McQuay WGS130AA: WGS140AW, Operating Manual

Page: 67 / 98

OM WGS-5 WGS 130A to 190A 67
Remote evaporative condensers require their own self-contained, on-board, head pressure
controls. See page 51.
Fan Stages
There are up to 6 stages of Fantrol available. See the table below:
Table 22, Fantrol Staging
Fantrol Stage Fans On
1 1
2 1,2
3 1,2,3
4 1,2,3,4
5 1,2,4,5,6
6 1,2,3,4,5,6
Staging Up
The controller looks at high the discharge pressure is and how quickly it is rising by
accumulating “points” in a collection bin called the “Stage Up Accumulator”.
There are four Stage Up deadbands that apply to the Fantrol stages. Stages one through
three use their respective deadbands. Stage four to six share the fourth Stage Up dead band.
When the saturated condenser temperature is above the the active deadband, a
Stage Up Error is accumulated.
Stage Up Error Step = Saturated Condenser Refrigerant temperature – ( Stage Up
dead band). If the condensing temperature is high (rising quickly) relative to the Target plus
the dead band, multiple Stage Up Error Steps will be accumulated. This will start a fan
sooner than if the temperature is rising slowly.
The Stage Up Error Step is added to Stage Up Accumulator once every Stage Up Error
Delay seconds. When the Stage Up Error Accumulator is greater than the Stage Up Error
Setpoint, another stage is added.
When a stage up occurs
or the saturated condenser temperature falls back within the Stage
Up deadband, the Stage Up Accumulator is reset to zero.
Forced Fan Stage At Start
Fans may be started simultaneously with the compressor based on outdoor ambient
temperature. When the compressor starts, a Fantrol stage is forced based on the following
table. Since this logic applies only to Fantrol fans, the VFD fan is not affected.
Table 23, Forced Fan Staging
OAT Fantrol Stage At Start
> 75
o
F Forced Fantrol 1 SP
> 90
o
F Forced Fantrol 2 SP
> 105
o
F Forced Fantrol 3 SP
Staging Down
There are four Stage Down dead bands. Stages one through three use their respective
deadbands. Stages four to eight share the fourth Stage Down deadband.
The Stage Down process is similar to the Stage Up except in reverse.
When the saturated condenser refrigerant temperature is below the Target – the active
deadband, a Stage Down error is accumulated.
Stage Down Error Step = (Target - Stage Down dead band) - Saturated Condenser
Refrigerant temperature.