Operating instructions
IMM AGSB-60 65
Summary:
At 100% slide position, in Subcool control, the DSH should be as high as possible with suction
pressure at a operable value based on water/glycol mixture. At 100% load, in Subcool control,
the DSH and suction pressure need to be balanced.
Example: Running circuit 1 at 100% slide target, with water only in the loop, set the low
evaporator pressure unload to 28psi (32°sat.) and the low evaporator pressure hold to 30psi.
Run the suction pressure at approximately 32psi at full load. This should allow room for 25-
30° DSH. As a rule of thumb, as outdoor air temperature drops, it becomes more difficult to
maintain minimum DSH with a given charge amount, due to higher refrigerant flows.
Figure 32, Discharge Superheat vs. Pressure Lift at Full Circuit Load
For Various Chilled Water Temperatures
Discharge superheat is directly related to the amount of liquid carried from the evaporator and
amount of motor heat rejected into the refrigerant.
Higher pressure lifts will result in higher discharge superheats.
More liquid carry over will result in lower discharge superheats, less liquid carry over will
result in higher discharge superheats.
More liquid carry over will occur when:
1. The refrigerant circuit is overcharged.
2. Excessive oil is in the evaporator.
3. Mass flow rate of compressor is increased.
4. Oil return solenoid is energized or leaking (more liquid injection than oil return).
5. Evaporator tubes fouled or are plugged.
Discharge Superheat vs. Pressure Lift
15
20
25
30
35
40
45
50
50.0 70.0 90.0 110.0 130.0 150.0 170.0 190.0 210.0 230.0 250.0
Pressure Lift, PSI (Discharge Pressure - Suction Pressure) (PSI = kPa/6.9)
Discharge Superheat, (F = C/0.55))
34/44 44/54
54/64 64/74