The methods used to humidify air include
• Direct spray of recirculated water into the airstream (air washer) reduces the dry-bulb temperature while maintaining an almost constant wet bulb in an adiabatic process [see Figure 3, Paths (1) to (3)]. The air may also be cooled and dehumidified, or heated and humidified by changing the temperature of the spray.
In one variation, the surface area of water exposed to the air is increased by spraying water onto a cooling/heating coil. The coil surface temperature determines the leaving air conditions. Another method is to spray or distribute water over a porous medium, such as those in evaporative coolers and commercial greenhouses. This method requires careful monitoring of the water condition to keep biological contaminants from the airstream (Figure 6).
• Compressed air that forces water through a nozzle into the airstream is essentially a constant wet-bulb (adiabatic) process. The water must be treated to keep particulates from entering the airstream and contaminating or coating equipment and furnishings.
Many types of nozzles are available.
• Steam injection, which is a constant dry-bulb process (Figure 7). owever, as the steam injected becomes superheated, the leaving dry-bulb temperature increases. If live steam is injected into the airstream, the boiler water treatment chemical must be nontoxic to the occupants and, if the air is supplying a laboratory, to the research under way.
Moisture condenses on a cooling coil when its surface temperature is below the dew point of the air, thus reducing the humidity of the air. In a similar manner, air will also be dehumidified if a fluid with a temperature below the airstream dew point is sprayed into the airstream. The process is identical to that shown in Figure 2, except that the moisture condensed from the airstream condenses on, and dissolves in, the spray droplets instead of on the solid coil surface. Chemical dehumidification involves either passing air over a solid desiccant or spraying the air with a solution of the desiccant and water. Both of these processes add heat, often called the latent heat of wetting, to the air being dehumidified. Usually about 465 J/kg of moisture is removed (Figure 8).
