The significance of the humidity factor for the capacity calculation of evaporators in GPC
When air cools down at cold surfaces while the temperature falls below the dewpoint of the air, humidity condenses. If this is the case for temperatures above 0 °C, humidity precipitates in form of liquid water. In case the air temperature is below 0 °C, snow, frost or ice will form on the cold surface. It does not matter if humidity precipitates in the form of liquid water or solid ice or snow: In both cases, heat of condensation is released (latent heat). In addition, the energy for the sensible cooling of the air is required. This means that the sum of both forms of energy (latent + sensible) is given off to the cold fin surface and hence to the refrigerant.
The absolute moisture content of the air decreases with falling temperatures. This means that much more humidity precipitates in NT rooms than in LT rooms, and that the air cooler and the connected refrigerating plant need to provide a higher overall cooling capacity. Therefore, evaporators – to name but one example – must always be designed in a way that their cooling capacity is higher for real-life, humid ambient conditions than for very dry air temperatures as prescribed by, for example, the standard test conditions according to EN 328. This standard describes procedures for measuring evaporator and air cooler capacities for which the dewpoint of the air always has to be at least 2 K below the coldest temperature in the evaporator (evaporating temperature) or in the air cooler (fluid temperature at the inlet). This standard describes and approves only the capacity in dry operation. As only the dry cooling capacity's measurement can be well controlled and reproduced, this method has found its way into the standard.
To determine the actually required cooling capacity, generally accepted correction factors have to be considered. These correction factors depend on the air inlet temperature and are illustrated in the chart below:
This implies that the actual cooling capacity is obtained from the capacity in dry operation multiplied by the humidity correction factor illustrated in the chart.