1988). In addition, humidifier moisture should never wet nearby filters. Water spray humidifiers in AHUs must be fitted with downstream demisters or eliminator plates to remove carryover of unevaporated droplets (Ager and Tickner, 1983).

Water sumps in cold water humidifiers must be fastidiously maintained in order to prevent the amplification of microorganisms that can cause building-related illnesses such as hypersensitivity pneumonitis, humidifier fever, and asthma (Morey et al., 1986). Humidifiers that use steam require less maintenance than those that use cold water. However, steam emitted into the supply airstream should not contain corrosion inhibitors such as volatile amines, because they can be nitrosated and are potentially toxic to occupants in the humidified zones (NRC, 1983a). The temperature of the moisture emitted by steam humidifiers is biocidal. Humidifiers that function by evaporating or emitting water molecules (only) are also not direct emission sources of bioaerosols.

Humidifiers that emit water droplets may do so by discharging all water from a supply line or only a portion of the water from a recirculation system. The potential for bioaerosol emission from these humidifiers is directly related to the microbiological contamination in the water supply that is aerosolized. Emission of microorganisms from spray-type humidifiers that use recirculated water is greatly reduced by installation of highly efficient upstream filters that remove dusts that would otherwise enter the humidifier and serve as growth nutrients.

ASHRAE Standard 62-1989 (ASHRAE, 1989b, Section 5.12) recommends steam as the preferred moisture source for humidifiers. However, if cold water humidifiers are used, water should originate from a potable source and units that use recirculated water should be subject to frequent maintenance and blow-down. The specific protocol for maintenance of humidifiers that use recirculated water is not specified in ASHRAE Standard 62-1989.

The Nordic Committee on Building Regulations (NKB, 1990) provides advice with regard to humidification of the air. Section 4.6.6 of this standard recommends that where humidification is required, a type of device "which does not involve the risk of microorganisms being released into the air shall be chosen."

AIR SUPPLY PLENUM AND DUCT WORK

After passing through the heat exchanger and supply fan (the location of the fan may be upstream or downstream of the heat exchanger), conditioned air is distributed through a system of ducts to the occupied spaces. The AHU plenum housing the fan is usually thermally insulated on its internal surfaces with a fiberglass lining that also acts as a sound attenuator.



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