Sizing Drive Motors for Industrial Compressors, Blowers and Fans - Different conditions effecting required Shaft Power PDF Print E-mail
Written by Gert Dam   
Thursday, 01 December 2011 22:23
Article Index
Sizing Drive Motors for Industrial Compressors, Blowers and Fans
Different conditions effecting required Shaft Power
Performance curves & Motor sizing
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Effect of Inlet Pressure on Shaft Power Required:

Process compressors and blowers designed for relatively low inlet pressures (near atmospheric pressure) are very much impacted by seemingly small changes in inlet pressure. The following three tables show power relative to atmospheric air at a 14.7 PSIA / 20 degr.C inlet condition (i.e. 100%).

Effect of Inlet Pressure on Horsepower
Percent of HP at 14.7 PSIA Pinlet, for a single stage

Pamb= 14.7 PSIA
at constant pressure- ratio

Pinlet
PSIG
HP Ratio
-10
32%
-5
66%
0
100%
2
114%
5
134%
7
148%
10
168%
15
202%
20
236%
40
372%
80
644%
100
780%
150
1120%
200
1461%


If the machine inlet will normally run sub-atmospheric, can the motor run with an ambient inlet ?

Effect of Inlet Temperature on Shaft Power Required:

Inlet temperature also affects density, which affects power required for compression or conveying.

For air compressors, if the summer and winter conditions are not given to the vendor, then the motor may be overload in the winter (often, managed by an amp limiter on the flow control valve) and the plant may run out of air (SCFM or pph) in the summer.

For heater or scrubber blowers that normally run with an elevated inlet temperature (eg. 250 degr.F or 120 degr.C), is the motor large enough to start the machine in the "cold" condition ? On the coldest day ? More, in the sections below.

Effect of Inlet Temperature on Horsepower
Percent of HP at 20 degr.C Tinlet, for a single stage

at constant pressure- ratio

Tinlet
degr.C
HP Ratio
-40
126%
-20
116%
0
107%
20
100%
40
94%
60
88%
80
83%
100
79%
150
69%
200
62%

 

Effect of Molecular Weight on Shaft Power Required:

Inlet molecular weight also affects inlet density and power. If the machine was designed for H2, can it compress air or N2 ? Or if it was designed for air, can it compress a high M.W. gas ?

Effect of Molecular Weight on Horsepower
Percent of HP at 28.96 Inlet M.W., for a single stage

at constant pressure- ratio

Molecular
Weight
HP Ratio
2
7%
10
35%
20
69%
28.96
100%
40
138%
50
173%
60
207%


If a motor is chosen that cannot run the machine with air or N2 at normal ambient air temperature, and there are occasions when it will be run this way, then the machine will have to be run with an orifice plate or throttling valve at the inlet to limit inlet density and prevent overloading the motor.

Also, if there is margin in the design to increase speed or capacity of a compressor, blower, or fan, and operations believes they may use this at some time, buying a larger motor upfront (for a relatively small cost adder) in anticipation of this can save money in the long run.