11-1
Solutions Manual
for
Heat and Mass Transfer: Fundamentals & Applications
Fourth Edition
Yunus A. Cengel & Afshin J. Ghajar
McGraw-Hill, 2011
Chapter 11
HEAT EXCHANGERS
PROPRIETARY AND CONFIDENTIAL
This Manual is the proprietary property of The McGraw-Hill Companies, Inc. (“McGraw-Hill”) and
protected by copyright and other state and federal laws. By opening and using this Manual the user
agrees to the following restrictions, and if the recipient does not agree to these restrictions, the Manual
should be promptly returned unopened to McGraw-Hill: This Manual is being provided only to
authorized professors and instructors for use in preparing for the classes using the affiliated
textbook. No other use or distribution of this Manual is permitted. This Manual may not be sold
and may not be distributed to or used by any student or other third party. No part of this Manual
may be reproduced, displayed or distributed in any form or by any means, electronic or
otherwise, without the prior written permission of McGraw-Hill.
PROPRIETARY MATERIAL. © 2011 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course
preparation. If you are a student using this Manual, you are using it without permission.
, 11-2
Types of Heat Exchangers
11-1C Heat exchangers are classified according to the flow type as parallel flow, counter flow, and cross-flow arrangement.
In parallel flow, both the hot and cold fluids enter the heat exchanger at the same end and move in the same direction. In
counter-flow, the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite direction. In cross-flow,
the hot and cold fluid streams move perpendicular to each other.
11-2C A heat exchanger is classified as being compact if β > 700 m2/m3 or (200 ft2/ft3) where β is the ratio of the heat
transfer surface area to its volume which is called the area density. The area density for double-pipe heat exchanger can not
be in the order of 700. Therefore, it can not be classified as a compact heat exchanger.
11-3C Regenerative heat exchanger involves the alternate passage of the hot and cold fluid streams through the same flow
area. The static type regenerative heat exchanger is basically a porous mass which has a large heat storage capacity, such as
a ceramic wire mash. Hot and cold fluids flow through this porous mass alternately. Heat is transferred from the hot fluid to
the matrix of the regenerator during the flow of the hot fluid and from the matrix to the cold fluid. Thus the matrix serves as
a temporary heat storage medium. The dynamic type regenerator involves a rotating drum and continuous flow of the hot
and cold fluid through different portions of the drum so that any portion of the drum passes periodically through the hot
stream, storing heat and then through the cold stream, rejecting this stored heat. Again the drum serves as the medium to
transport the heat from the hot to the cold fluid stream.
11-4C In the shell and tube exchangers, baffles are commonly placed in the shell to force the shell side fluid to flow across
the shell to enhance heat transfer and to maintain uniform spacing between the tubes. Baffles disrupt the flow of fluid, and an
increased pumping power will be needed to maintain flow. On the other hand, baffles eliminate dead spots and increase heat
transfer rate.
11-5C Using six-tube passes in a shell and tube heat exchanger increases the heat transfer surface area, and the rate of heat
transfer increases. But it also increases the manufacturing costs.
11-6C Using so many tubes increases the heat transfer surface area which in turn increases the rate of heat transfer.
11-7C In counter-flow heat exchangers, the hot and the cold fluids move parallel to each other but both enter the heat
exchanger at opposite ends and flow in opposite direction. In cross-flow heat exchangers, the two fluids usually move
perpendicular to each other. The cross-flow is said to be unmixed when the plate fins force the fluid to flow through a
particular interfin spacing and prevent it from moving in the transverse direction. When the fluid is free to move in the
transverse direction, the cross-flow is said to be mixed.
PROPRIETARY MATERIAL. © 2011 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course
preparation. If you are a student using this Manual, you are using it without permission.
, 11-3
The Overall Heat Transfer Coefficient
11-8C Heat is first transferred from the hot liquid to the wall by convection, through the wall by conduction and from the
wall to the cold liquid again by convection.
11-9C When the wall thickness of the tube is small and the thermal conductivity of the tube material is high, which is usually
the case, the thermal resistance of the tube is negligible.
11-10C The heat transfer surface areas are Ai = πD1 L and Ao = πD 2 L . When the thickness of inner tube is small, it is
reasonable to assume Ai ≅ Ao ≅ As .
11-11C The effect of fouling on a heat transfer is represented by a fouling factor Rf. Its effect on the heat transfer coefficient
is accounted for by introducing a thermal resistance Rf /As. The fouling increases with increasing temperature and decreasing
velocity.
11-12C None.
11-13C When one of the convection coefficients is much smaller than the other hi << ho , and Ai ≈ A0 ≈ As . Then we have
( 1/hi >> 1/ho ) and thus U i = U 0 = U ≅ hi .
11-14C The most common type of fouling is the precipitation of solid deposits in a fluid on the heat transfer surfaces.
Another form of fouling is corrosion and other chemical fouling. Heat exchangers may also be fouled by the growth of algae
in warm fluids. This type of fouling is called the biological fouling. Fouling represents additional resistance to heat transfer
and causes the rate of heat transfer in a heat exchanger to decrease, and the pressure drop to increase.
11-15C When the wall thickness of the tube is small and the thermal conductivity of the tube material is high, the thermal
resistance of the tube is negligible and the inner and the outer surfaces of the tube are almost identical ( Ai ≅ Ao ≅ As ). Then
the overall heat transfer coefficient of a heat exchanger can be determined to from U = (1/hi + 1/ho)-1
PROPRIETARY MATERIAL. © 2011 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course
preparation. If you are a student using this Manual, you are using it without permission.
, 11-4
11-16 The heat transfer coefficients and the fouling factors on tube and shell side of a heat exchanger are given. The thermal
resistance and the overall heat transfer coefficients based on the inner and outer areas are to be determined.
Assumptions 1 The heat transfer coefficients and the fouling factors are constant and uniform.
Analysis (a) The total thermal resistance of the heat exchanger per unit length is
1 R fi ln( Do / Di ) R fo 1
R= + + + +
hi Ai Ai 2πkL Ao ho Ao
1 (0.0005 m 2 .°C/W)
R= +
(800 W/m 2 .°C)[π (0.012 m)(1 m)] [π (0.012 m)(1 m)]
ln(1..2) (0.0002 m 2 .°C/W)
+ +
2π (380 W/m.°C)(1 m) [π (0.016 m)(1 m)]
1 Outer surface
+ 2 D0, A0, h0, U0 , Rf0
(240 W/m .°C)[π (0.016 m)(1 m)]
Inner surface
= 0.1334°C/W Di, Ai, hi, Ui , Rfi
(b) The overall heat transfer coefficient based on the inner and the outer surface
areas of the tube per length are
1 1 1
R= = =
UA U i Ai U o Ao
1 1
Ui = = = 199 W/m 2 .°C
RAi (0.1334 °C/W)[π (0.012 m)(1 m)]
1 1
Uo = = = 149 W/m 2 .°C
RAo (0.1334 °C/W)[π (0.016 m)(1 m)]
PROPRIETARY MATERIAL. © 2011 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course
preparation. If you are a student using this Manual, you are using it without permission.
Solutions Manual
for
Heat and Mass Transfer: Fundamentals & Applications
Fourth Edition
Yunus A. Cengel & Afshin J. Ghajar
McGraw-Hill, 2011
Chapter 11
HEAT EXCHANGERS
PROPRIETARY AND CONFIDENTIAL
This Manual is the proprietary property of The McGraw-Hill Companies, Inc. (“McGraw-Hill”) and
protected by copyright and other state and federal laws. By opening and using this Manual the user
agrees to the following restrictions, and if the recipient does not agree to these restrictions, the Manual
should be promptly returned unopened to McGraw-Hill: This Manual is being provided only to
authorized professors and instructors for use in preparing for the classes using the affiliated
textbook. No other use or distribution of this Manual is permitted. This Manual may not be sold
and may not be distributed to or used by any student or other third party. No part of this Manual
may be reproduced, displayed or distributed in any form or by any means, electronic or
otherwise, without the prior written permission of McGraw-Hill.
PROPRIETARY MATERIAL. © 2011 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course
preparation. If you are a student using this Manual, you are using it without permission.
, 11-2
Types of Heat Exchangers
11-1C Heat exchangers are classified according to the flow type as parallel flow, counter flow, and cross-flow arrangement.
In parallel flow, both the hot and cold fluids enter the heat exchanger at the same end and move in the same direction. In
counter-flow, the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite direction. In cross-flow,
the hot and cold fluid streams move perpendicular to each other.
11-2C A heat exchanger is classified as being compact if β > 700 m2/m3 or (200 ft2/ft3) where β is the ratio of the heat
transfer surface area to its volume which is called the area density. The area density for double-pipe heat exchanger can not
be in the order of 700. Therefore, it can not be classified as a compact heat exchanger.
11-3C Regenerative heat exchanger involves the alternate passage of the hot and cold fluid streams through the same flow
area. The static type regenerative heat exchanger is basically a porous mass which has a large heat storage capacity, such as
a ceramic wire mash. Hot and cold fluids flow through this porous mass alternately. Heat is transferred from the hot fluid to
the matrix of the regenerator during the flow of the hot fluid and from the matrix to the cold fluid. Thus the matrix serves as
a temporary heat storage medium. The dynamic type regenerator involves a rotating drum and continuous flow of the hot
and cold fluid through different portions of the drum so that any portion of the drum passes periodically through the hot
stream, storing heat and then through the cold stream, rejecting this stored heat. Again the drum serves as the medium to
transport the heat from the hot to the cold fluid stream.
11-4C In the shell and tube exchangers, baffles are commonly placed in the shell to force the shell side fluid to flow across
the shell to enhance heat transfer and to maintain uniform spacing between the tubes. Baffles disrupt the flow of fluid, and an
increased pumping power will be needed to maintain flow. On the other hand, baffles eliminate dead spots and increase heat
transfer rate.
11-5C Using six-tube passes in a shell and tube heat exchanger increases the heat transfer surface area, and the rate of heat
transfer increases. But it also increases the manufacturing costs.
11-6C Using so many tubes increases the heat transfer surface area which in turn increases the rate of heat transfer.
11-7C In counter-flow heat exchangers, the hot and the cold fluids move parallel to each other but both enter the heat
exchanger at opposite ends and flow in opposite direction. In cross-flow heat exchangers, the two fluids usually move
perpendicular to each other. The cross-flow is said to be unmixed when the plate fins force the fluid to flow through a
particular interfin spacing and prevent it from moving in the transverse direction. When the fluid is free to move in the
transverse direction, the cross-flow is said to be mixed.
PROPRIETARY MATERIAL. © 2011 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course
preparation. If you are a student using this Manual, you are using it without permission.
, 11-3
The Overall Heat Transfer Coefficient
11-8C Heat is first transferred from the hot liquid to the wall by convection, through the wall by conduction and from the
wall to the cold liquid again by convection.
11-9C When the wall thickness of the tube is small and the thermal conductivity of the tube material is high, which is usually
the case, the thermal resistance of the tube is negligible.
11-10C The heat transfer surface areas are Ai = πD1 L and Ao = πD 2 L . When the thickness of inner tube is small, it is
reasonable to assume Ai ≅ Ao ≅ As .
11-11C The effect of fouling on a heat transfer is represented by a fouling factor Rf. Its effect on the heat transfer coefficient
is accounted for by introducing a thermal resistance Rf /As. The fouling increases with increasing temperature and decreasing
velocity.
11-12C None.
11-13C When one of the convection coefficients is much smaller than the other hi << ho , and Ai ≈ A0 ≈ As . Then we have
( 1/hi >> 1/ho ) and thus U i = U 0 = U ≅ hi .
11-14C The most common type of fouling is the precipitation of solid deposits in a fluid on the heat transfer surfaces.
Another form of fouling is corrosion and other chemical fouling. Heat exchangers may also be fouled by the growth of algae
in warm fluids. This type of fouling is called the biological fouling. Fouling represents additional resistance to heat transfer
and causes the rate of heat transfer in a heat exchanger to decrease, and the pressure drop to increase.
11-15C When the wall thickness of the tube is small and the thermal conductivity of the tube material is high, the thermal
resistance of the tube is negligible and the inner and the outer surfaces of the tube are almost identical ( Ai ≅ Ao ≅ As ). Then
the overall heat transfer coefficient of a heat exchanger can be determined to from U = (1/hi + 1/ho)-1
PROPRIETARY MATERIAL. © 2011 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course
preparation. If you are a student using this Manual, you are using it without permission.
, 11-4
11-16 The heat transfer coefficients and the fouling factors on tube and shell side of a heat exchanger are given. The thermal
resistance and the overall heat transfer coefficients based on the inner and outer areas are to be determined.
Assumptions 1 The heat transfer coefficients and the fouling factors are constant and uniform.
Analysis (a) The total thermal resistance of the heat exchanger per unit length is
1 R fi ln( Do / Di ) R fo 1
R= + + + +
hi Ai Ai 2πkL Ao ho Ao
1 (0.0005 m 2 .°C/W)
R= +
(800 W/m 2 .°C)[π (0.012 m)(1 m)] [π (0.012 m)(1 m)]
ln(1..2) (0.0002 m 2 .°C/W)
+ +
2π (380 W/m.°C)(1 m) [π (0.016 m)(1 m)]
1 Outer surface
+ 2 D0, A0, h0, U0 , Rf0
(240 W/m .°C)[π (0.016 m)(1 m)]
Inner surface
= 0.1334°C/W Di, Ai, hi, Ui , Rfi
(b) The overall heat transfer coefficient based on the inner and the outer surface
areas of the tube per length are
1 1 1
R= = =
UA U i Ai U o Ao
1 1
Ui = = = 199 W/m 2 .°C
RAi (0.1334 °C/W)[π (0.012 m)(1 m)]
1 1
Uo = = = 149 W/m 2 .°C
RAo (0.1334 °C/W)[π (0.016 m)(1 m)]
PROPRIETARY MATERIAL. © 2011 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course
preparation. If you are a student using this Manual, you are using it without permission.
