POWER UP A WINNER - Plant Services

0 13 25 48 50 63 75 88 100
% Power (kW)
Note:
Efficiency / Compressors
1. These operating characteristics will vary with motor type,
basic design, motor efficiency, constant or variable speed, etc.
2. Amperage will vary indirectly with changes in rated voltage.
It is best to measure kW directly.
3. 87% amp = 80% power
Input power estimates flow
% kW power
110
100
90
80
70
60
50
40
30
20
10
0
Actual modulation
performace curve
No load
69% power
85% power
35% flow
% Flow
100% load
100% power
Estimated flow = 215 X .35 = 75 acfm
Note: The actual no load modulation point
will vary with each compressor. To establish
actual curve, measure kW at full load at rated
pressure and kW at point where valve closed
before blowdown. Draw appropriate curve.
10 20 30 40 50 60 70 80 90 100
Figure 3. This modulation performance curve applies to a singlestage,
50-hp class, lubricant-cooled rotary screw compressor with
modulation control.
If greater than 100 hp, use 1.2 times horsepower. Figure 2
shows typical motor performance curves and their relationships
to each other. The specific curves vary by motor type,
but the relationships that apply to a standard 1,800-rpm
induction motor are similar. In our example, the measured
current is 87% of the measured full-load amperage which,
in this case, reflects about 80% of full-load power (kW). The
no-load current is 84% of FLA, which reflects about 60% of
full-load power (kW). This no-load power percent is used to
draw the “power-to-flow curve.”
Proceed as follows to construct the curve.
• Make simultaneous measurements of current and voltage
at full-load and at no-load conditions.
• The no-load current (before blow down) of 102.6 represents
84% of full-load amps and, 69% of full-load power.
• Draw a performance curve with the correct slope
through the no-load point at 69%
• At 80% power, you have 35% flow (75 acfm out of 215 acfm).
This method is relatively accurate and sufficient for making a
reasonable flow estimate. One caveat: Too many times the percent
of full-load amps is used instead of percent of load. If the
87% of full-load amps had been incorrectly interpreted as 87%
power, the estimated flow would have been almost 60% of full
flow, or 129 cfm, a figure that’s almost twice the real value.
Hank van Ormer owns Air Power USA Inc. in Pickerington, Ohio.
Contact him at HankvanOrmer@aol.com and (740) 862-4112.
34 APRIL 2009 www.PLANTSERVICES.com