Use of elastomers in plate and frame Heat exchangers exchangers

Plate-and-frame Heat exchangers exchangers 

Of all compact types, the largest share in terms of number of units and dollar volume is represented by plate-and-frame exchangers. Developed late last century, the model made its commercial debut in the food industry in the 1920s. Advances in design, materials of construction and the manufacture of plate models, have made this configuration a viable option for a range of applications in the chemical and hydrocarbon process industries.

Plate exchangers are composed of corrugated alloy plates that, when stacked and compressed together, form an alternating series of hot and cold channels. The thin plates and very high turbulence produced by the corrugations, and normally countercurrent flow, provide substantially higher coefficients than can be achieved with shell-and-tube exchangers.

There are other advantages to plate exchangers. Among them are a small footprint to accommodate tight spaces, expansion capability for future capacity increases, and the ability to handle deep temperature crosses in a single frame -- eliminating stacked shells with a maze of piping, heavy foundations and large holdup volumes. The ability to achieve extremely close approaches permits the use of much-warmer cooling waters, even tempered water- or seawater-systems in areas where fresh water is scarce.

While plate exchangers have pressure drops comparable to those found in shell-and-tube units, they are generally confined to operation at lower pressures and temperatures due to the use of elastomer gaskets for sealing. Commonly stated limits have been 150 degrees Celsius and 20 bars.

If the elastomer is incompatible with the fluid to be handled, even small concentrations of a pernicious material may wreak havoc on the exchanger. Much work has been done to develop elastomer formulations that increase the temperature range and chemical resistance of gaskets. Several versions of nitrile, ethylene-propylene-diene monomer and Viton fluoroelastomer, a copolymer of vinylidene fluoride and hexafluororopropylene, are available, depending on the application. Potential buyers should work with a reputable vendor who not only understands heat exchanger equipment, but also the process in which it is to be used.

Questions are often asked about the use of materials other than elastomers for gaskets. In general, other materials require much higher sealing force, which makes it difficult to get a positive seal at elevated pressures, especially along the long peripheral gasket sections. The use of polytetrafluoroethylene (PTFE) encapsulated elastomers has met with some success, but is limited to small exchangers with surface areas of about 1 ft²/plate or less, and to the ring gaskets of semi-welded plate exchangers. Gaskets made of expanded PTFE have been introduced recently, but are too new to be fully assessed.