Building Design and Construction Handbook - Merritt - Ventech!

13.88 SECTION THIRTEEN
FIGURE 13.43 Control circuit for a cooling-tower-type HVAC system.
In buildings where one tower and pump are used to provide water for many
units, pressure switches may be used in series with the holding coil of each compressor
motor starter. No interconnecting wiring is necessary, for the compressor
will not be able to run unless the pump is in operation and provides the necessary
pressure to close the contacts on the pressure switch.
Some city ordinances require that indirect-expansion, or chilled-water, systems
be installed in public buildings, such as theaters, night clubs, hotels, and depots.
This type of system is illustrated in Fig. 13.30. The refrigerant is used to cool water
and the water is circulated through the cooling coil to cool the air. The water
temperature should be between 40 and 45�F, depending on whether the occupancy
is a high latent load or a low latent load. Because the cost of equipment is increased
and the capacity decreased as water temperature is lowered, most designers use
45�F water as a basis for design and use a much deeper cooling coil for high-latentload
installations.
The amount of chilled water in gallons per minute to be circulated may be
obtained from
24 � tons of refrigeration
Q � (13.37)
�T
where �T is the temperature rise of water on passing through the cooling coil,
usually 8 or 10�F. The smaller the temperature change, the more water to be circulated
and the greater the pumping cost, but the better will be the heat transfer
through the water chiller and cooling coil.
Coil manufacturers’ catalogs usually give the procedure for picking the number
of rows of coil necessary to do the required cooling with the water temperatures
available.
The water chiller should be sized to cool the required flow of water from the
temperature leaving the cooling coil to that required by the coil at a given suction
temperature.
Assuming 45�F water supplied to the cooling coil and a temperature rise of 8�F,
the water leaving the coil would be at 53�F. The chiller would then be picked to
cool the required flow from 53 to 45�F at37�F suction. The lower the suction