McQuay AGR 070AS, Operation Manual

Page: 96 / 100

96 AGR 070A through 100A IOMM AGR-1
Evaporator
The evaporator is the direct expansion, shell-and-tube type with refrigerant flowing through the tubes
and water flowing through the shell over the tubes. The tubes are internally finned to provide
extended surface as well as turbulent flow of refrigeration through the tubes. Normally no service
work is required on the evaporator.
Refrigerant Charging
AGR air-cooled chillers are shipped factory charged with a full operating charge of refrigerant but
there may be times that a unit must be recharged at the job site. Follow these recommendations when
field charging. Refer to the unit operating charge found in the Physical Data Tables.
Unit charging can be done at any steady load condition (preferably at 75 to 100% load) and at any
outdoor temperature (preferably higher than 70
°
F (21.1
°
C). Unit must be allowed to run 5 minutes or
longer so that the condenser fan staging is stabilized at normal operating discharge pressure. For best
results charge with two or more condenser fans operating on each refrigerant circuit.
The AGR units have a condenser coil design with approximately 15% of the coil tubes located in a
subcooler section of the coil to achieve liquid cooling to within 5
°
F (3
°
C) of the outdoor air
temperature when all condenser fans are operating. This is equal to about 15
°
F-20
°
F (8.3
°
C-11.1
°
C)
subcooling below the saturated condensing temperature when the pressure is read at the liquid valve
between the condenser coil and the liquid line filter drier. Once the subcooler is filled, extra charge
will not lower the liquid temperature and does not help system capacity or efficiency. However, a
little extra (10-15 lbs) will make the system less sensitive.
Note: As the unit changes load or fans cycle on and off, the subcooling will vary but should recover
within several minutes and should never be below 6
°
F (3.3
°
C) subcooling at any steady state
condition. Subcooling will vary somewhat with evaporator leaving water temperature and suction
superheat. As the evaporator superheat decreases the subcooling will drop slightly.
One of the following three scenarios will be experienced with an undercharged unit:
1. If the unit is slightly undercharged the unit will show bubbles in the sightglass. Recharge the unit
as described in the charging procedure below.
2. If the unit is moderately undercharged it will normally trip on freeze protection. (Freezestat trips
can also be an indication of low flow or poor heat transfer due to fouled tubes. Anti-freeze
solutions can also cause freezestat trips.) Recharge the unit as described in the charging procedure
below.
3. If the unit is severely undercharged the unit will trip due to lack of liquid flow to the expansion
valve. In this case either remove the remaining charge by means of a proper reclamation system
and recharge the unit with the proper amount of refrigerant as stamped on the unit nameplate, or
add refrigerant slowly through the suction valve on the compressor. Feed liquid into the suction
valve when the compressor is running. If the unit is severely undercharged the unit may nuisance
trip during this charging procedure. If this happens close off the refrigerant from the tank and
restart the unit. Once the unit has enough charge so that it does not trip out, continue with step 2 of
the charging procedure below.
Procedure to charge a moderately undercharged AGR unit:
1. If a unit is low on refrigerant you must first determine the cause before attempting to recharge the
unit. Locate and repair any refrigerant leak. Evidence of oil is a good indicator of leakage,
however oil may not be visible at all leaks. Liquid leak detector fluids work well to show bubbles
at medium size leaks but electronic leak detectors may be needed to locate small leaks.
2. Add the charge to the system through the suction shutoff valve or through the Schrader fitting on
the tube entering the evaporator between the compressor and the evaporator head.