12 heat exchanger heat exchanger.pdfA shell-and-tube heat exchanger must be designed to heat 2.5...

Heat Exchangers Chee 318 1

Lecture 12

Heat Exchangers


Heat Exchangers

• A heat exchanger is used to exchange heat between two fluids of

different temperatures, which are separated by a solid wall.

• Heat exchangers are ubiquitous to energy conversion and utilization.

They encompass a wide range of flow configurations.

• Applications in heating and air conditioning, power production, waste

heat recovery, chemical processing, food processing, sterilization in

bio-processes.

• Heat exchangers are classified according to flow arrangement and

type of construction.

� All principles that we have learned previously apply.

� In this chapter we will learn how our previous knowledge can be

applied to do heat exchanger calculations, discuss methodologies for

design and introduce performance parameters.


Design Process

• Rating of a heat exchanger:

� For a specific duty we need to specify the necessary area (or length)

and to decide what type of heat exchanger we need.

� Design of a heat exchanger is sometimes referred to as an “art”

(Perry)

• MUST READ: 38-40 heat exchanger lecture notes by Bob Heaslip

about design process steps.


Concentric Tube Construction

Parallel FlowParallel Flow

• - :• :

CounterflowCounterflow


Heat Exchanger Analysis

Recall from Chapter 8

• Expression for convection heat transfer for flow of a fluid inside a tube:

)( ,, imompconv TTcmq −= &

• For case 3 involving constant surrounding fluid temperature:

lms TAUq ∆=)/ln( io

iolm

TT

TTT

∆∆

∆−∆=∆


Heat Exchanger Analysis

In a two-fluid heat exchanger, consider the hot and cold fluids separately:

)(

)(

,,,

,,,

icoccpcc

ohihhphh

TTcmq

TTcmq

−=

−=

&

&

lmTUAq ∆=and

� The usual design goal is to determine the

required area A for a heating duty q

� Combine eqs. (11.1) and (11.2) and solve for A

� Need to determine U and ∆Tlm

(11.1) (11.2)


∆∆∆∆Tlm: 1. Parallel-Flow Heat Exchangers

where

Parallel FlowParallel Flow

lmTUAq ∆=

)/ln( 12

12

TT

TTTlm

∆∆

∆−∆=∆

ocoh

icih

TTT

TTT

,,2

,,1

−=∆

−=∆

∆T1 ∆T2


∆∆∆∆Tlm: 2. Counter-Flow Heat Exchangers

where

lmTUAq ∆=

)/ln( 12

12

TT

TTTlm

∆∆

∆−∆=∆

icoh

ocih

TTT

TTT

,,2

,,1

−=∆

−=∆

∆T1 ∆T2


Overall Heat Transfer Coefficient

• For tubular heat exchangers we must take into account the conduction

resistance in the wall and convection resistances of the fluids at the inner

and outer tube surfaces.

kL

DDR

AhR

AhUA

iocond

oo

cond

ii

π=

++=

2

111

)/ln(

where inner tube surface

outer tube surface LDA

LDA

oo

ii

π=

π=

(11.3)

ooii AUAUUA

111==


Fouling

• Heat exchanger surfaces are subject to fouling by fluid impurities,

rust formation, or other reactions between the fluid and the wall

material. The subsequent deposition of a film or scale on the surface

can greatly increase the resistance to heat transfer between the fluids.

• An additional thermal resistance, can be introduced: The Fouling

factor, Rf.

� Depends on operating temperature, fluid velocity and length of service of

heat exchanger. It is variable during heat exchanger operation.

• Fouling factors can be found in Table 11.1 textbook (SI units) or p. 51

heat exchanger lecture notes (EE units)


Overall Heat Transfer Coefficient

•The overall heat transfer coefficient can be written:

ooo

of

cond

i

if

iiooii AhA

RR

A

R

AhAUAUUA

11111++++===

"

,

"

, (11.4a)

o

ofcondo

i

ifo

ii

o

o

hRRA

A

RA

Ah

AU

1

1

++++

=

"

,

"

, (11.4b)


Determination of ho• Approach 1: Using correlations from Chapter 7

• Approach 2: Using chart by Kern, p. 56 heat exchanger lecture notes


Determination of tube side film coefficient, hi• Approach 1: Using correlations from Chapter 8

• Approach 2: Sieder and Tate relationship, p. 53 heat exchanger

lecture notes


Determination of Conduction Resistance

• Recall that

)/ln(

)/ln(

ioo

condo

iocond

DDk

DRA

kL

DDR

2

2

=

π=

• In EE units

)/ln( io

w

ocondow DD

k

DRAr

24==


Example 11.1

A counterflow, concentric tube heat exchanger is used to cool the

lubricating oil for a large industrial gas turbine engine. The flow rate of

cooling water through the inner tube (Di=25 mm) is 0.2 kg/s, while the

flow rate of oil through the outer annulus (Do=45 mm) is 0.1 kg/s. The

oil and water enter at temperatures of 100 and 30°C respectively. How

long must the tube be made if the outlet temperature of the oil is to be

60°C?


Table 8.2


Shell-and-Tube Heat Exchangers

Baffles are used to

establish a cross-flow and

to induce turbulent mixing

of the shell-side fluid, both

of which enhance

convection.

� The number of tube and

shell passes may be variedOne Shell Pass and One Tube Pass

One Shell Pass,

Two Tube PassesTwo Shell Passes,

Four Tube Passes


Some design tips

� A listing of common heat exchanger tube dimensions is included on

page 14, heat exchanger lecture notes.

� See p. 22-23 for useful information on baffle design


Multipass and Cross-Flow Heat Exchangers

To account for complex flow conditions in multipass, shell and tube

and cross-flow heat exchangers, the log-mean temperature difference

can be modified:

CFlmlm TFT ,∆=∆

where F=correction factor

Section 11.3.4 5th edition

For 6th edition see supplemental material (available on the

course website)


Correction Factor

where t is the tube-

side fluid

temperature


Example

A shell-and-tube heat exchanger must be designed to heat 2.5 kg/s of water

from 15 to 85°C. The heating is to be accomplished by passing hot engine

oil, which is available at 160°C, through the shell side of the exchanger. The

oil is known to provide an average convection coefficient of ho=400 W/m2.K

on the outside of the tubes. Ten tubes pass the water through the shell.

Each tube is thin walled, of diameter D=25 mm, and makes eight passes

through the shell. If the oil leaves the exchanger at 100°C, what is the flow

rate? How long must the tubes be to accomplish the desired heating?


Pressure Drop

In practice there can be a significant pressure drop along the pipes of

a multipass heat exchanger.

� Results in property changes

• Pressure drop must be accounted for in real design situations.

• See pages 67-69 in heat exchanger lecture notes.

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