12
PFS 155C - 315C
OM 135
A feature of the electronic expansion valve is a maximum operating pressure setting (MOP). This
setting limits the load on the compressor during start-up periods where high return evaporator water
temperatures may be present. The valve will limit the maximum suction pressure at start-up to
approximately 85 psig (586 kPa). The valve will close to a point necessary to maintain the 85 psig (586
kPa). During this time the superheat will rise above 12°F (6.7°C) and not drop below 12°F (6.7°C) until
the suction pressure drops below 85 psig (586 kPa). The valve will maintain evaporator pressure close
to 85 psig (586 kPa) until the evaporator water temperature decreases to approximately 55°-60°F (12.8°-
15.6°C).
The valve remains closed at start and opens as the pressure between the condenser and evaporator
rises to control at a pressure ratio that assures good liquid feed to the compressors. At the end of the
cooling cycle the valve closes, partially pumping down the evporator. The valve closes at the rate of
approximately 55 steps per second, or from full open to full closed in approximately 14 seconds.
Evaporator
The evaporator is a flooded, shell-and-tube type with water flowing through the tubes and refrigerant
flowing through the shell over the tubes. Normally no service work is required on the evaporator.
When a tube must be replaced, the old tube can be removed and replaced. Follow the requirements
set forth by the EPA for the pumpdown and recovery of refrigerant.
Condenser
The condenser is a shell-and-tube type with water flowing through the tubes and refrigerant in the
shell. External finned condenser tubes are rolled into steel tube sheets. Integral subcoolers are
incorporated on all units. All condensers are equipped with two relief valves and a shutoff valve.
Either end on the condenser can be easily removed in the field.
Refrigerant Charging
PFS water-cooled screw chillers are shipped with a full operating charge of refrigerant. If a unit must
be field charged, follow these recommendations.
PFS water-cooled screw chillers are more sensitive to undercharging than to overcharging.
Therefore, it is preferable to be slightly overcharged rather than undercharged on a circuit. The
optimum charge is the charge that allows the unit to run with a solid stream of liquid in the liquid line
at all operating conditions. When the liquid line temperature does not drop with the addition of 5-10
lb. (2.3-4.5 kg) of charge then the subcooler is nearly full and proper charge has been reached. If the
liquid line temperature does not drop and the discharge pressure goes up 3-5 psig (21-35 kPa) as 5-10
lb. (2.3-4.5 kg) of refrigerant is added the correct maximum charge has been reached.
Unit charging can be done at any steady load condition. Unit must be allowed to run 5 minutes or
longer.
Note:
As the unit changes load the subcooling will vary but should recover within several
minutes and should never show below 6°F (3.4°C) subcooling at any steady state run
condition. Subcooling will vary somewhat with evaporator leaving water temperature and
suction superheat. As the evaporator superheat goes lower the subcooling will drop slightly.
Excessive refrigerant losses can also leak oil from the system. When adding charge, add four
percent oil by weight. Use PLANETELF ACD 68AW oil.
A refrigerant leak in the unit could be very small and have little effect on system operation or
could be severe enough to cause the unit to shut down on a safety trip.
1.
If the unit is slightly undercharged the unit will show bubbles in the sightglass. Recharge the
unit as described in the following charging procedure.
