Section 6 • Compressor Control

Load Anticipating

The purpose of the load anticipating algorithm is to re- duce the amount of overshoot of the capacity slide posi- tion while the compressor attempts to meet the control setpoint. This advanced feature of the Vission 20/20 closely monitors the rate of change of the process vari- able and compares it to the control setpoint. If the pro- cess variable is changing in the direction of the control setpoint at the specified rate or greater, then the normal command to move the capacity slide is interrupted. The rate is calculated between time intervals set in the pro- portional control section of this screen.

Enable Load Anticipation Algorithm:

•Allows the operator to choose if the load anticipation algorithm runs.

Rate Dead Band:

•Defines the rate at which the capacity slide move- ment will be interrupted. This value is an absolute val- ue of the process variable. For example, the default value is 0.25. If the control mode is suction pressure, then this value is 0.25 Psig or if process temperature is the control mode then the value would be 0.25°F.

Oil Control

These setpoints determine how the Vission 20/20 will manage the oil of the compressor.

Oil Pump Press Restart Ratio:

•The on and off setpoints define when the oil pump will cycle on and off if the oil pump is selected to cycle from the configuration screen.

Oil Separator Heater Temp:

•When the oil temperature falls below this setpoint the oil heater will turn on. Note, there is a 5°F differen- tial associated with this setpoint. For example, when set at 100°F, the heater will turn on at 95°F and off at 105°F.

Liquid Injection

The setpoints is this section are to control the behavior of the liquid refrigerant injected into the compressor for oil cooling purposes. The liquid injection solenoid con- trol is based off of discharge temperature whether the compressor uses just an injection solenoid or a motor- ized valve in conjunction with the solenoid.

Liquid Injection Solenoid Control ONLY

•When using only the liquid injection solenoid, the so- lenoid is activated once the value of discharge tem- perature meets or exceeds the value of “Liquid inj. Setpoint 1” and the value of oil separator tempera- ture meets or exceeds the value of “Oil Sep. Temp. Override”. The injection solenoid will deactivate if either of setpoints are not met. This will prevent situ- ations where the discharge temperature may rise quickly, but the oil temperature is still very cold. By preventing the liquid injection solenoid from turning on at this point, the oil separator will not be subjected to additional liquid refrigerant, that would cool the oil even further.

Liquid Injection Control using a 4-20ma motorized valve.

•When a motorized valve is used to control the amount of liquid being injected into the compressor the pre- viously mentioned setpoints have a slightly different function. The Oil Sep. Temp. Override is still used in controlling the injection solenoid, however the Liquid Inj. Setpoint 1 is now used as the target temperature for the PID Algorithm that controls the position of the motorized valve. The algorithm compares the actual discharge temperature against the Liquid Inj. Setpoint 1. The difference between these is the error. The PID algorithm tries to drive the error to “zero” by moving the positioning valve to allow more or less liquid refrigerant to be injected into the compressor.

•PID algorithm can be notoriously hard to tune. As a result the Vission 20/20 offers a couple of addi- tional features to help control wild fluctuations in oil temperatures that could result in the compres- sor tripping off. The operator can choose to enable the minimum value position that automatically sets the liquid injection motorized value to the specified value whenever the discharge temperature has fallen below the Liquid inj. Setpoint 1. This feature nearly eliminates the overshoot of the PID in the downward direction and reduces the chance of the compressor tripping off due to low oil temperature. The operator can also choose to use an average of the discharge temperature and the oil manifold temperature as the control variable. The discharge temperature can vary quite drastically forcing the PID algorithm to drasti- cally adjust the motorized value. By averaging the more stable oil manifold temperature and discharge temperature, the control variable stabilizes and the PID is more easily tuned.

•Please note that as stated above, PID algorithms can be difficult to tune and there is no one set of PID val- ues that will work. The work required for a compres- sor to meet the requirement of its installation vary greatly and therefore the amount of heat transfered to the oil varies just as greatly. We recommend the

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Emerson 20/20 service manual Load Anticipating, Oil Control, Liquid Injection
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20/20 specifications

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