IMM AGS-2
79
Evaporator
The evaporator is a two-circuit, direct expansion, shell-and-tube type with water flowing
through the shell and refrigerant flowing in one pass through the tubes. The tubes are
internally enhanced to provide extended heat transfer surface. Normally, no service work is
required on the evaporator other than cleaning the water side in the event of improper water
treatment or contamination.
Charging Refrigerant
Note:
It is a good idea to record the normal values of refrigerant pressures, subcooling, superheat,
and evaporator and condenser approach temperatures during startup by the McQuay service
technician. This makes it easier to spot errant unit behavior.
Indications of a low refrigerant R-134a charge:
•
Condenser subcoolong approaching 0 degrees F.
•
Suction superheat higher than 10 to 12 degrees F.
•
Bubbles in the sight glass.
Indications of a high refrigerant R-134a charge:
•
Condenser pressure is abnormally high.
•
Subcooling is abnormally high. Take note of the subcooling on the unit at startup and
use this value as a benchmark.
•
EXV is at minimum position and discharge superheat is low (below 22 degrees F). The
circuit controller View Refrigerant Screen #7 displays the valve position and the valve
range. The minimum position occurs when the valve position value remains at the
lower limit of the range displayed.
AGS air-cooled screw compressor chillers are shipped factory-charged with a full operating
charge of refrigerant; but there can be times when 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 beginning on page 26 for packaged units and page 60 for
remote evaporator units. An initial charge of 80% to 90% of the nameplate is assumed.
Unit charge adjustment should be done at 100% load, at normal cooling outdoor
temperature (preferably higher than 75
°
F (24
°
C), and with all fans on. Unit must be
allowed to run 15 minutes or longer so that the condenser fan staging and load is stabilized
at normal operating discharge pressure. For best results, charge with condenser pressure at
design conditions.
Each circuit of the evaporator has a sight glass located in the liquid line. If the unit can be
run at close to ARI conditions (95
°
F ambient temperature and 44
°
F chilled water), there
should be no bubbles in the sight glass, but this does not necessarily mean that the unit is
correctly charged. Charge until the superheat and subcooling temperatures are within
range. The discharge superheat should be above 22 degrees F.
Procedure to charge an undercharged AGS unit:
1.
If a unit is low on refrigerant, 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 can be needed
to locate small leaks. Do not use oil/refrigerant detection additives.
2.
Add the charge to the system only through the evaporator charging valve.
3.
The charge must be added at the 100% slide valve position and above conditions.
