McQuay AGR 070AS, Operation Manual

Page: 42 / 100

42 AGR 070A through 100A IOMM AGR-1
System Adjustment
To maintain peak performance at full load operation, the system superheat and liquid subcooling may
require adjustment. Read the following subsections closely to determine if adjustment is required.
Liquid Sightglass and Moisture Indicator
The moisture indicator color is an indication of the dryness of the system and is extremely important
when the system has been serviced. Immediately after the system has been opened for service, the
element may indicate a wet condition. It is recommended that the unit operate for about 12 hours to
allow the system to reach equilibrium before deciding if the system requires a change of drier cores.
Bubbles in the sightglass to expansion valve at constant full load conditions indicates a shortage of
refrigerant, a plugged filter-drier, or a restriction in the liquid line. However, it is not unusual to see
bubbles in the sightglass during changing load conditions.
Refrigerant Charging
Liquid line subcooling at the liquid shut-off valve should be between 15° and 20°F at full load. If the
unit is at steady full load operation and bubbles appear in the sightglass, check liquid subcooling.
Thermostatic Expansion Valve
The expansion valve performs one specific function. It keeps the evaporator supplied with the proper
amount of refrigerant to satisfy the load conditions.
The sensing bulb of the expansion valve is installed in the closest straight run of suction line from the
evaporator. The bulb is held on by clamps around the suction line and is insulated to reduce the effect
of surrounding ambient temperatures. In case the bulb must be removed, simply slit the insulation on
each side of the bulb, remove the clamps and then remove the capillary tubing that runs along the
suction line from the valve.
The power element is removable from the valve body without removing the valve from the line.
Superheat and Subcooling
For optimum operation, set the evaporator superheat for 8 to 12°F (4.4 to 6.6°C) and condenser
subcooling (at the liquid shutoff valves) to 15°F to 20°F (8.3°C to 11.11°C).
CAUTION
Before adjusting superheat, check that unit charge is correct and liquid line sightglass is
full with no bubbles.
The suction superheat for the suction leaving the evaporator is set at the factory for 8° to 12°F at full
load. To have full rated unit performance the superheat must be about 8°F at 95°F outdoor ambient.
Crankcase Heaters
The reciprocating compressors are equipped with compressor crankcase heaters located at the oil
sump level. The function of the heater is to keep the temperature in the crankcase high enough to
prevent refrigerant from migrating to the crankcase and condensing in the oil during off-cycle.
Power must be supplied to the heaters 24 hours before starting the compressors.
Water Cooler
The water cooler is of the direct expansion type with removable internally finned tubes. The copper
tubes are individually rolled into heavy duty steel tube sheets and sealed by a steel refrigerant head.
The water connection nozzles which enter and leave the shell are on the same side of the unit. No
special attention is required for the cooler except that clean, filtered water should be supplied.