Since its inception in the 1920s for commercial usage, primarily in the dairy industry, the traditional plate-and-frame heat exchanger has evolved over the last several decades and variant models of PHEs have been developed. Although some of these modifications have been driven by new strategies for making more compact equipment, others have focused on overcoming some of the disadvantages and expanding the applications spectrum of PHEs. This evolution has typically manifested either as an altered structure or construction of the PHE or as variations in the plate surfaces corrugation patterns. In the former category during the last several decades, brazed, semi-welded, fully welded, wide-gap, double-wall PHEs, among many others, where the gasketing has been eliminated, have been developed.
Brazed plate heat exchanger
The brazed plate heat exchanger (BPHE) is essentially made up of a pack of thin corrugated stainless steel plates that are brazed together using copper as a brazing material to form a self-contained unit. Brazing eliminates the need of either frames or gaskets and results in a very compact exchanger. Also, instead of copper, where its use presents a compatibility problem with a process stream (e.g. ammonia), nickel or some other brazing material is used.
Because the plates are brazed to each other and there are no frames or gaskets, BHEs can handle higher pressure and temperatures than plate-and-frame heat exchangers, e.g. situations with pressure up to 30 bar and temperatures up to 400C. They are also characterized by very low weight due to the absence of frames. However, the exchanger length is usually less than 1m because of the brazing furnace size limitation, where their capacity is restricted to a single BPHE unit. Typical applications include heating and cooling (sensible or with phase change) in the process industry, evaporation and condensation in refrigeration systems, and other HVAC installations.
Semi-welded plate heat exchanger
By welding heat exchanger plates in pairs, to make what are commercially called twin plates, a semi-welded PHE is configured by assembing them in a plate-and-frame pack with gaskets only in the plate channels that handle the alternate fluid stream. This design is especially useful for handling relatively corrosive media, which flows in the welded twin-plate channels. The only gaskets in contact with this medium are two circular porthole gaskets between the welded plate pairs that are typically available in highly resistant elastomer and non-elastomer materials. The channels containing the non-corrosive, non-aggressive, secondary heating or cooling fluid medium flows are sealed using traditional elastomer gaskets.
The semi-welded PHEs can withstand pressures up to 30 bar on the welded twin-plate fluid side, though it should be pointed out that frames are still needed to hold the plate pack. The relatively higher pressure operation extends its applications to include evaporation and condensation in refrigeration and air-conditioning systems, among others.
Fully welded plate heat exchanger
The fully welded PHE is a gasket-free version, where a completely welded plate pack is bolted between the two end plates in a conventional frame. By joining the plates at their edges and eliminating the gaskets, the structural integrity of the plate pack is significantly enhanced, and so are the operating temperature and pressure limits of the gasketed PHEs. The laser welds are applied in two spatial dimensions along the edges in the plane of the plates. This allows the plate pack to expand and contract along its length as temperature and pressure changes take place, thereby making the pack more fatigue resistant. Consequently, they are particularly attractive for applications where the heat transfer for thermal processing undergoes rapid changes in temperature and /or pressure. However, fully welded PHEs, unlike gasketed and semi-welded models, lose the flexibility of either expanding or decreasing their surface area by adding or removing plates for meeting varing heat load requirements. Also, they cannot be cleaned readily by mechanical methods, and only chemical cleaning methods can be employed.
The fully welded PHEs are intended for thermal processes with severe duty requirements that often involve handling of highly aggressive or corrosive fluids. They can withstand temperatures up to 350C and pressure up to 40 bar. Typical applications included exchangers for desuperheating in heat recovery systems, refrigeration interchangers, and heaters of organic chemicals such as solvents, vegetable oils, steam, and batch reactors, among others.
Wide-gap plate heat exchanger
PHEs with wide-gap plate packs provide larger free-flow area channels for handling fluids containing fibres or coarse particles and high-viscous fluids, which normally clog or cannot be satisfactorily treated in other types of PHEs and still retain some of enhanced thermal-hydraulic performance characteristics. The plate-surface corrugations and gaskets are designed such that in the inter-plate channels the flow cross-section has a maximum gap of up to 16mm. The plate corrugations still provide an effective area enlargement and promote swirl flows to effect high heat transfer coefficients; the wider flow gap, however, tends to reduce the pressure drop penalty.
Typical applications include heating of raw, limed, and mixed juice in sugar mills, cooling and blenching of plant filtrate in pulp and paper mills, and sanitization of fibrous food product slurries.
Double-wall plate heat exchangers
The double-wall PHE is designed for use with either a reacting media or when product contamination between the two fluid steams must be avoided 'fail safe'. Double plates, sealed by conventional gaskets, replace the single plate that normally separates the two fluid media. In the event of the media reacting with the corroding the surface of the double-wall plates, the leakage is directed in the passage between the double plates. This essentially minimizes the possiblity of inter-fluid contamination, and the leakage also becomes easily visible on the outside of the heat exchanger. The more common applications of this type of PHE include, among others, heating and cooling of drinking water, pharmaceutical media, lubricating oil, and transformer oil.
Diabon graphite plate heat exchanger
The Diabon graphite PHE employs graphite plates which are developed for thermal processing of media that is normally too corrosive for plates made of exotic metals and alloys. The Diabon F100, or NSI graphite, is a composite material made up of graphite and fluoroplastics. The material is compressed into the shape of surface corrugated plates, and the formed plates are fitted with thin, flat, corrosion-resistant gaskets. Besides being corrosion resistant and capable of with-standing high temperatures, graphite plates offer good heat transfer characteristics in combination with low thermal expansion and high-pressure operation.
Some of the prevalent applications of PHEs with graphite plates include heating of pickling baths, surface treatment of metals, hydrochloric acid production, and flue-gas waste-heat recovery.
Minex plate heat exchanger
The design of the Minex PHE is a 'miniaturization' of the conventional plate-and-frame heat exchanger. The end frames and carrying or guide bars are eliminated, and tightening bolts are located with the outline dimensions of the heat transfer plates instead. This configuration makes it possible to combine a compact design with the PHE flexibility options of manual cleaning and ease of surface are expansion or reduction to meet changing application requirements. However, because only tightening bolts are used to keep the gasketed plate pack together, the small and compact Minex PHE is generally used in heat transfer duties where the required capacity is also very small. This type of PHE is intended for general-purpose, fixed load applications, and constitutes an attractive alternative to larger exchangers.
Several other types and modifications of PHEs are available commercially, and the above selections represent the more prominent sampling. While these new developments have overcome many of the deficiencies of the traditional plate-and-frame heat exchangers, the newer models that discard gaskets lose the advantages of easy cleaning and flexibility of adjusting or altering heat exchanger area. Nevertheless, the family of PHEs available has enlarged considerably with these new developments, which provides more competitive functional and cost attributes, and expands their domain in modern industrial applications.
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