Can LMTD be applied in multi pass heat exchangers?

In shell and tube exchangers, the flow pattern is a mixture of cocurrent, counter current, and crossflow, so the LMTD does not directly apply.

What are the limitations of lmtd method?

Assumptions and limitations However, if the specific heat changes, the LMTD approach will no longer be accurate. A particular case for the LMTD are condensers and reboilers, where the latent heat associated to phase change is a special case of the hypothesis.

What does it mean if a heat exchanger is a multipass exchanger?

Heat exchangers with multi pass flows means that the fluids pass through the heat exchangers several times before exiting. This type of flow creates a thermally longer heat exchanger, while still maintaining the same compact product footprint.

Where do we need to use LMTD correction factor and explain why?

The F-LMTD method is widely used in heat exchanger analysis, particularly for heat exchanger selection, (sizing problems) when as a result of the process requirements the temperatures are known and the size of the heat exchanger is required.

Why LMTD is used in heat exchanger?

The log mean temperature difference (LMTD) is used to determine the temperature driving force for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger.

What is the difference between LMTD and NTU method?

The LMTD method is convenient for determining the overall heat transfer coefficient based on the measured inlet and outlet fluid temperatures. The ε-NTU method is more convenient for prediction of the outlet fluid temperatures if the heat transfer coefficient and the inlet temperatures are known.

What is multipass shell and tube heat exchanger?

Multipass tubes are used to increase tube-side fluid velocity and, as a result, heat transfer efficiency. Baffles are used on the shell-side to increase velocity by making the fluid take a longer path between the inlet and outlet.

Which is not associated with a heat exchanger?

Which of the following is not associated with heat exchanger? Solution: The correction factor i.e. Mc Adam’s is associated with laminar film condensation on a vertical plate.

What happens when lmtd is zero?

Zero LMTD means no further heat transfer is possible and if heat transfer is to be done than infinite area is required, which is not possible.

Which of the following statements are not true about fouling in a heat exchanger?

Which of the following statements are not true about fouling in a Heat Exchanger? Explanation: When a heat exchanger faces fouling, its heat transfer coefficient decreases, pressure drop increases due to scaling, efficiency decreases and net heat transferred too decreases. 14.

What is importance of LMTD?

Significance of the LMTD The LMTD is the driven force for the heat exchange between the two fluids. As the LMTD value increases, the amounts of heat transfer between the two fluids also increase. The LMTD value is used for calculating the heat duty of the heat exchanger.

Can we use LMTD when one pass heat exchanger used?

It means you can use LMTD when one pass heat exchanger used because in multipass there is a combination of co-current and counter-current flow. Here you can see in one pass there is co-current flow or counter-current flow. Here you can see that in multipass heat exchanger there is a combination of co-current and counter-current flow.

Can I use a single LMTD for evaporation?

Because evaporation is taking place at constant temperature, it cannot be used a single LMTD. In this case the heat exchanger has to be treated as a combination of two or three (when superheat occurs) heat exchangers. Consider a parallel-flow heat exchanger, which is used to cool oil from 70°C to 40°C using water available at 30°C.

What is the difference between uu and LMTD?

U is the overall heat transfer coefficient for that exchanger A overall is the overall effective heat transfer area between the hot and cold sides of the exchanger LMTD is the logarithmic mean of temperature differences as discussed above.

What does LMTD stand for?

Engineers often use a logarithmic mean temperature difference (LMTD), which is used to determine the temperature driving force for heat transfer in heat exchangers. Thermal Engineering