There is no mention on most psychrometric charts of a property that is important in its own right, but also provides understanding for events that occur at many locations throughout the refrigeration facility. The dew-point temperature of air is defined as the temperature that results if the air is cooled with no change in its moisture content until the air reaches the saturation conditions. In Fig. 6.24, the temperature of air at condition a is reduced until the air is saturated. The temperature at which saturation is reached, tdp, is the dew-point temperature. One significance of the dew-point temperature is that if air at condition X is in contact with a surface having a temperature of tdp, condensation of moisture will occur on that surface.
Example 6.4. The condition of air in a meat-processing room is 12°C (53.6°F) and the relative humidity is 70%. The surface temperature of the support metal for the cooling coils is 4°C (39.2°F). What is the potential operating problem that exists?
Solution. The dew-point temperature of the air in the process room, found by moving horizontally on the psychrometric chart from the condition of the air to the saturation line is 6.8°C (44.2°F). Since some metal surfaces exist at the lower temperature of 4°C (39.2°F), some moisture will condense on these surfaces. The drip of moisture on the food products is prohibited.