Chapter 11
Heat Exchangers
Chapter Objectives
◼ To introduce performance parameters for
assessing the efficacy of a H.E.
◼ To develop methodology for designing a H.E.
◼ To predicting the performance of an existing
exchanger
What is heat exchanger?
 A device that exchange heat between two
fluids that are at different temperature and
separated by a solid wall is termed a heat
exchanger.
 Heat exchangers are used in space heating
and air-conditioning, power production, waste
heat recovery, and chemical processing.
Heat Exchanger Types
Concentric
tube heat
exchangers
Parallel Flow
➢ Simplest configuration.
Counterflow
➢ Superior performance associated with counter flow.
Cross-flow
heat
exchangers
Finned-Both Fluids Unmixed
Unfinned-One Fluid Mixed
the Other Unmixed
Heat Exchanger Types
One Shell Pass and One Tube Pass
Shell-andTube 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.
One Shell Pass, Two Tube Passes
Two Shell Passes, Four Tube Passes
Heat Exchanger Types
• 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, small
flow passages, and laminar flow.
(a)
(b)
(c)
(d)
(e)
Fin-tube (flat tubes, continuous plate fins)
Fin-tube (circular tubes, continuous plate fins)
Fin-tube (circular tubes, circular fins)
Plate-fin (single pass)
Plate-fin (multipass)
Overall Heat Transfer Coefficient
• An essential requirement for heat exchanger design or performance calculations.
• Contributing factors include convection and conduction associated with the
two fluids and the intermediate solid, as well as the potential use of fins on both
sides and the effects of time-dependent surface fouling.
• With subscripts c and h used to designate the cold and hot fluids, respectively,
the most general expression for the overall coefficient is:
1 = 1 = 1
UA (UA)c (UA)h
=
Rf ,c
Rf ,h
1
1
+
+ Rw +
+
(o hA)c (o A)c
(o A)h (o hA)h
(11.1)
Overall Heat Transfer Coefficient
2
➢ Rf → Fouling factor for a unit surface area (m  K/W)
→ Table 11.1
➢ Rw → Wall conduction resistance (K/W)
➢ o → Overall surface efficiency of fin array (Section 3.6.5)
 A

o,c or h = 1 − f (1 −  f ) 
A

c or h
A = At → total surface area (fins and exposed base)
A f → surface area of fins only
Heat Exchanger Analysis (1) LMTD
- The Log Mean Temperature Difference (LMTD) Method • A form of Newton’s law of cooling may be applied to heat exchangers by
using a log-mean value of the temperature difference between the two fluids:
q = U A T m
T m =
T1 − T2
1n ( T1 / T2 )
Evaluation of T1 and T2 depends on the heat exchanger type.
• Counter-Flow Heat Exchanger:
T1  Th ,1 − Tc ,1
= Th ,i − Tc ,o
T2  Th ,2 − Tc ,2
= Th ,o − Tc ,i
(11.14)
(11.15)
Heat Exchanger Analysis (1) LMTD
• Parallel-Flow Heat Exchanger:
T1  Th ,1 − Tc ,1
= Th ,i − Tc ,i
T2  Th ,2 − Tc ,2
= Th ,o − Tc ,o
과제: 예제 11.1에서 바깥쪽 환상공간의 직경을 45mm에서 40mm로 줄일
경우, 어떤 변화가 생기겠는가?
Heat Exchanger Analysis (2) The
Effectiveness-NTU Method
General Considerations
• Computational Features/Limitations of the LMTD Method:
➢ The LMTD method may be applied to design problems for
which the fluid flow rates and inlet temperatures, as well as
a desired outlet temperature, are prescribed. For a specified
HX type, the required size (surface area), as well as the other
outlet temperature, are readily determined.
➢ If the LMTD method is used in performance calculations for which
both outlet temperatures must be determined from knowledge of the
inlet temperatures, the solution procedure is iterative.
➢ For both design and performance calculations, the effectiveness-NTU
method may be used without iteration.
Heat Exchanger Analysis (2) The
Effectiveness-NTU Method
• Heat exchanger effectiveness,  :
q
0   1
=
qmax
(11.19)
• Maximum possible heat rate:
qmax = Cmin (Th,i − Tc,i )
Cmin
(11.18)
Ch if Ch  Cc
= or
 Cc if Cc  Ch
• Performance Calculations:
➢
 = f ( NTU, Cmin / Cmax )
Cr
예제 11.3
예제 11.5
➢ Relations → Table 11.3 or Figs. 11.10 - 11.15