Psychrometry and Humidity Charts

Study Notes

A psychrometric chart is a chart of humidity against temperature, with temperature (dry bulb) and humidity (absolute) as the two main axes.
The saturation curve (Ts, Ys) is plotted on the chart, dividing it into an unsaturated and a two-phase region.
The chart can be used to calculate relative humidities and other properties.

The wet-bulb temperature is the temperature reached by a water surface, such as a thermometer bulb surrounded by a wet wick, when exposed to air passing over it.
The wet-bulb temperature is lower than the dry-bulb temperature, due to the heat transfer from the warmer air to the wick and the heat transfer needed for evaporation of water from the wick into the air stream.
The wet-bulb depression is the difference between the wet-bulb and dry-bulb temperatures, which increases as the relative humidity of the air decreases.

The adiabatic saturation condition is the situation reached by a stream of water in contact with humid air.
The condition is reached when the heat lost by the humid air on cooling is equal to the heat of evaporation of the water leaving the stream of water by evaporation.
The total enthalpy change (kJ kg-1 dry air) is equal to zero, and the humid heat of the air (cs) is equal to -hc/kg'.

The Lewis number (hc/kg') is the ratio of the heat transfer coefficient to the mass transfer coefficient.
For the water/air system at normal working temperatures and pressures, the Lewis number is approximately equal to 1.
When the Lewis number is equal to 1, the wet bulb line and the adiabatic saturation line coincide.

The relative humidity, enthalpy, and specific volume of the air can be calculated using the psychrometric chart.
Example 7.7: Estimating relative humidity, enthalpy, and specific volume of air using the humidity chart.
Example 7.8: Calculating the total enthalpy change and the humid heat of the air using the adiabatic saturation condition.

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