Handbook of air conditioning and refrigeration / Shan K

27.22 CHAPTER TWENTY-SEVEN
coil and rotary wheel combination. The heat transfer between two airstreams by means of a spraying
coil-coil combination is always higher than that of a coil-to-coil arrangement, which is commonly
located inside the outdoor air and exhaust airstreams or between makeup air and exhaust
airstreams, and is often called a runaround system.
Tower, Plate-and-Frame Heat Exchanger, and Coil Combination
When the outdoor wet-bulb temperature drops below a certain value (about 40°F, or 4.4°C), it is
possible to use the condenser water from the cooling tower to cool the air and thus replace all or
part of the chilled water from the refrigeration plant, as shown in Fig. 27.7. To prevent the dirt and
solid matter contained in the open-circuit tower condenser water from scaling the inner surface of
the chilled water coil, a plate-and-frame heat exchanger is often used so that the tower condenser
water does not enter the coil directly.
In Fig. 27.7, the condenser water from the tower at 45°F (7.2°C) enters the plate-and-frame heat
exchanger. It cools all or part of the chilled water returning from the cooling coil to 50°F (10.0°C).
Chilled water from the plate-and-frame heat exchanger is then mixed with the water from the centrifugal
chiller and enters the chilled water cooling coil at 45°F (7.2°C).
To cool the chilled water at the heat exchanger when the outdoor wet-bulb temperature is 40°F
(4.4°C), the approach of the cooling tower should be no greater than 5°F (2.8°C) and the temperature
difference at the heat exchanger between the condenser water from the tower and the chilled
FIGURE 27.7 Tower, plate-and-frame heat exchanger, and coil combination.