Humidification of air by use of ultrasonic devices operating within an air handler are well known in the art. See, for example, U.S. Pat. Nos. B1 4,042,016, 4,118,945 and 4,564,375. Typically, these types of systems generate a fog using an ultrasonic device for breaking up a stream of air and water into fine droplets. These droplets preferably evaporate before the air carrying them along reaches filters or other downstream devices, such as cooling coils and the like. The distribution of the fog is normally selected to minimize the amount of air that can bypass mixing with the fog. In some air handlers, the water droplets still reach downstream devices, thereby wetting these with some further humidification arising from the evaporation from the wetted devices. In such case, it is still necessary to provide drainage for water dripping down from the wetted devices. Generally, however, wetting is preferably avoided for bacterial growth prevention and unnecessary loss of water.
In the ultrasonic humidification of the air in or before an air handler, the humidification of the air inside the air handler often achieves levels substantially higher than those within the building areas that are intended to be humidified, so that the humidification level inside the building achieves a desired level. Although saturation levels can occur inside the air handler, saturation preferably is avoided, primarily to prevent condensation on cool metal surfaces and prevent the creation of stagnant pools of water.
U.S. Pat. No. 2,720,939 describes a process of recovering aerosol solids by introducing a liquid such as water as a stable fog or mist so as to cause an agglomeration of the solid particles with water. Agglomeration is enhanced by use of various techniques such as sonic or electrostatic forces. The moisture-laden particles are then condensed out of the air stream by cooling the stream.
U.S. Pat. No. 3,473,298 describes an apparatus for removing air polluting contaminants and condensable vapors from exhaust gas. This involves a spray chamber for generating water in a finely-divided form, a demister for separating water droplets and a condensing tube structure to condense water vapor.
U.S. Pat. No. 3,494,099 describes an apparatus for removing contaminants from a polluted gas stream using an ultrasonic generator which forms a fog in a first zone. An electric field and sound waves are used to cause a three-dimensional oscillatory motion of the vibrating fog droplets. These droplets then are given increased contact time and area to remove contaminants from the polluted gas stream. In a second zone, fm modulated ultrasonic waves are generated to cause an agglomeration of fog droplets and thus removal of solid particles.
U.S. Pat. No. 3,593,496 describes a system for purifying air of organic pollutants by first humidifying the air and mixing this with an aerosol containing pollutants in a mixing chamber. The mixture is subjected to aerosol removal to produce purified air. Other wet scrubber-type contaminants removers are described in U.S. Pat. Nos. 3,704,570 and 3,755,990.
Many of these prior art wet scrubbers require enormous quantities of water to achieve a level of effectiveness. Typically, air washers require spray densities of 1 to 5 gallons per minute per square foot of crossectional area per bank, see the 1988 ASHRAE Equipment handbook on air washers. This, in turn, imposes a greater burden on the disposal or processing of the waste water produced by these wet scrubbers.
Removal from air streams of contaminants such as volatile organic compounds (VOC) is typically accomplished with activated carbon bed filtration systems. These are expensive and complex to use because the activated carbon bed immediately begins to lose effectiveness and needs to be replaced on a regular basis. In a well-maintained, actively-used carbon system, the carbon bed may need to be replaced on a weekly basis. The used carbon bed can be reactivated; however, after repeated activation cycles, the carbon tends to crack and degenerate leading to a loss of a useful, expensive carbon bed and a slip of unabsorbed VOC's.