Performance of Electric Systems

Since the vast majority of cooling equipment is driven with electric motors, the most common performance measure is kW/ton. This can be calculated for any given chiller by simply taking the manufacturer's estimated power use and dividing it by the cooling system capacity expressed in tons. For example, if the manufacturer estimates the peak power required for a 250 ton chiller is 150 kW, the chiller will use 150 kW/250 tons or 0.6 kW/ton on the design day.

But what is included in this 150 kW? Is it the chiller alone, or the pumps and fans in the remainder of the system? If this power estimate was for a packaged rooftop unit supplying air to the building, the power estimate is clearly the total kW/ton. However, in buildings where a centrally located water chiller is used, there are also chilled water and condenser water pumps as well as cooling tower fans. Were these included in the kW/ton performance measure? Not usually! And, in these cases, they must be added in to get the total kW per ton performance.

Design professionals usually evaluate chiller performance by comparing kW/ton for the chiller itself plus the heat rejection circuit. After all, for any given building, the chilled water distribution system alternatives would be identical. The differences between low and high efficiency cooling system alternatives shows up in the kW/ton for the chiller plus the heat rejection circuit. Therefore, use this total when comparing the kW/ton for chiller alternatives such as:

1. Air cooled versus water cooled

2. Low vs. high efficiency

3. Gas vs. electric driven (since gas equipment also has electrical requirements).

Obviously, gas equipment also has fuel consumption which should be factored into the comparison.

So far, we have compared equipment on the design day -- the relatively few hottest and/or most humid days in your area. These are certainly important in determining the electrical demand charges. But, in most parts of the United States, these conditions occur only a few hours a year. It is true they may occur one or more days in each of the warmest months (and thereby set a demand charge for those months), but this still doesn't represent the most common operating environment.

The HVAC industry recognized this and characterized these more common situations using the Integrated Part Load Value (IPLV) which is a defined number of hours at representative chiller operating conditions based on an ARI standard, Actual Part Load Value (APLV) is determined by the design community for the specific building situation at hand.

The specific kW/ton performance for each type of electric (and fuel-fired) system is discussed in the specific segment on that chiller design. Suffice it to say that most large cooling systems become more efficient at part load conditions, while small reciprocating compressor systems become less efficient at typical part load conditions.

Select from these areas of interest . . .