Building Design and Construction Handbook - Merritt - Ventech!

HEATING, VENTILATION, AND AIR CONDITIONING 13.73
The capacity of heat rejection equipment, such as towers or evaporative condensers,
depends on the wet-bulb temperature. The capacity of these units decreases
as the wet-bulb temperature increases.
Such equipment should be sized for a wet-bulb temperature a few degrees above
that used for sizing air-conditioning equipment.
As an example, consider an area where the design wet-bulb temperature is 75�F.
If we size the air-conditioning equipment for this condition, we shall be able to
maintain design inside conditions when the outside conditions happen to be 75�F
WB. There will be a few days a year, however, when the outside air may register
79 or 80�F WB. During the higher wet-bulb days, with the air-conditioning equipment
in operation, we shall balance out at a relative humidity above design. For
example, if the design relative humidity is 50%, we may balance out at 55% or
higher.
However, if the water towers and evaporative condensers are designed for 75�F
WB, then at a wet-bulb temperature above 75, the equipment capacity would be
decreased and the compressor head pressure would build up too high, overload the
motor, and kick out on the overload relays. So we use 78�F WB for the design of
heat rejection equipment. Now, on a 78�F WB day, the compressor head pressure
would be at design pressure, the equipment will be in operation, although maintaining
room conditions a little less comfortable than desired. At 80�F WB, the
compressor head pressure will build up above design pressure and the motor will
be drawing more than the design current but usually less than the overload rating
of the safety contact heaters.
Water condensers are sized for 104�F refrigerant temperature and 95�F water
leaving.
The amount of water in gallons per minute required for condensers is subject
to the type of heat rejection equipment used and manufacturer’s data should be
used. A rule of thumb is 3 gpm/ton per 10�F water-temperature rise.
When choosing a condenser for water-tower use, we must determine the water
temperature available from the cooling tower with 95�F water to the tower. Check
the tower manufacturer for the capacity required at the design WB, and 95�F water
to the sprays, and the appropriate wet-bulb approach. (Wet-bulb approach is equal
to the number of degrees the temperature of the water leaving the tower is above
the wet-bulb temperature. This should be for an economic arrangement about 7�F.)
Thus, for 78�F, the water leaving the tower would be 85�F, and the condenser would
be designed for a 10�F water-temperature rise, i.e., for water at 95�F to the tower
and 85�F leaving the tower.
Evaporative condensers should be picked for the required capacity at a design
wet-bulb temperature for the area in which they are to be installed. Manufacturers’
ratings should be checked before the equipment is ordered.
For small- and medium-size cooling systems, air-cooled condensers are available
for outdoor installation with propeller fans or for indoor installation usually with
centrifugal blowers for forcing outdoor air through ducts to and from the condensers.
13.24 COMPRESSOR-MOTOR UNITS
Compressor capacity decreases with increase in head pressure (or temperature) and
fall in suction pressure (or temperature). Therefore, in choosing a compressor-motor