Efficient Chillers

Available chiller operating efficiencies have improved significantly in the past few years. Design changes to the machines have been driven by both the demand for higher efficiency due to higher energy costs, and the legislated requirement to operate on new non-CFC refrigerants. Efficiency improvements have been achieved through the use of enlarged condensers and evaporators, and improved design of compressors (including more efficient methods of capacity control and variable speed drive). Related improvements in sizing of applications and the use of variable pumping on both the chilled water and condenser water circuits have also contributed to improved overall cooling plant efficiency.

The minimum efficiency recommended by ASHRAE for water-cooled chillers over 300 ton capacity is 0.64 kW/ton, while the minimum efficiency recommended by the Federal Energy Management Program (FEMP) is 0.56 kW/ton. Efficiencies available in new chillers today are typically in the 0.47 kW/ton range at standard ARI conditions.

Even higher efficiencies are possible with some of these new chillers at more typical local conditions. An example is where chilled water is required year-round. Centrifugal chillers equipped with variable speed drives can operate at an efficiency as high as 0.25 kw/ton. This is made possible by the machine's efficient capacity control system combined with a refrigerant circuit designed to utilize the low condenser water temperatures available in the winter season. One result of this new capability is that it can now be more energy efficient to operate a chiller of this type than to try to provide so-called "free cooling" using high pumping rates and air-flow rates.

Chillers over 20 years old are good candidates for replacement because they are nearing the end of their useful life and are significantly less efficient than new models. Chillers less than 20 years old may be improved designs, and should be considered on a case-by-case basis. Chillers with higher operating hours per year are better economic candidates for replacement. If a chiller is due for an overhaul or refrigerant replacement, chances are that a new energy efficient machine is cost effective.