Application Engineering Basics

Physical Laws for Blower Applications


In the following formulas these symbols are used:
P -
CFM -
RPM -
D -
H -
SG -
 Pressure in pounds per square inch (PSI) or inches of mercury column (inches Hg)
Volume in cubic feet per minute
Speed in revolutions per minute
Density in pounds per cubic foot (lbs./cu. ft.)
Height of air or gas column (ft.)
Specific Gravity (ratio of density of gas to the density of air)

"Standard Air" - Air at 68°F (absolute temperature 528°) and 29.92" Hg. (barometric pressure at sea level). The density of such air is 0.075 lbs./cu. ft. and the specific volume is 13.29 cu. ft./lb. The specific gravity is 1.0.

The outlet pressure of a blower depends on the condition of the air or gas at the inlet. The inlet condition is influenced by:
a - Specific gravity (The ratio of density of the gas to density of standard air)
b - Altitude (location of blower)
c - Temperature of inlet air

Basic Fan Laws Chart

VARIABLE VOLUME PRESSURE HORSEPOWER
WHEN SPEED CHANGES

Varies DIRECT
with Speed Ratio

 Varies with SQUARE of Speed Ratio
 Varies with CUBE of Speed Ratio
WHEN DENSITY CHANGES Does Not Change Varies DIRECT with Density Ratio
 Varies DIRECT with Density Ratio
Volume
The Volume changes in direct ratio to the speed.
Example - A blower is operating at 3500 RPM and delivering 1000 CFM. If the speed is reduced to 3000 RPM, what is the new volume?

V = Original Volume (1000 CFM)
V = New Volume
RPM = Original Speed (3500 RPM)
RPM = New Speed (3000 RPM)

Pressure
Pressure (barometric) varies in direct proportion to altitude.
Example - A blower is to operate at an elevation of 6000 feet and is to deliver 3 PSI pressure. What pressure (standard air) blower is required?

If it is desired to determine what pressure a 3 lb. (standard air) blower will deliver at 6000 feet -

When a blower is to operate at a high altitude it is frequently specified that the blower be capable of handling a given volume of "standard air". It is then necessary to determine the equivalent volume of air at the higher altitude. Example - A blower is to operate 6000 feet altitude and is to handle 1000 CFM of standard air. What is the CFM of air the blower must handle at 6000 feet altitude?

Let:
V = Volume of standard air (1000 CFM)
V = Volume of thinner air
Hg = Barometric pressure sea level (29.92)
Hg = Barometric pressure 6000' (23.98)

The pressure changes as the square of the speed ratio.
Example - A blower is operating at a speed of 3500 rpm and delivering air at 5.0 pounds pressure. If the speed is reduced to 3000 rpm, what is the new pressure?

P = Original Pressure (5 lbs.)
P = New Pressure
RPM = Original Speed (3500 RPM)
RPM = New Speed (3000 RPM)

 

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