78
IMM AGS-2
The following is the procedure for changing the filter-drier core:
The standard unit pumpdown is set to stop pumpdown when 20 psig (138 kPa) suction
pressure is reached. To fully pump down a circuit beyond 20 psig (138 kPa) for service
purposes, a "Full Pumpdown" service mode can be activated using the keypad.
With Full Pumpdown = Yes, then the next time the circuit is pumped down, the pumpdown
will continue until the evaporator pressure reaches 15 psig (103 kPa) or 120 seconds have
elapsed, whichever occurs first. Upon completing the pumpdown, the "FullPumpDwn"
setpoint is automatically changed back to "No".
The procedure to perform a full service pumpdown for changing the filter-drier core is as
follows:
1.
Under the "Alarm Spts", change the "FullPumpDwn" setpoint from "No" to "Yes".
2.
Move the circuit switch to the OFF position. The compressor will unload to minimum
slide position and the unit will pump down.
3.
Upon completing the full pumpdown per step 3, the "FullPumpDwn" setpoint is
automatically changed back to "No" which reverts back to standard 20 psig (138 kPa)
pumpdown stop pressure.
4.
If the pumpdown does not go to 15 psig (103 kPa) on the first attempt, one more
attempt can be made by repeating the above steps. Do not repeat "FullPumpDwn" more
than once to avoid excessive screw temperature rise under this abnormal condition.
5.
The circuit is now in the deepest pumpdown that can be achieved by the use of the
compressor. Close the two liquid line shutoff valves upstream of the filter-drier, on the
circuit to be serviced plus the optional suction shutoff valve. Manually open the EXV,
then remove the remaining refrigerant from the evaporator by the use of a refrigerant
recovery unit.
6.
Loosen the cover bolts, remove the cap and replace the filters.
7.
Evacuate and open valves.
Evacuate the lines through the liquid line manual shutoff valve(s) to remove
noncondensables that could have entered during filter replacement. Perform a leak check
before returning the unit to operation.
Compressor Slide Valves
The slide valves used for unloading the compressor are hydraulically actuated by pulses
from the load/unload solenoid as controlled by the circuit controller. See OM AGS for
details on the operation.
Electronic Expansion Valve (EXV)
The electronic expansion valve is located in each circuit’s liquid line entering the
evaporator.
The expansion valve meters the amount of refrigerant entering the evaporator to match the
cooling load. It does this by maintaining constant suction superheat. (Superheat is the
difference between the actual refrigerant temperature of the gas as it leaves the evaporator
and the saturation temperature corresponding to the evaporating pressure.) The EXV logic
controls the superheat between 4
°
F at 0% slide position and 8
°
F at 100% slide position.
The position of the valve can be viewed at any time by using the MicroTech II controller
keypad through the View Refrigerant menus. There are 6386 steps between closed and full
open. There is also a sight glass on the EXV to observe valve movement and to check if it
is open or closed visually.
