Under Pressure

When I’m working on the design of a powered ventilation system for a building, I’m reminded of the song by David Bowie and Freddie Mercury called Under Pressure. The song deals with the daily pressures of life and love, but it makes me think about whether the building should have positive or negative air pressure. In this article, I’ll discuss the benefits of both, some of the factors that should be considered, and how the ventilation equipment should be sized to achieve the desired pressure.

Benefits

A positive pressure system has two primary benefits. The first is that the source of supply air to the building can be controlled so there is no air infiltration through openings such as cracks or doors. The second is that it enhances the flow of consumption air flow to boilers, compressors and engines. A negative pressure system also has two primary benefits. The first is that the location of exhaust air can be controlled so that no air escapes the area except through the fans. The second is that truck and man door openings can be used as points of entry for supply air.

Factors

The most important factor as to whether to use a positive or negative pressure system is the type of activities or processes that are conducted inside the building. If any of the activities require filtered air or low humidity, such as food processing or cold storage, a positive pressure system is required. Boilers and engines located in confined spaces also require a positive pressure system. If any of the activities are going to create fumes or dust, such as welding or conveying, these require a negative pressure system. Negative pressure systems are also good when offices are attached to reduce fume, smoke or dust infiltration.

Desired Result

A positive pressure system is created when residual pressure of the supply air is greater than the residual pressure of the exhaust air. Negative pressure is created when the opposite exists. It is very important to accurately calculate all of the air flows, such as dust collectors and combustion air, and the total static pressure of the ventilation system. The desired result is a slightly negative or positive pressure. Too much pressure may cause doors to fly open or slam shut resulting in injuries.

If you are under pressure to get your ventilation system properly pressurized, let us help with the design and equipment selection.