What psychrometrics is
Psychrometrics is the study of moist air: air mixed with water vapour. It is easy to confuse with psychometrics, the measurement of mental abilities, which is unrelated.
The idea underneath everything
Air always carries some water vapour. How much it can carry depends strongly on temperature: the limit roughly doubles for every 12 °C or so. Air that holds as much water as it can is saturated, and any further cooling makes water appear as dew, fog or condensation. Almost every process that heats, cools, dries or humidifies air follows from this.
Three changes that matter in a plant
Take air at 25 °C and 50 % relative humidity at sea level:
- Cooling. The water content stays the same until the air reaches saturation. That temperature is the dew point, 13.9 °C here. Any pipe, product or coil colder than that sweats.
- Heating. The water content still does not change, but relative humidity falls: heated to 40 °C, the same air is at 21 %. This is how every dryer works: heat the air so it can take up moisture.
- Evaporation. Air blown over a wet surface cools it to the wet bulb temperature, 17.9 °C here. Nothing cooled by evaporation gets colder than this, which is why the wet bulb governs cooling towers and evaporative coolers.
Relative humidity on its own is misleading
The same air shows a different relative humidity every time its temperature changes. Engineers care about how much water the air actually holds and when it will condense.
What each quantity means
| Quantity | Meaning | Why a plant cares |
|---|---|---|
| Relative humidity | How full the air is, as a percentage of its limit | What most sensors read |
| Humidity ratio | Mass of water per kilogram of dry air | The real water content, used in drying and mass balances |
| Absolute humidity | Mass of water per cubic metre | Cleanrooms and sensor datasheets |
| Dew point | The temperature at which condensation starts | Sweating pipes, wet products, corrosion |
| Frost point | The temperature at which ice forms, below 0 °C | Cold rooms and evaporator coils icing up |
| Pressure dew point | The dew point inside compressed air | Compressed air dryer performance |
| Wet bulb | The lowest temperature evaporation can reach | Cooling towers, evaporative coolers, heat stress |
| Enthalpy | The heat energy the air carries | Heating and cooling loads |
| Specific volume, density | Volume per kilogram, and mass per cubic metre | Turning fan flow into mass flow |
| Vapour pressure deficit | How strongly air draws water from leaves | Greenhouse climate control |
Why pressure matters
At altitude the air is thinner, so the same water vapour is a larger share of each kilogram of air. At 25 °C and 50 % relative humidity, air at sea level holds 9.9 g of water per kg of dry air; in Nairobi, at about 82.5 kPa, it holds 12.2 g, nearly a quarter more. The dew point, 13.9 °C, is the same in both places, because it does not depend on pressure. A calculation that assumes sea level gives wrong drying and energy figures at altitude.
Where it is used
- Air conditioning and cleanrooms.
- Cold rooms and cold chains, where frost and condensation matter.
- Drying of tea, coffee and grain.
- Greenhouse climate control.
- Compressed air systems.
- Cooling towers.
- Direct air capture, where the humidity of incoming air affects sorbent performance and energy use.