ME421Lec10p3

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ME421

Heat Exchanger andSteam Generator Design

Lecture Notes 10 Part 3

Condensers and Evaporators

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Condensers• Condenser: a two-phase flow HEX where the heat is

generated from the conversion of vapor into liquid, and theheat generated is removed from the system by a coolant

• In general, two types (later, specific for AC/refrigeration):

– Indirect contact: condensing vapor and coolant are separated by asolid surface

• shell-and-tube: condensation inside or outside. vertical or

horizontal• plate: limited applications

• air-cooled: condensation in tubes, air blown over tubes (usuallyfinned)

– Direct contact: condensing vapor and coolant are in direct contact

• vapor bubbled into a pool of liquid

• liquid sprayed into vapor

• packed-column: liquid flows as a film over a "packing material"against upward flow of vapor

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Condensers• Most important feature: must have a vent for noncondensible

gas removal.• Noncondensibles reduce the condensing temperature, thus

the temperature difference between streams

• Noncondensible gas accumulation cannot be tolerated duringcondenser operation

• A vent must always be provided

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Shell-and-Tube CondensersHorizontal Shell-side Condensers

• E, G, H, J , and X-type can be used, F-type rarely

E-type

• one-pass shell

• simplest, with two outlet nozzles: one for condensate, onefor vapor vent.

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Shell-and-Tube Condensers J -type

• Advantages over E-type

– two inlet nozzles for the vapor to accommodate a large vapor volume

– vapor flow length is halved - reduced pressure drop

• Heat loads in both halves must be kept the same to preventnon-condensed vapor coming from one end to meet subcooledliquid coming from the other, which could result in periodicviolent vapor collapse (rapid condensation) and possibleexchanger damage due to vibration.

• To prevent this, more than one tube pass

is used when the tube-side fluid has a

large temperature variation

• Baffles same as E-type, except a full-circletube support plate at the center of the HEX

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Shell-and-Tube CondensersF-, G-, and H-type

• Have longitudinal baffles• Full-circle tube support plates in line with the inlet nozzles• H-type has a full-circle tube support plate at the center as well,

so any other segmented baffles may be unnecessary; thuslower pressure drop

• Vents placed above the condensate outlet nozzle, below

longitudinal baffle

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Shell-and-Tube CondensersX-type

• Useful for vacuum operation and large vapor volumes

• Large flow area and low pressure drop - very important forvacuum operation (to avoid reducing the saturation temperature

thus temperature difference)• Tube support to prevent vibration and good vapor distribution

are important

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Shell-and-Tube Condensers• Baffles in condensers are usually arranged with the cut vertical

so that the vapor flows from side to side, and not top to bottom.• Baffles are notched (with a hole) at the bottom to allow the

drainage of the condensate from one compartment to the next,

and finally to the condensate outlet nozzle.• Baffles close together increases shell-side velocities, and thus

pressure drop and vibration

• A design with no tubes in window allows for additionalintermediate baffles for tube support without significant effecton the flow, but this is an expensive design (large volume of

empty shell)• An impingement plate (next slide) is used below the vapor inletnozzle on a shell-side condenser to prevent tube erosion fromthe high velocity of the incoming vapor.

• When flow-induced vibration is critical, an extra tube supportplate is inserted near the nozzle (next slide).

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Shell-and-Tube Condensers

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Shell-and-Tube CondensersVertical Shell-side Condensers

• Advantageous for condensing a wide condensing rangemixture

• Provides good mixing of the condensate and the vapor

• Hard to clean the insideof tubes, not appropriate

for fouling coolants

Sh ll d T b C d

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Shell-and-Tube CondensersVertical Tube-side Condensers

• Can be designed for downflow or upflow (reflux mode) of vapor, with the condensate draining down

• Downflow:

– Vapor enters at top, flows down inside tubes, with condensate drainingfrom the tubes with gravity and vapor shear

– Fixed-tube sheet construction possible for chemical shell-side cleaning

– Two tube side passes (U-tube) possible with upward flow in the first

pass and downward flow in the second pass• Upflow:

– Vapor enters at bottom, flows upwards inside the tubes, withcondensate draining down the tubes with gravity only

– Flooding must be avoided, otherwise capacity will be limited when tubesare flooded

– Tubes extend below the bottom tube sheet and are cut at an angle to

allow dripping of the condensate– In general HEX is short (2-5 m) with large diameters

– Vapor velocity must be low to allow for the draining of condensates

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Vertical in-tube downflow condenser Reflux condenser

Sh ll d T b C d

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Shell-and-Tube CondensersHorizontal In-tube Condensers

• Common in air-cooled condensers and kettle reboilers (K-type)• When U-tube is used, different lengths of inner and outer tubeswill result in different condensation than a straight-tube, single-pass bundle

• Liquid-vapor mixture exiting from the first pass into theturnaround header has maldistribution into second pass tubes

Shell and Tube Condensers

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Shell-and-Tube Condensers• As a result of maldistribution, heat transfer and fluid flow

characteristics of these tubes are altered; thus the netaverage effective heat transfer coefficient for this pass isreduced

• Vent must be located where noncondensibles areconcentrated

• Due to possible stratification effects, this is not a veryeffective configuration for gravity controlled (low velocity)flows

• Subcooling is important to prevent flashing in the pipingcircuit and the equipment downstream from the condenser

• With vertical tubes, this can be achieved by a liquid levelcontrol and running the tubes full of liquid up to a certainheight

Steam Turbine Exhaust Condensers

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Steam Turbine Exhaust Condensers• Same as shell-side condensers (particularly X-type)

• Often referred to as surface condensers• Large heat duties and low condensing temperatures for

highest possible power-station efficiency

• Aim is to operate with the condensing temperature slightlyabove cooling water temperature

• Typically, coolant ~20oC, condensation ~30oC, with

saturation pressure about 0.042 bar-absolute; thus littlepressure available for pressure drop in the unit

• Good venting and low pressure drop are the governingfactors in surface condenser design

• Shells can be box-shaped or for low surface areas, cylindrical

• To provide subcooling, often a separate HEX is designed asa subcooler or sometimes over-design is done for thecondenser, subcooler as a second pass

Plate Condensers

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Plate Condensers• Gasketed-plate HEX are not suitable for phase change

operations• Spiral plate HEX are used as condensers, with pressures up

to 20 bar and temperatures up to 400oC

• Plate-fin condensers are used in low temperature (cryogenic)applications, where the temperature difference betweenstreams is small (1-5oC), but pressure drop is high andcleaning is difficult. Also, uniform flow distribution isimportant. If the flow rate is low in adjacent channels, partialcondensation can occur, which decreases the unit efficiencyand can lead to operating problems.

Ai -Cooled Condense s

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Ai -Cooled Condense s• Used when water is in short supply

• Tubes are usually finned on the outside

• Flow is forced draught (figure) or induced draught (suction)

• Air-cooled condensers are economical when condensation is20oC above ambient temperature

• Disadvantages: Relatively large floor area required, noisefrom fans, freezing of condensate in low temperature climates

Direct Contact Condensers

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Direct Contact Condensers• Inexpensive and simple, but limited applications due to

mixing of coolant and condensate• No fouling and high heat transfer rates per unit volume

• Three types:

– Pool condensers: Vapor is injected into a pool of liquid, where thecoolant may be a process fluid to be heated.

– Spray condensers: Subcooled liquid is sprayed into the vapor in alarge vessel. Often used for condensing steam with water as the

coolant, thus mixing is not a problem, otherwise a separator isrequired. Very suitable to replace vacuum condensers with a simplespray condenser and a compact single-phase cooler.

– Tray condensers: Used with dirty coolants, when spray nozzles may

be blocked. Trays are tilted slightly. Some degree of countercurrentflow is achieved.

Thermal Design of

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Thermal Design of Shell-and-Tube Condensers

• Design and rating methods introduced in Chapter 8

• Two approaches in the design of two-phase flow HEX:

– The HEX is taken as a single control volume with an

average U and two inlets and two outlets (lumped analysis)– The HEX is divided into segments or multiple control

volumes, with the outlet of one control volume being the

inlet of the next. Heat duty is obtained by integrating thelocal (segmental) values.

• First approach is in general satisfactory

• Heat duty is

( ) side)condensing(iimQ oihh −= &

( ) side)(coolant T TcmQ 1c2ccpc −= &

Thermal Design of

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• Also,

where Um is the mean overall heat transfer coefficient basedon the outside surface area (includes heat transfer

coefficients, wall resistance, fouling, as necessary), and ∆ Tlm

is the usual log mean temperature difference

• Heat transfer coefficient of the two-phase side must be

calculated by selecting a suitable correlation for the fluid• If the overall heat transfer coefficient varies considerably

along the HEX, then

• For variable U, local heat transfer from a differential surface

area, dA is δQ = UdAdT

• If U varies linearly, then , where U1 and U2

are at the inlet and outlet

∫=oAo

m UdAA

1

U

lmom TAUQ ∆=

( )21m UU21U +=

Thermal Design of

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• Study design example 11.1 in book

• Iterative procedure, with the iterations tabulated

• Inlet and outlet overall heat transfer coefficients are calculated

separately and the results are averaged for the whole HEX• Corrections on p.433, first paragraph:

The following tables summarize the results of this iteration for

the inlet and the outlet of the condenser when ∆ Tin = 25.8oCand ∆ Tout = 15.8oC. The initial guesses of ∆ Tw are 10oC and6oC for the inlet and outlet, respectively.

Table 11.1: first item should be ∆ Tw, oC (similar to Table 11.2)• Other minor typos

• No return pressure drop calculation required (correction)

Thermal Design of

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• Best choice in design will also take into consideration theeconomical evaluation of several designs– The condenser must be operable in a range of conditions, which

include the design conditions– The design must meet all specific physical limitations imposed, such as

weight, size, coolant process constraints, material constraints, etc.

– The design must allow standard and/or economical fabrication

– Operation and maintenance expenses must not exceed economicallimits

Design and Operational Considerations

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Design and Operational Considerationsfor Condensers

• Condensation Modes: Design is made for filmwisecondensation

• Condensation Regimes: Gravity controlled (low vapor

velocity) or vapor shear controlled (high vapor velocities)• Desuperheating: If superheated steam enters condenser,

single-phase process must be considered for this part

• Subcooling (if desirable): Single-phase process as well• Construction Considerations:

– Vertical in-tube condensation: very effective, but tube length is limited,

or use large shell diameters– Horizontal tube-side condensation: less effective, but stratification maybe a problem; use inclined tubes

– Horizontal shell-side condensation: very popular, easier to predict and

design, large surfaces, low pressure drop (example X-type shell)


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• If noncondensibles are present, condensing heat transfercoefficient may vary by an order of magnitude between theinlet and the outlet; thus, a stepwise calculation is required

• Venting of gases must be provided during construction of theunit


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Main Reasons for Failure

• More fouling than expected

• Improper draining of the condensate and flooding of the tubes

– condensate outlet is too small or too high• Inadequate venting of noncondensibles

• Design based on end temperatures only

• Flooding with backflow of liquid against upward vapor flow• Excessive fogging when condensing high molecular weight

vapors with noncondensible gases

• Maldistribution of vapor and liquid in parallel tubes

Condensers for Refrigeration and A/C

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• Condenser rejects heat and discharges high temperature andpressure liquid refrigerant

• Most common forms of condensers in this industry are– Water-cooled condensers

• Horizontal shell-and-tube• Vertical shell-and-tube

• Shell-and-coil: one or more continuous or assembled coils

• Double-pipe

– Air-cooled condensers: desuperheating, condensing, and subcoolingphases

– Evaporative condensers (air- and water-cooled): conservative design

though the combination of a condenser and cooling tower in oneequipment

• Read Section 11.9 for more detailed information

Evaporators for Refrigeration and A/C

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apo a o s o e ge a o a d /C

• Evaporator vaporizes the refrigerant

• Most common forms of evaporators are:– Water-cooling evaporators: In general, refrigerant boils inside the tubes

(direct expansion type), in which number of tube passes and type of

tubes is important. Other times, flooded evaporators are used, wherethe refrigerant is on the shell-side, and the tubes are covered (flooded)with a saturated mixture of liquid and vapor

Flooded Evaporator


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p g /

– Air-cooling evaporators:

Dry expansion coil is the most

widely used type. Refrigerant

enters the coil and cools the air

outside the coil; air flow can benatural or forced convection.

Standards for Condensers and Evaporators

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p

• The designer must be aware of the standards for the designand rating of air-conditioning evaporators and condensers (forexample ASHRAE – American Society of Heating,Refrigerating, and Air-Conditioning Engineers)

• The capacity of a HEX is determined by following thesestandards for testing. Then, the ratings published bymanufacturers can be compared on a common basis.

See Example 11.2 for the design of a water-cooled, shell-and-tube condenser

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ME421Lec10p3