Handbook of air conditioning and refrigeration / Shan K

16.22 CHAPTER SIXTEEN
16.22
If the required supply air volume flow rate deviates from the nominal rated value in percentage,
as shown below, then the cooling capacity and sensible cooling capacity can be roughly multiplied
by a multiplier because of the change of the heat-transfer coefficient as follows:
2. Calculate the heating coil load at the winter design condition or at the warm-up period. Determine
the capacity of the gas-fired furnace, electric heater, water or steam heating coil. For a
packaged heat pump, calculate the supplementary heating capacity of the gas-fired or electric
heater.
3. Evaluate the external total pressure loss of the duct system, terminal, and space diffusion devices
(see Chaps. 17 and 18). Determine the speed of the supply fan, relief fan, exhaust fan, or return
fan, such that the volume flow and the fan total pressure of the supply fan, supply fan/return
fan combination is equal to or greater than the sum of the external total pressure loss and the total
pressure loss in the PU.
For fans for which only a fan static pressure is given, roughly a 0.4-in. WC (100-Pa) of velocity
pressure can be added to the fan static pressure for purposes of rough estimation of fan total
pressure.
Check that the face velocity of the DX coil does not exceed 550 fpm (2.75 m/s) so that condensate
will not carry over.
4. For small PUs, a medium-efficiency air filter of lower pressure drop, such as a final pressure
drop between 0.4 and 0.6 in. WC (100 to 150 Pa), should be selected.
Example 16.1. Select an AHU or a rooftop PU for a typical floor in a commercial building with
the following operating characteristics:
Supply volume flow rate 16,000 cfm (7550 L/s)
Cooling coil load 520,000 Btu/h or 43 tons (152,360 W)
Sensible cooling coil load 364,000 Btu/h (106,650 W)
Outdoor air temperature 95°F (35°C)
Entering coil dry-bulb temperature 80°F (26.7°C)
Entering coil wet-bulb temperature 67°F (19.4°C)
External pressure drop 2.0 in. WC (500 Pa)
Total pressure loss of AHU or PU 2.25 in. WC (563 Pa)
For a 4-row, 12-fins/in. water-colling coil in an AHU, chilled water enters the coil at 45°F
(7.2°C) and is expected to leave the coil at 55°F (12.7°C).
Solution:
Volume flow � 20% � 10% 0% �10% �20%
Cooling capacity 0.965 0.985 1.0 1.015 1.025
Sensible cooling 0.94 0.97 1.0 1.03 1.06
1. Divide the supply volume flow rate by 500 fpm, that is, 16,000/500 � 32. From Tables 16.1
and 16.2, select an AHU of size 30, which gives a maximum static pressure up to 8 in. WG and a
face area of cooling coil of 29 ft2 . The actual face velocity of cooling coil is
vcoil � 16,000
� 551 fpm (2.75 m /s)
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