Revised December 2013
How do I calculate/convert a ton-hour of chilled water to its kWh or Btu value and what should I know about efficient chiller operation?
- 1 Ton of cooling capacity = 12,000 Btu/hr
- 1 kWh = 3412 Btu
One ton of cooling is 12,000 Btuh and 3.516 kWh. So, a 500-ton chiller is able to provide a maximum peak load of 6,000 kBtuh. But, in reality, the chiller rarely, if ever, operates at full load for a significant period of time. Engineers usually size the primary equipment with a 20% safety factor, then, since chillers come in finite sizes, select the next larger size. The electrical service to the chiller needs to match that of the manufacturer’s listing.
If your setup has constant volume pumping, actual energy use can be obtained by measuring the supply and return water temperatures of both the condenser and chilled water. Trending these temperatures with current transformers will allow for development of an energy use profile for your specific operation. To get a full load operation energy use, set all spaces to 55 degrees, maximizing the chiller output.
There are many ways to operate a chiller efficiently. An assessment will provide valuable information about staging, reset controls, cooling tower operation (for water cooled chillers), actuation and sequencing. Some changes that will reduce energy consumption include:
- Convert all three way valves to two way. This measure will allow a VSD on the pump to only pump as much water as necessary to meet the load. Only the coil that is the farthest away should have a 3-way valve so the pump is not dead-headed. Set the speed of the pump to the minimum pressure needed for the worst the condition where most of the 2-way valves are closed. Any other condition will require a lower pressure, allowing the pump to modulate down.
- Select the most efficient cooling equipment.Chillers are tested by AHRI and given a performance value that we can use for comparative purposes.
- Use water cooled chillers.For chillers at 100 tons or more, converting to a water cooled chiller will save energy. Air cooled chillers operate most efficiently in the cold weather and less efficiently in the warm weather. This is counter to the needs of the building. Using a water cooled chiller provides a more efficient operation all year, and also allows for chiller bypass (water side economizer) during mild weather conditions.
- Use VSD’s on cooling tower pumps and fans. A control sequence for a cooling tower that provides for the most efficient operation is to modulate the condenser pump based on maintaining a supply water temperature at about 3 to 7 degrees of wet-bulb temperature. And, the fans should stage on and modulate to maintain 10 to 15 degree difference between the supply and return water temperatures.
- Use primary pumping instead of primary/secondary. Some chillers allow for variable flow. Converting primary/secondary systems to primary only with a VSD will save energy.
- Size multiple chillers for a fraction of the load. Most installations have a chilled water load that varies throughout the year. An example of installations that may not is a data center. But, for most installations, the cooling load varies throughout the year, depending on weather and operation conditions. The energy use is higher for a chiller cycling on and off, operating part load. Verify staging of chillers and water flow optimize the cooling while minimizing the energy use.
- Verify proper coordination between components. Setpoints should not adversely affect comfort and should be set up to use the least amount of energy needed to maintain comfort. For example, the water temperature should be cool enough to remove moisture even though operating the chiller at higher temperatures will save energy.
PECI. Functional Testing and Design Guides
National Renewable Energy Laboratory (NREL).