Time Proportioning Control

Assume that Suction Pressure is the selected parameter to control compressor capacity. Time proportioning control is the method used to control the compressor slide valve position in response to a setpoint.

The setpoints are;
• Pressure Control Setpoint	(This is the target set-point.)

•Proportional band

•Dead-band

•Interval time

Example:

Setpoint = 20 psig

High Proportional Band = 4 psig

High Dead-band region = 4 x 0.10 = 0.4 psig Low Proportional Band = 3 psig

Low Dead-band region = 3 x 0.10 = 0.3 psig

Proportional Band

When the suction pressure exceeds the dead-band in either direction, it then enters the “proportional band” area. The Capacity Increase/Decrease Proportional Band defines the range of pressure in which corrective action will be taken to bring the suction pressure back to within the setpoint dead-band. This is done by pulsing the appropriate capacity motor. The length of the pulse is in direct proportion to the amount of deviation that the suction pressure is from the setpoint dead-band.

Dead Band

A region around the setpoint automatically exists and defines a range around the setpoint where the pressure is allowed to deviate without the program taking any corrective action. This allows small variances of pressure around the setpoint, and reduces capacity hunting from occurring. The dead-band regions are always defined as 10% of the proportional band region values.

Interval Time

The Capacity Increase/Decrease Interval Time is the time interval that the program will use to look and determine if a capacity increase or decrease pulse is required to bring the Suction Pressure back to the setpoint dead-band area. The length of the pulse is in direct proportion to the amount of deviation that the suction pressure is from the setpoint dead-band. The formula used to determine the length of pulse is described as:

•0.5 second pulse for every 10% away from dead-band region.

•0 second pulse = suction pressure within set-point dead-band

•0.5 second pulse = suction pressure is in the proportional band region and 10% or less from the dead-band region.

•1 second pulse = suction pressure is in the proportional band region and is > 10% and < 20% from the dead- band region.

•1.5 second pulse = suction pressure is in the proportional band region and is > 20% and < 30% from the dead- band region.

•2 second pulse = suction pressure is in the proportional band region and is > 30% and < 40% from the dead- band region.

•2.5 second pulse = suction pressure is in the proportional band region and is > 40% and < 50% from the dead- band region.

•3 second pulse = suction pressure is in the proportional band region and is > 50% and < 60% from the dead- band region.