Handbook of air conditioning and refrigeration / Shan K

27.20 CHAPTER TWENTY-SEVEN
If the electric power rate is $0.05/kWh, the number of payback hours for an indirect-direct
evaporative cooler added onto a DX coil packaged system is 7948; if the rate is $0.075/kWh, the
number of payback hours is 5295; and if the rate is $0.1/kWh, the number of payback hours is
3974 h.
Tower Coil and Rotary Wheel Combination
When a cooling tower is connected to a water cooling/heating coil as shown in Fig. 27.6a, the
tower coil becomes an indirect evaporative cooler. During the cooling season, the condenser water
from the tower is forced through the coil to cool the air flowing over it. Such a cooling coil is often
used as a precooling coil, because there may be another cooling coil downstream. In the heating
season, hot water from the condenser may flow through the coil and heat the air.
Using water from the cooling tower to cool the air by means of a precooling coil (to replace all
or part of the refrigeration) is often called a water-economizing process. The combination of a
cooling tower and the connected water cooling coil is called a water economizer and is shown in
Fig. 27.6a.
If the tower coil is in series with a direct evaporative cooler using a rotary wheel, the combination
is actually an indirect-direct cooler which can maintain space conditions in summer similar to
those achieved with refrigeration for areas where the outdoor wet-bulb temperature is below 65°C
(18.3°C), as discussed in Sec. 27.4. Water returns from the precooling coil, typically at a temperature
of 78°F (25.6°C), enters the cooling tower, and is evaporatively cooled to about 70°F (21.1°C).
Water is then drawn through the precooling coil, where it absorbs the heat from the air flowing over
the coil. It is then pumped back to the tower at a temperature of about 78°F (25.6°C) to be evaporatively
cooled again.
Outdoor air at a dry-bulb temperature of 100°F (37.8°C) and a wet-bulb temperature of 65°F
(18.3°C) is drawn through the precooling coil by the supply fan and is sensibly cooled to 75°F
(23.9°C). It then flows through a rotary wheel type of direct cooler and is evaporatively cooled,
typically to 57.5°F (14.2°C) dry-bulb and 56°F (13.6°C) wet-bulb temperatures. After that, air is
supplied to the conditioned space. Recirculating air may be used instead of outdoor air when it is
more beneficial.
In an effective tower coil, the approach of the cooling tower should be around 5°F (2.8°C). In
order to have such an approach, the cooling tower must be 60 percent larger than a tower with a
10°F (5.6°C) approach. The row depth and fin spacing of the precooling coil should be selected to
sensibly cool the outdoor air from 100°F (37.8°C) to 75°F (23.9°C) at an entering water temperature
of 70°F (21.1°C).
Field experience and tests have shown that a low face velocity through the wetted media of the
rotary wheel results in a saturation efficiency above 0.90 and prevents carryover. In these tests, the
air velocity was about 700 fpm (3.5 m/s) with a pressure drop of 0.25 in. WC (63 Pa). The rotary
wheel revolved at a rate of approximately 1.5 r/min.
In a system with an add-on evaporative cooler, the horsepower and number of operating hours of
the fan always far exceed those of the water pump. To save energy and reduce operating costs for a
large unit, a two-speed (or even three-speed) fan is often economical. The efficiency of the cooler
that consists of cooling tower, cooling coil, and rotary wheel depends largely on the conditions of
the fills in the tower, the inner surface of the cooling coil, and the wetted media in the rotary wheel.
If the wetted surfaces are clogged with dirt and scale, the efficiency will decrease proportionally to
the resulting drop in evaporation and airflow. Periodic bleedoff and other necessary water treatments
are essential for good performance.
Another type of spraying coil–rotary wheel combination is shown in Fig. 27.6b. When a spraying
coil is connected to a cooling coil, water that has been evaporatively cooled in the former can be
pumped to the cooling coil to absorb heat from the ambient air. Such a spraying coil-coil combination
is actually an indirect cooler. If this indirect cooler is connected with a rotary wheel or other
type of direct cooler, the resulting combination has a system performance similar to that of a tower-