Page 231 - Design and Operation of Heat Exchangers and their Networks

220 Design and operation of heat exchangers and their networks

The cost data for the total annual cost of heat exchangers were modeled
2
by Hadidi (2015) with C A ¼C a C A,tot , C A,tot ¼90 $/m , n A ¼0.6,
0
C el ¼20 $/MWh, τ¼5000h/yr, and η p ¼0.6, and the annuity factor for
fixed annual repayment of loan is given by (from Eq. 6.59)

r 0
0
C ¼ (5.100)
a n
0
1 1+ r Þ
ð
in which the rate of interest for loan r ¼0.1 and the plan lifetime n¼10yr.
0
It should be pointed out that in the previous designs, the fin efficiency has
not been mentioned. This might yield a wrong design with insufficient heat
transfer area. Because the fin layers are limited, the optimization would result
in large fin height and thin fin thickness.

Example 5.2 Design a plate-fin heat exchanger
Design an aluminum gas-to-air crossflow plate-fin heat exchanger using
offset-strip fins. The molar components of the flue gas are the following:
77% N 2 , 12% CO 2 ,7%O 2 , and 4% H 2 O. The design parameters and
their ranges are shown in Table 5.2 except for the fluid properties that
shall be calculated according to the fluid temperature. The heat loss to
the surrounding and the effect of heat conduction in fins and plates in
the flow direction are neglected. The exchanger shall have the minimum
2
total annual cost evaluated by Eqs. (5.97)–(5.99) with C A ¼100 $/m ,
n A ¼0.6, C el ¼30 $/MWh, τ¼6500h/yr, and η p ¼0.5. The area
reservation is not considered.
Solution
(1) Calculation of fluid properties
The fluid properties are calculated by RefProp. By assuming the outlet
temperatures and outlet pressures of the flue gas and air flows be their
inlet values, respectively,

5
T h,m ¼ T h,in + T h,out Þ=2 ¼ 513 K, p h,m ¼ p h,in + p h,out Þ=2 ¼ 10 Pa
ð
ð
5
T c,m ¼ T c,in + T c,out Þ=2 ¼ 277 K, p c,m ¼ p c,in + p c,out Þ=2 ¼ 10 Pa
ð
ð
we have
ρ ¼ ρ h,in ¼ ρ h,out ¼ 0:6988 kg=m , c p,h ¼ 1068 J=kgK
3
h
λ h ¼ 0:03909 W=mK, μ ¼ μ h,in ¼ μ h,out ¼ 2:672 10 5 sPa,
h
Pr h ¼ 0:7301
ρ ¼ ρ c,in ¼ ρ c,out ¼ 1:258 kg=m , c p,c ¼ 1006 J=kgK
3
c
λ c ¼ 0:02465 W=mK, μ ¼ μ ¼ μ ¼ 1:741 10 5 sPa,
c c,in c,out
Pr c ¼ 0:7102