Emerson VISSION 20/20 service manual Pull-down

•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.

Number of steps = (∆ 60 psig change * 1 step/5 psig) = 12 steps

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.

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