Many plants operate in geographic regions where ambient temperatures fall to near or below freezing. Two measures to prevent the spray water from freezing in such instances are:
(1) to locate the sump in a warm area, as in Fig. 7.20
(2) to drain the water from the condenser and operate the condenser dry.
The indoor sump in Fig. 7.20 must be able to accommodate all the water normally in suspension in the condenser during operation. The cost of pumping the spray water will be slightly higher than experienced when the sump is integral to the condenser because of the additional head attributable to the difference in elevation between the condenser and the sump.
The indoor sump will prevent freezing of the main body of water in extremely cold weather, but the water droplets that drift out of the condenser may freeze close to the condenser causing icing conditions. An alternative to the indoor sump is to drain the condenser and operate dry. The heat-transfer capacity of a condenser operating dry is strikingly less than when the condenser operates with the water sprays. Figure 7.21 shows relative capacities of a condenser operating dry compared to wet operation. It is significant that the dry condenser does not duplicate the wet capacity until the ambient dry-bulb temperature is about -30°C (-22° F). In many industrial refrigeration plants the load drops off as the ambient temperature drops, so the condenser can be shifted to dry operation when the ambient temperature drops below freezing. Such is not the case, however, for a plant with a predominantly process load which is only slightly affected by the ambient conditions. The sharp reduction in heat-transfer capacity under dry operation emphasizes that the evaporation process is the dominant one for transfer of heat to air in the evaporative condenser.
One incentive for a plant to shift to dry operation is that the costs of water treatment are usually a function of operating time, and this cost can be eliminated by dry operation.