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How to Use the Heat of Rejection Calculator
Type the cooling capacity from the equipment data or a load calculation and pick BTU/hr, kW or tons. This is the heat absorbed at the evaporator.
Use COP, EER or the measured compressor input power. For COP and EER, set the compressor share of the rated input if the rating includes fan power.
Choose air-cooled, water-cooled or evaporative. For air or water, keep or change the temperature rise across the condenser so the calculator can size the airflow or water flow.
The results give the heat of rejection in BTU/hr, kW and tons, the rejection ratio, the heat of compression and the airflow, water flow or evaporation rate.
Why heat of rejection is larger than cooling capacity
Every refrigeration and air conditioning system rejects more heat at the condenser than it absorbs at the evaporator. The refrigerant picks up the cooling load in the evaporator, the compressor then adds its own work, and the condenser has to give up both. The calculator finds that total from the cooling capacity and the compressor's share of the electrical input.
The heat balance behind the calculation
Heat of rejection equals cooling capacity plus the heat of compression. A 36,000 BTU/hr system with a COP of 3.0 uses 12,000 BTU/hr of compressor work, 3.52 kW, and so rejects 48,000 BTU/hr, which is 4.00 tons or 14.07 kW. The rejection ratio, 1.33 here, is 1 plus 1 divided by the COP, so it shrinks as efficiency rises: 1.20 at a COP of 5.0 and 1.40 at 2.5.
Compressor power, and why fan power stays out
Only the compressor's input power becomes refrigerant heat. Fan motors heat the air stream, and that heat never reaches the refrigerant, so it is not added to the coil load. This matters because nameplate COP and EER figures normally include the indoor and outdoor fans. The compressor share field lets you take them out: a 36,000 BTU/hr unit rated at EER 12 draws 3.00 kW, and at a 90% compressor share the heat of compression is 9,212 BTU/hr, giving 45,212 BTU/hr of rejection instead of 46,236. If you have measured the compressor power directly, use the power mode and the share field is not needed.
COP, EER and SEER
COP is cooling output divided by input power in the same units, and EER is the same ratio in BTU/hr per watt, so COP = EER ÷ 3.412. Use a rated EER here, not a SEER, because SEER averages performance over a season and says little about the peak hour that sets the condenser size. The EER calculator finds the EER from capacity and input power when the nameplate does not list it.
Rated conditions and nameplate data
Rated capacity and efficiency come from a standard test, and for unitary air conditioners and air-source heat pumps under 65,000 BTU/h the rating method is AHRI 210/240. A nameplate figure is correct only at its rating conditions, so on a hotter day the capacity is lower and the COP is lower, and the rejection ratio is higher than the rating suggests. If the COP falls from 3.5 to 2.5 the ratio rises from 1.29 to 1.40, which sends about 9% more heat to the condenser for the same cooling.
Canadian equipment requirements
Natural Resources Canada sets the minimum efficiency and the test method for central air conditioners and heat pumps sold in the country, and the NRCan page for central air conditioners and heat pumps lists the current requirements in SEER2. Those minimums are seasonal values, so use them to compare equipment and not to find the peak condenser load, which needs the COP or EER at your design outdoor temperature.
Air-cooled condensers: airflow from the rejection
Airflow in CFM is the heat of rejection divided by 1.08 times the air temperature rise in °F, which is the sensible heat of standard air. The 48,000 BTU/hr example at a 15°F rise needs 2,963 CFM, a figure that looks right for a 3 ton condenser fan. A smaller rise needs more air and runs a cooler coil, and a larger rise needs less air but pushes the condensing temperature up. Take the load from here into the condenser sizing calculator to size the coil.
Water-cooled and evaporative condensers
Water flow in US GPM is the heat of rejection divided by 500 times the water temperature rise in °F. A 350 kW chiller with a COP of 5.5 rejects 1,411,385 BTU/hr, 413.6 kW or 117.6 tons, and needs 282.3 GPM at a 10°F rise. That is the load a cooling tower or fluid cooler has to carry, and the cooling tower calculator sizes the tower for it. An evaporative condenser rejects heat by evaporating water, and the calculator gives an upper estimate from a latent heat of 1,050 BTU/lb, which is 5.5 gal/h for the 48,000 BTU/hr example. It is highest on dry days and lowest in humid weather, because humid air limits the cooling that evaporation can provide.
Frequently Asked Questions
Heat of rejection is the total heat the condenser must dissipate. It equals the cooling capacity absorbed at the evaporator plus the heat of compression added by the compressor, so Heat of Rejection = Cooling Capacity × (1 + 1/COP). A 36,000 BTU/hr (3 ton) system with a COP of 3.0 has a compressor heat of 12,000 BTU/hr and rejects 48,000 BTU/hr, or 4.00 tons, which is 1.33 times its cooling capacity. A COP of 5.0 gives a ratio of 1.20 and a COP of 2.5 gives 1.40, so the condenser is always larger than the evaporator duty.
No. The condenser fan heats the air stream, but that heat is never carried by the refrigerant, so it does not add to the coil load that the condenser has to reject. What does matter is how much of the rated input power goes to the compressor. A nameplate COP or EER usually includes indoor and outdoor fans, so the calculator has a compressor share field. A 36,000 BTU/hr unit rated at EER 12 draws 3.00 kW in total, and at a 90% compressor share the heat of compression is 9,212 BTU/hr, against 10,236 BTU/hr at 100%.
A hotter outdoor temperature raises the condensing temperature and the pressure ratio, so the compressor draws more power for the same cooling and the COP falls. If the COP of a system drops from 3.5 to 2.5, the rejection ratio rises from 1.29 to 1.40, so the same cooling load sends about 9% more heat to the condenser. Enter the COP that applies at your design outdoor temperature, not the rating-point value, and size the condenser for that hotter day.
For an air-cooled condenser, airflow in CFM equals the heat of rejection in BTU/hr divided by 1.08 times the air temperature rise in °F. For the 48,000 BTU/hr example at a 15°F rise that is 2,963 CFM. For a water-cooled condenser, flow in US GPM equals the heat of rejection divided by 500 times the water temperature rise in °F. A 350 kW chiller with a COP of 5.5 rejects 1,411,385 BTU/hr, which is 282.3 GPM at a 10°F rise. The calculator shows both, and an upper estimate of water evaporated for an evaporative condenser.