OM AGSB-5
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5. Check to see that pumpdown switches PS1 and PS2 are in the "Pumpdown and Stop"
(open) position. Throw the S1 switch to the "auto" position.
6. Under the "Control Mode" menu of the keypad, place the unit into the automatic cool
mode.
7. Start the system by moving pumpdown switch PS1 to the "auto" position.
8. Repeat steps 8 and 9 for PS2.
Expansion Valve Operation
Pre-open:
At the time of a start request, the EXV will perform a pre-open function. This process
insures that sufficient liquid refrigerant resides 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 set point before the compressor will
start. The timer is adjustable from 20 to 120 seconds, and has a default setting of 60
seconds.
Pressure Control:
The EXV will be in pressure control mode after startup. The EXV controls the evaporator
pressure to a calculated pressure target. The base pressure target is calculated using the
following formula:
Base target = 2/3LWT – 8
The base target is limited to a range from the low pressure inhibit set point plus 2 psi, up to
52 psi.
The base pressure 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 pressure target is adjusted
down one psi. Similarly, for every degree above the maximum, the pressure target is
adjusted up one psi. At any time, the adjusted target pressure cannot go below the low
pressure inhibit set point plus 2 psi, or above 52 psi.
EXV will transition from pressure control to subcool control when all of the following are
true:
Discharge Superheat > 22°F for at least 3 minutes while in pressure control
LWT <= 60°F
Subcool > current base subcool value OR EXV steps > (max EXV steps – 100)
Unit mode = Cool (includes Available Modes: Cool w/ Glycol. Subcool control is not
used in Ice Mode)
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,
A low evaporator pressure unload occurs.
The suction pressure drops below the low evaporator pressure unload setpoint, the
controller will switch from subcool control to pressure control, until sufficient
subcooling is established.
Summary of Contents for AGS 206A
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