The most simple, straightforward way to obtain dry air is to use desiccants-that is, adsorbants or materials that have a natural affinity for water. A desiccant is able to take up the additional moisture given up by the air without changing its size or shape. So as air stream can pass through a desiccant and become significantly drier without elaborate cooling, compression, cooling water, or other complex systems or controls. After the drying task is complete, the desiccant is regenerated via heat. Then the desiccant is ready to dry more air.

Lowering air temperature decreases the air's ability to hold moisture. Thus, the air can be made drier by cooling it. However cooling air just to dry it is usually not practical. An exception might be when cool air is needed anyhow, that air's dryness satisfies the needed moisture conditions, and enough conditioned air is available. Normally, this method is reserved for applications where outdoor air is being dried to levels only slightly lower than the incoming ambient-that is, the system air.

To remove large amounts of water by cooling the air, over-cooling and subsequent reheating air required. But such procedures typically have problems with operation and maintenance, as well as cycle and control; the method is unsuitable for producing large quantities of dry air. Another limitation to this technique is the freezing point of water. When air is dried via refrigeration, the cooling surfaces of the coils may reach sub-freezing temperatures. This causes ice to form, which, in turn, reduces the efficiency of the cooling system. So anti-icing devices or duel systems and defrost cycles may be required.

To prevent such cooling coil icing, a brine spray is commonly used. The brine must be reconstituted periodically or continuously. This requires additional equipment, maintenance and operating costs. Although this strategy is workable and often satisfactory, the complexities associated with cycling and controlling are detracting factors.

A special case involves a brine spray that can pick up moisture from the air at normal temperatures. This brine must be cooled and regenerated or reconcentrated either continuously or periodically. To deliver air at very low moisture, such a system is necessarily complex. For example, the brine must be mechanically refrigerated, and at all levels of drying, cooling must be used during the moisture absorbing cycle and after the regenerating or reconstituting cycles.

Anyone who has suffered the discomforts of hot, humid summer weather understands that it is not just the heat, but also the humidity that makes the air feel so miserable. That “muggy” feeling comes from the relative humidity or saturation level– that is, the amount of water contained by a pound of air at a specific temperature and atmospheric pressure.

When air has 50% relative humidity (RH), we say it is 50% saturated (the terms are numerically so close that we use them interchangeably). The air contains about half the water it could hold at the same temperature and pressure. Obviously, as air approaches 100% saturation, it can take on less and less water until at 100% RH, the air cannot hold more water.

Relative humidity is determined by comparing the “wet-bulb” and “dry-bulb” readings of a humidity measuring device- a hygrometer (see the table below). Once known, these values identify a point on the psychometric chart (see Appendix I, page 35) where air vapor mixture properties can be read directly.

The following hygrometers can measure the humidity or hygrometric state of the air:
A psychrometer consists of two thermometers (matched in type, scale and range), one of which has a cloth wick–a “sock” applied to its bulb. To use, the wick is wetted with distilled water and ventilated with air moving at a recommended 900 to 1000 feet per minute (fpm) or more at right angles to the instrument.

Dew-point hygrometers visually note when humidity-that is, water in the air-condenses on a cooled metallic surface. The temperature at which this condensation or dew-point occurs can help determine other air properties via charts and tables. Several types of dew-point hygrometers are widely used.

These hygrometers may be mechanical, electrical, electrolytic or gravimetric in nature. However, no organic material consistently reproduces its action over an extended time, especially in extremes of humidity or temperature. So this category is of limited value.