Question: What Are The Limitations Of Lmtd Method?

What is effectiveness of fin?

Effectiveness of fin (εfin): It is defined as the ratio of the actual heat transfer that takes place from the fin to the heat that would be dissipated from the same surface area without fin..

Can log mean temperature difference be negative?

According to the definition modeled above, LMTD cannot be negative.

What is NTU in heat exchanger?

NTU is defined as:[11.74]NTU=UACminwhere U is the overall heat transfer coefficient (kW/m2K), A is the heat transfer area of the heat exchanger (m2) and Cmin is the smallest thermal capacity of the two fluids (kW/K).

What are the drawbacks 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 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. …

How do you calculate log mean?

In mathematics, the logarithmic mean is a function of two non-negative numbers which is equal to their difference divided by the logarithm of their quotient. This calculation is applicable in engineering problems involving heat and mass transfer.

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.

Why is NTU more accurate than lmtd?

A more convenient method for predicting the outlet temperatures is the effectiveness- NTU method. This method can be derived from the LMTD method without introducing any additional assumptions. Therefore, the effectiveness-NTU and LMTD methods are equivalent.

What happens when lmtd is zero?

What is the physical significance when LMTD becomes zero in a heat exchanger? … A liquid can be heated, cooled or stabilized by transferring it through a heat exchanger. When the Log Mean Temperature Difference is zero there is no temperature exchange taking place.

What is difference between scaling and fouling?

When cooling systems operate, water with fouling factors flow through the system: Hard water causes scaling, whereas seawater can cause biofilm, or growth of micro and macro species. … As fouling increases, two things occur: First, the volume of water that can flow through the heat exchanger is reduced.

Which is more efficient parallel flow or counterflow?

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.

What is FT correction factor?

A ‘ft correction factor’ is defined as a ratio of the true mean temperature difference to the log-mean temperature difference (see Eq(2)). The ‘ft correction factor’ value must be greater than 0.75 for a heat exchanger to be feasible.

Why do we use correction factor in heat exchanger?

It is a measure that heat exchangers away from the ideal behavior of a flow heat exchanger that has the same terminal temperatures. Log Average temperature difference The correction factor F depends on the efficiency of the temperature P and the ratio of R heat capacity for a particular flow arrangement.

Why is lmtd used?

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?

by definition given above, LMTD for counter current flow = (60-50) / ln(60/50) = 10 / 0.1823 = 54.850C. For co-current heat exchanger, ΔT1 = TH1 – TC1 = 100 – 30 = 700C (At first end hot and cold fluids enter the heat exchanger.) (At the other end hot and cold fluids exit the heat exchanger.)

What type of flow has better heat transfer in a coil?

High flow turbulence between plates results in a greater transfer of heat and a decrease in pressure. 4.

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 lmtd and NTU?

The Number of Transfer Units (NTU) Method is used to calculate the rate of heat transfer in heat exchangers (especially counter current exchangers) when there is insufficient information to calculate the Log-Mean Temperature Difference (LMTD).

What does NTU mean?

Nephelometric Turbidity unitNTU stands for Nephelometric Turbidity unit, i.e. the unit used to measure the turbidity of a fluid or the presence of suspended particles in water. The higher the concentration of suspended solids in the water is, the dirtier it looks and the higher the turbidity is.