Putting It Into Practice

Controlling the Environment

HVAC Systems

A basic HVAC system consists of:

• A heat source (steam, hot water, electric)
• A cooling source (chilled water cooled by some type of refrigeration system)
• Air handling units (for circulating the air, these include fans and dampers—as well as filters and heating/cooling coils)
• Ductwork (through which air moves to and from spaces in the building)
• Equipment for filtering the air
• Control systems (usually computerized)

Temperature and Humidity

In very general terms, an HVAC system controls temperature by passing air over a heating coil within the air handling unit. Air is humidified by introducing water vapor into the stream of air before it passes into the space. It is dehumidified by passing air over a cooling coil. The temperature of the cooling coil is lower than the dew point temperature of the air, so moisture condenses on the coil and is then drained away. The air is then reheated to the desired temperature and sent into the space.

Pollutants and Air Circulation

Particulates are removed by using various grades of filters within the air handling unit. Electrostatic precipitators can be effective, but should not be used because they generate ozone. Gaseous pollutants are removed by using adsorbers within the air handling units (beds or canisters containing granular agents such as activated carbon).

Since filtration is expensive, it is also important to reduce the amount of pollutants that enter the building. Locate air intakes away from sources of pollutants such as vehicles, industrial processes, and laboratories. The smallest possible amount of outdoor air should be used within the HVAC system; as much air should be recirculated as possible. Recirculated air should also be filtered. Air should be circulated at a constant volume throughout collection spaces, and a slight positive pressure should be maintained in storage spaces to prevent untreated air from entering them.