Chiller kW per Ton Explained

1. What kW per ton means

kW/ton is the electrical power a chiller draws for each ton of cooling it delivers. One ton of refrigeration is 12,000 Btu/h, the heat needed to melt one short ton of ice in 24 hours.

Unlike COP and EER, lower kW/ton is better. A chiller at 0.55 kW/ton uses about 39% less electricity than one at 0.90 kW/ton for the same cooling.

2. Converting kW/ton to COP and EER

3.517 is the number of kilowatts in one ton of refrigeration. Common values:

3. How to measure kW/ton on a running chiller

You need three measurements taken at the same time:

1. Chilled water flow in gpm, from a flow meter or the pump curve and differential pressure.
2. Chilled water ΔT: return temperature minus supply temperature, in °F.
3. Chiller input power in kW, from the chiller panel or a power meter on the feeder.

The 24 comes from 500 × gpm × ΔT ÷ 12,000. Example: a chiller cooling 720 gpm from 54°F to 44°F delivers 720 × 10 ÷ 24 = 300 tons. If it draws 195 kW, it is running at 195 ÷ 300 = 0.65 kW/ton.

Small sensor errors matter. At a 10°F ΔT, a 0.5°F error in one temperature sensor moves the tonnage, and so the kW/ton, by about 5%. Calibrate or check the sensors against each other before trusting a trend.

For glycol loops, the constant changes because glycol carries less heat per gallon than water. The glycol chiller derating guide explains the correction.

4. Full load vs part load: IPLV and NPLV

Chillers spend most of the year below full load, and many are more efficient at part load. AHRI Standard 550/590 defines a single weighted figure, the Integrated Part Load Value (IPLV), from efficiency at 100%, 75%, 50% and 25% load. When kW/ton is used, the weighting is applied to the reciprocals:

Here A, B, C and D are the kW/ton at 100%, 75%, 50% and 25% load. A chiller rated 0.58, 0.50, 0.42 and 0.45 kW/ton at those points has an IPLV of about 0.456 kW/ton, far better than its full-load rating suggests. NPLV is the same calculation at your own design conditions rather than AHRI standard conditions.

IPLV is a comparison tool, not a forecast. Its weights describe a typical building-style load profile. A process chiller that runs near full load around the clock should be judged mostly on its full-load kW/ton.

5. Typical kW/ton by chiller type

• Water-cooled centrifugal: commonly around 0.5 to 0.6 kW/ton at full load. Variable-speed and magnetic-bearing machines can be much lower at part load.
• Water-cooled screw: commonly around 0.6 to 0.75 kW/ton at full load.
• Air-cooled screw or scroll: commonly around 1.0 to 1.3 kW/ton including condenser fans, because rejecting heat to dry air needs a higher condensing temperature.

These are broad ranges, not minimum standards. For code minimums, check the edition of ASHRAE 90.1 or the energy code your jurisdiction has adopted. The air-cooled vs water-cooled chiller comparison covers the trade-offs, including the water and cooling tower costs that come with water-cooled plants.

6. What a better kW/ton is worth

Take a plant with an average load of 300 tons for 4,000 hours a year at $0.12/kWh:

Each 0.01 kW/ton is worth about $1,440 a year in this example. That is why a chiller with a higher price and a better kW/ton often has the lower total cost of ownership. The chiller ROI and energy savings guide shows how to work out the payback.

7. Why kW/ton creeps up, and how to bring it down

• Condenser tube fouling: scale and biofilm raise the condensing temperature. A rising condenser approach temperature (condensing temperature minus leaving condenser water temperature) is the usual warning sign. Good water treatment and the right cooling tower cycles of concentration keep tubes clean.
• Chilled water setpoint: running colder than the process needs makes the compressor work harder. A common rule of thumb is roughly 1% to 2% of chiller energy for each °F, so raise the setpoint where the process allows.
• Condenser water temperature: cooler condenser water lowers kW/ton, within the chiller manufacturer's minimum limits. Tower fan control and condenser water reset strategies use this.
• Low load on a big chiller: a chiller that is far too large for the load cycles or runs at its least efficient point. See chiller sizing mistakes.
• Refrigerant charge and non-condensables: both a low charge and air in the condenser raise power. Your service contractor checks these during the annual inspection.

Frequently asked questions

Sources and further reading

• AHRI Standard 550/590: Performance Rating of Water-Chilling and Heat Pump Water-Heating Packages
• ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings (chiller efficiency tables)
• U.S. Department of Energy, Federal Energy Management Program: chiller and chilled water plant guidance