McQuay ACZ 045A, Installation, Operation And Maintenance Manual

Page: 101 / 104

IOMM ACZ/AGZ-1 101
Evaporator
The remote evaporator on AGZ-AM units and CDE evaporators are 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
AGZ-AM and ACZ units are shipped with a factory holding charge of refrigerant and it is necessary
to charge them at the job site after leak testing and evacuating the field installed refrigerant lines.
Follow these recommendations when field charging. Refer to the unit operating charge found in the
“Charging Section”.
Final 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 AGZ and ACZ 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 degrees F-20 degrees
F (8.3 degrees C-11.1 degrees 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. Recharge the unit as
described in the charging procedure below. However, freezestat trips can also be an indication of
low flow or poor heat transfer due to tube fouling. Anti-freeze solutions may also cause freezestat
trips.
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 through the suction valve on the compressor slowly. 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 AGZ 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.