Dew point. The temperature at which cooled air begins to condense; a coil surface below it dries the air, one above it does not. The coil model solves the partly wet case in between.
The dew point is the temperature at which moist air, cooled at constant pressure, becomes saturated and its water begins to condense; it is fixed by the moisture content alone, so a room at 24 °C and 50 % relative humidity has a dew point of about 12.9 °C whatever its dry-bulb temperature does. A cooling coil whose surface sits above the entering dew point cools the air dry; one whose surface is below it condenses, and a coil in between runs partly wet — this model solves that partially wet case as two sections meeting where the surface reaches the dew point, so the duty is continuous across the change.
A dedicated outside-air unit is specified on the supply dew point it holds, because the moisture it delivers is what sets the room's humidity once the sensible load is carried by other machines. The saturation line the dew point is read from is pure water's vapour pressure with a small enhancement factor for the presence of air; this solver uses Buck's factor as a function of pressure and temperature, held within 0.1 % of ASHRAE's published table, and the same line is drawn by the free psychrometric chart.
Open the builder — the examples in the Design tab's sources list include a machine of this kind, and the Simulate tab runs it against a year of weather.
DOAS — dedicated outside-air system · SZVAV — single-zone variable air volume · VAV terminal box · Economiser · Waterside economiser · Demand-controlled ventilation · Face-and-bypass · Head-pressure control · Humidity ceiling and floor · Cycling and modulation · Coupling · Priority · Category · Nested system · Design point and TMY year · Superheat · Subcooling · Sensible heat ratio (SHR) · Bypass factor · Wet-bulb temperature · COP and EER · Compressor map · Evaporating and condensing temperature · Range and approach · Heat-recovery (three-pipe) VRF · Typical meteorological year (TMY)