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How to Calculate LMTD

The Log Mean Temperature Difference (also known as the "LMTD" or the "LMT Difference") is a value, measured in degrees Fahrenheit, that gauges the difference in temperature between the hot and cold ends of a heat exchanger. Although the computation for the LMTD is somewhat involved, it is a very useful tool for measuring how much heat is transferred in a heat exchange--the higher the LMTD value, the more heat is being transferred in the heat exchange.

Calculator

Instructions

- 1
  Gather the four values that you need to compute the LMTD. You need the temperatures in degrees Fahrenheit of the hot stream inlet (the HSI), the hot stream outlet (the HSO), the cold stream inlet (the CSI) and the cold stream outlet (the CSO).
- 2
  Compute the numerator. Computing the numerator involves using each of the four values that you gathered in Step 1 a single time. The numerator is computed using the following equation:
  (HSI - CSO) - (HSO - CSI)
- 3
  Compute the denominator. Like the numerator, the denominator is computed by using each of the four values in Step 1 a single time. First, divide (HSI - CSO) by (HSO - CSI). Second, use a calculator to take the natural log (with the "LN" key on your calculator) of this value. This is the denominator.
- 4
  Take the numerator value from Step 2 and divide it by the denominator value from Step 3. This new value is the Log Mean Temperature Difference.

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