Chillers: Understanding Summer Energy Usage

21st August, 2009

As the weather begins to heat up, it is a prime time to get an understanding of chillers and how they impact your electric bill. Also, it is a good time to make sure you have completed some simple maintenance steps to ensure that you are getting the most out of your chiller.

Chillers are used for conditioning a medium (air, water or other substance depending on how the chiller is being used). In this case, we are going to focus on water chillers, but the same applies for other types chillers as well. In commercial buildings and many manufacturing facilities, chillers are used to remove heat and humidity and to meet room conditions. Another use for chilled water in a production facility may be to cool equipment or remove heat from products.

Basic Chiller Operation

Chillers have a few basic parts that allow the chiller to use a refrigerant to cool the medium to the appropriate levels.

The compressor takes cool refrigerant vapor at a low pressure and compresses it to a high pressure. In the process, heat develops in the refrigerant so that the vapor at the compressor outlet is quite hot. This hot refrigerant vapor moves to the condenser, where it is cooled. The condenser is simply a heat exchanger that uses air or water (generally from a cooling tower) to cool the vapor, causing it to condense or turn back into its liquid state. At this point, the refrigerant is a high pressure, warm liquid.

Simple Estimates of Chiller Energy Usage

If your chiller is being used primarily for facility cooling, the effect of outside weather can be seen in your electric bill. The following chart represents a facility’s energy usage over 12 months. You can see the increased usage in the winter and summer months that is related to heating and air conditioning.

To develop such an estimate for your facility, you can use the following method. If your facility does not have electric heat, take either January or December as the baseline electric usage. Subtract this baseline usage from your usage throughout the entire year to determine the amount of electric energy being consumed by the chiller.

If your facility does have electric heat, use the low months in the spring and fall to estimate the baseline. Use 85 percent of the lowest month(s) usage as the baseline and perform the same estimate as above. For the low months, assume the amount above the baseline is split equally between heating and cooling. (See chart.)

There are several things that can be done to reduce the amount of energy consumed by the chiller. Matching the chilled water set point to the desired room or processing conditions is one of the most effective. For each degree that you "over chill" the water, you consume 1.5 percent more energy. By raising the chilled water set point as much as possible (while still meeting the cooling need), you can appreciably lower chiller energy consumption.

Some energy management systems use an adaptive control scheme called chilled water reset. They continuously monitor the valve position of each chilled water coil throughout the building and ensure that one or more of them are open 90 percent or more. If not, the chilled water is raised incrementally. The controller knows that if all of the chilled water valves are somewhat closed, then the chilled water temperature can be raised until one or more of the coils need nearly full flow. This saves energy by continually adjusting the chiller to just meet the required operating conditions without over-chilling the water.

Another thing to focus on is chiller maintenance. It is important to purchase energy efficient chillers, but that efficiency gain is wasted if the chiller is not properly maintained. Recommended maintenance items for peak energy efficiency include:

  1. Inspect the chiller as recommended by the chiller manufacturer. Typically, this should be done at least quarterly.
  2. Routinely check refrigerant and inspect for refrigerant leaks.
  3. Check compressor operating pressures.
  4. Check all oil levels and pressures.
  5. Check motor voltages and amps for balanced loading.
  6. Check all electrical starters, contactors and relays.
  7. Check un-loader operation.
  8. Check water flow rates.
  9. Review water chemistry to ensure proper heat transfer.
  10. Review cooling tower operation.

As the summer temperature and humidity rise, you can expect the energy consumption of your chiller to rise. By using the information in this article you can have a better understanding of how to track and reduce that energy and get the most from your chiller systems.

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