Transcript Chapter 6

Chapter 11
Heat Exchangers (11.1-11.3)
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Case Study: Milk Pasteurization
• Every particle of milk or milk product must be heated to a specific
temperature for a specified period of time to inactivate bacteria that
may be harmful to health and cause spoilage.
– Public Health aspect
– Quality Considerations
• The extend of microorganism inactivation depends on the combination
of temperature and holding time.
• Ontario regulations:
– Milk: 63° C for not less than 30 min., 72° C for not less than 16 sec.
– Frozen dairy dessert mix (ice cream or ice milk, egg nog): at least 69° C
for not less than 30 min; at least 80° C for not less than 25 sec;
– Milk based products- with 10% mf or higher, or added sugar (cream,
chocolate milk, etc) 66° C/30 min, 75° C/16 sec
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Methods of Pasteurization
• Batch Method:
Uses a jacketed vat,
surrounded by
circulating water, steam,
or heating coils of water
or steam.
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Methods of Pasteurization
• Continuous process:
HTST (High
temperature short
time) pasteurization
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Continuous Process
Heat treatment is accomplished
using a plate heat exchanger.
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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.
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Concentric Tube Construction
Parallel
FlowFlow
Parallel
•
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-
Counterflow
Counterflow
:
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Cross-Flow Heat Exchangers
Finned-Both Fluids
Unmixed
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Unfinned-One Fluid Mixed
the Other Unmixed
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Shell-and-Tube Heat Exchangers
Baffles are used to establish a
cross-flow and to induce
turbulent mixing of the shellside fluid, both of which
enhance convection.
 The number of tube and shell
passes may be varied
One Shell Pass and One Tube Pass
One Shell Pass,
Two Tube Passes
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Two Shell Passes,
Four Tube Passes
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Compact Heat Exchangers
•
•
Widely used to achieve large heat rates per unit volume, particularly when one
or both fluids is a gas.
Characterized by large heat transfer surface areas per unit volume (>700
m2/m3), small flow passages, and laminar flow.
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Heat Exchanger Analysis
Recall from Chapter 8
• Expression for convection heat transfer for flow of a fluid inside a tube:
 c p (Tm,o  Tm,i )
qconv  m
• For case 3 involving constant surrounding fluid temperature:
To  Ti
q  U As Tlm
Tlm 
ln(To / Ti )
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Heat Exchanger Analysis
In a two-fluid heat exchanger, consider the hot and cold fluids separately:
qh  m h c p ,h (Th ,i  Th ,o )
qc  m c c p ,c (Tc,o  Tc,i )
(11.1) and
q  UA Tlm
(11.2)
 Need to define U and Tlm
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Overall Heat Transfer Coefficient
Parallel Flow
Counterflow
• 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.
1
1
ln(Do / Di )
1



UA hi Ai
2kL
ho Ao
(11.3a)
Note that:
1
1
1


UA Ui Ai Uo Ao
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where inner tube surface
outer tube surface
Ai  Di L
Ao  Do L
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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.
 Typical values in Table 11.1.
• The overall heat transfer coefficient can be written:
R "f ,i
R "f ,o
ln(Do / Di )
1
1
1





UA hi Ai
Ai
2kL
Ao
ho Ao
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(11.3b)
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Fin (extended surface) effects
• Fins reduce the resistance to convection heat transfer, by increasing
surface area.
• Expression for overall heat transfer coefficient includes overall surface
efficiency, or temperature effectiveness, ho, of the finned surface,
which depends on the type of fin (see also Ch. 3.6.4)
1
1
1



UA U c Ac U h Ah
R"f ,c
R"f ,h
1
1


 Rconduction 

(ho hA) c (ho A) c
(ho A) h (ho hA) h
(11.3c)
where c is for cold and h for hot fluids respectively
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Tlm: 1. Parallel-Flow Heat Exchangers
T1
T2
Parallel Flow
q  UA Tlm
Tlm 
T2  T1
ln(TCounterflow
2 / T1 )
where
T1  Th,i  Tc,i
T2  Th,o  Tc,o
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Tlm: 2. Counter-Flow Heat Exchangers
T1
T2
Counterflow
q  UA Tlm
Tlm 
T2  T1
ln(T2 / T1 )
where
T1  Th,i  Tc,o
T2  Th,o  Tc,i
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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?
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Special Operating Conditions
Condenser:
Hot fluid is
condensing
vapor (eg. steam)
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Evaporator/boiler:
Cold fluid is
evaporating liquid
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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:
Tlm  FTlm,CF
where F=correction factor (Figures 11.10-11.13) and
T1  Th,i  Tc,o
T2  Th,o  Tc,i
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Correction Factor
where t is the tubeside fluid
temperature
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Example 11.2
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?
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