When outlet temperatures are unknown, the effectiveness–NTU method predicts how well a given heat exchanger will perform. Effectiveness is the fraction of the maximum possible heat transfer that actually happens.
How to use the Effectiveness–NTU Calculator
- Exchanger type
- Hot fluid mass flow
- Hot fluid specific heat
- Hot inlet temperature
- Cold fluid mass flow
- Cold fluid specific heat
- Cold inlet temperature
- UA (W/K)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the exchanger type, enter both streams’ mass flow, specific heat and inlet temperature, and the product UA.
Tips
- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.
Common mistakes to avoid
- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).
Inputs explained
| Field | What to enter |
|---|---|
| Exchanger type | Choose one: Parallel flow, Counter flow, Shell and tube (one shell pass), Cross flow, both fluids unmixed … |
| Hot fluid mass flow | Units: kg/s, kg/h, g/s, lb/s, lb/min |
| Hot fluid specific heat | Units: J/(kg·K), kJ/(kg·K), J/(g·K), BTU/(lb·°F), cal/(g·°C) |
| Hot inlet temperature | Units: °C, °F, K |
| Cold fluid mass flow | Units: kg/s, kg/h, g/s, lb/s, lb/min |
| Cold fluid specific heat | Units: J/(kg·K), kJ/(kg·K), J/(g·K), BTU/(lb·°F), cal/(g·°C) |
| Cold inlet temperature | Units: °C, °F, K |
| UA (W/K) | A number |
What each result means
- Heat capacity ratio C_r = [Cmin]÷max([Ch], [Cc])
- NTU = [UA (W÷K)]÷[Cmin]
- Heat transfer rate = [Effectiveness ε]×[Cmin]×([Hot inlet temperature] – [Cold inlet temperature])÷1000
- Hot outlet temperature = [Hot inlet temperature] – [Heat transfer rate]×1000÷[Ch]
- Cold outlet temperature = [Cold inlet temperature] + [Heat transfer rate]×1000÷[Cc]
Example values
How the main result changes when Hot fluid mass flow changes and everything else stays at the example values above:
| Hot fluid mass flow (kg/s) | Effectiveness ε |
|---|---|
| 0.25 | 0.9 |
| 0.375 | 0.7684 |
| 0.5 | 0.6552 |
| 0.625 | 0.5661 |
| 0.75 | 0.4964 |
| 1 | 0.4955 |
Formula
C = ṁ·c_p, C_r = C_min/C_max, NTU = UA/C_min · Counter-flow: ε = (1 − e^{−NTU(1−C_r)})/(1 − C_r·e^{−NTU(1−C_r)})q = ε·C_min·(T_h,in − T_c,in), T_h,out = T_h,in − q/C_h, T_c,out = T_c,in + q/C_cWorked example
Inputs — Exchanger type: Counter flow; Hot fluid mass flow: 0.5 kg/s; Hot fluid specific heat: 4,182 J/(kg·K); Hot inlet temperature: 90 °C; Cold fluid mass flow: 0.8 kg/s; Cold fluid specific heat: 4,182 J/(kg·K); Cold inlet temperature: 15 °C; UA (W/K): 3,000.
Results — Heat capacity ratio C_r = 0.625; NTU = 1.4347; Effectiveness ε = 0.65521; Heat transfer rate = 102.75302 kW; Hot outlet temperature = 40.8594 °C; Cold outlet temperature = 45.7129 °C.
Key terms
- Effectiveness
- Actual heat transfer divided by the maximum possible.
Frequently asked questions
What is NTU?
UA/C_min – the "thermal size" of the exchanger.
Which type is most effective?
Counter-flow, for the same NTU.
When is the ε–NTU method used?
When outlet temperatures are unknown (rating problems).
What is C_r?
Ratio of the smaller to the larger heat capacity rate.
What formula does the Effectiveness–NTU Calculator use?
C = ṁ·c_p, C_r = C_min/C_max, NTU = UA/C_min · Counter-flow: ε = (1 − e^{−NTU(1−C_r)})/(1 − C_r·e^{−NTU(1−C_r)}).