ELI5: Explain Like I'm 5

Log mean temperature difference

When we heat or cool something, we often use devices like exchangers or radiators. These devices are designed to transfer heat from one fluid to another fluid. For instance, we may want to heat water using steam, or we may want to cool down air using water.

To understand how much heat can be transferred between two fluids, we need to consider the temperature difference between them. The greater the temperature difference, the greater the amount of heat that can be transferred. But since the temperature of the fluids may change as they move through the device, we need to calculate an average temperature difference over the entire length of the device.

This is where the log mean temperature difference (LMTD) comes in. The LMTD takes into account the fact that the temperature difference between the fluids is not constant over the length of the device. It is calculated by taking the logarithmic average of the temperature differences at the inlet and outlet of the device.

To put it in simple terms, imagine you have a bowl of hot soup and a bowl of cold salad. You want to mix them together to get a bowl of warm soup. You start by pouring some soup into the salad bowl and stirring them together. After a few minutes, you pour the mixture back into the soup bowl and stir again.

The temperature of the soup and salad will have changed during this process, so you need to calculate the average temperature difference between the two bowls. To do this, you take the temperature difference between the soup and salad when you pour them together, and the temperature difference between them when you pour them back. You then take the logarithmic average of these two differences to get the overall LMTD.

This may seem like a simple example, but the LMTD is used in many industrial processes, such as power generation, chemical production, and HVAC systems. By using the LMTD to calculate the amount of heat that can be transferred between two fluids, engineers can design more efficient and effective heat exchangers and other devices.