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M&M

EFRIGERATION

. 3-13

Chapter 3: Capacity Control

Current, the CCS informs the master sequencer that no further capacity increases are

possible and additional compressors may be needed.

Running in Full Load Mode

When the Full Load command is received the CCS disables all capacity control and

locks the slide valve at the fully loaded position. However, normal limiting and unload-

ing functions will still be active.

Anti-Cycle in Sequenced Mode

While in the Sequenced mode the anti-cycle time takes on a new role. If the master

sequencer requests a screw compressor to stop but time is still remaining on the anti-

cycle timer, the compressor will not stop. The compressor will remain running, at its

minimum slide valve position, until the anti-cycle timer expires. This procedure

ensures that the compressor is always available for the next sequenced start. If the

compressor should be stopped manually, or due to a failure, with time remaining on

the anti-cycle timer it would then be unavailable for sequencing until the timer expires.

Forced Unloading & Limiting

Forced Unloading and Limiting are performed to protect the system under extreme condi-

tions or to slow-down the rate of capacity increase. The CCS may unload the compressor

or limit loading of the compressor due to high motor current or high discharge pressure. In

the Manual mode of operation, only high motor current limiting and forced unloading is

enabled. Forced unloading and limiting will cancel any ongoing capacity control move-

ments of the slide valve. If a parameter exceeds the high limit setpoint the compressor

will be limited (not allowed to load). If a parameter exceeds the high unload setpoint the

compressor will be forced to unload until the parameter is reduced.

Ramp Start

Ramp Start adjusts the forced unloading and limiting setpoints during a period of time

after the compressor is started. Ramp start is used to reduce the demand power require-

ments during compressor startup. Each high unload and high limit setpoint has associated

with it a similar ramp unload and ramp limit setpoint. Also associated with each different

parameter is a ramp duration time. After the compressor has started, the forced unloading

and limiting setpoints are gradually changed from the ramp setpoints to the high setpoints

during the ramp duration time. The Discharge Pressure Ramp Start parameters are setup

on the Discharge Pressure Setpoints screen under the SETPT hot key. The Motor Current

Ramp Start parameters are setup on the Motor Current Setpoints screen under the

SETPT hot key.

The relationship of the ramp start feature to unloading and limiting can be seen in the fol-

lowing figure.

M&M RWB series User Manual

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