Handbook of air conditioning and refrigeration / Shan K

15.54 CHAPTER FIFTEEN
Using an air-side heat-transfer coefficient h o � 13.4 Btu/h�ft 2 �°F as in Example 15.1, from Eq.
(10.21), the heat-transfer coefficient for the wet surface is
Then
3. On the water side,
As in Example 15.1, B � 11.4 � 1.15 � 13.1, from Eq. (15.43), for 1 ft 2 of outer surface area at
the dry-wet boundary,
From Eq. (15.44),
hwet � m�
ho � 2.84 � 13.4 � 38.06 Btu/h�ft
cpa 2�°F (216 W/m2 ��C)
Re D � �v wD i
� w
According to the above simultaneous equations,
The dry-wet boundary can therefore be determined as shown in Fig. 15.31.
4. From Eq. (15.45), the sensible cooling capacity of the dry part of the dry-wet coil is
Q cs � 60V˙ a� ac pa(T ae � T ab)
The average temperature of the airstream in the dry part is
�
Pr � � wc pw
k w
h i � 0.023 Re D 0.8 Pr 0.4 � 0.023 � (12,020) 0.8 (9.12) 0.4
� 838.6 Btu/h�ft 2 �°F
h wet� s(T ab � T sb) � h i
B (T sb � T wb)
38.06 � 0.76 (T ab � 60) � 838.6
13.1 (60 � T wb)
V˙ a� ac pa(T ae � T ab) � 8.33V˙ galc pw(T wl � T wb)
11,000 � 0.075 � 0.243(80 � T ab) � 8.33 � 74 � 1(55 � T wb)
T ab � 74.8�F and T wb � 53.3�F (11.83�C)
� 11,000 � 60 � 0.075 � 0.243 � (80 � 74.8) � 62,548 Btu/h (18,327 W)
T ad, m � T ae � T ab
2
The average temperature of the water stream corresponding to the dry part of the dry-wet coil is
T wd,m � T wl � T wb
2
62.4 � 4 � 3600 � 0.496
3.09 � 12
�
�
3.09 � 1
0.339
80 � 74.8
2
� 9.12
� 77.4�F
55 � 53.3
� � 54.15�F
2
� 12,020