HEAT EXCHANGERS AND LMTD

HEAT EXCHANGERS

In day to day life the use of heat exchangers is unavoidable. Not only in domestic but also commercially it has high viability. In air conditioning, refrigeration, chemical and production plants etc it is used since decades.


When two fluid are at different temperature then the appliance known as heat exchangers are used to facilitate heat transfer in between these two fluid. In heat exchanger there is a wall which separates the two fluids. Both conduction and convection of heat transfer phenomenon occur in the appliance. Conduction occurs through the wall separating the two fluids and convection occurs in fluid layer.
The construction of heat exchanger is very simple. A very simple heat exchanger consists of two concentric tubes in which hot fluid is allowed to flow through inner tube and cold fluid flows in outer concentric tube.

TYPES OF HEAT EXCHANGERS ON BASIS OF FLOW OF FLUID:-


  • Parallel flow heat exchanger
  • Counter flow heat exchanger
  • Cross flow heat exchanger
Depending on the mode of flow of fluid or we can say depending on the direction of hot and cold fluid type of heat exchangers are characterized. In parallel flow heat exchangers the hot and cold fluid in the concentric tube move in same direction. In counter flow heat exchangers the case is reverse, here the fluid enters from opposite ends and the flow is is in opposite direction. Both the hot and cold fluid moves in opposite direction. Such heat exchangers provides greater heat transfer per unit volume.

Other type of its classification is cross flow heat exchanger in which fluid flow is perpendicular to each other. A best heat exchanger is one which provides maximum surface area for heat transfer and minimum resistance in the flow of fluid, otherwise the rate of heat transfer may reduce.


fig:-heat exchanger

Area density of heat exchanger is nothing but the ratio of heat transferring area of heat exchanger to the volume. Area density decides the compactness of heat exchangers. When the area density is greater than 750 m^2/m^3.


LOGARITHMIC MEAN TEMPERATURE DIFFERENCE (LMTD)


Transfer of heat from one fluid to another is given by Q = UA (LMTD). Where "U" is overall heat transfer coefficient. "A" is the area cross section and the mean temperature difference is indicated by LMTD.


Consider a parallel flow heat exchanger of length  "L" . The temperature distribution of cold and hot flow along the length is given by logarithmic mean temperature difference (LMTD).


                      LMTD = (ΔT1-ΔT2)/ln(T1/T2)


Where  ΔT1 and ΔT2 are temperature difference of fluid at entry and exit. The average temperature difference is ( ΔT1 + ΔT2)/2.


The overall heat transfer  "U" depends on the flow velocity and not on the direction of flow. The overall heat transfer coefficient across the flow in a heat exchanger is given by





where "hi" is convective heat transfer coefficient at inside the surface of wall, "ho" is heat transfer coefficient outside of wall surface of pipe and thickness be delta. "k" is coefficient of thermal conductivity of material.


NTU method of effectiveness of heat exchanger:-

Heat exchanger effectiveness is defined as ratio of actual heat transfer to maximum possible heat that can be transferred. 

E = actual heat transfer/maximum heat that can be transferred


Larger is the value of NTU, depicts greater the size of heat exchanger.


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