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How to Calculate a Mean Temperature Difference

The mean temperature difference is used to better understand the heat transfer rate in flow systems. The larger the value for MTD, the more heat transfer that is occurring. Two forms of mean temperature difference exist, arithmetic mean temperature difference (AMTD) and logarithmic mean temperature difference (LMTD). LMTD is a nonlinear approximation of a heat transfer curve, while AMTD is a linear approximation. This method is used by engineers to better understand heat transfer in systems such as heat exchangers. It allows them to see how heat will dissipate and move throughout any system involving heating and cooling, such as in a radiator.

Calculator
Graph or data of system temperature

Instructions

- 1
  Determine type of flow in the system. Flow in the same direction is parallel flow. Flow in the opposite directions is counter-current flow. Flow perpendicular to each other is called cross flow. Knowing the type of flow lets you set the correct values for the inlet and outlet temperatures.
- 2
  Calculate logarithmic mean temperature difference using a log function.
  Assume you have an inlet, X, and an outlet,Y.
  A = change in temperature at outlet X
  B = change in temperature at outlet Y
  LMNT = (A-B)/[ln(A/B)] = A minus B over the natural log of A divided by B
- 3
  Calculate the arithmetic mean temperature difference using a linear function.
  Assume you have an inlet, X, and an outlet, Y.
  A= initial inlet temperature + initial outlet temperature
  B= final inlet temperature + final outlet temperature
  AMTD= A/2 - B/2 = A divided by 2 minus B divided by 2
- 4
  Calculate the AMTD/LMTD quickly with the calculator on the Engineering Toolbox website in the Resources section.

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