Effective use of negative- and positive-pressured areas in a building can be a smart way to conserve energy. Potential applications are restaurants, chemistry labs, and other areas that generate odors or fumes that need to be kept from adjacent areas.
The usual solution in such areas is to exhaust all the air. This can be expensive, though, because not only has the air been cooled or heated and dehumidified, but also outside air must be treated to replace it. This is not to imply that air shouldn’t be exhausted, only that advanced controls make it possible to vent less air and still maintain comfortable, healthy working conditions.
If the area to be exhausted is separated from the rest of the occupied space, such as a kitchen from the dining area, global controls can minimize space-conditioning costs. Some workspaces in the kitchen need a specific exhaust rate, such as at the grille or the fryers. Instead of operating vent fans at these stations all the time, run them only when cooking limits the exhausting of conditioned air. Maintaining a positive pressure of the dining area relative to the kitchen effectively minimizes any fume migration.
Positive pressure occurs any time more air is supplied to a zone than is returned to the air handler. When 20 percent outside air is supplied to the return-air stream, for example, the conditioned area is always at a slightly higher pressure than the outside. This limits the infiltration of outside air through the building envelope, and in humid climates reduces the latent load on the cooling coils by keeping the hot, humid air outside. This same effect is possible between two conditioned zones. Inexpensive static-pressure sensors can measure the pressure in each zone and either exhaust more air from the kitchen, for example, by a variable-speed fan or supply extra outside air to the dining area by a damper actuator in the outside-air intake to the air handler serving
that zone.
A new control device measures the air flowing through an opening in the wall separating the two zones and electronically actuates a damper, or fan motor to maintain the setpoint pressure. These are dependable devices, called differential-pressure sensors, that are accurate enough to be used in hospital isolation rooms and tuberculosis wards.
The net effect of zone pressure control is to maintain a static-pressure barrier between two zones, so that a higher level of contamination can be allowed in the highly exhausted area without causing discomfort to the occupants or contaminating adjacent sensitive occupied areas.
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Excerpt from Energy Conservation Projects.