10
CMC TECHNICAL REFERENCE MANUAL
1X36003 Version 2.52
1996-1999 Ingersoll-Rand Company
Date of Issue: 18-Oct-1999
The following table presents seven capacity requirements for a plant air system. At each of
the capacities, the table shows the compressor output, valve position, discharge pressure
and power. Each of these values represents a percentage and is only an example. P2 is the
specified discharge pressure and P0 is the barometric pressure.
From the table above, once the system required capacity moves below 75 percent, the
compressor still produces 75 percent capacity with 80 percent of the power. If the system
needs only 25 percent capacity, it will still have to pay for 80 percent of the power. This is
why it is important to open the bypass valve at the last possible moment; therefore, setting
MinLoad
properly is critical for efficient energy management.
Surge Control
Surge is the reversal of flow within a dynamic compressor that takes place when the
capacity being handled is reduced to a point where insufficient pressure is being generated
to maintain flow. This condition can potentially damage the compressor if it is severe and is
allowed to remain in that state for a prolonged period; therefore, control and prevention is
required.
Control Methodology
Surge prevention is accomplished by opening the bypass valve prior to reaching the surge
point. The point at which the bypass valve opens is
MinLoad
. By blowing a portion of the air
to the atmosphere, the compressed air system gets the air that it demands. The
compressor avoids surge because it is still producing a constant air capacity.
Surge Detection
Even though the CMC controls to prevent surge, it can still occur. Insufficient rise to surge,
rapid changes in system discharge pressure, and various other reasons exist for a
compressor to surge.
Insufficient Rise To Surge
Rise to surge is the percentage of the compressor's surge pressure to discharge pressure
(see Figure 8). When an insufficient rise to surge situation exists, small fluctuations in the
air system demand and ambient temperature can cause the compressor to surge.
From Figure 8, when T=cold, there is sufficient rise to surge. As the ambient temperature
increases to T=hot, the amount of rise to surge decreases because the discharge pressure
is remaining constant and the natural curve is changing with temperature.
System
Compressor
Compress
or
Open Position
Require
d
Capacity
Operating
State
Output
Capacity
Inlet
Valve
Bypass
Valve
Discharge
Pressure
Power
0
Off
0
0
100
0
0
0
Unloaded
10
10
100
>P
0
20
100
Full Load
100
100
0
P
2
100
75
MinLoad
75
70
0
P
2
80
50
MinLoad
75
70
25
P
2
80
25
MinLoad
75
70
50
P
2
80
0
MinLoad
75
70
100
P
2
80
