Operating instructions

Variable Speed Pumping
Variable water flow involves lowering 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.
System Water Volume Considerations
All chillers need adequate time to recognize a load change, respond to the change and stabilize
without short cycling the compressor. The water volume in the system and the size of the piping
loop is a critical consideration. Good engineering practice is to have a minimum water volume of
four times the flow rate (GPM) for comfort cooling applications. For process applications where the
load can change quickly, contact the local McQuay sales office for recommendations. A water
storage tank (provided by others) can be required to increase the system water volume in some
systems.
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. We believe that these guidelines should be an industry standard
and not just recommendations from McQuay.
Glycol Solutions
The use of a glycol/water mixture in the evaporator to prevent freezing will reduce system capacity
and efficiency and increase pressure drop. The system capacity, required glycol solution flow rate,
and pressure drop with glycol can be calculated using the following formulas and tables.
1. Capacity – Multiply the capacity based on water by the Capacity correction factor from
Table 8
through
Table 11.
2. Flow – Multiply the water evaporator flow by the Flow correction factor from
Table 8 through
Table 11 to determine the increased evaporator flow due to glycol.
If the flow is unknown, it can be calculated from the following equation:
FactorCorrectionFlow
TDelta
glycolCapacityTons
×
−
×
=
)(24
(gpm) Flow Glycol
3. Pressure drop -- Multiply the water pressure drop from page 25 by Pressure Drop correction
factor from
Table 8 through Table 11. High concentrations of propylene glycol at low
temperatures can cause unacceptably high-pressure drops.
4. Power -- Multiply the water system power by Power correction factor from
Table 8 through
Table 11.
Test coolant with a clean, accurate glycol solution hydrometer (similar to that found in service
stations) to determine the freezing point. Obtain percent glycol from the freezing point table below.
It is recommended that a minimum of 25% solution by weight be used for protection against
corrosion or that additional compatible inhibitors be added.
IMM ACZ/AGZ-4 ACZ / AGZ-BM 17