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has opened, the Condenser Refrigerant Control ramp-up period begins as described in

. While both compressors are running, the variable geometry

diffusers are modulated according to the stall activity and pre-rotation vanes position. There is no

VGD response to surge activity.
When both compressors have been brought online, run for at least 30 minutes, unless the anti-

recycle feature has been disabled, a fault is detected, or a Local or Remote stop is applied. After the

30 minute waiting period, a soft shutdown, as described in

Compressor shutdown

, is performed on

the lag compressor if the lead compressor motor current goes below the

Low Load setpoint

(20%

to 60%; default 45%) for a period equal to the

Low Load Time setpoint

(1 to 20 minutes; default 5).

Lag compressor start with high head

When operating at high head conditions, surges can occur while going from one compressor to two

compressor operation. A high-head condition is detected by measuring the Delta P/P [

(Condenser

pressure – Evaporator pressure) / Evaporator pressure

] and comparing it to the

High Head DP/P Limit setpoint. By closing the lead compressor’s pre-rotation vanes during a lag

compressor start when a high head condition is present, surge conditions can be minimized.
With the lead compressor running and it is determined the lag compressor is required to run, the

Delta P/P is measured. If it is > the High Head DP/P Limit setpoint, a close signal is applied to the

lead compressor’s pre-rotation vanes coincident with the start of the lag compressor’s pre-lube.

They can open when all the following conditions are met:

•

The lag compressor is running

•

The lag discharge valve is open

•

The difference between the lead and lag motor currents is < 5%

An LED on the Surge – Dual Compressor Setup screen and the Capacity Compressor Cycling

screen illuminates whenever a high head Lag compressor start condition exists. Also, the message

Warning – Head Pressure – High Head Lag Start Limit

appears on the System Status line

of the display during this condition.

Condenser refrigerant level control – lag start

When operating at low condenser liquid temperatures, a Low Evaporator Pressure trip can occur

when going from one compressor to two compressor operation.
During startup of the lag compressor, as the lag discharge valve opens, the system will see a step

change in refrigerant flow. Although the Condenser Refrigerant Level Control keeps the refrigerant

level stable during normal operation, the variable orifice does not open quickly enough when the

lag compressor is started. This causes the refrigerant to stack in the in the condenser, starving the

evaporator, and causing a low evaporator pressure shutdown.
The condenser Refrigerant Level Control Lag Start mode is initiated when the lag compressor

enters prelube and is terminated 2 minutes after the lag discharge valve has opened. During this

mode, if the refrigerant level is > 20%, the level control lower (valve open) output is turned on for 5

seconds every 10 seconds until the level is less than 20%. If the refrigerant level is < 20%, the level

control is held in place.
After the Lag Start mode has ended, a linearly increasing ramp limit, called the Refrigerant Level

Target, is applied to the Refrigerant Level setpoint. This ramp allows the level to go from the

present level (20%) to the programmed Refrigerant Level setpoint over a period equal to one-half of

the Valve Preset Time setpoint. During this ramp-up period, the Refrigerant Level target is used to

control the refrigerant level in the condenser. The Refrigerant Level setpoint is then used to control

the level for the remainder of the run period.
On the Refrigerant Level Control screen, the Refrigerant Level Override Mode displays

Valve

Preset

during the pre-positioning of the refrigerant level control valve while bringing on the lead