Psychrometrics for drying or ventilation processes

Psychrometric Chart


Reference Pressure: Default value of 101325 Pa represents the standard atmospheric pressure (1 atm).

Relative Humidity: Ratio between the amount of water in the air and maximum water the air could potentially hold.

Absolute Humidity: Total mass of water vapor present in a given mass of air. Also sometimes called Humidity Ratio.

Condensed Water: Amount of water that condensed due to cooling.

Interior Temperature and Relative Humidity: Values of Temperature and Relative Humidity inside the storage room.

Exterior Temperature and Relative Humidity: Ambient Temperature and Relative Humidity. This is the conditions of the air fed to the ventilation system.

Cooling Temperature: Temperature to which the ventilation system cools the ambient air.

Reheating Temperature: Temperature to which the ventilation system reheats the air after cooling. This is the temperature the systems feeds to the storage room.

Wet Bulb Temperature: Temperature read by a thermometer covered in water-soaked cloth (wet-bulb thermometer) over which air is passed. At 100 % relative humidity, the wet-bulb temperature is equal to the air temperature (dry-bulb temperature). At lower humidity the wet-bulb temperature is lower than dry-bulb temperature because of evaporative cooling.

Dew Point Temperature: Temperature to which air must be cooled to become saturated with water vapor. When cooled further, the water vapor will condense to form liquid water.

Vapor Pressure: Pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system.

Specific Volume: Ratio of a substance's volume to its mass. It is the reciprocal of density.

Specific Enthalpy: Ratio of a system's enthalpy to its mass.


D. Meyer and D. Thevenard (2019). PsychroLib: a library of psychrometric functions to calculate thermodynamic properties of air. Journal of Open Source Software, 4(33), 1137

American Society of Heating, Refrigerating and Air-Conditioning Engineers (2017). ASHRAE handbook fundamentals 2017: SI edition. Atlanta, GA: ASHRAE.