Refrigerant Charge - McQuay SWP 018 Installation And Maintenance Manual

as low as 6°F to 8°F. Insufficient superheat will cause liquid
floodback to the compressor and possible liquid slugging.
Excessive superheat will reduce system performance and
shorten compressor life. Verify that the sensing bulb is prop-
erly located (see Figure 23) and securely strapped to the
refrigerant line. Turn the adjusting stem clockwise to
increase superheat. Adjust the stem (maximum one turn at a
time) and observe the superheat. Allow up to 30 minutes for
the system to rebalance at the final setting.

Refrigerant Charge

Units are shipped with a full operating charge of refrigerant
and oil. However, in the event of a leak in the system, some
added charge may be required. If an undercharged situation
occurs, any of the following may be experienced:
1. If a circuit is slightly undercharged, bubbles will show
in the sightglass.
2. If a circuit is moderately undercharged, it may trip on its
frost protection sensor.
3. If the circuit is severely undercharged, it may trip on its
low pressure safety.
If any of these conditions occur, first identify and correct the
source of the leak and then follow the charging procedure
described below.
Using the liquid line sight-glass as the sole means of meter-
ing additional refrigerant charge into a self-contained unit, or
any AC unit, will not always provide the desired result.
Depending on the load conditions experienced by the equip-
ment during the charging process, adding refrigerant until
the sight-glass is clear of all bubbles may over charge the
system and cause future operating issues. The better way to
charge a circuit is to use liquid sub-cooling and suction line
superheat as indicators, using the following procedure:
1. Verify that superheat is set per the System Check, Test
& Start procedures in this manual.
2. Measure the discharge pressure reading and convert it to
a discharge temperature.
3. Measure and record the circuit's liquid line temperature.
4. Measure and record the entering condenser water tem-
perature using the MicroTech II display.
5. Calculate liquid subcooling: subcooling = discharge
temperature - liquid line temperature
6. If the calculated subcooling value is less than 8ºF, refrig-
erant needs to be added.
7. Monitoring discharge pressure and liquid line tempera-
ture, add refrigerant until the discharge temperature
minus the liquid line temperature is equal to 8ºF ± 2ºF.
If the system is running at light load conditions, sub-
cooling should be at the low end of the range. If the sys-
tem is running near design conditions, subcooling
should be near the upper end of the range.
IM708
8. Verify that superheat is still in the prescribed range.
Following this method should prevent over charging of
the circuit.
Figure 29. Water and Refrigerant Piping Schematic
Note: Water Temperature Sensor to be installed by others
in field after the condenser water connections are
made.
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