Specifications

50 Catalog WSC/WDC-4
Application Considerations
Optimum Water Temperatures and Flow Rates
A key to improving energy efficiency for any chiller is minimizing the lift, or pressure difference,
between the compressor suction and discharge pressures. Reducing the lift reduces the
compressor work, and hence its energy consumption per unit of output. The chiller typically has
the largest motor of any component in a chilled water system.
Higher leaving chilled water temperatures
Warmer leaving chilled water temperatures will raise the compressor’s suction pressure and
decrease the lift, improving efficiency. Using 45°F (7.0°C) leaving water instead of 42°F (5.5°C)
will make a significant improvement.
Evaporator temperature drop
The industry standard has been a ten-degree temperature drop in the evaporator. Increasing the
drop to 12 or 14 degrees will improve the evaporator heat transfer, raise the suction pressure, and
improve chiller efficiency. Chilled water pump energy will also be reduced.
Condenser entering water temperature
As a general rule, a one-degree drop in condenser entering water temperature will reduce chiller
energy consumption by two percent. Cooler water lowers the condensing pressure and reduces
compressor work. One or two degrees can make a noticeable difference. The incremental cost of
a larger tower can be small and provide a good return on investment.
Condenser water temperature rise
The industry standard of 3 gpm/ton or about a 9.5-degree delta-T seems to work well for most
applications. Reducing condenser water flow to lower pumping energy will increase the water
temperature rise, resulting in an increase in the compressor’s condensing pressure and energy
consumption. This is usually not a productive strategy.
System analysis
Although McQuay is a proponent of analyzing the entire system, it is generally effective to place
the chiller in the most efficient mode because it is, by far, a larger energy consumer than pumps.
The McQuay Energy Analyzer™ program is an excellent tool to investigate the entire system
efficiency, quickly and accurately. It is especially good at comparing different system types and
operating parameters. Contact you local McQuay sales office for assistance on your particular
application.
For Best Chiller Efficiency
Vessel Activity Example
Evaporator Higher leaving water Temperatures
44
°F instead of 42°F
Evaporator Higher water temperature drops 12 degrees F instead of 10 degrees
Evaporator Lower flow rates 2.4 gpm/ton instead of 3.0 gpm/ton
Condenser Lower entering water temperature
84
°F instead of 85°F
Condenser Higher flow rates (3.0 gpm/ton or higher) 3.0 gpm/ton instead of 2.5 gpm/ton
The designer must determine the proper chiller efficiency for a given application. The most
efficient chiller is not always the best. A life cycle analysis (as performed by McQuay’s Energy
Analyzer program, for example) is the only way to be sure of the best selection. Utility costs, load
factors, maintenance costs, cost of capital, tax bracket; in other words, all the factors affecting
owning cost, must be considered.
Generally, the attempts to save the last few full load kW are very costly. For example, the cost to
go from 0.58 to 0.57 kW/ton could be very costly because of the large number of copper tubes that
would have to be added to the heat exchangers.