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LMTD Calculator

Compute parallel-flow or counterflow LMTD, handle equal terminal differences, and apply a user-entered correction factor.

LMTD inputs

Result section

warningcounterflow

Log mean temperature difference

42.4509 ΔK

Secondary results

Corrected LMTD: 42.4509 ΔK
Terminal difference 1: 45 ΔK
Terminal difference 2: 40 ΔK
Equal-difference limit: Not used

Unit conversion summary

Unit-normalization summary: SI base units; counterflow

Warnings

Thermal properties can vary with temperature, pressure, composition, grade, direction, phase, surface condition, manufacturing process, and data source. Verify all values for the actual operating condition.

Assumptions

Calculations are performed locally in your browser and are not submitted to a third-party calculation API.

Calculation steps

Compute terminal differences and logarithmic mean42.4509 ΔK

Log mean temperature difference

LMTD = (deltaT1 - deltaT2) / ln(deltaT1 / deltaT2)

Technical visual

Calculate LMTD and corrected LMTD from heat-exchanger terminal temperatures.

Engineering interpretation

Positive terminal differences produce a valid LMTD for the selected arrangement.

What is log mean temperature difference and how does this LMTD Calculator work?

Log mean temperature difference, or LMTD, is the effective average thermal driving force through a heat exchanger when terminal temperature differences vary along its length. Parallel-flow and counterflow arrangements use different end differences, and multipass layouts may need a correction factor.

LMTD Calculator applies this concept to its defined inputs and workflow. This LMTD calculator converts absolute inlet and outlet temperatures to terminal differences and applies the logarithmic mean temperature formula. Its governing relationship or workflow is stated as follows: Parallel-flow or counterflow log mean temperature difference with equal-difference limit handling.

The calculator evaluates Calculation mode, Temperature difference, and Display precision and returns LMTD, base-unit normalization, formula substitution, and calculation steps using the stated method. The selected units and sign or reference conventions must describe one consistent physical case. Users should interpret the output within the assumptions shown on the page and repeat the check with trusted reference values when the result affects engineering or research decisions. Thermal properties are treated as constant user-entered values.

How to use the lmtd calculator

Choose parallel flow or counterflow.
Enter hot and cold inlet and outlet temperatures.
Review terminal differences, LMTD, corrected LMTD, and invalid crossing warnings.

Input explanations

All four temperatures are absolute-temperature inputs.
Both terminal temperature differences must remain positive for standard LMTD.
Correction factor is user-entered and does not come from shell-and-tube charts.

Notation

Formula notation
Symbol	Meaning	Unit
delta T1, delta T2	Terminal temperature differences	K
F	Correction factor	-
LMTD	Log mean temperature difference	K

Worked examples

For counterflow with 90/60 C hot and 20/45 C cold, LMTD is about 42.46 K.

Cross-tool workflow

The LMTD calculator converts absolute inlet and outlet temperatures to terminal differences and applies the logarithmic mean temperature formula.

Use LMTD with overall U Compare with effectiveness NTU

Common mistakes

Using the arithmetic mean instead of logarithmic mean for unequal terminal differences.
Ignoring temperature crossing.
Using a correction factor without checking the applicable exchanger arrangement.

Limitations

Thermal properties are treated as constant user-entered values.
The tool does not select material, fluid, steam, refrigerant, phase-change, or fouling data automatically.
Results are preliminary engineering estimates and do not certify code compliance or safety.
Multidimensional effects, transient storage, contact details, and property variation must be evaluated separately when important.
No shell-and-tube or crossflow correction-factor chart system is included.
Temperature crossing is reported as invalid for standard LMTD.

References

Fundamentals of Heat and Mass Transfer: Incropera, DeWitt, Bergman, and Lavine, Fundamentals of Heat and Mass Transfer.
Heat and Mass Transfer: Cengel and Ghajar, Heat and Mass Transfer: Fundamentals and Applications.
Heat Transfer: Holman, Heat Transfer.
Compact Heat Exchangers: Kays and London, Compact Heat Exchangers.
SI units: BIPM, The International System of Units (SI Brochure).
NIST unit guidance: NIST Special Publication 811, Guide for the Use of the International System of Units.

What assumptions does the LMTD calculator use?

It uses the stated steady or lumped formula with user-entered properties and does not infer hidden losses, standards factors, or temperature-dependent property data.

Are Celsius and Fahrenheit temperature differences handled as offsets?

No. Absolute temperature and temperature difference are separate quantities. Temperature differences use delta units without absolute-temperature offsets.

Can I use the result for final design approval?

No. Use the result for education, estimation, and preliminary review only. Critical work must be checked with standards, validated software, measured properties, and qualified engineering judgment.

Does the calculation run on a remote thermal-property API?

No. Calculations are performed locally in your browser and are not submitted to a third-party calculation API.

Engineering disclaimer

This tool is intended for educational, estimation, and preliminary engineering use. Always verify critical heat-transfer calculations with applicable standards, validated software, measured properties, and qualified professional judgment before using results in real design, manufacturing, construction, safety, or compliance decisions.

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LMTD inputs

Result section

warningcounterflow

Log mean temperature difference

42.4509 ΔK

Secondary results

Corrected LMTD: 42.4509 ΔK
Terminal difference 1: 45 ΔK
Terminal difference 2: 40 ΔK
Equal-difference limit: Not used

Unit conversion summary

Unit-normalization summary: SI base units; counterflow

Warnings

Thermal properties can vary with temperature, pressure, composition, grade, direction, phase, surface condition, manufacturing process, and data source. Verify all values for the actual operating condition.

Assumptions

Calculations are performed locally in your browser and are not submitted to a third-party calculation API.

Calculation steps

Compute terminal differences and logarithmic mean42.4509 ΔK

Log mean temperature difference

LMTD = (deltaT1 - deltaT2) / ln(deltaT1 / deltaT2)

What is log mean temperature difference and how does this LMTD Calculator work?

Symbol

Meaning

Unit

delta T1, delta T2

Terminal temperature differences

Correction factor

LMTD

Log mean temperature difference

Limitations

Thermal properties are treated as constant user-entered values.

The tool does not select material, fluid, steam, refrigerant, phase-change, or fouling data automatically.

Results are preliminary engineering estimates and do not certify code compliance or safety.

Multidimensional effects, transient storage, contact details, and property variation must be evaluated separately when important.

No shell-and-tube or crossflow correction-factor chart system is included.

Temperature crossing is reported as invalid for standard LMTD.

References

Fundamentals of Heat and Mass Transfer: Incropera, DeWitt, Bergman, and Lavine, Fundamentals of Heat and Mass Transfer.

Heat and Mass Transfer: Cengel and Ghajar, Heat and Mass Transfer: Fundamentals and Applications.

Heat Transfer: Holman, Heat Transfer.

Compact Heat Exchangers: Kays and London, Compact Heat Exchangers.

SI units: BIPM, The International System of Units (SI Brochure).

NIST unit guidance: NIST Special Publication 811, Guide for the Use of the International System of Units.

Engineering disclaimer

LMTD inputs

Calculation mode

Terminal temperatures

Result section

Secondary results

Unit conversion summary

Warnings

Assumptions

Calculation steps

Log mean temperature difference

Technical visual

Engineering interpretation

What is log mean temperature difference and how does this LMTD Calculator work?

How to use the lmtd calculator

Input explanations

Notation

Worked examples

Cross-tool workflow

Common mistakes

Limitations

References

Related tools

Print summary

Engineering disclaimer

Need Professional Support?

LMTD inputs

Calculation mode

Terminal temperatures

Result section

Secondary results

Unit conversion summary

Warnings

Assumptions

Calculation steps

Log mean temperature difference

Technical visual

Engineering interpretation

What is log mean temperature difference and how does this LMTD Calculator work?

How to use the lmtd calculator

Input explanations

Notation

Worked examples

Cross-tool workflow

Common mistakes

Limitations

References

Related tools

Print summary

Engineering disclaimer

Need Professional Support?