How does the magnitude of LMTD compare with the arithmetic average temperature?

The Logarithmic Mean Temperature Difference is always less than the Arithmetic Mean Temperature Difference.

How does the LMTD for a heat exchanger differ from the arithmetic mean temperature difference?

In arithmetic mean temperature difference you have to find temperature difference on whole lengh of heat exchanger. Then, find add them and divide by total number of readings and you will get your answer. In Logarithmic mean temperature difference you have to find temperature on inlet and outlet point.

Under what conditions is the effectiveness NTU method definitely preferred over the lmtd method in the analysis of a heat exchanger?

In heat exchanger analysis, if the fluid inlet and outlet temperatures are specified or can be determined by simple energy balance, the LMTD method can be used; but when these temperatures are not available The NTU or The Effectiveness method is used.

What is the difference between LMTD and NTU methods?

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.

How LMTD varies for the same inlet and exit temperatures of the hot and cold fluids?

For the same inlet and exit temperatures of the hot and cold fluids, the Log mean temperature difference (LMTD) is. Greater for counter flow heat exchanger than the parallel flow heat exchanger. Same for both parallel and counter flow heat exchangers. Depending on the properties of fluid.

When can LMTD not be used?

A particular case where the LMTD is not applicable is condensers and reboilers, where the latent heat associated to phase change makes the hypothesis invalid. It has also been assumed that the heat-transfer coefficient (U) is constant and not a function of temperature.

What is NTU effectiveness?

Effectiveness-NTU (number of transfer units) methods are popular in heat exchanger design [1,2]. They are simple in form. The number of transfer units (NTU = UA/(mcp)) itself is a combination of overall heat transfer coefficients, transfer area, fluid flow rate and heat capacity.

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.

Which heat exchanger is more efficient?

Counter flow heat exchangers are inherently more efficient than parallel flow heat exchangers because they create a more uniform temperature difference between the fluids, over the entire length of the fluid path.

When to use LMTD and when to use AMTD?

In other words, if there is a significant difference between range mean terminal temperature of ΔTinletinlet and ΔToutletoutlet, we should use LMTD. The table and the graph shows the variation of the LMTD and the AMTD for a range mean terminal temperature.

What is the formula for LMTD?

LMTD Formula: LMTD = [ΔT1- ΔT2]/[lnΔT1 / lnΔT2 ] LMTD Equation. where ΔT1 and ΔT2 as defined below for the LMTD counter-current flow and LMTD parallel flow: LMTD Counter-current flow exchanger. LMTD Counter Flow Formula. LMTD Parallel flow Exchanger LMTD Parallel Flow Formula

When to use AMTD in heat exchanger?

The AMTD introduced with the assumption that the fluid temperature change in the two flows sides of the heat exchanger are small the use of the AMTD is justified. In other word, if there is no significance diffrence between range mean terminal temperature of ΔTinletinlet and ΔToutletoutlet, we can use AMTD

What is arithmetic mean temperature difference (arithmetic MDD)?

Arithmetic mean temperature difference is used when there is constant temperature difference across heat exchanger at each interval is maintained. If not, you need to use LMTD. LMTD is nothing but you are finding out temperature difference across heat exchanger at fixed intervals and finding out the average of these difference.