Heat exchange – Casing or tank enclosed conduit assembly
Patent
1993-08-20
1996-02-27
Flanigan, Allen J.
Heat exchange
Casing or tank enclosed conduit assembly
165166, 29890042, F28F 314
Patent
active
054941008
DESCRIPTION:
BRIEF SUMMARY
The invention described herein addresses the field of heat exchanger and heat exchanger component engineering and construction.
Heat exchangers developed so far fall into two major categories: Tube heat exchangers and bonded or welded plate fin heat exchangers.
Heat exchangers of either type are manufactured by several manufacturers. Though satisfactory in operation, each design has certain major drawbacks limiting performance and reliability.
The efficiency of the tube heat exchanger is low. The design requires large areas of heat transfer surface to offset low heat transfer rates. In addition, tube heat exchangers are heavy, bulky and costly.
Bonded plate fin heat exchangers consist of stacked, shaped and ribbed plates arranged within top and bottom casing channels held together by tie-rods. Supply and return passages are created by stacking and bonding shaped plates. Such heat exchangers are compact and have good thermal performances. Their reliability, however, is a function of bonding and the temperature and pressure of the product carried. Also, other factors such as aging affect bond life.
Welded plate fin heat exchangers have the advantages and not the drawbacks of bonded plate fin heat exchangers, but are more expensive. Yet several design and manufacturing disadvantages remain, which are recognized by their manufacturers.
The first drawback, i.e., residual and other stresses, results from deep-drawing the plates to the required shape.
The second drawback, cavities, occurs where plates are bonded together or bonded to the fluid manifolds.
FIGS. 1 and 2 show deep-drawn plates (1-2) welded according to the prior art and where the welded areas (3) are liable to cavities. FIG. 3 shows pressure-induced distortion. It is found that distortion favors the formation of cavities (4) and is moreover inherent in the manufacturing process used: Plates are drawn first and welded next, favoring small stagnant chloride pools that cause corrosion, ultimately destroying stainless steel heat exchangers.
Expansion stress is a third drawback of plate fin heat exchangers.
In the case of parallelopipedic or cubical heat exchangers featuring welded twin-plate units stacked to create the core, stress caused by significantly different core top and bottom temperatures is taken up by a casing or box-like structure. The casing or box is subject to core homothetic temperatures differing as a function of casing gage. Temperature differences cause individual plates to expand at different rates, progressively causing casings or boxes to distort into truncated pyramid shapes. The core, then, is subject to temperature-induced stress on the one hand and stress caused by the deep-drawing process on the other.
In the case of self-supporting parallelopipedic heat exchanger cores with heavy-gage external top and bottom casing plates designed to take up pressure stresses and a core design featuring two outside walls integral to the self-supporting headers at either end, which, because they are only partly involved in the heat transfer process, take on temperatures differing from the other core plates--contact points are insufficient to ensure uniform temperatures between header walls because of the fact that the convection heat transfer rate is usually higher than the conduction heat transfer rate--which causes heavy stresses in the headers.
Such stresses can cause failure or leaks at the header or manifold welds.
A fourth drawback of the welded plate fin heat design is that such heat exchangers cannot be X-rayed or accessed completely. Certain supply and return parts cannot, therefore, be inspected, especially where the core is assembled to the casing.
The purpose of the invention is to overcome the drawbacks common to tube and welded plate fin heat exchangers described herein.
The object is to design a compact heat exchanger with the performances of the welded plate type and the reliability of the tube heat exchanger.
A further object is to design a heat exchanger free from the stresses caused by its manufacturing process.
A further ob
REFERENCES:
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patent: 2690002 (1954-09-01), Grenell
patent: 2941787 (1960-06-01), Ramen
patent: 3129756 (1964-04-01), Ramen
patent: 4688631 (1987-08-01), Peze et al.
Manufacturing Technology Note--U.S. Army Material Development and Readiness Command, Computer-Controlled Laser Welding, Jun. 1984.
JP 58128236, Patent Abstracts of Japan, Manufacture of Heat Collector.
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