If a water-cooled condenser is employed to remove heat from the refrigerant, a water tower (Fig. 13.31) may be used to cool the condenser discharge water, which can then be recirculated back to the condenser.
Where practical, the water condenser and tower can be replaced by an evaporative condenser as in Fig. 13.32.
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 75F.
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 75F WB. There will be a few days a year, however, when the outside air may register 79 or 80F 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 75F 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 78F WB for the design of heat rejection equipment. Now, on a 78F 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 80F 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 104F refrigerant temperature and 95F 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 10F water-temperature rise.
When choosing a condenser for water-tower use, we must determine the water temperature available from the cooling tower with 95F water to the tower. Check the tower manufacturer for the capacity required at the design WB, and 95F 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 7F.)
Thus, for 78F, the water leaving the tower would be 85F, and the condenser would be designed for a 10F water-temperature rise, i.e., for water at 95F to the tower and 85F 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.