Aluminum alloy plate fin heat exchangers have been used in the aircraft industry for 50 years and in cryogenics and chemical plant for 35 years. They are also used in railway engines and motor cars.
Stainless steel plate fins have been used in aircraft for 30 years and are now becoming established in chemical plant.
The concept is shown in Figure 1 . Corrugated metal fins are placed between flat plates. The structure is joined together by brazing (see later). The fins have the dual purpose of holding the plates together, thus containing pressure, and of forming a secondary (fin) surface for heat transfer. At the edges of the plates are bars, which contain each fluid within the space between adjacent plates.
The heights of corrugations and bars may vary between plates, as shown. For a liquid stream we can use a low height corrugation, matching high heat transfer coefficient with lesser surface area while for a low pressure stream we can use a high corrugation height, matching low coefficient with higher surface area but also giving larger through area to achieve lower pressure drop. An industrial unit contains about 1000 m2 of surface per cubic meter.
Aluminum units can be made up to 1.2 m × 1.2 m cross section and 6.2 m long.
Stainless steel units can be made up to 500 mm × 500 mm cross section × 1500 mm length.
Plain corrugation is the basic form and is used normally for low pressure drop streams.
Perforated corrugation shows a slight increase in performance over plain corrugation, but this is reduced by the loss of area due to perforation. The main use is to permit migration of fluid across fin channels, usually in boiling duties.
Serrated corrugation is made by cutting the fins every 3.2 mm and displacing the second fin to a point half way between the preceding fins. This gives a dramatic increase in heat transfer.
Herringbone corrugation is made by displacing the fins sideways every 9.5 mm to give a zig-zag path. Performance is intermediate between the plain and serrated forms. The friction factor continues to fall at high Reynolds numbers, unlike the serrated, showing advantages at higher velocities and pressures.
The designer can, therefore, vary fin heights, fin pitch and fin thickness together with four standard fin types giving great versatility of design.
Plate-fin units are normally arranged for counterflow heat exchange. Cross flow units are used for vehicle radiators and cross counterflow is used for liquid subcoolers.
Figure 4 shows the typical layer arrangements for a three-stream heat exchanger. A two-stream exchanger can be constructed by using the first of the arrangements shown for the hot stream, alternating with the second arrangement shown for the cold stream.
In this way, the heat exchanger is built up to the appropriate height. It is then brazed together; headers and pipework are welded over the inlet and outlet parts of each stream to give a finished unit. Layers are normally arranged with alternating hot and cold streams, as below:
An alternative system, called double banking, is sometimes used, as below:
A three-stream unit is made by using all the fin arrangements shown in Figure 4.
Figure 5 gives a drawing of a five-stream heat exchanger. This has "cut away" sections to show components, one of which shows the method used for stream distribution using an end entry inlet. When several cold streams are used in one unit the layers of each stream should be evenly disposed across the stack height.
Figure 6 shows a method by which a stream which occupies layers at one end of a block can be taken out at part length and replaced by a second stream at the other end of the block. This can be repeated and in ethylene exchangers up to 5 streams occupy the same layers for different lengths.
A variation of the system allows part of a stream to be taken out (or added) at part length. The use of all these features permits a high degree of process intensification in a single block. The maximum number of streams in one block, so far, is ten, of which three had partway take-offs or additions.
Aluminum units use material AS3003 in the exchanger block. Braze material is AS3003 + silicon. Plates are purchased with a thin film of braze metal on both sides. The unit is built and placed in a vacuum furnace. The braze takes place under vacuum and at a temperature of 580°C. The parts of the block are then firmly attached together.
AS5083 is used for headers and piping below 65°C. Above this temperature AS5454 is used.
Stainless steel units are made of AISI type 321. Braze material is essentially nickel and can be applied to the plates by spraying. Brazing takes place under vacuum at temperatures up to 1050°C.
Aluminum units operate with design pressures up to 100 bars and at temperatures from absolute zero to 65°C. Above 65°C a change of header material will allow operation to 120°C with reduced design pressures. Stainless steel units are currently limited to 50 bars design pressure and temperatures up to 750°C.