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PD Blower System Terminology

Are you fairly new to positive displacement blowers and wondering what all the different terms and acronyms stand for? Here is a list of the common PD blower terminology to become familiar with and hopefully clear up any confusion you may have.

SCFM – Standard CFM. It is defined as the measured flow of free air and converted to a standard set of reference conditions (14.7 PSIA, 68°F, and 0% relative humidity).

ACFM – Actual cubic feet per minute represents the actual flow condition seen by the blower at a specified operating condition at the inlet. ACFM depends on the pressure, temperature and humidity of the actual air.

ICFM – Inlet cubic feet per minute is used to establish conditions in front of additional equipment like inlet filter, blower or booster.

CFM – Best thought of as the amount of intake volume that makes it through the machine (not leaks through seals). This term should be avoided if possible as it is vague and could mean ACFM, ICFM or SCFM.

CFR – Cubic feet per revolution is the amount of volume displaced in one revolution. PD blowers have a CFR rating typically found on pressure & vacuum performance curves.

SLIP – Occurrence of a certain amount of air/gas that “slips” back between the operating clearances of rotors and housing. Slip is expressed in RPM and is the speed the blower must turn to compensate for leakage past the designed clearances. Slip is dependent on the geometry of the blower and the discharge pressure. The higher the discharge pressure, the greater the amount of slip.

SLIP FACTOR – measured by testing. The blower discharge opening is blocked off and a pressure gauge or manometer is connected at the discharge. Blower speed is gradually increased until a defined pressure is reached defining the Slip Factor for the blower at the given pressure.

BHP – brake horsepower is the total horsepower that is required at the input shaft of the blower.

PSIA – pounds per square inch absolute and relative to a full vacuum. Absolute pressure is zero referenced against a perfect vacuum so it is equal to the sum of gauge pressure plus atmosphere pressure.

PSIG – pounds per square inch gauge which is the pressure from gauge. It is measured relative to atmospheric pressure. It is referenced against ambient air pressure so it is equal to absolute pressure minus atmospheric pressure. Gauge pressure is positive for pressure above atmospheric pressure and negative for pressure below it. Negative signs are usually omitted. 0.0 PSIG = 14.7 PSIA.

Atmospheric Pressure – Also known as barometric pressure above perfect vacuum, it is the pressure within the atmosphere of Earth. It is the force per unit area exerted by the weight of the atmosphere. The atmospheric pressure at sea level is 1 atm, 760 mmHg (Torr), 29.92 inHg or 14.696 PSI.

Temperature rise – the difference between the inlet and discharge temperature.

Discharge temperature – temperature measured right at the blower discharge port.

Differential pressure – difference in pressure between two points such as the inlet and discharge ports of a PD blower. PD blowers will have a maximum differential pressure rating.

Working pressure – this is the inlet pressure plus the differential pressure. Blowers will have a maximum allowed working pressure rating.

Vacuum – expressed as a positive number in units of Torr or inHg and is the low-pressure area (also called a depression) below the atmospheric level. Gauge vacuum conditions are usually referred to in units of inHg while absolute vacuum is expressed in units of Torr or mmHg.

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