McQuay AGS 225D, Operating Manual

Page: 42 / 82

42 OM AGSD-1
Compressor Capacity Control
Compressor capacity is determined by calculating a slide position target. Adjustment to the
slide target for normal running conditions occurs every 5 seconds. For loading a maximum
change of 1% is allowed, and for unloading a maximum change of 2% is allowed.
Required parameters
Count In Load Balance status for each compressor
Slide targets of all compressors
LWT error
LWT slope
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 cir 2 cir 3 target)/3)
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 evaporator low-pressure alarms at the start of a circuit in cold climates, a
low OAT start logic is introduced. It allows the circuit to operate at low evaporator
pressures for a longer time than normal as well as multiple start attempts.
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 this 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.