20 IMM AGS-2
9. In the event glycol is added to the water system as a late addition for freeze protection,
recognize that the refrigerant suction pressure will be lower, cooling performance less,
and water side pressure drop greater. If the percentage of glycol is large, or if
propylene is employed in lieu of ethylene glycol, the added pressure drop and loss of
performance could be substantial.
10. For ice making or low temperature glycol operation, a different freezestat pressure
value is usually required. The freezestat setting can be manually changed through the
MicroTech II controller.
Make a preliminary leak check prior to insulating the water piping and filling the system.
Include a vapor barrier with the piping insulation to prevent moisture condensation and
possible damage to the building structure. It is important to have the vapor barrier on the
outside of the insulation to prevent condensation within the insulation on the cold surface
of the pipe.
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 chilled water 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, a
minimum system volume of three minutes times the flow rate (gpm) is recommend. For
example, if the design chiller flow rate is 400 gpm, we recommend a minimum total system
volume of 1200 gallons (400 gpm x 3 minutes).
For process applications, such as a quenching tank, where the cooling load can change
rapidly, additional system water volume is needed. 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 can need 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 reducing the water flow through the evaporator as the load
decreases. 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 25. 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.
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