Question: Why Lmtd Is Used In Heat Exchanger?

What are the limitations of lmtd method?

Cause LMTD method can only be used in design of heat exchanger, when all terminal temperature are known or easily determined.

The difficulty arises if temperature of fluid leaving are not known or possible to determined..

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 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).

How do you calculate effective heat exchanger?

If one of the fluids is undergoing a change of phase throughout the entire heat exchanger and at a constant temperature (i.e. heat capacity is infinite), the thermal efficiency is simply Eff=1-exp(-U.A/Cmin), where Cmin is the heat capacity (m x cp) for the single phase fluid, U-overall heat transfer coeffcient and A …

What is the most efficient heat exchanger?

Plate exchanger is the most efficient due to turbulent flow on both sides. High heat-transfer coefficient and high turbulence due to even flow distribution are important. However, a plate heat exchanger regenerator is restricted to low viscosities. With high viscosities, a special tubular may be required.

Why is lmtd needed?

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 use of the LMTD arises, straightforwardly, from the analysis of a heat exchanger with constant flow rate and fluid thermal properties.

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)

What happens when lmtd is zero?

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 parallel flow heat exchanger?

In heat exchanger. This flow arrangement is called parallel flow. Heat is transferred from the warm fluid through the wall of the inner tube (the so-called heating surface) to the cold fluid. A heat exchanger can also be operated in counterflow, in which the two fluids flow in parallel but opposite directions.

How are heat exchangers classified?

Heat exchangers are classified according to transfer processes into indirect- and direct- contact types. FIGURE 1.2 (a) Classification according to process function; (b) classification of condensers; (c) classification of liquid-to-vapor phase-change exchangers. exchanger as a regenerator.

What is effectiveness NTU method?

called the effectiveness–NTU method, which greatly simplified. heat exchanger analysis. ❑ This method is based on a dimensionless parameter called the. heat transfer effectiveness, defined as. The Effectiveness – NTU Method.

Can log mean temperature difference be negative?

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

How do you calculate lmtd in heat exchanger?

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.)

When or why NTU E method is used rather than lmtd?

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 is effectiveness in 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.