56
OM
AGSB-5
Subcool Control:
After completing pressure control, the EXV transitions to subcool control, which is used
only when the unit mode is Cool. In this mode, subcooling is controlled by the EXV, with
adjustments to the subcool target based on discharge superheat. The base subcool target
varies linearly from 9 to 20
o
F as slide position changes from 0 to 100%. The subcool target
will vary from 5 to 20, if 20°< DSH < 40°.
The base subcool target value is adjusted when the discharge superheat is less than 22
°
F, or
greater than 40°F. For every degree below the minimum, the subcool target is adjusted up
one degree. Similarly, for every degree above the maximum, the subcool target is adjusted
down one degree. The maximum offset to the base subcool value is 15
°
F, and the adjusted
subcool target is limited to a minimum of 2°F.
The EXV may transition from subcool control back to pressure control. This occurs when
the LWT is greater than 63°F, the unit mode is switched from Cool to Ice, or a low
evaporator pressure unload occurs.
Temporary Shutdown
Move pumpdown switches PS1 and PS2 to the "Pumpdown and Stop" position. After the
compressors have pumped down, turn off the chilled water pump.
CAUTION
Do not turn the unit off using the "S1" switch, without first moving PS1 and PS2 to
the "Stop" position, unless it is an emergency, as this will prevent the unit from going
through a proper shutdown/pumpdown sequence.
CAUTION
The unit has a one-time pumpdown operation. When PS1 and PS2 are in the
"Pumpdown and Stop" position the unit will pumpdown once and not run again until
the PS1 and PS2 switches are moved to the auto position. If PS1 and PS2 are in the
auto position and the load has been satisfied, the unit will go into one-time
pumpdown and will remain off until the MicroTech II control senses a call for cooling
and starts the unit. Under no circumstance use the compressors for pumpdown of
the system with the liquid line valves closed.
CAUTION
It is important that the water flow to the unit is not interrupted before the
compressors pump down to avoid freeze-up in the evaporator.
CAUTION
If all power is turned off to the unit, the compressor heaters will become inoperable.
Once power is resumed to the unit, it is important that the compressor and oil
separator heaters are energized a minimum of 12 hours before attempting to start the
unit. Failure to do so could damage the compressors due to excessive accumulation
of liquid in the compressor.
Summary of Contents for AGS 206A
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