Materials for plate heat exchangers focuses primarily upon the plates and gaskets. Since these items significantly effect first cost and equipment life, this procedure should receive special attention.
Plates
One of the features that makes plate type heat exchangers so attractive for geothermal application is the availability of a wide variety of corrosion- resistant alloy for construction of the heat transfer surfaces. Most manufacturers offer the alloys listed below:
304 Stainless steel
316 Stainless steel
37 Stainless steel
Titanium
Tantalum
Incaloy 825
Hastelloy
Inconel
Aluminium Bronze
Monel.
In addition to these, a larger number of optional alloys are available by special order. Most manufacturers will quote either 304 or 316 stainless steel as the basic material.
For direct use geothermal applications the choice of material is generally a selection between 304 stainless, 316 stainless and Titanium. The selection between 304 stainless and 316 is most often based up on a combination of temperature and chloride content of the geothermal fluid. Combinations of temperature and chloride content that are located above the threshold offer the potential for localized pitting and crevice corrosion. Fluid characteristic above the threshold for a particular alloy do not guarantee that corrosion will absolutely occur. However this curve, based on oxygen free environments does provide a useful guide for plate selection. Should oxygen be present in as little as parts per billion in concentrations, the rates of localized corrosion would be significantly increased. Should the system for which the heat exchanger is being selected offer the potential for oxygen from entering the circuit, a more conservative approach to materials selection is recommended.
Titanium is rarely required for direct use applications. In applications where the temperature or chloride requirements are in excess of the capabilities of 316 stainless steels, titanium generally offers the least cost alternative.
Austenitic stainless alloys with higher chromium and molybdenum contents could be recommended for this application also. These alloys however are generally not available as standard plate materials as is titanium.
A typical application in which titanium has been employed is in geothermal systems that serve loads in which the secondary fluid is heavily chlorinated. The most common of these is swimming pools. The nature of swimming pools is such that the pool water is both high in chloride and oxygen content. As a result titanium is the alloy generally selected. Plates made of 316 stainless steel, in the heat exchanger serving the swimming pool at Oregon institute of technology failed in less than two years as a result of localized corrosion. The first cost premium for titanium over stainless steel plates is approximately 50%.
The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations and write ups are only for reference and educational purpose.