Air is a mixture of dry air and water vapour, and the part that makes refrigeration calculations difficult is the water vapour. Psychrometrics describes this mixture with six quantities: dry-bulb temperature (the air temperature we know), wet-bulb temperature, relative humidity, dew point, absolute humidity (grams of water per kilogram of dry air) and enthalpy (kJ per kilogram, the total heat content of the air). At a given pressure, if any two of these are known the rest are determined; the psychrometric chart shows this relationship on a single page.
An example reading: air at 30 °C and 50% relative humidity contains 13.3 g of water per kilogram, its dew point is 18.4 °C, its wet-bulb temperature 22 °C and its enthalpy 64 kJ/kg. Cold room air at 0 °C and 90% humidity, on the other hand, contains 3.4 g/kg of water and has an enthalpy of 8.5 kJ/kg. So every kilogram of air that comes in from outside brings about 55 kJ of heat and 10 g of water to the room. Door infiltration, frosting and defrost calculations all rest on these two differences.
The most useful conclusions for the refrigeration engineer are these. If air touches a surface colder than its dew point it gives up moisture; the evaporator surface is always like this, so every evaporator is also a dehumidifier. The greater the temperature difference (TD) between the evaporator and the room, the more the air dries: a difference of 4–6 K gives a room humidity above 90%, a difference of 10–12 K gives 70–75%. The same rule applies to the outer surface of the panel: if the surface temperature falls below the dew point of the ambient air, the panel sweats.
Application notes: charts are drawn for a specific air pressure; a sea-level chart gives small deviations in Konya, at an altitude of 1,000 m, and for precise calculation a chart or software suited to the altitude is used. Below zero the water that air can carry falls sharply (saturated air at −20 °C holds only 0.6 g/kg), so in a frozen room even a small entry of moisture shows up as frost. The air infiltration item in the cooling load calculator is based on this enthalpy difference; the unit converter converts temperature and energy units.