9. Psychrometry

Psychrometry is concerned with the behaviour of humid air and the prediction of its properties. More strictly it covers the behaviour of any vapour (not just water vapour) when mixed with a gas (not just air). However, because the air/water system is of huge importance, not least to food processing, and is the most commonly encountered gas/vapour mixture, the terminology employed often appears to be specific to air and water. Prediction of the properties of moist air, for example, humidity, maximum possible humidity, temperature, density and so on, is especially important in drying operations. The psychrometric chart is a simple graphical method of presenting this information, and the principles behind the chart are explained in this chapter. The outcome of a drying process can often be followed more easily by monitoring the condition of the air with the aid of a psychrometric chart than by directly measuring the moisture content of the substance being dried. Of equal interest is the establishment of the correct atmospheric conditions for food storage.