pump Cross flow

Due to the eccentric position of the impeller relative to the casing and liquid ring, the spaces between the impeller blades fill with liquid during rotation and any air or gas trapped in the impeller space or cell is compressed and discharged from the casing through the outlet port. This leaves the cell available to receive air or gas as it is presented to the inlet port on the next revolution. A small portion of the seal water is discharged with the vapor, and a constant supply of fresh seal must be maintained (30-50 gal/mm (6.8-11.4 m3/h)).

In addition to being the compressing medium, the liquid ring absorbs the heat generated by compression and friction, absorbs any liquid slugs or vapor entering with the gas stream, and condenses water vapor entering with the gas.

The functions of the heat exchangers in the plant

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Should be Prepared by MOPCO

Procedure to Place Heat exchanger in Service

A heat exchanger is a pressure vessel designed for operation at certain specific limits of pressure and temperature, and the system must be safeguarded with safety valves and controls so that these design conditions are not exceeded and that all operating personnel are alerted.

When placing a unit in operation, open the vent connections and start to circulate the cold medium only. Be sure that the passages in the exchanger are entirely filled with the cold fluid before closing the vents. The hot medium should then be introduced gradually until all passages are filled with liquid. Then, close vents and slowly bring the unit up to temperature.

1- Start operation gradually. Do not admit hot fluid to the unit suddenly when it is empty or cold. Do not shock unit with cold fluid when it is hot.

2- In shutting down, flow of hot medium should be shut off first. If it is necessary to stop circulation of cooling medium, the circulation of hot medium should also be stopped by by-passing or otherwise.

3- Do not operate equipment under conditions in excess of those specified on nameplate.

4- In all installations, there should be no pulsation of fluids since this causes vibration and strain with resulting leaks.

5- All gasketed joints should be rechecked for tightness after the unit has been heated to prevent leaks and blowing out gaskets.

6- Units with packing rings may require adjustment from time to time to eliminate slight leakage. As joint containing packing rings requires only a small amount of bolting pressure to seal tight.

Many heat exchangers handle fluids which are irritating or dangerous to the human system and could cause problems if bolted and packed joints are not maintained in a leak tight condition both at operating pressures and temperatures, and also at no flow, ambient conditions.

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If fluid are not irritating or dangerous a leak will at least cause a slippery situation on the floor below.

Since one fluid in the exchanger is at a higher temperature, any leaks might cause burns.

If leakage should appear at the packed end joint after the cooler is placed in operation, the bolting should be pulled up only enough to stop it. This can be accomplished by taking a one-half turn on each successive bolt starting at one point and continuing around the cooler until all leakage has been eliminated. Do not tighten this joint any more than is required to stop the initial leakage.

When the packing has been repeatedly tightened to the point where there is almost a metal to metal contact between the bonnet (or channel) and the shell flanges, the two packing rings should be replaced.

Be sure that all parts of the system are clean and in proper operating condition. An exchanger cannot perform properly unless all connected equipment is functioning properly, yet, the exchanger is frequently blamed for non performance when the actual trouble is elsewhere in the system.

Observe the following precautions to obtain maximum performance:

A) Exchanger must be full of fluid in both shell and tube sides.B) Provide periodic venting if air tends to accumulate in systemC) Maintain rated flow of both mediums.D) Avoid excessive flow of cooling water in exchangers used as

coolers. It is a frequent cause of tube failure through erosion, and may decrease cooling efficiency, especially with heavy oils.

E) Inspect exchanger periodically and clean thoroughly when necessary, especially inside tubes.

Procedure to Take a Heat Exchanger out of Service

1. The hot fluid must be shut off before the cold fluid. This should be done slowly to allow the exchanger to cool down. The cold fluid must not be shut off first. Otherwise, the heat from the hot side will cause the cold fluid to increase in temperature and as

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there is no place for the expansion, the pressure would build up and cause exchanger ruptures.

2. After the hot fluid has been shut off, both on inlet and outlet of the exchangers and the temperature has cooled to that of the cold fluid, then the cold fluid can be shut off on both inlet and outlet valves.

3. Both shell and tube side should now be pumped out to slop or drained down.

4. Both inlet and outlet lines should be blanked off for safety.

5. If the exchanger is in sour oil service or any iron sulphide scale is expected, the exchanger should be water washed before opening to the atmosphere.

Normal operation and monitoring the performance of heat exchangers

Since the heat demands of the process are not constant, and the heat content of the two fluids is not constant either, the heat exchanger must be designed for the worst case and must be controlled to make it operate at the particular rate required by the process at every moment in time. The heat exchanger itself is not constant. Its characteristic changes with time. The most common change is a reduction in the heat transfer rate due to fouling of the surfaces. Exchangers are initially oversized to allow for the fouling which gradually builds up during use until the exchanger is no longer capable of performing its duty. Once it has been cleaned it is again oversized.

Where Do We Measure?

At the fundamental level, there is only one variable that can be controlled , the amount of heat being exchanged. In practical situations it is not possible to measure heat flux. It is always the temperature of one fluid or the other which is being measured and controlled. It is not possible to control both since the heat added from one is taken from the other. Therefore the first consideration is to specify the place at which the temperature is to be kept constant. This is usually within a piece of equipment somewhere downstream of the outlet of one of the fluids. Assuming there is not much temperature change along the piping, the measurement may be anywhere between the outlet itself and the point of interest,

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perhaps at the base of a distillation tower. In cases where the measurement is being made downstream of a bypass valve, the further downstream, the better the mixing will be, and the more representative the measurement. On the other hand, too far down-stream may result in process dead time that can make control difficult. In cases where the "other" fluid is the one being manipulated, it is often quite sufficient to make the measurement directly downstream of the outlet nozzle of the exchanger.

Which Stream Do We Manipulate?

The second consideration is which stream to manipulate. The complications arise from the fact that exchangers have four ports and involve two different fluids, either of which may change phase. The former feature alone allows eight different valve arrangements. The diagram assumes that it is the fluid on the shell side whose temperature is being controlled. As likely as not, it is the one on the tube side. It doesn't really make any difference to the control strategy. The real issue is which fluid is to be manipulated by the valves. For the sake of discussion we will term the two streams the "process" side and the "heat exchange medium" side. A complete tabulation of all the possibilities is:

a - Process side, outlet throttling.b - Process side, inlet throttling.c - Process side, bypass with outlet restriction.d - Process side, bypass with inlet restriction.e - Medium side, outlet throttling.f - Medium side, inlet throttling.g - Medium side, bypass with outlet restriction.h - Medium side, bypass with inlet restriction.

Figure 17 A Shell and Tube Heat Exchanger

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Throttling The Process Fluid. ?

It is quite meaningless to attempt to control the process temperature by throttling either the inlet or the outlet of the process fluid. The desired process flow rate is set by other requirements and these would be interfered with by manipulating the process flow. Temperature will change somewhat since flow reduction increases the residence time of the fluid and the outlet temperature will more closely approach the inlet temperature of the medium. On the other hand, variations in process flow, caused by some external influence, is one of the major causes of temperature variation. It is often the reason why we must manipulate some other parameter to maintain constant temperature.

Bypassing The Process Fluid.

Process temperature can be controlled by manipulating process flow if a bypass is installed. As the outlet temperature rises (assume this is a heater), more fluid is bypassed around the exchanger without being heated. As the two streams are blended together again, the correct temperature is achieved.

Split Range

Bypass manipulation sounds simple but there are a few tricks to it. Firstly, there are two ways of arranging the valve controls: We can attempt to minimize pressure drop at all times, or we can attempt to keep the pressure drop constant. In neither case do we want to interrupt the total flow. If we wish to minimize pressure drop, a butterfly valve is the likeliest choice. However, even a wide open butterfly has some pressure drop. It may be greater than that of the heat exchanger itself. This means that even when the valve is wide open only half, or less, will bypass the exchanger. To accomplish a greater degree of bypass, a restriction must be placed on the flow through the exchanger.

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Figure 18

Process Side Bypass with Restriction on the Outlet

The restriction should be adjustable since conditions change and we do not want more restriction than necessary. The easiest way to do this is with a hand valve. Since these valves are often in relatively inaccessible places, remote actuators may be added. Once that is done it becomes an obvious matter to arrange automatic controls so that once the bypass is fully open, the restriction valve starts to close, and vice versa.

Medium Side Throttling

Avoid using a process side bypass valve with fluids that are being heated and have a tendency to break down or scorch. These include many food products and also petroleum products or other chemicals that may polymerize or coke at high temperatures. The problem is that the outlet temperature is a blend of the bypass stream and the stream through the heater. The peak temperature to which any part of the stream is exposed may considerably exceed that of the combined outlet. Over-done and half-baked don't average out! The extreme case is when the exchanger is on full bypass. The fluid trapped inside the heater will then be at the temperature of the heating medium.The solution is to control the process temperature by throttling the heat exchange medium. In this case the heat available to the process is manipulated.

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Figure 19

Medium Flow Tube Side Outlet Throttling

The example is a simple and rather straightforward application. Hot oil is being supplied to heat a process stream. It is desired to keep the process stream at a constant temperature. There is no reason to maintain the flow of oil in excess of what is needed , it can be throttled to control the temperature. In this case the valve is placed on the outlet of the exchanger. The valve is not expected to handle a large pressure drop nor is tight shutoff of any particular value. Therefore a butterfly valve is quite acceptable. Furthermore its low pressure drop (high C,) when wide open is an advantage.

The effects of inlet and outlet throttling are about the same, so secondary considerations come into play. In general it is a good idea to keep the pressure on a hot fluid to reduce any chance of dissolved gases bubbling out. A valve on the cooler end may be cheaper and will probably last longer. Leaks are less likely as the fluid will be more viscous than on the hot side. Thus it is best to throttle a heating medium on the outlet side.It is rare that a heat exchanger cooling the process should have a reason not to use bypass control on the process side. Also, it is usually undesirable to throttle cooling water since it is at least mildly corrosive and is seldom clean. For this reason it is usually put through the tubes. In order to improve heat exchange and also to avoid the build-up of deposits and fouling, it is best to maintain its velocity.If it should be necessary to throttle the cooling medium, consideration must be given to the possibility of boiling. (Remember that the cooling medium is the one being heated.) Assuming that boiling is not intended, but that the possibility exists, the valve should be placed on the outlet in order to maintain pressure on the fluid. In other words, inlet throttling is rarely used with single phase fluids.

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Cross Exchangers.

When the heat in one process stream is to be exchanged with another process stream, the flow on neither side may be interfered with while controlling the temperature. An example is when distillation tower bottoms are cross exchanged with the tower feed. The tower requires a high temperature at the bottom in order to function but the heat is not "consumed" by the process nor is it needed in the product. It is returned from the bottom product back to the feed. This is a common and extremely effective energy conservation measure.

As with all energy recovery arrangements, the key to success is to control the heat recovery without disturbing the process. That is, the flow of neither of the two process streams may be interfered with. The solution is to manipulate the heat transfer by bypassing one of the two streams around the exchanger. Most often control is exercised on the tube side. The failure modes of the valves are chosen to prevent overheating and flow blockage.

Uncontrolled Heat Exchange. In some cross exchange applications it is desired to recover all the heat (or cold) content of the product stream and to transfer it to the feed. In such cases the exchanger needs no controls at all. The feed stream usually has a second exchanger downstream of the first to boost the temperature to the required level. This exchanger is the one that is manipulated.

Aerial Coolers.

As mentioned earlier, aerial coolers can be considered a special type of shell and tube exchanger in which the shell is the shell of the cooler. A large fan is used to blow air, usually from below, past the tubes. As with other exchangers it is possible to control the temperature by manipulating the process or the medium flows. The normal way to provide accurate temperature control is to use process flow bypass valves. In addition there are three means of manipulating the medium: Louver or damper control, fan pitch control and variable speed.

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Fan Pitch Control.

This is an obvious means of controlling the temperature. It has the advantage of reducing horsepower as the cooling demand is reduced. As with every control technique, there are limitations. Firstly, the turndown is rather poor. This is especially important in a northern climate where it may not be possible to turn down the fans sufficiently in winter. The spinning blades still stir up the air even when the pitch is zero. Natural draft alone may provide more cooling than is required. Secondly, the pitch control mechanism can be a maintenance headache.

Variable Speed.

Fully variable fan speed control is becoming more common on aerial coolers. The fan motors are often quite numerous but not extremely large. However, the cost of the electronics has come down considerably in recent years. On the other hand, two-speed fans have always been quite common. This is especially true in climates with extreme seasonal temperature swings. Reducing the fan to half speed results in an 85% reduction in electric power demand. (Remember that electrical power varies as the cube of fan speed.) Cutting the speed of an electric motor in half requires only a reconnection of the wiring to a multipole stator. This can be accomplished by having two electrical starters wired in different ways. The increase in cost is not very large.A variation of the two-speed motor is to arrange for reverse flow. This can be extremely useful in climates where icing is a problem. Reversing the flow blows warm air through the inlet louvers and serves to melt any accumulated ice. This should be done before ice build-up is too large or large chunks of ice may be sent crashing down onto other pieces of equipment or even personnel.

Louver Control.

Automatic louver control has similar problems as fan pitch control. There is an additional problem of hysteresis. The louvers seldom move smoothly for long and it becomes very difficult to maintain stable control as dirt and wear accumulate over time, especially in sandy or dusty environments.

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In climates with strong seasonal temperature swings, it is possible to stabilize the air temperature by controlling internal recirculation. The exchanger is fitted with a duct leading from the top outlet to the bottom inlet of the unit. Dampers are placed in this duct and at the air intake. A temperature controller senses the air above the fan and controls it by opening the recirculation duct and simultaneously closing the intake. A possible arrangement using both outlet louver control from the process and recirculation control off the internal air temperature is presented in the following figure.

Figure 20 Aerial Cooler

Louver Controls

Feed forward. Large heat exchangers have both dead time and considerable thermal inertia. These two factors can make control difficult. Feed forward can be usefully applied if load changes are a problem. Since the heat demand is proportional to the process flow rate, other things being equal, a flow rate measurement can be used. If the exchanger is very large, it may be necessary to insert a lag or other form of delay into the flow signal to prevent it from acting too soon and causing a reverse spike to appear in the temperature. Note that the dead time is inversely proportional to the flow rate and some "typical" value must be used.

Figure 21 Feed Forward Control

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Combination Control.

Sometimes a heat exchanger is used to heat, or cool, a fluid whose total flow is being controlled by some other parameter. The most straightforward way of controlling this is to use a three-way valve, or two butterflies, to control the heat exchange and to use another valve to control the total flow. The flow control valve must be on the common line either upstream or downstream of the exchanger. This arrangement has two valves in series and cries out for a way of eliminating one of them. If the positions of the inlet and bypass valves are controlled separately so that the total Cv is controlled by the Flow Controller and the difference between the C„s is controlled by the Temperature Controller, complete control can be achieved with only two valves. The example uses boiler feedwater to cool a sulphur condenser at the same time the water is being preheated. The sulphur vapour, being the more difficult fluid, is in the tubes. The water is in the shell. Since we want to make certain that the water does not boil, we will put the valves on the outlet side. The valve controlling the outlet of the exchanger receives a signal equal to half the sum of the two controller outputs. The valve controlling the bypass receives half of the difference between the two controller outputs. Assuming that the installed characteristic of both valves is linear, the combined flow of the two valves is then dependent entirely on the Flow Controller. The difference between the two flows is dependent on the Temperature Controller. In this particular situation it is desirable that both valves are fail open. If the failure mode of either, or both, valves is fail closed, the signs of the summing/scalers UY-A and UY-B will have to be changed to give the proper result.

Figure 22 Temperature and Flow control

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Equipment Protection

Severe corrosion is sometimes a problem. If so, corrosion detection devices may be installed. These consist of a thin wire or film of the same material as the exchanger. The wire is held in a holder that is inserted through a nozzle into the exchanger. Two electrical contacts are accessible from the outside. When the resistance is measured, the extent of corrosion can be determined directly. These devices are not normally connected into a data logging network. The usual practice is to make the measurements with a portable monitor on a regular basis. Intrinsically safe monitors are available for hazardous locations.

Safety

Overpressure is the only common safety issue affecting shell and tube heat exchangers. Pressure relief must be provided for both the shell and tube sides. If the source of overpressure is from upstream, the relief valve for that stream is best placed on the inlet. Otherwise it does not matter much whether it is on the inlet or outlet so long as they are inside any control or isolation valves. It is not sufficient to put minimum stops on the valves as these are easily altered. Even if the stops are welded in place, the valve may be replaced at some future date and the modification forgotten. If careful analysis shows that there are no process, fire, or failure conditions that could possibly require relief valves, it is still strongly advised to install thermal relief's on both sides of any exchanger that is capable of being blocked in. It may be argued that the fluid is gas or that the process is not capable of adding heat to the blocked in exchanger. This argument overlooks the various unanticipated conditions that may arise during testing and maintenance. A worst case scenario: A cooler was taken out of service and steam cleaned. No one had drained the cooling water which expanded in the tubes and ruptured the joints. True, good maintenance practice would have prevented this incident. But then an 3/4 relief valve would have provided a permanent solution and would have cost a lot less than the damage caused by its absence.

Parallel Heat Exchangers

Aerial coolers may be viewed as a number of coolers in parallel. A single thermometer at the inlet is sufficient but a separate one at each outlet to the header is essential. There is absolutely no other

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way to identify individual plugged or fouled sections without taking the whole thing apart.Heat exchangers in parallel do not share the flow equally. Symmetrical piping is like the perfect life: at best a pious intention. The problem is that an initial flow imbalance can grow. If a reduction in flow causes fouling which further restricts the flow, a positive feed-back loop is set up which can cut one exchanger entirely out of circulation. Some way must be found to force the flows to balance. Unfortunately, it is meaningless to attempt to control any variable without measurement, and flow measurement is expensive. Not only are the instruments expensive in terms of installed cost and maintenance, the required piping arrangement is also expensive. Thus automatic flow balancing is rarely installed except in extremely critical service such as a furnace with multiple tube passes.

If a bank of parallel exchangers is controlled using a bypass it is only meaningful to have a single bypass for the entire bank. If the temperature of stream A is being controlled by throttling stream B, every exchanger must have its own valve or imbalance is sure to result. In fact, a separate control loop on every exchanger is probably a good idea. Care must be taken in the location of the temperature sensors so that each senses only the contribution of the exchanger it is controlling.

Fouling and parameters affecting the fouling formation

Fouling

Fouling is defined as the formation on heat transfer surfaces of undesired deposits, which impede the heat transfer and increase the resistance to fluid flow, resulting in higher pressure drop. Industrial heat exchangers rarely operate with nonfouling fluids. Low-temperature cryogenic heat exchangers are perhaps the only exception. The growth of the deposits causes the thermohydraulic performance of heat exchanger to degrade with time. Fouling affects the energy consumption of industrial processes, and it can also decide the amount of extra material required to provide extra heat-transfer surface employed in heat exchangers to compensate for the effects of fouling. In addition, where the heat flux is high, as in steam generators, fouling can lead to local hot spots and ultimately it may result in mechanical failure of the heat-transfer surface. The progression of fouling with time and its effect on

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thermal performance are shown schematically in Fig. 23 give general and specific information on fouling.

Figure 23 Effect of fouling.

EFFECT OF FOULING ON THE THERMOHYDRAULIC PERFORMANCE OF HEAT EXCHANGERS

Effects can include:

1. The fouling layers on the inside and the outside surfaces are known generally to increase with time as the heat exchanger is operated. Since the fouling layers normally have lower thermal conductivity than the fluids or the conduction wall, they increase the overall thermal resistance.2- There is an increase of the surface roughness, thus increasing frictional resistance to flow, and fouling blocks flow passages; due to these effects, the pressure drop across the heat exchanger increases.

3. Fouling may create a localized environment where corrosion is promoted.

4. Fouling will reduce the thermal effectiveness of heat exchangers, which in turn affect the subsequent processes or will increase the thermal load on the system.

5.An additional goal become prevention of contamination of a process fluid or product.

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COSTS OF HEAT EXCHANGER FOULING

Fouling of heat exchangers has been estimated to represent an annual expense in the United States of somewhere between $4.2 and $10 billion. Fouling is something that is unwanted and counterproductive. The presence of fouling on heat exchange surfaces causes additional costs due to the following reasons:

1. Increased capital expenditure due to oversizing.2. Energy losses associated with poor performance of the

equipment.3. Treatment cost to lessen corrosion and fouling.4. Lost production due to maintenance schedules.Economic considerations should be among the most influential parameters for determining appropriate allowances for fouling. It is important to determine a strategy as to whether first cost, operating and maintenance cost, or total cost over a period of years is the objective.

OversizingWhile sizing a heat exchanger it is a normal practice to oversize the heat-transfer surface area to account for fouling, and the oversizing is normally of the order of 2040%.

Additional Energy CostsSince fouling reduces the heat-transfer rates, additional energy is expended to increase the heat-transfer rate; fouling also increases the pressure drop across the core and hence more pumping power is required to meet the heat load.

Treatment Cost to Lessen Corrosion and Fouling

The formation of fouling deposits on heat-transfer surfaces necessitates periodical cleaning, which costs money for cleaning materials and process, personnel required, and recently environmental problems to discharge the effluents.

Lost Production Due to Maintenance Schedules and

Down Time for MaintenancePeriodical cleaning requires plant shutdown and hence the unavailability of system for productive purposes. Critical industries that cannot afford for downtime and loss in production will maintain

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standby units, which again raises additional capital costs for spares.

PARAMETERS THAT INFLUENCE FOULING RESISTANCESMany operational and design variables have been identified as having most pronounced and well-defined effects on fouling. These variables are reviewed here in principle to clarify the fouling problems and because the designer has an influence on their modification. Those parameters include the following:

1. Properties of fluids and their propensity for fouling2. Surface temperature3. Velocity and hydrodynamic effects4. Tube material5. Fluid purity and freedom from contamination6. Surface roughness7. Suspended solids8. Placing the more fouling fluid on the tube side9. Shell-side flow10. Type of heat exchanger11. Heat exchanger geometry and orientation12. Equipment design13. Seasonal temperate changes14. Heat-transfer processes like sensible heating, cooling,

condensation, vaporization, etc.

Properties of Fluids and Usual Propensity for Fouling

The most important consideration is the fluid and the conditions conducive for fouling. At times a process modification can result in conditions that are less likely to cause fouling.

Temperature

A good practical rule to follow is to expect more fouling as the temperature rises. This is due to a “baking on” effect, scaling tendencies, increased corrosion rate, faster reactions, crystal formation and polymerization, and loss in activity by some antifoulants. Lower temperatures produce slower fouling buildup, and usually deposits that are easily removable. However, for some process fluids, low surface temperature promotes crystallization and solidification fouling. For those applications, it is better to use

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an optimum surface temperature to overcome these problems. For cooling water with a potential to scaling, the desired maximum surface temperature is about 140°F (60°C). Biological fouling is a strong function of temperature. At higher temperatures, chemical and enzyme reactions proceed at a higher rate with a consequent increase in cell growth rate. For any biological organism, there is a temperature below which reproduction and growth rate are arrested and a temperature above which the organism becomes damaged or killed. If, however, the temperature rises to an even higher level, some heat-sensitive cells may die.

Velocity and Hydrodynamic Effects

Hydrodynamic effects such as flow velocity and shear stress at the surface influence fouling. Within the pressure drop considerations, the higher the velocity, higher will be the thermal performance of the exchanger and less will be the fouling. Uniform and constant flow of process fluids past the heat exchanger favors less fouling. Foulants suspended in the process fluids will deposit in low-velocity regions, particularly where the velocity changes quickly, as in heat exchanger water boxes and on the shell side.Higher shear stress promotes dislodging of deposits from surfaces. Maintain relatively uniform velocities across the heat exchanger to reduce the incidence of sedimentation and accumulation of deposits.

Tube Material

The selection of tube material is significant to deal with corrosion fouling.

Carbon steel is corrosive but least expensive.

Copper exhibits biocidal effects in water. However, its use is limited in certain applications:

(1) Copper is attacked by biological organisms including sulfate-reducing bacteria; this increases fouling. (2) Copper alloys are prohibited in high-pressure steam power plant heat exchangers, since the corrosion deposits of copper alloys are transported and deposited in high-pressure steam generators and subsequently block the turbine blades. (3) Environmental protection limits the use of copper in river, lake, and ocean waters, since copper is poisonous to aquatic life.

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Noncorrosive materials such as titanium and nickel will prevent corrosion, but they are expensive and have no biocidal effects.

Glass, graphite, and Teflon tubes often resist fouling and/or improve cleaning.

Although the construction material is more important to resist fouling, surface treatment by plastics, vitreous enamel, glass, and some polymers will minimize the accumulation of deposits.

Impurities

Seldom are fluids pure. Intrusion of minute amounts of impurities can initiate or substantially increase fouling. They can either deposit as a fouling layer or acts as catalysts to the fouling processes. For example, chemical reaction fouling or polymerization of refinery hydrocarbon streams is due to oxygen ingress and/or trace elements such as Va and MO.In crystallization fouling, the presence of small particles of impurities may initiate the deposition process by seeding. The properties of the impurities form the basis of many antifoulant chemicals. Sometimes impurities such as sand or other suspended particles in a cooling water may have a scouring action, which will reduce or remove deposits.

Surface Roughness

The surface roughness is supposed to have the following effects:

1. The provision of “nucleation sites” that encourage the laying down of the initial deposits.2. The creation of turbulence effects within the flowing fluid and, probably, instabilities in the viscous sublayer.Better surface finish has been shown to influence the delay of fouling and ease cleaning.

Similarly, nonwetting surfaces delay fouling. Rough surfaces encourage particulate deposition. After the initiation of fouling, the persistence of the roughness effects will be more a function of the deposit itself. Even smooth tubes may become rough in due course due to scale formation, formation of corrosion products, or erosion.

Suspended Solids

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Suspended solids promote particulate fouling by sedimentation or settling under gravitation onto the heat-transfer surfaces. Since particulate fouling is velocity dependent, prevention is achieved if stagnant areas are avoided. For water, high velocities (above 1 m/s) help prevent particulate fouling. Often it is economical to install an upstream filtration.

Placing the More Fouling Fluid on the Tube Side

As a general guideline, the fouling fluid is preferably placed on the tube side for ease of cleaning. Also, there is less probability for low-velocity or stagnant regions on the tube side.

Shell-Side Flow

Velocities are generally lower on the shell side than on the tube side, less uniform throughout the bundle, and limited by flow-induced vibration. Zero-or low-velocity regions on the shell side serve as ideal locations for the accumulation of foulants. If fouling is expected on the shell side, then attention should be paid to the selection of baffle design. Segmental baffles have the tendency for poor flow distribution if spacing or baffle cut ratio is not in correct proportions. Too low or too high a ratio results in an unfavorable flow regime that favors fouling (Fig. 24).

Figure 24 Effect of baffle spacing and cut on fouling. Top: Moderate baffle spacing and baffle cut; bottom, wide baffle

spacing and large baffle cut. Note: dark areas represent stagnant areas with heavy fouling.

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MATERIAL SELECTION PRINCIPLES

In engineering practice the selection of materials of construction depends on the equipment being designed and the service requirements imposed on them. Selection of materials involves the thorough understanding of their availability, source, lead time, product forms, and size. In simple terms, the following factors are to be considered while selecting the materials for heat exchanger and pressure vessel components:

1. Compatibility of the materials with the process fluids.2. Compatibility of the materials with the other component

materials.3. Ease of manufacture and fabrication by using standard methods

like machining, rolling, forging, forming, and metal joining methods such as welding, brazing, and soldering.

4. Strength and ability to withstand operating temperature and pressure.

5. cost.6. Availability.While selecting materials one has to balance many requirements; these requirements include the following:

Expected total life of plant or process.Expected service life of the material.Reliability (safety, hazard, and economic consequences of failure).Material costs.Fabrication costs.Maintenance and inspection costs.Availability in required size, shape, thickness, and so on, and delivery time.Return on investment.

Materials are selected based on past experience, corrosion tests, the literature, and the recommendations of material suppliers. The direct measure of success in materials selection and fabrication is reflected in the behavior of the equipment in service. In order to assure safe, reliable and uninterrupted service with monetary benefits, proper attention must be given to materials selection commencing from the design stage and must continue through fabrication, installation, and maintenance. Once onstream, the

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equipment should be monitored for specified performance. With this background information, the logical sequence of material selection for the construction of pressure vessels and heat exchangers is discussed next.

Materials SelectionWhile selecting materials for the construction of heat exchangers and pressure vessels, the following points may be considered for the desired performance and the life of the equipment

P I :1. Review of operating process.2. Review of design.3. Selection of material.4. Evaluation of material.5. Specification.Finally the material should be cost-effective. Do cost-benefit analysis for an optimum material selection.

Review of Operating Process

The foremost step in the material selection is the thorough review of the process environment and equipment operating conditions like operating temperature, pressure, and phases of the fluids. The following operating data are required by the design engineer:

1. Environment-nature and composition of fluid handled, water chemistry, steam quality, constituents concentration of a solution, conductivity, pH, aeration, impurities, and so on,

2. Pressure-average and range, constant or cyclic, internal and external loadings.

3. Temperature-average and range, constant or variable, thermal gradients, and thermal shock.

4. Velocity-flow rate, linear velocity, nominal and range, degree of agitation, turbulence, etc.

In addition to operating temperature and pressure, other factors such as the startups and shutdowns, intermittent operation, transients and pressure surges, and momentary failure of the system must be considered for the satisfactory performance of a material.

Review of Design

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After the review of the operating process, the type or design of the equipment and various assemblies and subassemblies, size, complexity, and criticality in service should be considered. Applicability of various fabrication techniques like forging, machining, welding, soldering, brazing, etc., is considered.

Selection of Material

The selection of proper materials for wetted and nonwetted parts, pressure and non-pressure parts, and equipment support is an important design step. Material Standards like ASTM, DIN, BS, ISO, and JIS give data on a large number of ferrous and nonferrous materials. Many of these materials have been adopted in the Pressure Vessel Codes by the country of origin in the Material Standards. Data from these material standards are generally restricted to the physical properties, such as Young’s modulus, yield strength, minimum tensile strength, maximum permissible operating temperature, elongation, coefficient of expansion, etc. But codes and standards do not explicitly express the suitability of the materials for the intended service environment, either internal or external. These details must be provided by the corrosion engineer or many sources are to be referred.

Corrosion

There are two main reasons for concern about and study of corrosion: (1) economics and (2) conservation of materials. Of these, the economic factors mostly favor study and research into the mechanisms of corrosion and the means of controlling corrosion. Economic reasons for corrosion study include:

1. Loss of efficiency: Corrosion can result in the build up of corrosion products and scale, which can cause a reduction in heat transfer as well as an increase in the power required to pump the fluid through the system.

2. Loss of product due to leakage: High fuel and energy costs as a result of leakage of steam, fuel, water, compressed air, or process fluid that absorbed energy.

3. Possible impact on the environment: If the leaking fluid is corrosive in nature, it will attack its surroundings, and if lethal or poisonous, it will create hazards and environmental problems.

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Discharge of copper and chromate-treated water is severely regulated to conserve aquatics and biosphere.

4. Lost production as a result of a failure.

5. High maintenance costs.

6. Warranty claims on corroded equipment and the consequent loss of customer confidence, sales, and reputation.

7. Contamination and loss of product quality, which can be detrimental to the product, such as foodstuffs, soap products, discoloration with dyes, etc.

8. Extra working capital to carry out maintenance operations and to stock spares to replace corroded components.

9. Overdesign: In many instances, when the corrosive effect of a system is known, additional thickness to components is provided for in the design. This is known as corrosion allowance and involves additional material cost and extra weight of new units.

10. Highly corrosive fluids may require the use of expensive materials such as titanium, nickel-base alloys, zirconium, tantalum, copper-nickels, etc. The use of these materials contributes to increased capital cost.

11. Damage to adjacent equipment and the system components.

Pressure drop and performance of heat exchangers

The term “pressure drop” refers to the pressure loss that is not recoverable in the circuit. The determination of pressure drop in a heat exchanger is essential for many applications for at least two reasons:1. The operating cost of a heat exchanger is primarily the cost of the power to run fluid moving devices such as pumps, fans, and blowers.2. The heat-transfer rate can be significantly influenced by the saturation temperature change for a condensing/evaporating fluid for a large pressure drop.

The pressure drop associated with a heat exchanger may be considered as having two major components:(1) pressure drop associated with the core or matrix, and (2) pressure drop in inlet and outlet headers, manifolds, nozzles, or ducting due to change in flow area, flow turning, etc. In this

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section, core pressure drop for extended surface exchangers, regenerators, and tubular exchangers is presented, followed by the pressure drop associated with bends and flow turnings.

Analysis of the operation problems

Generally, most problems arise from improper operation of the heat transfer system. Six specific causes are most often at the root of the problem.

One common cause is using a heat transfer fluid well beyond its condemning limits. If the fluid's saturation level for products of oxidation is exceeded, continued use of the fluid leads to carbonaceous sludge deposits. Implementing a fluid monitoring program will identify the timing for fluid change-out and prevent the fluid from being used beyond these condemning limits.

A second common cause is subjecting the fluid to temperatures that exceed the recommended maximums. If the fluid's flow rate is inadequate, its residence time in the heat source may be excessive. Or, if the system is operating correctly but the fluid is being heated beyond its recommended maximum, it may be inappropriate for the application. Simulated distillation of a sample of the system fluid using the gas chromatographic distillation (ASTM D 2887) method can identify any increased amounts of low and high boilers, relative to fresh fluid.

Another source of problems is inadequate cleaning. If sludge and degraded fluid remain in the system after cleaning, they can cross-contaminate the fresh fluid and shorten its life. Prior to cleaning, the system fluid should be analyzed to identify the extent of cleaning required. Also, the new fluid should be analyzed within one to two days after startup to establish a baseline for the newly charged system fluid and identify any cross-contamination.

Another common cause is extraneous contamination. Most often, extraneous contamination occurs when the wrong product is added during system fluid top-off, or when fluid from the process side leaks into the heat transfer system. The fluid should be analyzed to determine the extent of contamination and its impact on the fluid's heat transfer properties such as viscosity and the potential for solids deposit. If an adverse impact on system performance is likely, the fluid should be replaced.

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A fifth common cause of problems is water used during cleaning that is not removed during startup. When water is present in the system, rapidly heating the fluid to operating conditions can result in the expulsion of hot fluid from the expansion tank as the water expands during its conversion to steam. The proper procedure for system startup must include slow initial heating of the fluid to just above the boiling point of water. Then, the fluid should be circulated through the expansion tank until all water and vapors have been vented.

The final cause of problems is inadequate system or equipment design, or poor unit operation. To eliminate these causes, design and operating procedures must be changed to improve system operation and on-stream reliability.

Troubleshooting of the plant heat exchangers

To effectively troubleshoot poorly performing heat transfer systems, follow these steps:

• Clearly define the problem based on observations and accumulated information.

• Review available historical system operation data and fluid condition analyses.

• Identify and obtain any additional information and analysis that may be required.

• Identify and list potential root causes and consider each. • Deduce the root cause based on the accumulated information. • Execute corrective action.

Once the problem is corrected, the necessary changes in system design or operating procedures should be implemented to prevent a reoccurrence.

Troubleshooting can be made easier if the system's operating parameters and the fluid's condition are monitored and analyzed routinely. In addition, the following equipment and materials should be available to facilitate troubleshooting:

• Sufficient and properly located temperature and pressure gauges to monitor the fluid at the inlet and outlet of each heat-user and the heat source.

• An up-to-date process and instrument drawing of the system.

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• Historical operating and fluid analysis data. • Equipment specification sheets and fabrication drawings. • Process and mechanical design information about the heat

transfer system and users. • System operating data from the period prior to and during the

problem. This can be compared with normal and historical data. • Fluid analysis during the problem. • Documentation of previous system problems

The following conditions develop, it is important to evaluate the possible cause and implement corrective action.

Insufficient Heat At User

If users on the system are unable to get the required amount of heat, possible causes may be:

Fouled Heat Transfer Surfaces

Fouled heat transfer surfaces at the user are caused by deposits of resinous carbonaceous material. This can develop if the fluid is thermally cracked to form heavy boilers. Another possible cause is fluid oxidation that leads to the products of oxidation polymerizing and depositing on the heat exchange surface.

Fluid analysis for TAN, viscosity, solids and GCD readings can quickly identify if the fluid has been oxidized or thermally degraded. An oxidized fluid will have a TAN greater than one and exhibit increased viscosity and solids. A thermally degraded fluid exhibits reduced viscosity and GCD 10% point, but increased 90% point and solids content.

Checking the historical operating conditions around the fouled equipment can act as a cross-check to confirm fouling. If fluid flow to the user is constant but the differential temperature across the user is reduced, this indicates that the transfer surface is fouled.

Low Fluid Flow

Low fluid flow, due to partially plugged lines or filters as well as pump-related problems, also can lead to insufficient heat at the user. Checking the pressure differential across the pertinent piping or equipment sections will identify any blockages.

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Low Fluid Temperature from the Heat Source

Low fluid temperature from heat source often is caused by progressive fouling of the tubes or the heat source's electrical element. Other possible causes are reduced gas firing/watt setting or reduced fluid residence time in the heat source. Fouling in the heat source can be determined from a fluid analysis along with a review of the differential temperature across the source. The other possible causes can be identified by inspecting relevant temperature and flow controllers as well as monitoring devices.

If fouling is the cause, cleaning the heat source's heat exchange surface is the only way to restore system efficiency. If fluid oxidation has caused the fouling, the fluid must be changed. It is important to identify the cause of fouling: If the fluid is thermally degraded, for example, increasing the heat source's firing or watt setting to get more heat to the user will only compound the problem. Low fluid velocity or increased residence time in the heat source leads to heat transfer areas with high heat flux, and correspondingly high film temperatures, that result in thermal cracking and, ultimately, coke deposits.

Increased Viscosity

Caused by the continued use of an oxidized fluid, increased fluid viscosity also can result in insufficient heat to the user. An excessive buildup of the products of oxidation causes the fluid to thicken, and the higher viscosity fluid is less efficient at transferring heat. A 50% increase in a heat transfer fluid's viscosity results in approximately 20% reduction in the fluid's film coefficient. Inadvertently contaminating system fluid with a higher viscosity material also will increase its viscosity.

High Fluid Losses/Make-Up Rate

If the system constantly requires fluid additions, possible

causes are:

Vapor Leaks from System

Heat transfer fluids that exhibit high vapor pressure (chemical aromatics, for example) tend to leak from connections and fittings. If

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the system's operating pressure is not set above the fluid's vapor pressure, fluid vapors will vent continuously from the expansion tank, requiring frequent top-offs to maintain fluid levels. Low vapor pressure fluids are preferred for liquid-phase systems because they can be operated with essentially no pressure and minimum venting of vapors.

Leaks from Fittings and Connections

At the system operating temperature, metal fittings and connections can expand, causing leaks. Another common cause of leaks is using a seal material that is incompatible with the system fluid. To prevent leaks, flanges and connections should be tightened while the system is at the operating temperature. If a systems is frequently shut down and restarted, it is more likely to leak. Where flanged connections are necessary, a 300 lb raised face flange should be used to improve sealing and minimize leaks. If the seals are incompatible with the fluid or (in the case of mechanical seals) if there is insufficient cooling/ flushing, leaks will occur frequently.

Thermal Degradation and Venting of Lighter Components

If the fluid being used is not thermally suited for the application, it will thermally degrade. The low boilers produced will be vented via the expansion tank. This problem is compounded if the expansion tank is operated at elevated temperatures.

Thermal stability at the bulk operating temperature is an important criterion to consider when selecting a heat transfer fluid. It also is important to ensure that the fluid's maximum film temperature is never exceeded -- otherwise, the fluid will degrade and foul heat transfer surfaces, reducing efficiency.

High Fluid Level in Expansion Tank

If the fluid level in the expansion tank is too high, some fluid droplets will become entrained with the vapors being vented. The fluid level in an expansion tank should be 50 to 75% when the system fluid is at its normal operating temperature. The expansion tank's operating pressure should be higher than the fluid's vapor pressure to minimize venting of fluid vapors.

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Short Fluid Life

The main processes by which the life of a heat transfer fluid can be shortened are:

Oxidation

Oxidation is a result of the fluid coming in contact with atmospheric oxygen at elevated temperatures. The rate of fluid oxidation can be reduced by using a fluid with antioxidant additives and operating the expansion tank at atmospheric temperature with an inert gas blanket.

Thermal Degradation. Thermal degradation is the result of the fluid being subjected to excessively high temperatures, extended residence times in the heat source, or being unsuitable for the application. Flame impingement on heater tubes creates a localized hot spot with excessively high heat flux. This also causes thermal degradation.

Contamination

Contamination occurs when the wrong material is added as the top-off fluid, when the system is inadequately cleaned or when process fluid leaks into the heat transfer fluid.

Frequent Filter Plugging

Filter plugging reduces fluid flow and should be corrected. Possible causes are:

Polymerization

Frequent filter plugging indicates fluid polymerization and the buildup of resinous sludge. Most often, this occurs when a fluid has been used well beyond its condemning limits and has been extensively oxidized.

Fouling After Cleaning

Frequent filter plugging also can occur after an extensively fouled or coked system has been cleaned and new fluid added. Residual solid particles or sludge tend to accumulate on the filters during the initial operating period.

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Unsaturated Components. Heat transfer fluids produced via the solvent refining/dewaxing approach may contain some unsaturated components and can form sludge deposits when thermally cracked or oxidized. Synthetic-based fluids are highly aromatic and can form carbonaceous solids when thermally cracked or oxidized. Fluids produced from hydro-processed paraffinic base stocks that contain no unsaturated components are less likely to form deposits.

Increased Pressure Drop in the System

This can be caused by a restriction in the piping network, filter plugging or increased system fluid viscosity. If not addressed, it will lead to further problems.

Pump-Related Problems. Centrifugal pumps equipped with mechanical seals, water cooling on the bearings and seal flush system typically are used for fluid circulation in heat transfer systems. Possible problems include:

Cavitation/Vapor Locking. Cavitation will occur if the system pressure falls below the vapor pressure of the fluid being pumped, causing the fluid to vaporize and form pockets of vapor. Pockets of water that periodically come in contact with hot fluid also will create excessive system vapor.

The expansion tank should be installed at the system's highest point and should be piped upstream of the circulating pump to provide a positive suction head and allow venting of any vapors before they can cause pump cavitation. A dual-legged expansion tank is preferred because it allows the fluid's full flow to be diverted through the tank and allows more efficient venting of air, water vapors/steam and low boilers.

Leaks

Frequent leaks from a pump with a mechanical seal indicates inadequate cooling at the seal face. If the seal face temperature becomes excessively high, the fluid in contact with the seal face will thermally crack and leave a hard carbon deposit. The abrasive coke buildup will cause wear and eventually lead to fluid leaks. A constant flow of low-pressure steam at the seal face can prevent this problem.

Other Problems. Finally, other potential problems with a heat

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transfer system are:

Unable to Attain Desired Fluid Flow Rate.

• Insufficient net positive suction head (NPSH), which is the minimum suction pressure required by the pump to prevent cavitation.

• Plugged suction line. • Entrained air leak on pump suction. • Excessive low boilers in fluid to pump suction. • Impeller too small. • Increased fluid viscosity. • Pump speed too low.

Pump Running, But no Discharge.

• Pump not properly primed. • Blockage in impeller or suction line. • Rotation is in wrong direction and motor needs to be

reversed.

Pump Operates for Short Period, Then Loses Prime.

• Air leak on pump suction. • Insufficient NPSH.

Excessive Noise or Vibrations.

• Cavitation due to vapors or high viscosity fluid. • Diameter of suction piping too small. • Mechanical failure or misalignment. • Pump and/or piping not properly secured.

Noise in Pipes.

Most likely, noisy pipes are caused by water in the system at the operating temperature. Water can be removed by carefully circulating the hot system fluid through the expansion tank and allowing the water vapors to be vented slowly.

Plugged Tubes

• Back flushing• Air bumping

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• Acid cleaning vs. piping corrosion• Upsetting cooling tower pH due to acid cleaning

Hydrocarbon Leaks

• Watch for hydrocarbon haze• Rapid biological growth on cooling tower cell decks • Check for gas with test meter on cell decks• Vibrating cooling water lines • Check vent on exchanger channel head for leaks

Cooling Tower Deficiency

• Check wet bulb temperature• Inspect Interior for damaged fill• Eliminate large holes in distribution decks• Redistribute water to individual cells• Unplug distribution holes• Increase chlorination rate

High Exchanger Outlet Temperature

• Cooling water pump deficiency• Plugged cooling tower screen• Plugged exchanger tubes• Plugged floating head

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