Understanding Log Mean Temperature Difference in Process Engineering

LMTD is a way of measuring the average temperature difference between two fluids in a heat exchanger. A heat exchanger is a device that transfers heat from one fluid to another, for example, from hot water to cold air. The temperature difference between the fluids determines how much heat is transferred.

However, the temperature difference is not constant throughout the heat exchanger. It depends on the direction and speed of the fluids, the shape and size of the heat exchanger, and the properties of the fluids. Therefore, we need a way to calculate an effective temperature difference that represents the overall heat transfer process.

One way to do that is to use the LMTD. The LMTD is based on the idea that the heat transfer rate is proportional to the logarithm of the temperature difference. This means that a small change in temperature difference at a high value has more impact than a large change at a low value. For example, increasing the temperature difference from 10°C to 20°C will result in more heat transfer than increasing it from 100°C to 110°C.

To calculate the LMTD, we need to know the temperature difference at the inlet and outlet of the heat exchanger, where the fluids enter and exit. Then, we take the difference between these two values and divide it by the natural logarithm of their ratio. This gives us a single value that reflects the average temperature difference across the heat exchanger.

The LMTD can be used to estimate the heat transfer rate, the heat transfer area, or the overall heat transfer coefficient of the heat exchanger, depending on the information available. However, the LMTD is only valid for certain types of heat exchangers, such as parallel flow, counter flow, or cross flow. For more complex geometries, such as shell and tube or plate heat exchangers, we need to use a correction factor to adjust the LMTD.

Calculation of LMTD:

The formula to calculate LMTD depends on the type of heat exchanger flow arrangement. For a counterflow heat exchanger, the formula is:

LMTD = $\frac{\Delta T_{1} - \Delta T_{2}}{\ln(\frac{\Delta T_{1}}{\Delta T_{2}})}$

Where:

$\Delta T_{1}$ is the temperature difference at the hot fluid inlet and outlet.
$\Delta T_{2}$ is the temperature difference at the cold fluid inlet and outlet.

For a parallel flow heat exchanger, the formula is simpler:

LMTD = $\frac{\Delta T_{1} - \Delta T_{2}}{\ln(\frac{\Delta T_{1}}{\Delta T_{2}})}$

Example

Let’s consider a shell-and-tube heat exchanger used to cool a hot fluid from 150°C to 50°C using water entering at 20°C. The flow is counter-current, and the specific heat capacities are 4.18 kJ/kg°C for water and 2.5 kJ/kg°C for the fluid being cooled.

Given:

Inlet temperature of hot fluid ( $T_{h,in}$ ): 150°C
Outlet temperature of hot fluid ( $T_{h,out}$ ): 50°C
Inlet temperature of cold fluid ( $T_{c,in}$ ): 20°C

Calculation:

1. Calculate the temperature differences:

$\Delta T_{1} = T_{h,in} - T_{c,out}$
$\Delta T_{2} = T_{h,out} - T_{c,in}$
$\Delta T_{1} = 150°C - 20°C = 130°C$
$\Delta T_{2} = 50°C - 20°C = 30°C$

2. Calculate LMTD:

$LMTD = \frac{130°C - 30°C}{\ln(\frac{130°C}{30°C})}$
$LMTD ? \frac{100°C}{\ln(\frac{130°C}{30°C})}$
$LMTD ? \frac{100°C}{\ln(4.33)}$
$LMTD ? \frac{100°C}{1.467}$
$LMTD ? 68.12°C$

The Log Mean Temperature Difference ( $LMTD$ ) for this scenario is approximately $68.12°C$ . This value serves as a crucial parameter in heat exchanger design calculations, aiding engineers in optimizing heat transfer efficiency.

Understanding Log Mean Temperature Difference in Process Engineering

Calculation of LMTD:

Example

Calculation:

Published by Jack

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