Heat Exchanger LMTD Calculator (Log Mean Temperature Difference)

Heat Exchanger LMTD Calculator (Log Mean Temperature Difference) | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Math & Numbers
Input types: select, number
Output type: json
Sample coverage: 4
API ready: Yes

Overview

Calculate the log mean temperature difference (LMTD) for parallel-flow or counter-flow heat exchangers using four stream temperatures. The calculator reports LMTD in K and can also calculate heat transfer rate when overall heat transfer coefficient U and area A are provided.

When to use

Estimate the effective temperature driving force in a heat exchanger.
Compare LMTD for parallel-flow and counter-flow arrangements.
Calculate heat transfer rate from U, A, and LMTD during thermal design.

How it works

1Enter the hot-stream inlet and outlet temperatures and the cold-stream inlet and outlet temperatures.
2Select parallel flow or counter flow and choose K, °C, or °F as the input temperature unit.
3The calculator determines the two terminal temperature differences and applies the LMTD formula.
4Optionally enter U in W/(m²·K) and A in m² to calculate Q = U·A·LMTD in watts.

Use cases

Thermal design of shell-and-tube and other heat exchangers.
Preliminary sizing checks using an overall heat transfer coefficient and heat transfer area.
Engineering review of inlet and outlet temperature measurements for counter-flow or parallel-flow equipment.

Examples

1. Counter-flow LMTD calculation

Thermal engineer

Background

An engineer is checking the temperature driving force for a counter-flow heat exchanger with known stream inlet and outlet temperatures.

Problem

Find the two terminal temperature differences and the resulting LMTD.

How to use

Select Counter Flow, enter hot-stream temperatures of 100 °C and 60 °C, and cold-stream temperatures of 20 °C and 50 °C.

flowType: counter; hotInletTemp: 100; hotOutletTemp: 60; coldInletTemp: 20; coldOutletTemp: 50; tempUnit: °C; decimalPlaces: 4

Outcome

The terminal differences are 50 K and 40 K, producing an LMTD of approximately 44.8142 K.

2. Parallel-flow heat transfer rate

Process design engineer

Background

A process designer wants to estimate the heat transfer rate for a parallel-flow exchanger using its overall coefficient and area.

Problem

Calculate both LMTD and Q from the stream temperatures, U, and A.

How to use

Select Parallel Flow, enter hot-stream temperatures of 120 °C and 70 °C, cold-stream temperatures of 30 °C and 60 °C, U of 500 W/(m²·K), and A of 2 m².

FAQ

What does the LMTD calculator calculate?

It calculates the log mean temperature difference of a heat exchanger for parallel-flow or counter-flow operation.

What temperature inputs are required?

You need the hot-stream inlet and outlet temperatures and the cold-stream inlet and outlet temperatures.

What units does the calculator use for LMTD?

The result is reported in kelvin (K). Temperature differences in °C equal differences in K, while Fahrenheit differences are converted to K.

Can it calculate heat transfer rate?

Yes. Enter both U and A to calculate Q = U·A·LMTD in watts.

What happens if a terminal temperature difference is zero or negative?

The calculator rejects the input because a non-positive terminal difference indicates a physically impossible temperature cross.

flowType: parallel; hotInletTemp: 120; hotOutletTemp: 70; coldInletTemp: 30; coldOutletTemp: 60; tempUnit: °C; heatTransferCoeff: 500; area: 2; decimalPlaces: 4

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Counter-flow LMTD calculation

2. Parallel-flow heat transfer rate

FAQ

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