Operating instructions

IMM AGSB-60 21
System Water Volume
It is important to have adequate water volume in the system to provide an opportunity for the
chiller to sense a load change, adjust to the change and stabilize. As the expected load change
becomes more rapid, a greater water volume is needed. The system water volume is the total
amount of water in the evaporator, air handling products and associated piping. If the water
volume is too low, operational problems can occur including rapid compressor cycling, rapid
loading and unloading of compressors, erratic refrigerant flow in the chiller, improper motor
cooling, shortened equipment life and other undesirable consequences.
For normal comfort cooling applications where the cooling load changes relatively slowly, we
recommend a minimum system volume of three minutes times the flow rate (gpm). For
example, if the design chiller flow rate is 800 gpm, we recommend a minimum system volume
of 2400 gallons (800 gpm x 3 minutes).
For process applications where the cooling load can change rapidly, additional system water
volume is needed. A process example would be a quenching tank. The load would be very
stable until the hot material is immersed in the water tank. Then, the load would increase
drastically. For this type of application, system volume will have to be increased.
Since there are many other factors that can influence performance, systems can successfully
operate below these suggestions. However, as the water volume decreases below these
suggestions, the possibility of problems increases.
Variable Speed Pumping
Variable water flow involves changing the water flow through the evaporator as the load
changes. McQuay chillers are designed for this duty, provided that the rate of change in water
flow is slow and the minimum and maximum flow rates for the vessel are not exceeded.
The recommended maximum change in water flow is 10 percent of the change per minute.
The water flow through the vessel must remain between the minimum and maximum values
listed on page 26. If flow drops below the minimum allowable, large reductions in heat
transfer can occur. If the flow exceeds the maximum rate, excessive pressure drop and tube
erosion can occur.
Evaporator Freeze Protection
Flooded evaporators are popular with chiller manufacturers because of their inherent high
efficiency. Care must be exercised in the equipment design and in the operation of these
evaporators to prevent freezing between 32°F and -20°F.
For protection down to 0°F (-18°C), the AGS chillers are equipped with thermostatically
controlled evaporator heaters that help protect against freeze-up provided the chiller goes
through its normal pumpdown cycle. Several occurrences can prevent this normal pumpdown
from happening:
1. A power failure will prevent pumpdown and there is a potential for freezing outdoor
equipment in systems using 100 percent water as the chilled fluid.
2. Unit shutdown due to a fault will cause immediate compressor shutdown without the
pumpdown cycle. This situation can be remedied by correcting the fault, restarting the
unit, and allowing it to go through its normal shutdown pumpdown.