Operating instructions

IMM AGSB-60 63
extended surface and turbulent flow of water through the tubes. Normally no service work is
required on the evaporator other than cleaning the water (tube) side in the event of improper
water treatment or contamination.
Charging Refrigerant
Why does the AGS flooded evaporator use subcooling control?
Subcool control maintains proper evaporator level for efficiency and is the most stable value
with which to control a flooded evaporator chiller. Discharge superheat control is affected by
many variables such as motor heat, refrigerant flow, number of fans operating, amount of
refrigerant in the oil, etc. Additionally, the chiller cannot be controlled by the traditional
suction superheat control due to the saturated refrigerant entering the suction cooled motor.
Often this is a heavily saturated vapor that helps cool the motor and is not suitable for flow
control purposes.
Do not use the evaporator sight glasses to charge the unit.
Each circuit of the evaporator has a sight glass located on the side, halfway up and adjacent to
the internal tube sheet. There should be refrigerant level viewable in each circuit. A low level
combined with low evaporator pressure indicated by a LowEvapPressHold alarm indicates a
low refrigerant charge for the circuit.
Use these sight glasses for reference only. The expansion valve control varies with operating
conditions and may cause a higher or lower level based on control decisions. The sight glasses
can give you some relative information for decision making. If there is a considerable amount
of oil in the system, you may see oil floating on the evaporator liquid level and/or oil smearing
on the sight glass as the liquid level rises and falls.
Discharge superheat
The most important value to monitor while setting the charge on an AGS flooded evaporator
chiller is the discharge superheat (DSH), and especially at full load. Between 20 and 22
degrees F (11 and 12 degrees C) DSH, the compressor will hold its slide target and will not
load up. If the DSH drops below 20 degrees F (11 degrees C), it will unload. Excessive
refrigerant charge, excessive oil, a large amount of oil in circulation and a leaking or over
feeding evaporator solenoid valve will all cause low discharge superheat.
Approach temperatures:
Oil in the system will affect the condenser and evaporator approach temperatures. The design
approach (saturated discharge temperature minus ambient air temperature) on the condenser at
full load is 30 to 35 degrees F (16 to 19 degrees C). The evaporator approach should be 3 to
10 degrees F (1.6 to 5.5 degrees C), depending on conditions and percent of glycol, if used.
Oil in evaporator:
Oil in the evaporator will float on the liquid refrigerant and get pulled out with suction gas,
carrying liquid refrigerant with it and reducing the discharge superheat. The goal is to keep the
discharge superheat above 22 degrees F (12 degrees C), and ideally at 35 degrees F
(19 degrees C), while trying to get the compressor loaded up. The higher the refrigerant flow,
the quicker the oil will be recovered.
Evaporator Oil Return Line:
In some applications, the evaporator oil return line can cause low discharge superheat and
some oil loss into the system. It may be necessary to reduce the flow through the evaporator