Psychrometry & Drying Theory Notes

Last reviewed 1 Oct 2026 · 6 min read

Moist air — psychrometric properties

Air for drying is a mixture of dry air and water vapour. Atmospheric pressure $P$ (about 101.325 kPa at sea level) equals $p_{a} + p_{v}$ (partial pressures of dry air and vapour).

Property	Meaning
Dry-bulb temperature (DBT) $T$	Ordinary air temperature (thermometer)
Wet-bulb temperature (WBT) $T_{w b}$	Temperature of a thermometer covered by a wet wick in moving air; evaporation cools it; WBT ≤ DBT (equal at 100 % RH)
Dew-point temperature $T_{d p}$	Temperature at which vapour starts to condense when the air is cooled at constant pressure and humidity
Relative humidity (RH)	$\frac{p _{v}}{p _{s a t} ( T )} \times 100$
Humidity ratio (absolute humidity) $W$	kg of water vapour per kg of dry air
Specific enthalpy $h$	kJ per kg of dry air
Specific volume $v$	m³ per kg of dry air
Wet-bulb depression	$T - T_{w b}$ — a measure of the drying power of air

FormulaKey relations

W = 0.622 \frac{p _{v}}{P - p _{v}} p_{v} = R H \times p_{s a t} (T)

h = 1.006 T + W (2501 + 1.86 T) kJ/kg dry air (T in °C)

v = \frac{0.287 ( T + 273 )}{P} (1 + 1.608 W) m^{3} / kg dry air (P in kPa)

Worked ExampleExample — properties of ambient air

Air is at 30 °C and 60 % RH at 101.325 kPa. The saturation pressure at 30 °C is 4.246 kPa.

$p_{v} = 0.60 \times 4.246 = 2.548$ kPa.
$W = 0.622 \times \frac{2.548}{101.325 - 2.548} = 0.622 \times \frac{2.548}{98.777} = 0.0160$ kg/kg (16.0 g of vapour per kg of dry air).
$h = 1.006 \times 30 + 0.01604 \times (2501 + 1.86 \times 30) = 30.18 + 0.01604 \times 2556.8 = 30.18 + 41.0 = 71.2 kJ/kg$ .
Dew point: the temperature at which $p_{s a t} = 2.548$ kPa, about 21.4 °C (saturation pressure is 2.487 kPa at 21 °C and 2.645 kPa at 22 °C).

Psychrometric chart and processes

A psychrometric chart plots $W$ (vertical) against DBT (horizontal) with lines of constant RH, WBT, enthalpy and specific volume.

Process	Path on the chart	Example
Sensible heating	Horizontal line to the right ( $W$ constant); RH falls	Heating air in a dryer's heater
Sensible cooling	Horizontal line to the left until saturation (dew point)	Cooling coil
Adiabatic (evaporative) cooling / humidification	Along a constant-WBT (constant enthalpy) line, upward and to the left	Drying chamber, evaporative pad cooling
Cooling and dehumidification	Down to the saturation curve and along it	Refrigeration dehumidifier
Mixing of two air streams	Straight line between the two states (mass-weighted)	Recirculation in dryers

Worked ExampleExample — heating the same air for drying

The air above (30 °C, $W = 0.01604$ ) is heated sensibly to 60 °C: $W$ stays the same. The saturation pressure at 60 °C is 19.94 kPa, so the new RH $= \frac{2.548}{19.94} \times 100 = 12.8 %$ .

The heated air can now pick up much more water — it can absorb moisture until saturation or until the grain's equilibrium RH. For example, if it leaves the grain bed at 40 °C and 60 % RH (saturation pressure at 40 °C is 7.384 kPa, so $p_{v} = 0.60 \times 7.384 = 4.43$ kPa), then $W = 0.622 \times \frac{4.43}{101.325 - 4.43} = 0.0284$ and the moisture pickup is about $0.0284 - 0.0160 = 0.0124$ kg of water per kg of dry air.

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Moist air — psychrometric properties

Psychrometric chart and processes

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