Workplace Safety Guides

   Ventilation and Air Pressure

    

Air is important because it can transport contaminants to the worker's breathing zone. But it can also be used to help control exposures to contaminants. Air is a complex mixture of gasses and vapors like nitrogen, oxygen, water vapor, carbon dioxide, argon, and many others in trace quantities.

Air seems light and airy, but it's actually quite heavy, weighing about 75 pounds per 1,000 cubic feet at sea level. If we move 1,000 cubic feet of air per minute through duct work, we're moving about 75 pounds of air every minute. This requires choosing fans and motors large enough to handle the load.

The speed or velocity, called V, of air is measured in feet per minute or FPM. Air volume flow rate, called Q, is expressed in cubic feet per minute or CFM. We cannot directly measure Q. It is equal to the product of the average velocity and cross sectional area through which the air flows. That is Q=V*A.

Air is pressurized by gravity, the barometric pressure. At sea level the barometric pressure is about 29.9 inches of mercury as measured with a barometer. The barometric pressure is also called the absolute static pressure.

Air starts moving because there is a difference in pressure between two points. Air moves from an area of higher pressure to an area of lower pressure. The lower pressure in exhaust systems is generated by the fan and is called the static pressure. The static pressure created by the fan is less than barometric pressure on the suction or inlet of the fan. And greater than barometric pressure on the discharge or outlet side. It acts equally in all directions and can be measured used a water manometer.

Static pressure is expressed in inches of water. You might hear "the fan generates two inches of negative static pressure," which means that the pressure inside the duct on the suction side of the fan is two inches of water less than the pressure outside of the duct. The pressures encountered in ventilation applications are relatively small. The use of conventional units of pressure measurement such as inches of mercury or pounds per square inch would result in pressures expressed as very small numbers. It is more convenient to express pressure as the height of the water column that the pressure in the duct will support.

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