Standard Cubic Feet per Minute defines mass flow. Volume is calculated at prescribed set of conditions. The specifier must state standard conditions. The most prevalent standard conditions used in North America is 14.7 psia (barometric pressure at sea level), 68°F and 36% relative humidity. These are the conditions set forth by the ASME (American Society of Mechanical Engineers. API (American Petroleum Institute) is 14.7 psia, 60°F and 0% Relative Humidity.


Actual Cubic Feet per Minute defines volumetric flow at specific conditions. A mass flow must be known so that it can be converted into the actual flow rate.


Inlet Cubic Feet per Minute is typically used interchangeably with ACFM. It represents the volumetric flow at the inlet to the unit. This would take into account any losses from items such as filters, silencers or other fittings or appurtenances.


Cubic Feet per Revolution defines how much volume is displaced for one revolution of the shaft.

1 PSI Slip

1 PSI Slip is defined as the revolutions per minute that a blower must turn to generate 1 psig on a blanked off flange. This is a measure of how much internal clearance a positive displacement blower has. A low number represents very small clearances while a larger number represents more open clearances. More open clearances allow a unit to operate at higher differential temperatures, therefore higher pressure or vacuum.


Pounds per square inch Absolute is a measure of pressure on an absolute scale and is not the same as what you would read on a pressure gauge. Typical pressure gauges take ambient pressure into account.

Ambient Pressure

Ambient pressure is the absolute pressure that surrounds you and is measured as PSIA. Ambient pressure is typically the same as barometric pressure. However, if the equipment is located in either a vacuum chamber or pressure vessel, the ambient pressure will be that absolute pressure that is present in the chamber or vessel.

Square Load

Rotary lobe blower families are typically designed around a standard square load. Square load is calculated by dividing the differential pressure times the cylinder length by the gear diameter. The equation can be manipulated such that given any three of the parameters the fourth parameter can be calculated. Square load is typically expressed in pounds.
Square Load Equation


The impeller is also called a “lobe”. It typically is has the cross section of the number 8. The three most common styles are the involute, segment waist and cycloidal.


The cylinder encases the impellers and has the pipe connections. The most common material is cast iron, but they can be cast in many different materials.


Each blower has two headplates that bolt to the cylinder. The shafts that pass through each impeller also pass through the headplates. The bearings and seals are typically mounted in the headplates.


A blower is typically defined as a device that does not compress a gas but moves a volume of gas to overcome some sort of system resistance. A blower in the strictest sense does not reduce the volume of gas as it is moved by the impellers. The impellers merely sweep the gas from the inlet to the discharge. A blower is typically a machine that operates below 20 psig. Blowers can be stage to achieve higher discharge pressures, but no internal compression takes place. Blowers are typically defined as positive displacement type machines.

Positive Displacement Blower

This is a style of machine that moves a specific volume of gas with each revolution. A common application for positive displacement blower is as a meter. The specialized type positive displacement blowers typically have very tight clearances which create very low slip numbers. If you know the displacement (cfr) and the speed, the volume can be calculated very accurately.

Vertical Unit or Horizontal Unit

A vertical unit is typically a unit with horizontal gas flow. One can determine orientation by drawing a line between the two shaft centers. If the line is vertical, it is a vertical unit. If the line is horizontal, it is a horizontal unit. A horizontal unit typically has vertical air flow. While this is the common nomenclature in North America other regions tend to offer additional configurations such as a top inlet and side discharge.


Gas Horsepower is the amount of horsepower required to move the gas from the inlet to the discharge of the unit. It is typically a function of speed, displacement (cfr) and differential pressure.


Friction Horsepower is the amount of horsepower to overcome the mechical resistance of the machine. This includes bearings, seals and gear losses.


Brake Horsepower is GHP (Gas Horsepower) plus FHP (Friction Horsepower)

2/3rds Rule

This is typical rule in the blower industry that reduces the maximum allowable temperature rise of a blower. It is calculated by subtracting 2/3rds of the quantity of suction temperature minus ambient temperature from the maximum allowable temperature rise of the unit. In effect, if your ambient temperature is much lower then your gas temperature it reduces the maximum differential pressure of the blower.

Average Temperature Rule

The average temperature rise of the unit must be equal to or less than 250°F. The average temperature is calculated by adding the suction temperature to the discharge temperature and dividing the result by 2.

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