Handbook of air conditioning and refrigeration / Shan K

Example 10.1. A DX coil in a packaged conditioning system has a supply volume flow rate of
5500 cfm (2595 L/s). Air enters the coil at a dry-bulb temperature of 80°F (26.7°C) and a wet-bulb
temperature of 67°F (19.4°C). Calculate the total cooling capacity of this DX coil at full load and
the conditions of the air leaving the coil, using the following data:
● A three-row, 15-fins/in. (1.7-fins/mm) coil with the same coil construction parameters shown in
Table 10.1
● Boiling heat-transfer coefficient for HCFC-22 inside the copper tubes of 700 Btu/h�ft 2 �°F (3975
W/m 2 �°C) at T ev � 45°F (7.2°C)
● Surface effectiveness for wet fins � s � 0.76
● Heat-transfer coefficient of the outer surface of the coil (corrugated fin) h o � 12.90 Btu/h�ft 2 �°F
(73.25 W/m 2 �°C)
Solution. For a DX coil using corrugated plate fins, select a face velocity v a � 550 fpm (2.75
m/s) to provide an efficient heat transfer and a reasonable air-side pressure drop, and avoid condensate
carryover. Face area A a for the DX coil can then be calculated as
For a 15-fins/in. (1.7-fins/mm) DX coil, from Table 10.1, the coil core surface area parameter F s �
17.48. At full-load operation, for a face area of 10 ft 2 , and a multiplying factor of 1.15 for the corrugated
surface, the outer surface area of this DX coil is
A o � F s N r A a � 17.48 � 1.15 � 3 � 10 � 603 ft 2 (56 m 2 )
For corrugated fins, A o /A i � 14.03 � 1.15. The inner area is therefore
As given, for a wet surface � s � 0.76. Then, from Eq. (10.6),
U o A o �
�
The heat capacity rate for conditioned air is calculated as
The number of transfer units is
A i �
1
A a � V˙ s
v a
1/(� s h o A o) � 1/(h r A i)
C a � 60V˙ a� ac pa � 60 � 5500 � 0.075 � 0.243 � 6014 Btu / h ��F
NTU � U o A o
C a
� 5500
550
603
14.03 � 1.15 � 37.4 ft2 (3.5 m 2 )
1
1/(0.76 � 12.90 � 603) � 1/(700 � 37.4)
� 4995
6014
From Eq. (10.3), the effectiveness for the DX coil can be calculated as
� wet � 1 � exp(-NTU) � 1 � e �0.831 � 1 � 0.436 � 0.564
Considering the influence of the superheated region, effectiveness of the DX coil should be multiplied
by a degrading factor F super � 0.95. Then
� DX � 0.0564(0.95) � 0.536
REFRIGERATION SYSTEMS: COMPONENTS 10.11
� 10 ft2
� 0.831
� 4995 Btu / h��F