McQuay AGS 206A, Operation Manual

Page: 21 / 66

OM AGSB-5 21
EXV Control
Three different expansion valve (EXV) control modes are used. Any time the EXV is not in
one of these control modes, it will be closed. For any of the control modes, the EXV
position is limited to a range based on the slide position target. The minimum setting
provides sufficient flow for motor cooling, the maximum flow limit helps prevent flood
back to the compressor. The table below shows the EXV range for each size compressor, at
minimum and maximum capacity. The minimum and maximum values vary linearly with
slide position, defining a new EXV control range for every change in slide position.
Table 9, EXV Range
Compressor Size
EXV Slide %
205mm 220mm 235mm
Min 0 250 250 250
Max 0 3000 3000 3000
Min 100 870 1080 1300
Max 100 3400 4200 5000
Based on the values in the above table, the EXV control range varies as shown in the figure
below. The shaded area is the control range.
Figure 3, EXV Control Range
Max EXV
@ 100%
100
Max EXV
@ 0%
Min EXV
@ 0%
EXV Control Range
0
Slide Position (%)
EXV
Steps
Min EXV
@ 100%
1. Pre-Open
At the time of a start request, the EXV will perform a pre-open function. This is to provide
sufficient liquid refrigerant in the evaporator to avoid low pressure situations at startup.
During pre-open, the EXV will open to 3000 steps, while the saturated evaporator
temperature is less than the LWT. The EXV will move to 250 steps when the saturated
evaporator temperature is equal to, or greater than, the LWT.
The EXV must be in the pre-open state for a time equal to the pre-open timer setpoint
before the compressor will start. This state may be skipped if the start request occurs, and
the evaporator saturated temperature is greater than LWT + 5
°
F. In this case, the
compressor would start and the EXV would go straight to pressure control.
2. Pressure Control
The EXV will be in pressure control mode after startup, and always when in the ICE mode.
In this mode, the EXV controls discharge superheat with adjustments to the evaporator
pressure target. A proportional integral function is used to keep the evaporator pressure at
the target.