Handbook of air conditioning and refrigeration / Shan K

28.8 CHAPTER TWENTY-EIGHT
The nominal flow rate is the flow rate of a fan coil whose external pressure drop is at a specific value
when the fan switch is in the high position and the fan coil is at sea level. The higher the external pressure
drop, the lower the volume flow rate. If the external pressure drop increases from 0.06 to 0.3 in.
WC (15 to 75 Pa), the volume flow rate of the fan coil may decrease to 55 percent of its nominal value.
For a typical fan coil, the volume flow rate of fan switch position medium is about 80 percent of
the high value, and low is only 70 percent of the high value.
Cooling and Dehumidifying
Because the face velocity of a specific fan-coil unit at nominal volume flow rate is nearly the same
for various sizes, and because the outer surface area of a water cooling coil is divided into dry and
wet parts during cooling and dehumidifying, the coil capacity and the sensible heat ratio of the
cooling and dehumidifying process SHR c depend on the following factors:
● Dry- and wet-bulb temperatures of entering air
● Entering water temperature
● Water temperature rise in the coil
● The surface area and number of rows, including both pipe surface and fin area, of the coil
For a 04 fan-coil unit with an entering air temperature of 80°F (26.7°C) dry-bulb and 67°F
(19.4°C) wet-bulb, and an entering water temperature of 45°F (7.2°C), if the water temperature rise
is 10°F (5.6°C), then its cooling and dehumidifying capacity Q cc varies from 11 to 14 MBtu/h
(3.22 to 4.1 kW), and SHR c varies from 0.65 to 0.80.
If the volume flow rate of a fan-coil unit deviates from the nominal value because of a greater
external pressure drop or a higher altitude, its total cooling capacity should be corrected as
follows:
Qc,c � CpCaQc,r (28.1)
where Qc,c � corrected total cooling capacity, MBtu/h (kW)
Qc,r � manufacturer’s catalog listed cooling capacity at a specific nominal volume flow rate,
MBtu/h (kW)
Cp, Ca � total cooling capacity correction factor for excessive external pressure drop and high
altitude, respectively
The corrected sensible cooling capacity of the fan coil Q cs,c, in MBtu/h (kW), is
Qcs,c � CpsCasqcs,r (28.2)
where Qcs,r � manufacturer’s catalog-listed sensible cooling capacity at nominal volume flow rate,
MBtu/h (kW)
Cps, Cas � sensible cooling capacity correction factor for excessive external pressure and high
altitude, respectively
Correction factors C p, C ps, C a, and C as can be found in the manufacturer’s catalog. If these data are
not available, the following values can then be used for C p and C ps:
External pressure drop, in. WC (Pa) C p C ps
0.06 (15) 1 1
0.1 (25) 0.96 0.96
0.2 (50) 0.84 0.83
0.25 (63) 0.76 0.73