Handbook of air conditioning and refrigeration / Shan K

FIGURE 15.32 Psychrometric analysis of part-load operation of a dry-wet coil.
AIR SYSTEMS: COMPONENTS—FANS, COILS, FILTERS, AND HUMIDIFIERS 15.51
1. According to the required supply volume flow rate, choose an optimum face velocity v a; it is
usually not greater than 550 fpm (2.75 m/s) to prevent condensate carryover, wet surfaces and possible
mold growth in air system, and a 500 fpm (2.5 m/s) or less cooling coil face velocity is preferable.
Calculate the face area A a of the coil.
2. From the coil load, calculate the unit cooling and dehumidifying capacity per ft 2 face area
Q c/A a.
3. Determine the chilled water temperature entering the coil T we. This is closely related to the required
condition of air leaving the coil T al.
4. Select the optimum chilled water temperature rise �T w. A lower �T w means a greater water
flow rate, a higher water velocity in the tubes, and a greater heat-transfer coefficient. A lower �T w
also results in a greater pump power and a higher initial investment in the water system. Water temperature
rise �T w also influences air system and refrigeration system performance. It must be
matched with the expected temperature of conditioned air off the coil in the air system as well as
the chilled water temperature difference in the chiller.
The current practice is to use a �Tw from 10 to 20°F (5.6 to 11.1°C) in order to reduce water
flow and save pump power. When �Tw is determined, V˙
gal,
in gpm (L/s), can be calculated accordingly.