M&M
R
EFRIGERATION
I
NC
. 3-3
Chapter 3: Starting the Screw Compressor
Oil Pressure Calculations
The Oil Pressure and Oil Filter Pressure values are computed using a combination of the
Oil Pressure After Filter, Oil Pressure Before Filter, and Discharge Pressure transducers.
The following formulas are used:
For a Full Lube Oil Pump the oil pressure is always computed as:
Oil Pressure = Oil Pressure After Filter - Discharge Pressure.
For Cycling and Pre-Lube Pumps:
If the motor is off, Oil Pressure = Oil Pressure After Filter - Discharge Pressure.
If the motor is on, Oil Pressure = Oil Pressure After Filter - Suction Pressure
The Oil Filter Pressure is always computed as:
Oil Filter Pressure = Oil Pressure Before Filter - Oil Pressure After Filter.
Oil Level Indication
When a pre-lube or a cycling oil pump is used, an oil level indicator switch is installed in
the oil separator. This switch opens when the oil level is too low for compressor operation
and the compressor is failed. The level switch is required since the oil pressure is com-
puted as the difference between the oil pressure after filter sensor and suction pressure
sensor when the motor is running. This value could be at normal operating levels even if
oil is not present. When a full time oil pump is used the level switch is not used.
Starting the Screw Compressor
Once a start has been requested, the CCS will delay a minimum of eight (8) seconds before
starting the motor. The minimum start delay is in place to provide the operator an opportunity
to abort the start if unsafe conditions exist.
Once the compressor motor has started, the CCS will delay a minimum of fifteen (15) seconds
before transitioning into the running state. This “up to speed” delay is in place to assure the
motor is turning at full RPM, and allow the system to stabilize from the effects of starting the
motor.
All Alarms and Failures based on sensor values and sensor failures are inhibited during the
“up to speed” delay.
On RWB compressors, the standard oil pressure alarms and failures are inhibited for two (2)
minutes after the motor starts.
Once a start request is made, the microprocessor will unload the slide valve utilizing the
unload solenoid. The slide valve position must be below 5% before the compressor will be
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