Heat Exchanger Effectiveness (NTU) Calculator
Effectiveness, NTU, capacity ratio and duty of a counterflow heat exchanger.
Results
What this tool does
Effectiveness is the fraction of the theoretically possible heat that an exchanger actually moves, and it depends on only two dimensionless numbers: NTU, which measures how much surface you bought, and the capacity ratio, which measures how badly matched the two streams are. Doubling the area beyond NTU of about 3 buys very little, which is the practical reason exchangers are not made arbitrarily large.
Formula
NTU = UA/Cmin ; efficacy = [1 - e^(-NTU(1-Cr))] / [1 - Cr e^(-NTU(1-Cr))] (counter-flow)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ua | UA value | W/K |
cm | Smaller heat capacity rate | W/K |
cx | Larger heat capacity rate | W/K |
th | Hot side temperature | °C |
tc | Cold side temperature | °C |
EF | Effectiveness | % |
NT | Number of transfer units | — |
CR | Capacity ratio | — |
QM | Maximum possible transfer | kW |
QQ | Heat transferred | kW |
Worked example
- UA value5000 W/K
- Smaller heat capacity rate4000 W/K
- Larger heat capacity rate6000 W/K
- Hot side temperature90 °C
- Cold side temperature20 °C
- Effectiveness60.79 %
- Number of transfer units1.2500
- Capacity ratio0.6667
- Maximum possible transfer280.000 kW
- Heat transferred170.226 kW
Limitations
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
Frequently asked questions
When is NTU better than LMTD?
When you know the inlet temperatures and the exchanger, and want to find out what comes out. LMTD needs all four temperatures, so with two of them unknown it forces you to iterate. NTU goes straight to the answer. The reverse case — sizing an exchanger for known inlet and outlet temperatures — is where LMTD is the shorter road.