Dittus-Boelter Heat Transfer Calculator
Nusselt number and convective heat transfer coefficient for turbulent flow inside a tube.
Results
What this tool does
Sizing a heat exchanger always comes down to the film coefficient on each side, and for turbulent flow inside a plain round tube the Dittus-Boelter correlation is the one that has been used since 1930. It is deliberately simple — no property corrections at the wall, no entrance effects — which is why it is quoted as accurate to within about 25%. For a first pass at surface area, that is usually enough.
Formula
Nu = 0.023 Re^0.8 Pr^n ; h = Nu k / D
Variables
| Symbol | Meaning | Unit |
|---|---|---|
re | Reynolds number | — |
pr | Prandtl number | — |
kk | Thermal conductivity | W/(m·K) |
dd | Pipe diameter | mm |
nx | Prandtl exponent | — |
NU | Nusselt number | — |
HC | Overall heat transfer coefficient | W/(m²·K) |
RT | Inside surface resistance | m²·K/W |
Worked example
- Reynolds number50,000
- Prandtl number4
- Thermal conductivity0.6 W/(m·K)
- Pipe diameter25 mm
- Prandtl exponent0.4
- Nusselt number230.00
- Overall heat transfer coefficient5520.0 W/(m²·K)
- Inside surface resistance0.000181 m²·K/W
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.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
Frequently asked questions
Should the exponent be 0.4 or 0.3?
0.4 when the fluid is being heated, 0.3 when it is being cooled. The difference comes from how viscosity changes near the wall: a heated liquid is thinner at the wall and mixes better. The correlation itself assumes fully developed turbulent flow, Re above about 10000, Pr between 0.6 and 160, and a tube at least ten diameters long.