HEAT EXCHANGER

• A device whose primary purpose is the transfer of energy between two fluids is named a Heat Exchanger.

Heat Exchangers

Applications of Heat Exchangers

Heat Exchangers prevent car engine

overheating and increase efficiency

Heat exchangers are used in Industry for

heat transfer

Heat exchangers are used in AC and

furnaces

Analysis of heat exchanger

For analyzing of heat exchanger we will discuss the two methods.

• Log Mean Temperature Difference Method (LMTD)

• effectiveness- NTU method

Heat exchangers usually operate for long periods of time with no change in there operating condition. Therefore, they can be modeled as study flow device.

Assumptions

• The mass flow rate of each fluid remain constant.• The fluid properties such as temp. and velocity at inlet and outlet

remain same.• The fluid stream experienced little or no change in their velocities

and elevations, and thus the kinetic and potential energy changes are negligible.

• The specific heat of the fluid, in general, changes with temp. But, in a specified temperature range it can be treated as a constant at some average value with little loss in accuracy.

• Axial heat conduction along the tube is usually insignificant and can be considered negligible.

• Finally, the outer surface of the heat exchanger is assumed to be perfectly insulated.

Under these assumptions, the first law of thermodynamucs requires that the rate of heat transfer from the hot fluid be equal to the rate of heat transfer to the cold one.That is,

Principle of heat exchanger

Counter flow Parallel flow

HC CmCm HC CmCm

LMTD methods.The curse of the non-linear behavior

Due to the nonlinear behavior of the temperature difference cross the heat exchanger. An appropriate average temperature difference has to be adopted

The lmtd definition

The correction factor F for multi-pass and

cross-flow

• The standard lmtd formulation is limited to the simple cases of parallel and counter flow configurations.

• In more complex cases as cross flow and multi-pass the correction factor F has to be considered.

SELECTION OF HEAT EXCHANGERS

Heat exchangers are complicated devices and the result obtained with the simplified approach presented should be used with care.

It is natural to tend to overdesign the heat exchanger in order to avoid unpleasant surprises.

Engineers in industry often find themselves in a position to select heat exchangers to accompany certain heat transfer tasks.

Heat transfer enhancement in heat exchanger is usually accompanied by increased pressure drop and thus higher pumping power. Therefore any gain from enhancement should be weighed at the cost of the accompanying pressure drop.

Some thought should be given to which fluid should pass through the tube side and which through the shell side. Usually the more viscous fluid is suitable for shell side (larger passage area and thus lower pressure drop) and fluid with higher pressure for the tube side.

THE PROPER SELECTION DEPENDS ON FOLLOWING FACTORS:

•HEAT TRANSFER RATE

•SIZE AND WEIGHT

•COST

•PUMPING POWER

•MATERIAL

HEAT TRANSFER

The heat exchanger should be capable of transferring heat at the specified rate in order to achieve the desired temperature change of the fluid at the specified mass flow rate

SIZE AND WEIGHT

Normally the smaller and lighter heat exchanger is the better one. This is especially in the case of automotive and aerospace industries.

Larger heat exchangers carry higher price tag. The space available for the heat exchanger in some cases limits the length of the tube that can be used.

COST

Budgetary limitations plays an important role in the selection oh heat exchangers, except where money in not so important.

An Off-the-shelf heat exchanger has a definite cost advantage over those made to order as in the cases where heat exchangers are integral part of the overall device to be manufactured.

The operating and maintenance costs of the heat exchanger are also important considerations in assessing the overall cost

OPERATING COST = (PUMPING COST, kW) X (HOURS OF OPERATIONS, hrs) X (PRICE OF

ELECTRICITY, Rs/kWh)

PUMPING POWER

In heat exchangers, both fluids are usually forced to flow by pumps or fans that consume electrical power.

Pumping power is the total electrical power consumed by the motors of the pumps and fans.

MATERIALS

A temperature difference of 50oC or more between the tubes and the shell will probably pose differential thermal expansion problems and need to be considered. In case of corrosive fluids, we may have to select expensive corrosion resistance materials such as stainless steel.

FOULING FACTOR

The performance of heat exchanger usually deteriorates with time as a result of accumulation of deposits on heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer and causes the rate of heat transfer in a heat transfer to decrease. The net effect of these accumulation of heat transfer is represented by fouling factor which is a measure of thermal resistance induced by fouling.

Fouling should be considered in the design and selection of heat exchanger. In such application it may be necessary to select a larger and expensive heat exchanger.

The periodic cleaning of heat exchanger and the resulting down time are additional penalties associated with fouling.

The fouling factor depends on operating temperature and the velocities of fluids and length of service. Fouling increases with increasing temperature and decreasing velocities.

Fouling Factor

Precipitation of solid deposits

Corrosion and Chemicals fouling

Biological fouling

PRECIPITAION OF SOLID DEPOSITES

These is specially found in the areas where the water is hard. The scales of such deposits come off by scratching and the surfaces can be cleaned of such deposits by chemical treatment.

To avoid these problem, water in power and process plants is extensively heated and its solid contents are removed before it is allowed to circulates through the system.

The solid ash particles in the flue gases accumulating on the surfaces of air pre-heaters create similar problems.

CHEMICAL FOULING

In this case surfaces are fouled by the accumulation of the products of chemical reactions on the surfaces. This form of fouling can be avoided by coating metal pipes with glass or using plastic pipe instead of metal ones.

BIOLOGICAL FOULING

Heat exchangers may also be fouled by the growth of algae in warn water. This type of fouling can be prevented by chemical treatment.

• The closed-type exchanger is the most popular one.

• One example of this type is the Double pipe exchanger.

• In this type, the hot and cold fluid streams do not come into direct contact with each other. They are separated by a tube wall or flat plate.

Why shell-and-tube?

CEC survey: S&T accounted for 85% of new exchangers supplied to oil-refining, chemical, petrochemical and power companies in leading European countries. Why?

• Can be designed for almost any duty with a very wide range of temperatures and pressures

• Can be built in many materials• Many suppliers• Repair can be by non-specialists• Design methods and mechanical codes have been

established from many years of experience

Scope of shell-and-tube• Maximum pressure

– Shell 300 bar (4500 psia)– Tube 1400 bar (20000 psia)

• Temperature range– Maximum 600oC (1100oF) or even 650oC– Minimum -100oC (-150oF)

• Fluids – Subject to materials – Available in a wide range of materials

• Size per unit 100 - 10000 ft2 (10 - 1000 m2)

Can be extended with special designs/materialsCan be extended with special designs/materials

Construction• Bundle of tubes in large cylindrical shell• Baffles used both to support the tubes and to direct

into multiple cross flow• Gaps or clearances must be left between the baffle

and the shell and between the tubes and the baffle to enable assembly

Baffle

Shell

Tubes

Shell-side flow

Tube layouts

• Typically, 1 in tubes on a 1.25 in pitch or 0.75 in tubes on a 1 in pitch

• Triangular layouts give more tubes in a given shell• Square layouts give cleaning lanes with close pitch

pitchTriangular30o

Rotatedtriangular60o

Square90o

Rotatedsquare45o

TEMA standards

• The design and construction is usually based on TEMA 8th Edition 1998

• Supplements pressure vessel codes like ASME and BS 5500

• Sets out constructional details, recommended tube sizes, allowable clearances, terminology etc.

• Provides basis for contracts• Tends to be followed rigidly even when not strictly

necessary• Many users have their own additions to the

standard which suppliers must follow

TEMA terminology

• Letters given for the front end, shell and rear end types

• Exchanger given three letter designation

• Above is AEL

ShellFront endstationary head type

Rear endhead type

Front head type• A-type is standard for dirty tube side

• B-type for clean tube side duties. Use if possible since cheap and simple.

B

Channel and removable cover Bonnet (integral cover)

A

More front-end head types

• C-type with removable shell for hazardous tube-side fluids, heavy bundles or services that need frequent shell-side cleaning

• N-type for fixed for hazardous fluids on shell side

• D-type or welded to tube sheet bonnet for high pressure (over 150 bar)

B N D

Shell type• E-type shell should be used if possible but• F shell gives pure counter-current flow with two

tube passes (avoids very long exchangers)

E F

One-pass shell Two-pass shell

Longitudinal baffle

Note, longitudinal baffles are difficult to seal withNote, longitudinal baffles are difficult to seal with

the shell especially when reinserting the shell the shell especially when reinserting the shell afterafter

maintenancemaintenance

More shell types• G and H shells normally only used for

horizontal thermosyphon reboilers• J and X shells if allowable pressure drop can

not be achieved in an E shell

J

HG

X

Split flow Double split flow

Divided flow Cross flow

Longitudinalbaffles

Rear head typeThese fall into three general types• fixed tube sheet (L, M, N)• U-tube• floating head (P, S, T, W)

Use fixed tube sheet if T below 50oC, otherwise use other types to allow for differential thermal expansion

You can use bellows in shell to allow for expansion but these are special items which have pressure limitations (max. 35 bar)

Fixed rear head types

• L is a mirror of the A front end head

• M is a mirror of the bonnet (B) front end

• N is the mirror of the N front end

L

Fixed tube sheet

Floating heads and U tube

Allow bundle removal and mechanical cleaning on the shell side

• U tube is simple design but it is difficult to clean the tube side round the bend

Floating headsT S

Pull through floating headNote large shell/bundle gap

Similar to T but with smaller shell/bundle gap

Split backing ring

Other floating heads

• Not used often and then with small exchangersP W

Outside packing to give smaller shell/bundle gap

Externally sealed floating tube sheetmaximum of 2 tube passes

Shell-to-bundle clearance (on diameter)

0.5 1.0 1.5 2.0 2.50

Shell diameter, m

Cle

aran

ce,

mm

0

150

100

50

Fixed and U-tube

P and S

T

Example

• BES

• Bonnet front end, single shell pass and split backing ring floating head

What is this?

Allocation of fluids• Put dirty stream on the tube side - easier to

clean inside the tubes• Put high pressure stream in the tubes to avoid

thick, expensive shell• When special materials required for one stream,

put that one in the tubes to avoid expensive shell• Cross flow gives higher coefficients than in plane

tubes, hence put fluid with lowest coefficient on the shell side

• If no obvious benefit, try streams both ways and see which gives best design

Example 1Debutaniser overhead condenser

Hot side Cold side

Fluid Light hydrocarbon Cooling water

Corrosive No No

Pressure(bar) 4.9 5.0

Temp. In/Out (oC) 46 / 42 20/30

Vap. fract. In/Out 1 / 0 0 / 0

Fouling res. (m2K/W) 0.00009 0.00018

Example 2Crude tank outlet heater

Cold side Hot side

Fluid Crude oil Steam

Corrosive No No

Pressure(bar) 2.0 10

Temp. In/Out (oC) 10 / 75 180 / 180

Vap. fract. In/Out 0 / 0 1 / 0

Fouling res. (m2K/W) 0.0005 0.0001

Rule of thumb on costing• Price increases strongly with shell diameter/number of tubes because of

shell thickness and tube/tube-sheet fixing • Price increases little with tube length• Hence, long thin exchangers are usually best• Consider two exchangers with the same area: fixed tubesheet, 30 bar both

side, carbon steel, area 6060 ft2 (564 m2), 3/4 in (19 mm) tubes

Length Diameter Tubes Cost

10ft 60 in 3139 $112k (£70k)

60ft 25 in 523 $54k (£34k)

Shell thickness

p is the guage pressure in the shell

t is the shell wall thickness

is the stress in the shell

From a force balance

pDs

t

p

t

2t pDs tpDs2

hence

Typical maximum exchanger sizes

Floating Head Fixed head & U tube

Diameter 60 in (1524 mm) 80 in (2000 mm)

Length 30 ft (9 m) 40 ft (12 m)

Area 13 650 ft2 (1270 m2) 46 400 ft2 (4310 m2)

Note that, to remove bundle, you need to allow at least as much length as Note that, to remove bundle, you need to allow at least as much length as the length of the bundlethe length of the bundle

FoulingShell and tubes can handle fouling but it can be reduced

by• keeping velocities sufficiently high to avoid deposits• avoiding stagnant regions where dirt will collect• avoiding hot spots where coking or scaling might

occur• avoiding cold spots where liquids might freeze or

where corrosive products may condense for gases

High fouling resistances are a self-fulfilling prophecy

-

Velocity Velocity

Resonance Instability

Flow-induced vibrationTwo types - RESONANCE and INSTABILITY• Resonance occurs when the natural frequency

coincides with a resonant frequency• Fluid elastic instability

Both depend on span length and velocity

Tu

be

dis

pla

cem

ent

Avoiding vibration• Inlet support baffles - partial baffles in first

few tube rows under the nozzles• Double segmental baffles - approximately

halve cross flow velocity but also reduce heat transfer coefficients

• Patent tube-support devices• No tubes in the window (with intermediate

support baffles)• J-Shell - velocity is halved for same baffle

spacing as an E shell but decreased heat transfer coefficients

Avoiding vibration (cont.)

Inlet support baffles

Double-segmental baffles

No tubes in the window - with intermediate support baffles

TubesWindows with no tubes

Intermediate baffles

Shell-side enhancement• Usually done with integral, low-fin tubes

– 11 to 40 fpi (fins per inch). High end for condensation

– fin heights 0.8 to 1.5 mm

• Designed with o.d. (over the fin) to fit into the a standard shell-and-tube

• The enhancement for single phase arises from the extra surface area (50 to 250% extra area)

• Special surfaces have been developed for boiling and condensation

Low-finned Tubes• Flat end to go into tube sheet and

intermediate flat portions for baffle locations

• Available in variety of metals including stainless steel, titanium and inconels

Tube-side enhancement using inserts

Spiral wound wire and twisted tape• Increase tube side heat transfer coefficient

but at the cost of larger pressure drop (although exchanger can be reconfigured to allow for higher pressure drop)

• In some circumstances, they can significantly reduce fouling. In others they may make things worse

• Can be retrofittedTwisted tape

Wire-wound inserts (HiTRAN)• Both mixes the core (radial mixing) and

breaks up the boundary layer

• Available in range of wire densities for different duties

Problems of Conventional S & T

Zigzag path on shell side leads to• Poor use of shell-side pressure drop• Possible vibration from cross flow• Dead spots

– Poor heat transfer– Allows fouling

• Recirculation zones– Poor thermal effectiveness,

Conventional Shell-side Flow

Shell-side axial flowSome problems can be overcome by having axial

flow• Good heat transfer per unit pressure drop but

– for a given duty may get very long thin units– problems in supporting the tube

RODbaffles (Phillips petroleum)• introduced to avoid vibrations by providing

additional support for the tubes• also found other advantages

– low pressure drop– low fouling and easy to clean– high thermal effectiveness

RODbafflesTend to be about 10% more expensive

for the same shell diameter

Twisted tube (Brown Fintube)

• Tubes support each other

• Used for single phase and condensing duties in the power, chemical and pulp and paper industries

Shell-side helical flow (ABB Lummus)

Independently developed by two groups in Norway and Czech Republic

Comparison of shell side geometries

Twistedtube

Segmentalbaffles

Helicalbaffles

RODbaffles

Good p Y N Y YHigh shell N Y Y NLow fouling Y N Y YEasycleaning

Y With squarepitch

With squarepitch

Y

Tube-sideenhance.

Included With inserts With inserts With inserts

Can givehigh

Y N N Y

Lowvibration

Y With specialdesigns

With doublehelix

Y