Handbook of air conditioning and refrigeration / Shan K

(T w2 � T w1)/(h s � h a), or their average, 1/2[1/(h s86 � h a78) � 1/(h s85 � h a78)]. The tower coefficient
of the cooling tower that cools the condenser water from 95 to 85°F is the summation or
integration of the tower coefficient of 95 to 94°F,...,86 to 85°F.
From App. Table B.1, at a wet-bulb temperature of 78°F, the enthalpy of saturated air is 41.59
Btu/lb. Because the enthalpy of the saturated air film at a water temperature 85°F is 49.45 Btu/lb,
the enthalpy difference between the saturated air film at 85°F and air at 78°F wet-bulb temperature
is h s � h a � 49.45 � 41.59 � 7.86 Btu/lb, and 1/(h s � h a) � 0.1272 lb/Btu (0.0609 kg/kJ).
From Eq. (10.22), for each degree of increase in condenser water temperature, the corresponding
increase in the enthalpy of air is
Then at a condenser water temperature of 86°F, the enthalpy of saturated air film is 50.68 Btu/lb,
and the enthalpy of the corresponding air in contact with this condenser water is
h a � 41.59 � 1.2 � 42.79 Btu/lb
The enthalpy difference between the saturated air film and air at T w � 86°F is then
and
h s � h a � 50.68 � 42.79 � 7.89 Btu/lb
1
hs � ha dh a � m˙ w
m˙ a
� 1
7.89
REFRIGERATION SYSTEMS: COMPONENTS 10.45
c pw dT w � (1.2 � 1) dT w
� 0.1267 lb / Btu (0.0606 kg / kJ)
TABLE 10.2 Numerical Integration of a Counterflow Cooling Tower for Example 10.2
Cooling Enthalpy of
water saturated air Air enthalpy h s – h a,
temperature, °F film h s, Btu/lb h a, Btu/lb Btu/lb
1
,
hs � ha lb/Btu
85 49.45 41.59
Design conditions
7.86 0.1272
86 50.68 42.79 7.89 0.1267
87 51.95 43.99 7.96 0.1256
88 53.25 45.19 8.06 0.1241
89 54.58 46.39 8.19 0.1221
90 55.95 47.59 8.36 0.1196
91 57.36 48.79 8.57 0.1167
92 58.79 49.99 8.80 0.1136
93 60.27 51.19 9.08 0.1101
94 61.79 52.39 9.40 0.1064
95 63.34 53.59 9.75 0.1026
Part-load operation (50 percent design load)
82 45.91 41.59 4.32 0.2315
83 47.06 42.79 4.27 0.2341
84 48.23 43.99 4.24 0.2358
85 49.45 45.19 4.26 0.2347
86 50.68 46.39 4.29 0.2331
87 51.95 47.89 4.36 0.2294
Average
�Tw ,
hs � ha �� ,
lb/Btu�°F
�Tw hs � h �
a
0.1270 0.1270
0.1262 0.2532
0.1249 0.3781
0.1231 0.5012
0.1209 0.6221
0.1182 0.7403
0.1151 0.8554
0.1118 0.9672
0.1082 1.0754
0.1045 1.1799
0.2328 0.2328
0.2350 0.4678
0.2402 0.7080
0.2338 0.9418
0.2312 1.1730