this resulted in units of much lighter weight than could be obtained with tubular constructions . the growth of the aircraft industry brought even greater emphasis to the need for lightweight compact heat exchangers . during the 1930&apos;s , the secondary surface plate-and-corrugation construction became established for aero-engine radiators , using dip-soldered copper . the air and engine-coolant passages were separated by flat plates . the air passages were packed with corrugated foil bonded to the primary plates to provide the necessary surface area for heat transfer . the narrower coolant passages were also packed with foil , chiefly to provide sufficient support for the flat plates to withstand the coolant pressure loadings . the introduction of the aluminium alloy dip-brazing process in the early 1940&apos;s was quickly taken up for aircraft heat exchangers and led to substantial weight reductions as compared with copper construction . this development coincided with the introduction of pressurized aircraft cabins and the demand for air-to-air cabin coolers . although in this case the heat transfer coefficients on the two sides of the heat exchanger were of comparable magnitude , the use of secondary surface was still attractive , since the greater part of the surface area could be made up of fins only 0.006 in ( 0.15 mm ) thick . furthermore , developments in the detail form of the fins made possible a reduction in the total surface area required as compared with the use of smooth continuous passages for the same thermal duty and pressure losses . the properties of compact form , low weight , and design flexibility thus developed found ready application on a much larger scale with the introduction of tonnage scale air separation plants . 2.1 . methods of construction . the basic method of construction is both simple and extremely flexible . figure 3 illustrates the arrangement of a single passage . this can be extended in length and width up to the limit of manufacturing equipment available . the corrugation is machine-formed , thus ensuring a high standard of uniformity in height and fin pitch . a number of such passages may be combined to give either a cross or a countercurrent flow formation , as shown in figures 4 and 5 . the size and type of corrugation may be varied for each stream to suit the operating requirements and to provide a reasonable layout with minimum block volume and weight . typical corrugations are shown in figure 6 . the flow patterns may be further developed to provide multi-pass or multi-stream arrangements by the inclusion of suitable internal seals and distributors and the fitting of external header tanks , as indicated in figures 7-10 . with the simple cross-flow layout in figure 7 , the corrugations extend throughout the full length of each set of passages , and no internal distributors are required . this construction is appropriate when the temperature range in each stream does not exceed about one-half of the difference between the warm and cold inlet stream temperatures or , more generally , when the effective mean temperature difference in cross-flow is not significantly below the logarithmic mean temperature difference for countercurrent flow . on low temperature plants , this construction is sometimes useful for liquefiers , where the temperature changes little on the condensing side , and where a large throughput of low pressure gas as the warming stream calls for a large cross-section and short passage length . for higher duties , with temperature ranges in both streams up to 80 per cent or 90 per cent of the inlet temperature difference it is sometimes advantageous to use a multi-pass cross-countercurrent arrangement as shown in figure 8 . stream A flows straight through , while stream B is guided by means of internal seals and external tanks to make the required number of passes . the unit may thus be considered as comprising several cross-flow sections , assembled in counter-formation , such that the effective mean temperature difference approaches much more closely to countercurrent than to cross-flow conditions . this type of construction is used for gas-gas and gas-liquid applications . when very high thermal efficiencies of , say , 95-98 per cent are required , a pure countercurrent formation is invariably adopted . typical layouts are shown in figures 9 and 10 . the choice of headering is governed by several considerations , such as the operating pressures , the number of separate streams involved , and whether or not reversing duty is included . figure 9 shows an arrangement suitable for two-stream steady duty , in which stream A is at comparatively low pressure . a more general solution is shown in figure 10 . this is used if streams A and B are reversed periodically , the geometry of these streams being symmetrical to maintain constant flow characteristics . additional steady streams may be included as C , D , or E to suit requirements . this type of arrangement is also used when dealing with high pressure streams in all channels . in all countercurrent flow units , suitable distributors must be provided in the end regions , such that the flow of each stream is spread uniformly across the whole width of each layer throughout the length of the main zone . this problem is of great importance not only to the proper performance of the heat exchanger but also on manufacturing and mechanical strength considerations . further details are given in later sections of this review . the possibility of varying the geometry and type of corrugation in different layers has already been mentioned . for industrial applications , the height of corrugation normally lies in the range 0.15-0.47 in ( 3.8-12 mm approx ) , the thickness varies from 0.008 to 0.015 in ( 0.2 to 0.38 mm approx ) , and the fin pitching varies from 10 to 15 or 18 fins per inch ( 3.9 to 5.9 or 7.1 fins per centimetre ) depending upon the type of corrugation . the resulting total surface areas lie from about 300 to 450 square feet per cubic foot of block volume ( 1,000 to 1,500 square metres per cubic metre ) . the free cross-sectional area ratios lie between 0.70 and 0.80 . both the surface area and the free cross-sectional area must be suitably divided between the various streams . for instance , in a two-stream gas-gas heat exchanger , each stream may have a surface area of about 200 square feet per cubic foot of total block volume , with a free cross-section ratio of about 0.4 , while in a gas-liquid heat exchanger , the gas stream would have a surface area of about 300 square feet per cubic foot of total block volume , with a free cross-section ratio of about 0.6 . in general , the taller corrugations are used for gas streams , while those at 0.15 in to 0.25 in high are used for liquid streams and in condenser-reboilers . the use of plain continuous corrugations is chiefly limited to condenser-reboiler use , or to cases where the free passage of contaminating solids is desired . for most other applications , a reduction in the total surface area and block volume required can be achieved by the use of more complex types of corrugation such as the herringbone and multi-entry patterns shown in figure 6 . these are discussed in more detail in later sections . the manufacture of the heat exchangers involves several distinct stages , beginning with the assembly and dip-brazing of individual blocks , i.e tube plates , corrugations , and edge-seals only . each block is thoroughly cleaned after brazing and subjected to preliminary leak tests before the fitting of header tanks by argon-arc welding . the block is then tested hydraulically to its full design test pressure on each stream separately . in the case of multiple assemblies , each block may also be submitted to flow tests on each stream prior to selective assembly to ensure uniformly balanced flow distribution throughout the whole assembly . figure 11 shows a typical two-stream countercurrent block during manufacture , with header tanks fitted to one stream only . On completion , this type of block would be suitable for either steady or reversing operation . with existing brazing equipment , individual blocks are made up to 9 ft ( 2.75 m ) long with an overall cross-section of 17 in x 21 in ( 0.43 m x 0.53 m ) to give a total block volume of about 22 ft3 ( 0.62 m3 ) . by means of appropriate manifolding , a number of such blocks may be assembled together either in series or in parallel , or a combination of both , according to requirements . two blocks are shown as a series arrangement in figure 12 and sixteen blocks in parallel in figure 13 . a more complex arrangement is shown in figure 14 . this assembly contains three separate heat exchangers through which one stream is common on the low pressure side , while two of the high pressure streams are in parallel and the third high pressure stream in series . this complete assembly is welded up to form a single unit with flanged main connections and vents only . for multiple arrays it is generally preferred to assemble together a number of blocks and to weld up all the interconnecting pipework and manifolding , so as to limit the number of flanged joints on the plant . if aluminium pipework is used the flanged joints may be eliminated completely and the heat exchangers attached to the main pipework by site-welding . for very large assemblies , it may be necessary to split the design into several separate sub-assemblies suitable for transport and installation , and to connect these together at the erection stage either by site-welding or by flanged joints . 2.2 . mechanical design . mechanical design aspects must always be considered from the outset , since these may well influence the general layout and internal construction and so affect the basis for performance assessment . for the operating pressures and conditions required , two major factors are the internal pressure loading on the corrugations and associated brazed joints , and those on the header tanks , together with any external pipework loadings . for low-to-medium steady operating pressures in the range 0-100 lb &amp;sol; in2 ( gauge ) ( 1-8 kg &amp;sol; cm2 ( abs ) ) , the mechanical design does not generally present any serious problems . for higher pressures and for reversing duty the mechanical design requirements become of increasing importance , particularly in relation to the size and arrangement of header tanks . the internal plate and corrugation construction is adequate for static test pressures at room temperature of 600-1,000 lb &amp;sol; in2 ( gauge ) ( 42-70 kg &amp;sol; cm2 ( abs ) ) , depending upon the type and thickness of corrugation . for low temperature applications the corresponding rated maximum operating pressures would be 250-450 lb &amp;sol; in2 ( gauge ) for steady conditions , or 125-225 lb &amp;sol; in2 ( gauge ) for reversing applications . for such pressures , however , it is necessary to limit the span of the header tanks in order to avoid excessive peripheral loadings in the plane of attachment to the block . this means either that the block cross-section must be kept small , or that small tanks , such as those shown in figure 10 , must be fitted . the latter alternative is particularly suitable for reversing applications and for large scale steady operation . internal distributors are necessary to spread the flow across the whole width of each passage , and these must be adequate to withstand the internal pressure loadings . when considering the installation of a heat exchanger assembly for low temperature service , due consideration must be given to thermal contractions both in normal service and under any abnormal circumstances which might arise . the method of mounting and the external pipework must be sufficiently flexible to allow for such movements without imposing excessive loads on to the assembly . this precaution is , of course , common to all low temperature installations . in normal operation relative movements within the assembly should not generally provide any serious problem since the balancing of flows which is so important on performance considerations ensures that the temperature patterns , and hence the contraction effects , are also uniform across any section of the assembly . nevertheless , an adequate measure of flexibility is maintained between parallel assemblies to allow for any residual unbalance effects and for temporary effects which might arise during transient or abnormal operating conditions . 2.3 . performance . performance design calculations for any type of heat exchanger depend upon the process requirements , i.e flow rates , temperatures , and pressures of each stream , and upon the relevant heat transfer and friction factors for the type and arrangement of surface considered . the latter must generally be determined experimentally in the first instance . the broad subjects of heat transfer and heat exchanger design are well covered by McAdams and Jakob , while Kays and London give the results of extensive researches and experiments particularly related to compact forms of heat exchanger including various types of secondary surface construction . 