Air Balance Definitions

Air balance definitions explain the terminology which is commonly used in the air balance trade. This is the recognized vernacular of the industry.

CFM = Cubic feet of air per minute. This is standard air unless stated as otherwise. Standard air is substantially dry air at 70 degree F and a barometric pressure of 29.92" mercury.

TP = Total pressure of air that exists in the duct and is the sum total of static and velocity pressure. This pressure is normally obtained by placing a pitot tube directly in line with the air stream and reading on a pressure meter both pressures.

SP = Air balance definitions state that static pressure is the standing or bursting pressure in the duct. This pressure is normally obtained by placing a probe perpendicular to the direction of flow and reading on a pressure meter.

VP = Velocity pressure which is the pressure caused by the movement of air in the duct. This pressure cannot be obtained except by determining the total pressure and subtracting the static pressure. In fact with today's technology one uses a hot wire anemometer to measure velocity directly in fpm.

FSP = Fan static pressure which is the fan total pressure less the fan velocity pressure.

BHP = Brake horsepower is normally considered the total power which is applied to the drive shaft of any rotating piece of equipment. When brake horsepower is field checked, things such as belt losses, general transmission losses and motor efficiency or power factor are not taken into account.

RPM = Revolutions per minute is the number of times the shaft revolves in one minute.

NP = Nameplate refers to the actual listing by the manufacturer as to size, type, amperage, horsepower, voltages, operating pressures of motors, fans, pumps or any other piece of equipment. 

OA = Outdoor or fresh air.

RA = Air balance definitions state that return air or recirculated air is that being drawn from the space and resupplied to the space.

EA = Exhaust air which is being removed from the space and exhausted outside the building.

MA = Mixed air is a combination of outdoor and return air.

DT = Delta T which is a difference in temperature.

BTU = British thermal unit which is the amount of energy required to raise one pound of water one degree fahrenheit.

BTU/HR = British thermal units of heat loss or gain in one hour.

MBH = One thousand British thermal units per hour. This unit of measure is in common use to reduce the size figures involved.

GPM = Gallons per minute, normally used to describe water flow.

DP = Delta P which is a difference in pressure.

DB = Dry bulb temperature refers to the air temperature without regards to the moisture content. An ordinary thermometer records dry bulb.

WB = Wet bulb temperature is the temperature at which liquid or solid water, by evaporating into air, can bring the air to saturation at the same temperature.

RH = Relative humidity is the ratio of the existing moisture content of the air over the maximum moisture content this air could obtain at the same dry bulb temperature.

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