Operating instructions

64 IMM AGSB-60
oil return line by incrementally closing down the ball valve. This can help maintain oil in the
oil separator and higher DSH, if it is overfeeding and dropping the DSH too much. The
minimum superheat the control will allow is 35 degrees F (19 degrees C) to help ensure that
the DSH does not cause issues with limiting the compressor with low discharge superheat or
cause oil loss. Most of the oil recovery is done through carry-over through the suction line.
The evaporator oil return line is used more effectively for discharge temperature control, and a
by-product is that a small percentage of oil will be recovered.
Basic Charging Information:
Determine the following:
1. What control mode is the EXV in?
2. What is the circuit status?
3. What is the compressor slide position?
4. What is the DSH at 100% load?
5. What is the suction pressure at 100% load?
6. Is the evaporator oil return line (EORL) solenoid on?
7. What is the outdoor air temperature (OAT) and how many fans are on?
8. How does the discharge superheat compare to Figure 32 on page 65.
Details to Consider:
1. The unit must be in Subcool control before being able to fine-tune the charge. If the unit
has insufficient subcooling, it will not convert to Subcool control. At 100% load there
must be a minimum of 20 degrees F (11 degrees C) of liquid line subcooling before the
circuit will allow subcooling control, therefore you may need to add charge to get to this
point in the case of severely undercharged units.
2. Verify that the circuit is not limited on a capacity limit or inhibit event. Limitation of the
chiller on low DSH, high lift, or low evaporator pressure may be clues to help determine a
refrigerant or oil charging issue.
3. It is hard to determine proper charge amounts while at part loads. For best charging results
the slide target should be at 100%. If there is a significant over or under charge you may
have to make adjustments to get the compressor to full load. It may be necessary to revisit
a unit when it would be at full load, to check and fine tune the charge. Sequentially
shutting off all but one circuit may provide a full load on the remaining circuit.
4. In order to maintain oil integrity, the discharge superheat needs to be greater than
20 degrees F (11 degrees C). The compressor will unload below this value. Between
20 and 22 degrees F (11 and 12 degrees C) DSH, the compressor will not load up and will
be in a low discharge superheat inhibit event. At high refrigerant flows, more liquid carry-
over will occur and the DSH will be lower. This means the compressor will have to be at
l00% to set up the refrigerant charge correctly.
5. Typically the suction pressure will be near the Low PressureHold setpoint while at full
load. You may need to sacrifice some suction pressure by removing some refrigerant to
get the discharge superheat up.
6. See above note on evaporator oil return line.
7. The lower the OAT, and the lower the saturated condensing temperature is, the more
refrigerant flow there will be, increasing the possibility of more liquid carry over from the
evaporator.
8. Use Figure 32, Discharge Superheat vs. Pressure Lift chart to verify charge. For a given
lift, superheat above the curve indicates low charge, below indicates high charge.