Heat Exchanger Effectiveness NTU Calculator — Heat Transfer Rate & Outlet Temperature Validator
Calculate heat transfer rate using inlet temperatures, heat capacity rates, UA, and flow configuration based on the ε-NTU method.
Enter the known values and review the calculated result
Input parameters
Use consistent values and select the intended engineering units.
Hot fluid (stream 1)
Cold fluid (stream 2)
Heat exchanger properties
Results
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Method, application and limitations
Review the calculation method, intended application and engineering assumptions before using the result in a design decision.
Formula and calculation method
Heat transfer rate formula:
C₁ = ṁ₁ · cₚ₁
C₂ = ṁ₂ · cₚ₂
Cmin = min(C₁, C₂)
Cmax = max(C₁, C₂)
Cᵣ = Cmin / Cmax
NTU = UA / Cmin
Qmax = Cmin · (T₁,in − T₂,in)
For counterflow when |1 − Cᵣ| < 1e−6:
ε = NTU / (1 + NTU)
For counterflow when |1 − Cᵣ| ≥ 1e−6:
ε = (1 − exp(−NTU · (1 − Cᵣ))) / (1 − Cᵣ · exp(−NTU · (1 − Cᵣ)))
For parallel flow:
ε = (1 − exp(−NTU · (1 + Cᵣ))) / (1 + Cᵣ)
Q = ε · Qmax
T₁,out = T₁,in − Q / C₁
T₂,out = T₂,in + Q / C₂
where:
- Q — heat transfer rate (W)
- T₁,out — hot outlet temperature (K)
- T₂,out — cold outlet temperature (K)
- ṁ₁ — hot fluid mass flow rate (kg/s)
- ṁ₂ — cold fluid mass flow rate (kg/s)
- cₚ₁ — hot fluid specific heat capacity (J/kg·K)
- cₚ₂ — cold fluid specific heat capacity (J/kg·K)
- T₁,in — hot inlet temperature (K)
- T₂,in — cold inlet temperature (K)
- UA — overall conductance (W/K)
- C₁ — hot side heat capacity rate (W/K)
- C₂ — cold side heat capacity rate (W/K)
- Cmin — minimum heat capacity rate (W/K)
- Cmax — maximum heat capacity rate (W/K)
- Cᵣ — capacity ratio (-)
- NTU — number of transfer units (-)
- ε — effectiveness (-)
- Qmax — maximum heat transfer (W)
When to use this calculator
When to use this calculator:
- To calculate heat transfer rate from hot and cold inlet temperatures, mass flow rates, specific heat capacities, UA, and flow configuration.
- To calculate hot outlet temperature after heat is transferred from stream 1 to stream 2.
- To calculate cold outlet temperature after heat is received by stream 2.
- To compare counterflow and parallel flow results using the same inlet conditions and overall conductance.
- To evaluate whether the exchanger result satisfies positive temperature-difference and outlet-temperature feasibility checks.
How to interpret the result
Heat transfer rate is defined as the transferred thermal power from the hot stream to the cold stream.
The result depends on the inlet temperature difference, minimum heat capacity rate, overall conductance, capacity ratio, and flow configuration. Increasing the inlet temperature difference increases the maximum possible heat transfer. Increasing overall conductance increases NTU, which can increase effectiveness and therefore increase heat transfer rate.
- Safe — returned when the design feasibility checks pass, effectiveness is not greater than 0.9, and the capacity ratio is not below 1e−4.
- Warning — returned when the minimum temperature difference or either approach temperature is not positive, when effectiveness is greater than 0.9, or when the capacity ratio is below 1e−4.
- Invalid — returned when required positive inputs are not greater than zero, when T₁,in ≤ T₂,in, when flow configuration is not counterflow or parallel, when effectiveness is outside (0, 1], when Q > Qmax, or when outlet and pinch temperature checks fail.
The result is used to evaluate heat transfer rate, outlet temperatures, effectiveness, NTU level, sizing classification, and feasibility of the calculated exchanger state.
Calculation example
Example:
A user checks a counterflow heat exchanger where a hot stream enters at 360 K and a cold stream enters at 300 K.
- ṁ₁ = 1.2 kg/s
- cₚ₁ = 4180 J/kg·K
- T₁,in = 360 K
- ṁ₂ = 1.0 kg/s
- cₚ₂ = 4180 J/kg·K
- T₂,in = 300 K
- UA = 2500 W/K
- Flow configuration = counterflow
Q = 128522 W, T₁,out = 334.38 K, T₂,out = 330.75 K.
Assumptions and limitations
- The hot inlet temperature must be higher than the cold inlet temperature.
- The model uses positive mass flow rates, positive specific heat capacities, and positive overall conductance.
- The flow configuration is limited to counterflow or parallel flow.
- NTU is calculated as overall conductance divided by the minimum heat capacity rate.
- Effectiveness is calculated from the implemented ε-NTU equations for the selected flow configuration.
- Heat transfer rate cannot exceed the maximum heat transfer based on the minimum heat capacity rate and inlet temperature difference.