LMTD and NTU Methods

Introduction   

The “LMTD” is a logarithmic method that is utilized in order to find out the mean temperature difference. This is executed in order to evaluate the “temperature driving force” existing in the process of heat transfer within flow systems. It is viewed mostly in the heat exchangers. On the other hand, the “NTU” is recognized as a unit that is utilized in order to find out the amount of turbidity of fluids. Moreover, this method is also used in order to determine the existence of “suspended particles in water”. The turbidity is examined from the visual appearance of the dirt particles existing in the water. 

Significance of LMTD method

The rate of the heat transferred in “double pipe heat exchangers” is evaluated with the utilization of the LMTD method. In this regard, a specific formula is designed in order to execute the calculation. The formula is identified as “Q=UALMTD”, where U depicts the coefficient of heat transfer, A depicts the surface area of heat transfer, and LMTD stands for “Mean Temperature Difference”. The temperature difference is measured between a hot, as well as a cold fluid that passes through each of the ends of the Heat Exchanger pipe. The factors considered in this calculation are inferred to be the nature of the heat exchanger, as well as the fluid temperatures with regard to the four ends of the heat exchanger.

The four ends of the heat exchanger are the “Inlet temperature of the hot fluid”, “Inlet temperature of the cold fluid”, “Outlet temperature of the hot fluid”, and “Outlet temperature of the hot fluid”. In order to execute the calculation, the averages of the differences between the temperatures of cold, as well as hot feeds are considered. However, there are some limitations found in this method. In the case of “specific heat changes”, this method is not found to be accurate. This method is used for liquids in a steady-state and hence is not applicable in the case of dynamic evaluation. There is no change in the passes during the transfer of heat. Moreover, the avoidance of kinetic energy, as well as potential energy may be a barrier in providing an accurate solution.

Significance of NTU method

The “NTU method” is utilized in order to optimize the number of transferred units in the process of exchange of heat. This method is found to be applied in the case of insufficient data gathered from the calculation of LMTD.  In other words, it is stated that this method is helpful in the case of the unavailability of specific temperatures. This method is found to be effective in arranging the flow of liquid in addition to the existing parallel, as well as counter flow. The formula utilized in this calculation is depicted as “NTU=UA/mcp”, where ‘NTU’ stands for “Number of transfer units”, ‘U’ is the coefficient of heat transfer, ‘A’ is the “surface area of heat transfer”, and ‘mcp’ depicts the rate of fluid flow, as well as the capacity of heat.

Moreover, this method is also found to be utilized in the evaluation of the rate of mass exchange. This is a considerable approach towards building an association between the performances of heat, as well as mass transfer. The mass transfer is identified to be a complicated process as compared to the heat transfer method. There are many segments where this method is applicable in an effective way. In this regard, the “Parallel flow heat exchanger”, as well as the “Counterflow heat exchanger” are the perfect examples where the NTU method is applied.

Distinguishing factors between LMTD and NTU methods

In accordance with the LMTD method, it is found that this method is considered to be convenient for the determination of the overall amount of heat transfer. This is done on the basis of the obtained inlet, as well as outlet temperatures of the concerned fluid. On the other hand, in the case of the NTU method, it is optimized that this method is found to be convenient for predicting the temperature of the outlet of the concerned fluid. This is effective in the cases where the coefficient of heat transfer, as well as the inlet temperature of the fluid, is provided. In this regard, it is stated that the NTU method is considered to be more acceptable as compared to the LMTD method. Moreover, it is concluded from the study that the calculation involved in the determination of the concerned factors of heat exchange is found to be simpler as compared to the LMTD method. Furthermore, in the NTU method, a “large variety of heat exchange configurations is taken into consideration”. Hence, this method is effective in providing more authentic, as well as accurate results.  

Conclusion  

On the basis of the study, it is concluded that the LMTD is a simple, as well as the ideal approach in the course of analyzing the quantity of heat exchanged by the exchanger. The calculations involved in this measurement are considered to be simple to be executed at any phase of research. On the other hand, in the process of LMTD, whenever the data is insufficient to execute the calculation, the NTU method acts as a savior and completes the steps of the calculation. Hence, it is concluded that the data obtained in NTU is also helpful in the LMTD method.