Heat Exchangers

What are heat exchangers for?

- Heat exchangers are practical devices used to transfer

energy from one fluid to another

- To get fluid streams to the right temperature for the next

process

* reactions often require feeds at high temp.

- To condense vapours

- To evaporate liquids

- To recover heat to use elsewhere

- To reject low-grade heat

- To drive a power cycle

Application: Power cycle

Steam Turbine

Boiler CondenserFeed water

Heater

Main Categories Of Exchanger

Wall separating streams Direct contact

- Most heat exchangers have two streams, hot and cold, but some have more than two

Heat exchangers

Recuperators Regenerators

Wall separating streamsWall separating streams Direct contacts

Recuperators/Regenerators

- Recuperative:

Has separate flow paths for each fluid

which flow simultaneously through the

exchanger transferring

the streams

- Regenerative

heat between

Has a single flow path which the hot

and cold fluids alternately pass

through.

Double Pipe

- Simplest type has one tube inside another - inner tube may have longitudinal fins on the outside

- However, most have a number of tubesin the outer tube - can have very manytubes thus becoming a shell-and-tube

Shell and Tube

- Typical shell and tube exchanger as used in the process industry

Shell-Side Flow

Plate-Fin Exchanger

- Made up of flat plates (parting sheets) and corrugated sheets which form fins

- Brazed by heating in vacuum furnace

Configurations

T2

t1

T1

t2

ParallelFlowT1

T2

t1 t2

Position

Tem

pera

ture

T2

t2

T1

t1

CounterFlowT1

T2

t2 t1Tem

pera

ture

Position

Basic flow arrangement in tube in tube flow

Heat Exchanger Analysis

Log mean temperature difference (LMTD) method

.Want a relation Q=UAΔTm

Where ΔTmis some mean ΔT between hot and cold fluid

Counterflow

Note Th,out can be <Tc,out

Parallel flow

T ' s ' cannot cross

Energy balance (counterflow) on element shown

hhcc

ch

cc

c

hh

h

ch

ccchhh

cmcmQdTTd

cm

QddT

cm

QddT

TTUdAQd

c

m

dTcmdTcmQd

11

(1) fromNow

)2(

EquationRate

heat specific

fluid of rateflow mass

)1(

2121

11

22

and

ratefer heat trans Total

11ln

21 Integrate

11

(2), from Subtract

cccchhhh

hhccch

ch

hhccch

ch

TTcmQTTcmQ

cmcmUA

TT

TT

dAcmcm

UTT

TTd

Qd

• Remember – 1 and 2 are ends, not fluids

• Same formula for parallel flow (but ΔT’s are different)

•Counterflow has highest LMTD, for given T’s therefore smallest area for Q.

Difference eTemperaturMean Log is LMTD

LMTD

/ln

2

1

put and mfor Substitute

12

12

222

111

c

UAQ

TT

TTUAQ

ENDTTT

ENDTTT

ch

ch

Condenser Evaporator

Multipass HX Flow Arrangements

- In order to increase the surface area for convectionrelative to the fluid volume, it is common to design formultiple tubes within a single heat exchanger.

- With multiple tubes it is possible to arrange to flow so thatone region will be in parallel and another portion in counterflow.

1-2 pass heat exchanger,indicating that the shell sidefluid passes through the unitonce, the tube side twice. Byconvention the number of shellside passes is always listedfirst.

- The LMTD formulas developed earlier are no longer adequate formultipass heat exchangers. Normal practice is to calculate the LMTD forcounter flow, LMTDcf, and to apply a correction factor, FT, such that

=FT ⋅LMTDCFΔθeff

- The correction factors, FT, can be found theoretically and presentedin analytical form. The equation given below has been shown to beaccurate for any arrangement having 2, 4, 6, .....,2n tube passes pershell pass to within 2%.

1Rfor ,1

:essEffectiven/1

/1

shell

shell

N

N

XR

XP

1R1RP2

1R1RP2ln1R

PR1

P1ln1R

F

2

2

2

T

1Rfor ,

1NPN

PP

shelloshell

o

11

12o

tT

ttP

1

1

o

o

P

RPX

12

21 ratioCapacity tt

TTR

T,t = Shell / tube side; 1, 2 = inlet / outlet

R=0.1

1.0

0.50.0 1.0P

R=10.0

FT

Effectiveness-NTU MethodNumber of Transfer Units (NTU) Method

havecan ratecapacity heat

C ,C oflesser with fluid only thethen

since and

fluid One

H.Ex. long infinitelyan for is where

:esseffectiven Define

? conditionsinlet

given for perform Ex. H. existing willHow

max

BA

A

,,max

max

max

T

TCTC

TcmTcmQ

TTTT

Q

Q

Q

BBA

BBAA

incinh

actual

max

min

minmax

min

max

min

min

in.cin.hmin

in.cin.hmin

maxminmax

C

C1

C

UA-exp

C

C1

C

C1

C

UA-exp-1

......... )LMTD(UAQ intoback Substitute

sT'outlet contain not does which for expressionWant

TTCQ or,

TTC

Q and TCQ i.e.

min

max

min

HEx.) of (size units transfer of No. and

,

C

UANTU

C

CNTU

Charts for each Configuration

incinh TTCQ ..min

Procedure:

Determine Cmax, Cmin/Cmax

Get UA/Cmin, from chart

• NTUmax can be obtained from figures in

textbooks/handbooks First, however, we must determine

which fluid has Cmin

• For the type of HEX used in this problem

Examination of the last equation, subject to values given, indicated that gas will have Cmin.

U

CNTUA

C

UANTU minmax

min

max

Effectiveness can be calculated using

°C⎪=38.0m2

m2°C180 W

1.4⎪4,882W⎪

Umin=

NTUmaxCA=

→ NTUmax=1.4ε=0.649

min=0.467⎪Cmax

C

⎪→

⎠

⎛ ⎞

⎪⎝

⎬⎪⎪⎪⎪⎪⎭

⎪⎪

⎪⎫