Handbook of air conditioning and refrigeration / Shan K

15.50 CHAPTER FIFTEEN
Wet Part
coil Twd, m, in °F (°C), is (Twb � Twl). Unit sensible cooling capacity of each ft2 (m2 1�2 ) outer surface
area of the dry part of the water cooling coil, including the tube and fins, can then be calculated as
Because U o can be calculated from Eq. (15.27), the total outer surface area of the dry part of the
coil A o, d, in ft 2 (m 2 ) is
(15.46)
As for the DX coil, the cooling and dehumidifying capacity of the wet part of a dry-wet coil Q c, w,
in Btu/h (W), can be calculated as
(15.47)
where h s, we � enthalpy of saturated air film at chilled water entering temperature,°F (°C). The effectiveness
of the wet part of the dry-wet coil (wet, the number of transfer units (NTU), and the
heat capacity rates C a and C w can be similarly calculated by Eqs. (15.39), (15.40), (15.36), and
(15.37), respectively.
Dry-Wet Coil at Part-Load Operation
Selection of a Dry-Wet Coil
Q cs
A o, d
� U o(T ad, m � T wd, m)
Ao, d �
Uo (Tad, m � Twd, m)
Q c, w �� wet60V˙ a� a(h ab � h s,we)
When the dry-wet coil of a constant-volume system is at part-load operation, the reduction of the
space cooling load results in a drop in space temperature. This causes the control system to modulate
the two-way valve at the inlet of the water cooling coil to reduce the mass flow rate of chilled
water flowing through the coil, as shown in Fig. 15.32. Because of the reduction of the chilled water
flow rate, the following conditions occur:
● The velocity of chilled water inside the coil tubes is reduced, and therefore the rate of heat-transfer
between the air and water decreases.
● Although the reduction of the space cooling load tends to lower the temperature of conditioned
air leaving the coil T al, nonetheless T al increases because of the reduction of the chilled water flow
rate and has a greater influence than the drop of the space cooling load.
● From the psychrometric analysis shown in Fig. 15.32, the space conditioning line sp-rp at partload
operation is shorter because of the reduction of space cooling load.
● The chilled water flow rate is reduced so that the off-coil temperature T ccp and supply temperature
T sp match the reduction of space cooling load and maintain a space temperature T rp at part-load
operation.
● The higher dry-bulb and dew point temperatures of conditioned air leaving the coil cause an
increase in space relative humidity � r. Sometimes, � r may be raised to 70 percent during partload
operation. This is the main disadvantage in controlling space temperature by means of
chilled water flow rate modulation at part-load operation.
The construction of a water cooling coil, a dry-wet coil, is similar to that of a DX coil, covered in
Sec. 10.2. As for a DX coil, the procedure for selecting a water cooling coil according to the manufacturer’s
data is as follows:
Q cs