McQuay WGS190AW User Manual

OM WGS-5 WGS 130A to 190A 57
Every 12 seconds, the current LWT is subtracted from the value 12 seconds back. This
value is added to a buffer containing values calculated at the last five intervals. The final
result is a slope value that is an average over the past 60 seconds.
Count In Load Balance Flag
A circuit will be counted for load balancing only if all of the following are true:
 Compressor state is run
 Compressor is able to load up (no limits in effect due to pressure or superheat)
 Slide control is auto
 Slide position target is less than the max slide target setpoint
Cool Mode
When the chiller is in COOL mode, capacity of the compressor is adjusted to maintain
leaving water temperature at the Active LWT setpoint while balancing the load between
running circuits. Load balance offset, LWT error, and LWT slope are used to calculate a
change in slide position as described below.
Load balance offset:
IF other compressor is flagged to be counted in load balance
THEN Load Balance Offset = slide target - (cir 1 target + cir 2 target)/2
ELSE Load Balance Offset = 0
LWT Error:
LWT Error = (Leaving Evaporator Water Temp) – (Active LWT setpoint).
LWT slope:
Slope (deg/minute) = sum of last five LWT changes as calculated every 12 seconds
Slide target adjustment = [LWT Error + (LWT Slope x 4) – Load Balance Offset] / #
Compressors Running
Ice Mode
In ICE mode, the compressor capacity is increased at the maximum rate continuously until
reaching the maximum slide position. Load balancing, LWT error, and LWT slope are
ignored.
Low OAT Start Logic
In order to avoid low pressure alarms at the start of a circuit, low OAT start logic allows for
running at low pressures for a longer time than normal, as well as multiple start attempts.
This logic is only used for air-cooled operation.
A low OAT start is initiated if the condenser saturated temperature is less than 60F when
the compressor starts. Once this happens, the circuit is in the low OAT start state for a time
equal to the low OAT start timer setpoint. During this time, the freezestat logic and the low
pressure events are disabled. The absolute limit of –10 psi is still enforced.
At the end of the low OAT start, the evaporator pressure is checked. If the pressure is
greater than, or equal to, the low evaporator pressure unload setpoint, the start is considered
successful. If the pressure is less than the unload setpoint, the start is not successful and the
compressor will stop. Three start attempts are allowed before tripping on the restart alarm;
so if on the third attempt the start is not successful, the restart alarm is triggered.
The restart counter will be reset when either a start is successful or the circuit is off on an
alarm.

McQuay WGS190AW Specifications

General

Brand	McQuay
Model	WGS190AW
Category	Chiller
Language	English