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• Stop Delay = delays the compressor from stopping when the suction pressure is equal to or less than the
stop pressure.
• Minimum Slide Position = The minimum capacity slide position that the compressor is allowed to run at. By
forcing the compressor capacity to operate at a value above minimum, we insure that the suction pressure
will be pulled down to the Stop pressure setpoint value.
Pull-down
• The Pull-down set-points define a method of slowly pulling the suction pressure down from a high value.
This is sometimes required on systems that have liquid recirculation systems. On such systems, if the suc-
tion pressure is pulled down too fast, the pumps can cavitate, causing vibration and damage to the pumps.
Pull-down is also required for new plant startups. Pulling the temperature of new buildings down too quickly
can cause structural damage, so limiting the suction pressure pull-down rate will prevent this, allowing time
to de-humidify the rooms as the temperature in the rooms are pulled down.
• The pull-down feature provides a method to slowly pull the suction pressure down to operating conditions.
The pull-down method used is to step the suction pressure down over a defined time interval. When the
time interval expires, another step is taken.
Example:
Assume the suction pressure is at 80 psig and the set-point we want to get to is 20 psig. This defines a change of
60 psig. We want to allow 48 hours of pull-down time. Pick a reasonable step pressure of 5 psig for every step.
Numberofsteps=(∆60psigchange*1step/5psig)=12steps
Delay per step = (48 hours / 12 steps) = 4 hours/step
For the first 4 hours, the compressor runs at 75 psig.
next 4 hours @ 70 psig
next 4 hours @ 65 psig
and so forth. After the 12th step (running at 25 psig), 48 hours will have elapsed, and the new setpoint be-
comes 20 psig, achieving the 20 psig setpoint after 48 hours. After the pull-down setpoint equals or is less
than the control setpoint, the pull-down feature disables itself.
• Enable = Enables Pulldown control. Uncheck the box to disable the Pull-down setpoints.
• Step Pressure = This setpoint defines the step increments which the suction pressure will be controlled at.
• Delay Per Step = Defines the time increment at which the compressor will be controlled for each step.