Lecture 14 - Heat Exchangers1

Lecture 14

HEAT

EXCHANGERS

Lecture I

Lecture 14 - Heat Exchangers2

Heat Exchangers INTRODUCTION

Definition: A heat exchanger is a device

that facilitates transfer of heat from one

fluid stream to another.

Heat Exchangers are used in:

Power generation, refrigeration, heating, air-

conditioning, food processing, chemical

processing, oil refining and automobiles.

Heat Exchangers are classified into two

types:

1. Single-Stream Exchangers

The temperature of only one stream changes

in the exchanger (evaporators, condensers,..)

2. Two-Stream Exchangers

The temperature of both streams change in the

exchanger (radiators, oil coolers, ..)

Lecture 14 - Heat Exchangers3

Type of Exchangers

Single-Stream Two-Stream

Lecture 14 - Heat Exchangers4

Type of Exchangers, continued

Geometric Flow Configurations:

• Single-Stream: The temperature of only one

stream changes in the exchanger

• Parallel-Flow Two-Stream: Two fluid flows

parallel to each other in the same direction. It is

often constructed as a shell-and-tube exchanger

(Cocurrent)

• Counterflow Two-Stream: Two fluid flows

parallel to each other in opposite direction. It is

often constructed as a shell-and-tube exchanger.

(Counter-current)

Lecture 14 - Heat Exchangers5

Type of Exchangers, continued

Geometric Flow Configurations:

• Cross-flow Two-Stream: The two streams flow at

right angles to each other. Intermediate

Effectiveness. Example: Automobile Radiators.

• Cross-counterflow Two-Stream: The tubes can

pass twice or four times the shell. The more it

crosses the more effective it is.

•Multipass Two-Stream:When the tubes of a

shell-and-tube exchanger double back one or more

times inside the shell.

• Regenerators: The above configurations involve

steady flows and temperatures usually called

Recuperators. For Regenerators the two streams

flow alternately through a stored matrix of

substantial heat storage capacity. Regenerators can

have parallel, counter and cross flow configurations.

Lecture 14 - Heat Exchangers6

Lecture 14 - Heat Exchangers7

Fluid Temperature Behavior

Notations:

H denotes hot stream

C denotes cold stream,

TH and TC change depending on the configuration.

In parallel flow two stream (TH -TC) decreases

along the exchanger in the flow direction and

TC,out< TH,out.

In the Counterflow two-stream exchanger (TH -TC)

can increase, decrease or remain constant.

Lecture 14 - Heat Exchangers8

Lecture 14 - Heat Exchangers9

Energy Balance

The energy balance of a simple coaxial-tube parallel-

flow heat exchanger is:

)()(

: vapoursaturated withcondenser simplea For

direction of veirrespecti positive as ratesflow mass

thetakingexchanger couterflowfor applies also This

)()()()(

)()()()(

hen,constant t isheat specific theAssuming

stream. cold theand

streamhot thebetween ferefheat trans theis where

)()(

,,

,,,,

0,,,0,

0,,,0,

inCoutCCPfgHH

inCoutCCPoutHinHHP

CLCCPLHHHP

CLCCLHHH

TTcmhm

QTTcmTTcm

or

QTTcmTTcm

Q

Qhhmhhm

−=

=−=−

=−=−

=−=−

&&

&&&

&&&

&

&&&

Lecture 14 - Heat Exchangers10

Overall Heat Transfer Coefficient

Most heat exchangers involve tubes which

have an overall heat transfer coefficient, U.

perimeter tube theis where

21

2

)/ln(

211

,,

℘

πππ℘ ++=ooc

io

iic rhk

rr

rhU

A well known problem of heat exchangers

are fouling. Deposits of calcium or

magnesium on the surface alters the

conductivity of the surface and can also alter

the conductivity coefficient.

.resistance foulinghot and cold theare and where

11

fHR

fCR

RR

UU C

fC

H

fH

f ℑℑ℘℘ ++=

Lecture 14 - Heat Exchangers11

Fouling in a Finned Heat

Exchanger

For finned clean tube in a heat exchanger:

LAhLAhk

rr

rhU pocffoc

io

iic /)/(1

2

)/ln(

211

,,, +ηππ℘ ++=

Lecture 14 - Heat Exchangers12

Fouling Resistance for Heat

Exchangers

Lecture 14 - Heat Exchangers13

Approximate Overall H-T

Coefficient

Lecture 14 - Heat Exchangers14

fgCCoutHinHHp hmTTcm( && =− )()

:anceEnergy BalExchanger

,,

Single-Stream Steady-Flow ExchangerAnalysis of an Evaporator

Lecture 14 - Heat Exchangers15

To obtain gas temperature variation along the

exchanger we now consider a differential element

∆x.

Energy balance in this element shows that the heat

transfer across the tube wall must equal the gas flow

rate times its enthalpy decrease.

D

TTcmTTxUxxHxHpHHsatH

π=℘

−=−∆℘∆+

where

)||()( &

Dividing by ∆x , letting ∆x 0, and rearranging:

0)( =−+ ℘

satHcm

U

dx

dT TTpHH

H

&

Boundary conditions: x=0 : TH=TH,in

xcmU

satinHsatH

pHHeTTTT)/(

,)(

&℘−−=−

Single-Stream Steady-Flow ExchangerAnalysis of an Evaporator continued

Lecture 14 - Heat Exchangers16

Single-Stream Steady-Flow ExchangerAnalysis of an Evaporator continued

units.transfer

ofnumber theis and esseffectiven

exchanger theis where, 1

or

1,

:ratureexit tempe get the weLxFor

/

,

,

Ntu

e-ε

cme

TT

TT

-Ntu

LU

satinH

outHinH pHH

ε=

−=−

−

=

℘− &

The effectiveness is the actual heat transfer

rate divided by the maximum heat transfer

for an infinitely long exchanger. The larger

Ntu is the more effective the heat exchanger.

εεεε typically range from 0.6 to 0.9.

Lecture 14 - Heat Exchangers17

Example 8.4, page 780

In a pilot open-cycle ocean thermal

energy conservation plant, 1 kg/s of

warm sea water at 300 K enters an

evaporator maintained at 2619 Pa.

The water is injected through an array

of nozzles to give an estimated

transfer area and liquid-side heat

transfer coefficient of 0.80 m² and

17,000 W/m² K, respectively. At

what rate is vapor produced?

In Class Exercise