Recovery Of Process Heat

Since it is quite clear that never again will energy be as inexpensive as in the past, it therefore is necessary to conserve this natural resource—to recover more of the process heat that currently is dissipated to waterways and atmosphere. Some of this heat can be recovered with the aid of high-efficiency heat exchangers, which can operate economically with a close temperature approach at relatively low pumping powers. One type of unit that is particularly suited for this duty is the APV Paraflow plate heat exchanger. For many applications, this equipment can transfer heat with almost true countercurrent flow coupled with high coefficients to provide efficient and inexpensive heat transfer.

Unfortunately, the plate heat exchanger has been considered by many chemical engineers to be suitable only for hygienic heat-transfer duties. This, of course, is not so. Nowadays, many more plate type units are sold for chemical and industrial use than are sold for hygienic applications. A further mistake is the claim that the plate heat exchanger can be used only for duties when the volumetric flows of the two fluids are similar. Again, this is not true, although it must be stated that the plate heat exchanger is at its most efficient when flows are similar.

Basic Considerations

Since plate and tubular heat exchangers are the most widely used types of heat-transfer equipment, it is well to draw a brief comparison of their respective heat-recovery capabilities for the energy-conscious plant manager.

While the plate heat exchanger does have mechanical limitations with regard to withstanding high operating pressures above 300 psig, it is more efficient thermally than shell and tube units, especially for liquid—liquid duties. In many waste heat recovery applications, however, both pressure and temperature generally are moderate and the plate-type unit is an excellent choice since its thermal performance advantage becomes very significant for low-temperature approach duties. Higher overall-heat transfer coefficients are obtained with the plate unit for a similar loss of pressure because the shell side of a tubular basically is a poor design from a thermal point of view. Pressure drop is used without much benefit in heat transfer on the shell side due to the flow reversing direction after each cross pass. In addition, even in a well-designed tubular heat exchanger, large areas of tubes are partially bypassed by liquid and areas of low heat transfer thus are created. Conversely, bypassing of the heat transfer area is far less of a problem in a plate unit. The pressure loss is used more efficiently in producing heat transfer since the fluid flows at low velocity but with high turbulence in thin streams between the plates.

For most duties, the fluids also have to make fewer passes across the plates than would be required either through tubes or in passes across the shell. In many cases, the plate heat exchanger can carry out the duty with one pass for both fluids. Since there are fewer passes, less pressure is lost as a result of entrance and exit losses and the pressure is used more effectively.

A further advantage of the plate heat exchanger is that the effective mean temperature difference usually is higher than with the tubular. Since in multipass arrangements the tubular is always a mixture of cross and contraflow, substantial correction factors have to be applied to the log mean temperature difference. In the plate unit for applications where both fluids take the same number of passes through the exchanger, the LMTD correction factor approaches unity. This is particularly important when a close or even relatively close temperature approach is required.

Thermal Performance Data

Although the plate heat exchanger now is widely used throughout industry, precise thermal performance characteristics are proprietary and thus unavailable. It is possible, however, to size a unit approximately for turbulent flow liquid—liquid duties by use of generalized correlations that apply to a typical plate heat exchanger. The basis of this method is to calculate the heat-exchanger area required for a given duty by assuming that all the available pressure loss is consumed and that any size unit is available to provide this surface area.

For a typical plate heat exchanger, the heat transfer can be predicted in turbulent flow by the following equation

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