22
OM
AGSB-5
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 setpoint, plus 2 psi, up to
52 psi.
The pressure control target may be adjusted if the discharge superheat is not within an
acceptable range. If the superheat is less than 22
°
F, the base pressure target will be reduced
by a value equal to the low superheat error. If the superheat is more than 40
°
F, the base
pressure target will be increased by a value equal to the high superheat error. At any time,
the adjusted target pressure cannot go below the low pressure inhibit setpoint plus 2 psi, or
above 52 psi.
The 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
•
Unit mode = Cool (includes Available Modes: Cool w/ Glycol. Subcool control is not
used in Ice Mode)
The pressure target will ramp down when transition to pressure control occurs. The
pressure target will decrement 0.2 psi every second until it reaches the normal target. The
starting target is the current pressure at transition to pressure control, but is limited to a
maximum of 52 psi.
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 in F code, a
low evaporator pressure unload occurs. Starting with F code, when 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.
3. Subcool Control
After completing pressure control, the EXV transitions to the primary mode of operation,
subcool control. 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 5
degrees F to 20 degrees F as slide position changes from 0 to 100%.
The base subcool target value is adjusted when the discharge superheat is less than 22
degrees F, or greater than 40 degrees 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 degrees F, and the adjusted subcool target is limited to a minimum of 2 degrees F.
When the circuit initially enters subcool control, the subcool target is set to the current
subcool value less 2 degrees. This value may be above the normal 5 to 20 degree range.
The target subcool value is then reduced 0.1 degree every two seconds until it reaches the
calculated target value.
The EXV may transition from subcool control back to pressure control. This occurs when
the LWT is greater than 63
°
F or if the pressure drops to the LPUnload SP, while in subcool
control.
The EXV will transition from subcool control to pressure control if a low pressure unload
occurs. The normal rules to transition back to subcool control will apply.
Summary of Contents for AGS 206A
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