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1、Unit 19 Types of Heat ExchangersHeat exchangers are equipment primarily for transferring heat between hot and cold streams.They have separate passages for the two streams and operate continuously.The most versatile and widely used exchangers are the shell-and-tube types but various plate and other t
2、ypes are valuable and economically competitive or superior in some applications.These other types will be discussed briefly but most of the space following will be devoted to the shell-and-tube types primarily because of their importance but also because they are most completely documented in the li
3、terature.Thus they can be designed with a degree of confidence to fit into a process.The other types are largely proprietary and for the most part must be process designed by their manufacturers.Plate-and-Frame Exchangers Plate-and-frame exchangers are assemblies of pressed corrugated plates on a fr
4、ame. Gaskets in grooves around the periphery contain the fluids and direct the flows into and out of the spaces between the plates.Close spacing and the presence of the corrugations result in high coefficients on both sides several times those of shell-and- tube equipment and fouling factors are low
5、.he accessibility of the heat exchange surface for cleaning makes them particularly suitable for fouling services and where a high degree of sanitation is required as in food and pharmaceutical processing.Operating pressures and temperatures are limited by the natures of the available gasketing mate
6、rials with usual maxima of 300 psig and 400 F.Since plate-and-frame exchangers are made by comparatively few concerns most process design information about them is proprietary but may be made available to serious engineers.Friction factors and heat transfer coefficients vary with the plate spacing a
7、nd the kinds of corrugations.Pumping costs per unit of heat transfer are said to be lower than for shell-and-tube equipment.1n stainless steel construction the plate-and-frame construction cot is 50%-70% that of the shell-and-tube.Spiral Heat Exchangers In spiral heat exchangers the hot fluid enters
8、 at the center of the spiral element and flows to the periphery; flow of the cold liquid is countercurrent entering at the periphery and leaving at the center.Heat transfer coefficients are high on both sides and there is no correction to the log mean temperature difference because of the true count
9、ercurrentaction. These factors may lead to surface requirements 20% or so less than those of shell-and-tube exchangers. Spiral types generally may be superior with highly viscous fluids at moderate pressures.Compact (Plate-Fin) Exchangers Compact exchangers are used primarily for gas service.Typical
10、ly they have surfaces of the order of 1200 m2 /m3 corrugation height 3.8-11.8 mm corrugation thickness 0.2-0.6 mm and fin density 230-700 fins/m.The large extended surface permits about four times the heat transfer rate per unit volume that can be achieved with shell-and-tube construction.Units have
11、 been designed for pressiIres up to 80 atm or so.The close spacings militate against fouling service.Commercially compact exchangers are used in cryogenic services and also for heat recovery at high temperatures in connection with gas turbines.For mobile units as in motor vehicles compact exchangers
12、 have the great merits of compactness and light weight.Any kind of arrangement of cross and countercurrent flows is feasible and three or more different streams can be accommodated in the same equipment.Pressure drop heat transfer relations and other aspects of design are well documented.Air Coolers
13、 In such equipment the process fluid flows through finned tubes and cooling air is blown across them with fans. The economics of application of air coolers favors services that allow 25-40 1 temperature difference between ambient air and process outlet.In the range above 10 Mbtu/l air coolers can be
14、 economically compettve with watercoolers when water of adequate quality is available in su Hicient amountDouble-Pipe Exchangers This kind of exchanger conssts of a central pipe supported withn a larger one by packng glands. The straight length is limited to a maximum of about 20 ft;otherwise the ce
15、nter pipe wi1l sag and cause poor distribution in the annulus.It is customary to operate with the high pressure high temperature high density and corrosive fluid in the inner pipe and the less demanding one in the annulus. The inner surface can be provide with scrapers as in dewaxing of oils or crys
16、tallization from solutions.External longitudinal fins in the annular space can be used to improve heat transfer with gases or viscous fluids.When greater heat transfer surfaces are needed several double-pipes can be stacked in any combination of series or parallel.Double-pipe exchangers have largely lost out to shell-and-tube units in recent years.They may be worth considering in these situations:1. When the shell-side coefficient is less than half that of the tube side;the
