Quick Answer: Why Is Lmtd Needed?

How do you calculate transfer units?

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 is overall heat transfer coefficient?

The overall heat transfer coefficient, or U-value, refers to how well heat is conducted through over a series of resistant mediums. Its units are the W/(m2°C) [Btu/(hr-ft2°F)].

What is effectiveness NTU method?

The effectiveness-NTU method is very useful for all the flow arrangements (besides parallel flow and counterflow ones) because the effectiveness of all other types must be obtained by a numerical solution of the partial differential equations and there is no analytical equation for LMTD or effectiveness, but as a …

What is Amtd?

AMTD = Arithmetic Mean Temperature Difference (oF, oC) tpi = primary inlet temperature (oF, oC) tpo = primary outlet temperature (oF, oC) tsi = secondary inlet temperature (oF, oC) tso = secondary outlet temperature (oF, oC)

Which type of fluid flow arrangement heat exchanger has higher value of lmtd?

Most classical heat transfer textbooks state that under an identical situation a counter flow heat exchanger has a higher value of LMTD than a parallel flow one. Consequently, for exactly the same inlet and outlet hot and cold fluid temperatures, the counter flow heat exchanger requires a smaller heat transfer area.

What advantage does the effectiveness NTU method have over the lmtd method?

Therefore, the effectiveness-NTU and LMTD methods are equivalent. An advantage of the effectiveness-NTU method is its ability to predict the outlet temperatures without resorting to a numerical iterative solution of a system of nonlinear equations.

Which one is having the highest value of fouling factor?

Which one is having the highest value of fouling factor? Explanation: Fouling factor for engine exhaust is 0.002 while that of industrial liquids, fuel oil and turbine exhaust are 0.0002, 0.0010 and 0.0002 respectively.

What are the assumptions made during lmtd analysis?

One of the fundamental assumptions adopted in the derivation of the LMTD method is that the fluid specific heats are constant and the fluid temperature variations only result from heat exchange.

What is unit of lmtd?

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.

How do you calculate lmtd correction factor?

Hence a correction factor ‘F’ must be introduced in the general heat equation and the equation is modified as Q = UA (F) LMTD. This correction factor ‘F’ depends on the number of shells of the heat exchanger and on the terminal temperatures of the heat exchanger.

What is effective heat exchanger?

Effectiveness of a heat exchanger. The effectiveness (ϵ) of a heat exchanger is defined as the ratio of the actual heat transfer to the maximum possible heat transfer.

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. … The LMTD is a logarithmic average of the temperature difference between the hot and cold feeds .

Why is counter flow better than parallel flow?

The exchanger is performing at its best when the outlet temperatures are equal. 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.

Which mode is more dominant in heat conduction?

ConvectionConvection is usually the dominant form of heat transfer in liquids and gases. Although sometimes discussed as a third method of heat transfer, convection is usually used to describe the combined effects of heat conduction within the fluid (diffusion) and heat transference by bulk fluid flow streaming.

How do you calculate log mean?

Divide the difference of x and y by the difference of ln x and ln y. Make sure that x and y are in the same order in the quotient and denominator of the fraction. In the example problem, 90/0.38 = 236.84. The logarithmic mean is 236.84.

How do you find the overall heat transfer coefficient?

Composition= the overall heat transfer coefficient (W/(m2•K))= the contact area for each fluid side (m2) (with and expressing either surface)= the thermal conductivity of the material (W/(m·K))= the individual convection heat transfer coefficient for each fluid (W/(m2•K))= the wall thickness (m).

Why is NTU more accurate than lmtd?

The NTU method does not rely on using an numerical iterative solution to find the outlet temperatures of the heat exchanger, which would be the case if the LMTD method were used to find two outlet temperatures. Thus, the NTU would save much calculation time in this case.

What are the limitations of lmtd method?

Assumptions and limitations 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. For a condenser, the hot fluid inlet temperature is then equivalent to the hot fluid exit temperature.

What is lmtd formula?

Log Mean Temperature Difference (LMTD) is an indicator of the average temperature difference between the hot and cold fluids in a heat exchanger. It is calculated using the following equation – ΔT1 → the temperature difference between hot and cold fluids at one end of the heat exchanger.

What is effectiveness of fin?

Efficiency of fin (ηfin): The efficiency of a fin is defined as the ratio of the actual heat transfer from the fin to that the heat that would be dissipated if whole surface of the fin is maintained at base temperature.

What is fouling factor?

The fouling factor represents the theoretical resistance to heat flow due to a build-up of a layer of dirt or other fouling substance on the tube surfaces of the heat exchanger, but they are often overstated by the end user in an attempt to minimise the frequency of cleaning.