BIZOMAT MAKEOS · Hydraulics
Heat Exchanger LMTD and Area
What log mean temperature difference and area does this heat exchanger need for the duty?
Log mean temperature difference for counter-flow or parallel-flow exchange, the area needed for a heat duty at an overall coefficient, and a temperature-cross check.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Hot fluid in | °C | Inlet temperature of the hot fluid. |
| Hot fluid out | °C | Outlet temperature of the hot fluid. |
| Cold fluid in | °C | Inlet temperature of the cold fluid. |
| Cold fluid out | °C | Outlet temperature of the cold fluid. |
| Flow arrangement | choice | Flow arrangement (shell-and-tube with several passes needs a correction factor, not included). |
| Heat duty | kW | Heat to transfer. |
| Overall heat transfer coefficient | W/(m²·K) | Overall coefficient for the exchanger type and fluids (oil to water about 200–600 W/m²K). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Hot fluid in
- What it is
- The temperature of the hot fluid as it enters the heat exchanger.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -100 °C to 1000 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 70 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter degrees Celsius; only the differences matter in the result.
What this means: Hot fluid out
- What it is
- The temperature of the hot fluid as it leaves the heat exchanger, after giving up heat.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -100 °C to 1000 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must be lower than the hot inlet temperature.
What this means: Cold fluid in
- What it is
- The temperature of the cold fluid as it enters the heat exchanger.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -100 °C to 1000 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Cold fluid out
- What it is
- The temperature of the cold fluid as it leaves the heat exchanger, after picking up heat.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -100 °C to 1000 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 35 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- In parallel flow the cold outlet cannot exceed the hot outlet; the tool rejects temperatures that cross.
What this means: Flow arrangement
- What it is
- Whether the two fluids flow in opposite directions (counter-flow) or the same direction (parallel flow) through the exchanger.
- Units and format
- Choose one of 2 options from the list: Counter-flow; Parallel flow.
- Example
- The form opens with “Counter-flow” selected as a worked example. Choose the option that matches your case.
- Get it right
- Multi-pass shell-and-tube exchangers need a correction factor that this tool does not apply.
What this means: Heat duty
- What it is
- The heat that must pass from the hot fluid to the cold fluid.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 1000000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The area required rises in direct proportion to the duty.
- Get it right
- It should match the heat given up by the hot fluid and gained by the cold one.
What this means: Overall heat transfer coefficient
- What it is
- The overall heat transfer coefficient for the exchanger type and fluids, including wall and fouling resistances, from the exchanger maker or a heat transfer reference.
- Units and format
- Enter a number in W/(m²·K): watts per square metre per kelvin. The form accepts 0.000001 W/(m²·K) to 100000 W/(m²·K). Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 400 W/(m²·K) as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The area required falls in inverse proportion.
- Get it right
- Include fouling in this value; a clean coefficient overstates performance once in service.
Method and formulas
- Counter-flow: ΔT₁ = Th,in − Tc,out; ΔT₂ = Th,out − Tc,in. Parallel: ΔT₁ = Th,in − Tc,in; ΔT₂ = Th,out − Tc,out.
- LMTD = (ΔT₁ − ΔT₂) ÷ ln(ΔT₁ ÷ ΔT₂).
- Area = Q ÷ (U × LMTD).
- Example: the default temperatures give LMTD = 29.72 K and need 1.6824 m² for 20 kW.
Limitations
- Pure counter-flow or parallel flow; multi-pass exchangers need the F correction factor.
- Fouling must be included in U.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size an oil cooler or plate heat exchanger
- Check an existing exchanger for a new duty
- Compare counter-flow and parallel flow
Who it is for
- Process engineers
- Maintenance engineers
- Design engineers
What you get
- Terminal temperature differences
- LMTD
- Area required
Interpreting the result
- Counter-flow needs less area and allows the cold outlet to exceed the hot outlet.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 371–378. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Water in the flue gas and condensing heat recovery
What it covers: How much water burning gas makes, the flue gas dew point and how excess air moves it, recovering latent heat with a condensing exchanger, and a flue that was never meant to be wet.
Method explained here · BIZOMAT original book
Heat Transfer by the Numbers
What it covers: Calculate heat flows in plant: energy and power, conduction through walls and pipe insulation, convection, radiation, economic insulation, heat exchangers and heating and cooling times.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.