McQuay AWS, Operating Manual

Page: 35 / 68

KOMAC00607-09EN 35
When one fan is running, a fixed point is used in place of a deadband. When the
Saturated Condenser temperature drops below 70°F, stage down error is accumulated.
When a stage down occurs or the saturated temperature rises back within the Stage
Down dead band the Stage Down Error Accumulator is reset to zero. The accumulator
is held at zero after startup until either the outside ambient temperature is less than or
equal to 75°F, or the saturated condenser temperature is greater than the condenser
target less the active stage down deadband.
VFD
Condenser pressure trim control is accomplished using an optional VFD on the first fan.
This VFD control varies the fan speed to drive the saturated condenser temperature to a
target value. The target value is normally the same as the saturated condenser
temperature target.
VFD State
The VFD speed signal is always 0 when the fan stage is 0.
When the fan stage is greater than 0, the VFD speed signal is enabled and controls the
speed as needed.
Stage Up Compensation
In order to create a smoother transition when another fan is staged on, the VFD
compensates by slowing down initially. This is accomplished by adding the new fan
stage up deadband to the VFD target. The higher target causes the VFD logic to
decrease fan speed. Then, every 5 seconds, 0.1
°
F is subtracted from the VFD target
until it is equal to the saturated condenser temperature target set point. This will allow
the VFD to slowly bring the saturated condenser temperature back down.
EXV Control
The EXV is moved at a rate of 150 steps per second, with a total range of 3810 steps.
Positioning is determined as described in the following sections, with adjustments made
in increments of 0.1% of the total range.
Preopen Operation
The EXV control includes a preopen operation that is used only when the unit has
optional liquid line solenoids. The unit is configured for use with or without liquid line
solenoids via a set point.
When a circuit start is required, the EXV opens before the compressor starts. The
preopen position is defined by a set point. The time allowed for this preopen operation
is at least enough time for the EXV to open to the preopen position based on the
programmed movement rate of the EXV.
Startup Operation
When the compressor starts (if no liquid line solenoid valve is installed), the EXV will
start to open to an initial position that allows a safe start up. The value of LWT will
determine if it is possible to enter the normal operation. If it is higher than 68°F then a
pressostatic (constant pressure) 50.8 psi control will start to keep the compressor into the
envelope. It goes in normal operation as soon as the suction superheat drops below a
value equal to the suction superheat setpoint.
Normal Operation
Normal operation of the EXV is used when the circuit has completed startup operation
of the EXV and is not in a slide transition conditions.
During normal operation, the EXV controls suction superheat to a target that can vary
from 7.2 degrees F to 15.3 degrees F.
The EXV controls the suction superheat within 1.5 degrees F during stable operating
conditions (stable water loop, static compressor capacity, and stable condensing
temperature).