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Recuperator use – condensers and evaporators and boilers – fluids do not make contactwith each other. They exchange heat through walls of heat exchanger.
Regenerator use – preheating air in power station combustion using the exhaust gases.Rotating mesh or frame which alternately heats and cools, taking heat from one side tothe other.
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These are the simplest type to analyse, because we can estimate reasonably well thetemperature profile and hence the heat transfer coefficient.
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Energy balance is the overall achievement of the heat exchanger – it is the reason for it.The calculation depends however on the performance of the heat exchanger, which itdoes not control. The calculation of the heat exchanger performance is down to heattransfer.
Theory tells us how to get heat transfer across the surfaces. Complicated by thestreamwise gradients of temperature.
TA and TB are the temperatures of fluid A and B in the separated channels at a point in theheat exchanger.
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The LMTD formula is derived from consideration of elemental heat exchange. Theinterested student can refer to Rogers and Mayhew. Not necessary to know how to deriveit for this course.
All you need is the end temperature differences in the heat exchanger.
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This is what the LMTD is used for, with a known overall heat transfer coefficient, U.
U must be determined by calculation or from tabulated data from text book.
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Note: The parallel arrangement has the lower LMTD – therefore needs larger area forsame Q. This is one of the advantages of counter flow.
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i.e. the heat transfer coefficient may vary with fluid temperatures.
In a shell and tube heat exchanger, with the tube carrying hot fluid and the shell coldfluid, the conduction term can usually be neglected if the thickness of the wall is smalland the conductivity high.
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i.e. if temperatures are not known, iterative calculation is required.
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The shell flow passes over the tubes in cross flow several times. This example from theboiler room shows the tube side cap separating in and out-going tube fluid.
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Figure shows a matrix with cross flowing fluids, the resulting temperature profiles arecomplex.
The LMTD method relies on prior experimental data to cope with this.
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