The flow pattern through a heat exchanger affects the required heat exchanger surface. A counterflow heat exchanger needs the lowest heat transfer surface area. It gives a higher value for log mean temperature difference than either a parallel flow heat exchanger or a crossflow heat exchanger.
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A heat exchanger can have several different flow patterns. Counterflow, parallel flow, and crossflow are common heat exchanger types. A counterflow heat exchanger is the most efficient flow pattern of the three. It leads to the lowest required heat exchanger surface area because the log mean temperature drop is the highest for a counterflow heat exchanger
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The Counterflow Heat Exchanger
A counterflow heat exchanger has the hot fluid entering at one end of the heat exchanger flow path and the cold fluid entering at the other end of the flow path. Counter flow is the most common type of liquid-liquid heat exchanger, because it is the most efficient. A double pipe heat exchanger is usually operated as a counter flow heat exchanger, as shown in the diagram at the left. A picture of a double pipe heat exchanger is shown at the right. The flow pattern in a shell and tube heat exchanger with a single tube pass will be approximately counterflow if it is long in comparison with its diameter. Because of the baffles and the need to distribute the flow of the shell side fluid over the cross-section of the shell, the flow is not as close to counterflow in a shell and tube heat exchanger as it is in a double pipe heat exchanger. The bottom diagram on the left shows approximately counter flow in a straight tube, one tube pass shell, and tube heat exchanger.
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The Parallel Flow Heat Exchanger
A double pipe heat exchanger can be operated in parallel flow mode as shown in the diagram at the left. Similarly a shell and tube heat exchanger can be operated in approximately parallel flow by having both fluids enter at one end and exit at the other end. With parallel flow the temperature difference between the two fluids is large at the entrance end, but it becomes small at the exit end as the two fluid temperatures approach each other. The overall measure of heat transfer driving force, the log mean temperature difference is greater for counter flow, so the heat exchanger surface area requirement will be larger than for a counter flow heat exchanger with the same inlet and outlet temperatures for the hot and the cold fluid.
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The Crossflow Heat Exchanger
A car radiator and an air conditioner evaporator coil are examples of crossflow heat exchangers. In both cases heat transfer is taking place between a liquid flowing inside a tube or tubes and air flowing past the tubes. With a car radiator, the hot water in the tubes is being cooled by air flowing through the radiator between the tubes. With an air conditioner evaporator coil, air flowing past the evaporator coils is cooled by the cold refrigerant flowing inside the tube(s) of the coil. Crossflow heat exchangers are typically used for heat transfer between a gas and a liquid as in these two examples.
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Hybrid Flow Heat Exchangers
Shell and tube heat exchangers with two or four tube passes are common. They will have a hybrid flow pattern, where it might be approximately concurrent flow in some part of the heat exchanger and approximately parallel flow or crossflow in another part. Examples are the straight tube heat exchanger with 2 tube passes shown at the left and the two pass condenser shown at the right.
3. AC Evaporator Coil: http://www.residential.carrier.com/products/coils/evaporator/updowna.shtml
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About the Author
Dr. Harlan Bengtson is a registered professional engineer with 30 years of university teaching experience in engineering science and civil engineering. He holds a PhD in Chemical Engineering
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Types of Shell and Tube Heat Exchangers - This article provides information about the general configuration of a shell and tube heat exchanger and terminology used with it. U-tube, straight tube, and multipass shell and tube heat exchangers are discussed.
Preliminary Heat Exchanger Design Example - This article provides an example calculation of required heat transfer surface area based on a prescribed heat transfer rate, inlet and outlet fluid temperatures and estimated overall heat transfer coefficient.