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Calculate heat rejection in kW with our Heat Rejection Calculator. Find fluid heat loss, condenser heat rejection, and cooling-water flow with ease.
The Heat Rejection Calculator helps you find how much heat a system must remove or reject. It works with fluid flow, condenser load, and cooling-water flow.
You can use it for water, air, cooling systems, condensers, radiators, and other thermal systems. The calculator also supports common units such as kg/s, kg/min, kg/hr, lb/s, lb/min, lb/hr, GPM, kW, tons of refrigeration, BTU/hr, and HP.
It gives a quick and clear result without doing manual unit conversions.
The main rate of heat rejection formula is:
Heat Rejection = Mass Flow Rate × Specific Heat × Temperature Difference
In normal notation:
Q̇ = ṁ × Cp × ΔT
Where Q̇ is heat rejection in kW, ṁ is mass flow in kg/s, Cp is specific heat in kJ/kg·K, and ΔT is the temperature change in K or °C.
The temperature difference is:
ΔT = Outlet Temperature − Inlet Temperature
For water, this calculator uses Cp = 4.186 kJ/kg·K. For air, it uses Cp = 1.005 kJ/kg·K.
For condenser heat rejection, the calculator uses:
Qcond = Qevap + Wcomp
Here, Qcond is total condenser heat rejection, Qevap is the evaporator cooling load, and Wcomp is compressor power.
For required cooling-water flow, the formula is:
ṁ = Q̇ ÷ (Cp × ΔT)
For water, the calculator can also convert the result to US GPM.
These formulas also apply to many water heat rejection, radiator heat rejection, and cooling-system calculations. The exact method depends on the system and the data available.
Suppose water flows at 10 kg/s. Its specific heat is 4.186 kJ/kg·K. The water enters at 25°C and leaves at 30°C.
First, find the temperature difference:
ΔT = 30 − 25 = 5°C
Now apply the heat rejection formula:
Q̇ = 10 × 4.186 × 5
Q̇ = 209.3 kW
So, the heat rejection rate is 209.3000 kW.
For a condenser example, assume the evaporator load is 100 kW and compressor power is 25 kW.
Qcond = 100 + 25
Qcond = 125 kW
Thus, the condenser must reject 125 kW of heat.
For cooling-water flow, if the heat rejection is 125 kW and water temperature rises from 25°C to 30°C:
ṁ = 125 ÷ (4.186 × 5)
ṁ = 5.9737 kg/s
The calculator also gives about 94.674 GPM for water.
Heat rejection calculations are useful in many engineering systems. They can help estimate heat dissipation for electrical equipment, cooling-water demand, condenser loads, radiator performance, and engine cooling requirements.
A cooling tower heat rejection calculation can use the water-side heat balance when flow rate, specific heat, and temperature change are known. Likewise, an air conditioning heat rejection formula can help determine condenser heat rejection from cooling capacity and compressor input.
The term heat of reaction calculator is different. It normally refers to chemical reaction enthalpy, not thermal heat rejection. So it should not be confused with this calculator.
The Heat Rejection Calculator makes heat rejection calculations faster and easier. Its core methods are Q̇ = ṁCpΔT, Qcond = Qevap + Wcomp, and ṁ = Q̇/(CpΔT).
It handles common unit conversions and provides practical results for fluid cooling, condenser heat rejection, and required cooling-water flow.
The main formula is Q̇ = ṁ × Cp × ΔT.
Heat rejection rate is commonly measured in watts or kilowatts. This calculator reports the main heat-rejection result in kW.
Use Qcond = Qevap + Wcomp. Add the evaporator cooling load and compressor power after converting both to kW.
Use ṁ = Q̇ ÷ (Cp × ΔT). The result is mass flow in kg/s.
Yes. Select Water, enter the water flow, Cp, inlet temperature, and outlet temperature. The calculator uses Cp = 4.186 kJ/kg·K.
Both terms describe removing heat from a system. Heat rejection often refers to heat transferred out of a system, while heat dissipation is commonly used for heat released to the surrounding environment.