Rising energy costs have significantly increased the need for low cost, yet effective, heat exchangers since virtually every type of fuel consuming engine, power plant or industrial process gives off some recoverable heat capable of being converted to useful work. The cost of such exchangers has, however, in the past discouraged wide spread use of heat exchangers in certain applications. One well known type of low cost heat exchanger employs a plurality of stacked plates arranged to allow heat donative and heat recipient fluids to flow in heat exchange relationship on opposite sides of each plate. It has long been recognized that the efficiency of such primary surface heat exchangers is a direct function of the total surface area of the stacked plates and an inverse function of the wall thickness of the plates which separate the heat exchange fluids.
One technique for forming such heat exchanger plates, thus, includes forming a large number of corrugations or pleats in ductile sheet metal of relatively thin gauge. In order to prevent nesting of the plates when stacked, the corrugation pleats are given a wavy (or curvilinear) configuration in plan view. When thus constructed the pleat crests of one plate form at least some points of contact with the crests of the adjacent plates. An example of this type of corrugated heat exchanger plate is illustrated in U.S. Pat. No. 3,759,323, to Dawson et al.
Attempts to increase the heat transfer efficiency of corrugated plates of the type illustrated by U.S. Pat. No. 3,759,323, by metal gauge reduction and increased pleat density, have not always met with success. The structural rigidity of the corrugation pleats is decreased upon reduction in the gauge of metal forming the plate, and when such weakening is combined with an increase in the density of pleats, the chances of a flow passage becoming restricted or obstructed dramatically increases. In particular, weak walled, high density pleats are subject to mechanical distortion during the process of manufacture and are also subject to distortion and/or collapse from uneven temperature induced expansions and contractions.
In U.S. Pat. No. 3,892,119, it is noted that cost savings without reduced efficiency can be realized in the manufacture of heat exchangers formed of plates such as illustrated in U.S. Pat. No. 3,759,323 by increasing the height and number of pleats in each plate to permit reduction in the number of plates required for a given heat exchange capacity. An increase in the height of each pleat, however, has further aggravated the problem of undesired mechanical or temperature induced pleat wall distortions and has, up to the present, placed a practical limit on the efficiency which can be achieved by the use of primary surface heat exchangers employing pleated plates.