Handbook of air conditioning and refrigeration / Shan K

15.56 CHAPTER FIFTEEN
6. From the psychrometric chart, at a T ab of 74.8°F and a dew point of 60°F, h ab � 30.3 Btu/lb�
Also the enthalpy of saturated air film h s,we � 17.65 Btu/lb when the temperature T we is 45°F. Then
the cooling and dehumidifying capacity of the wet part of the dry-wet coil is
The cooling capacity of the dry-wet coil is
or 367,119/20 � 18.4 MBtu/h�ft 2 which is close to the value 19.1 MBtu/h�ft 2 given in the manufacturer’s
catalog.
7. If the water velocity inside the copper tubes is reduced to 63 percent of the full-load value
v w � 0.63 � 4 � 2.5 ft/s and the water temperature rise is increased to 16°F, from the performance
data shown in manufacturer’s catalog, at an entering dry-bulb temperature of 79°F, entering wetbulb
temperature of 68°F, and an enthalpy of 32.4 Btu/lb, the cooling capacity is reduced from
19.1 to 15.3 MBtu/h�ft 2 . Total cooling and dehumidifying capacity of the water cooling coil at part
load is
The reduction of enthalpy �h during the cooling and dehumidifying process at part-load operation is
The enthalpy of the air leaving the coil is
h al � 32.4 � 6.2 � 26.2 Btu/lb
At an entering dry-bulb temperature of 79°F and wet-bulb temperature of 68°F, from the cooling
and dehumidifying curve on the psychrometric chart, the condition of air off the coil at part-load
operation is 61.3°F dry-bulb and 59.5°F wet-bulb temperature. The load ratio at part load �
15.3/19.1 � 0.80.
If the sensible heat ratio and system heat gain at part-load operation are the same as in full-load
operation, the air conditioning cycle at part load can then be drawn on the psychrometric chart, as
shown by the dotted line in Fig. 15.32. The space relative humidity has been increased from 55
percent at full load to 59 percent at 80 percent part-load operation. Air conditioning processes and
cycles are discussed in Chap. 20.
15.11 COIL ACCESSORIES, CONDENSATE CONTROL, AND
FREEZE-UP PROTECTION
Coil Accessories
Q c,w �� wetV˙ a� a(h ab � h s,we)
� 0.4864 � 11,000 � 60 � 0.075(30.3 � 17.65) � 304,571 Btu/h
Q c � Q cs � Q c,w � 62,548 � 304,571 � 367,119 Btu/h (107,566 W)
Q c � 15.3 � 1000 � 60 � 20 � 306,000 Btu/h (89,660 W)
306,000
�h �
� 6.2 Btu/lb
11,000 � 60 � 0.075
Coil accessories include air vents, drain valves, isolation valves, flow metering valves, balancing
valves, thermometers, pressure gauge taps, condensate drain taps, and even distribution baffles.
Air Vents. These devices discharge air from the coils. Air vents are usually located at the highest
point of the coil and at the pipe risers. Manual vents are inexpensive and more reliable, whereas
automatic vents are more convenient to operate.