Generally speaking, we can say that heat exchager tubes fall into two broad categories: metallic and non metallic tubes.
Metallic tubes are secured to their tubesheets (or tubeplates, as they say in Britain), usually by expanding or by expanding followed by seal welding, depending on the pressure they must withstand. Some of the metals used as material of construction are carbon, low alloy and stainless steel, titanium, Admiralty brass, aluminum alloys, proprietary alloys (Inconel, Incoloy, Sandvik, Ferralium etc. etc.). Of course, the material is choosed based on the design pressure of the exchanger and the corrosive properties of the fluids.
Non metallic tubes are made, besides other less frequent materials, of graphite, porcelane and glass. Graphite and procelane tubes are usually glued (stuck) to their tubesheets by means of special adhesives. Glass, on the other hand, can not be glued because it has no pores, so it is secured by means of suitable packing (rubber or plastic) and packing glands. These heat exchangers are used in extremely corrosive services, that no metallic material will support. Example: phosphoric, sulphuric, hydrochloric and nitric acid plants.
A well designed heat exchangers does not have just the tubes it needs for the service it is supposed to render. Rather, it will have some percentage of extra tubes. This is because, as time passes, some of the tubes will corrode or break. As soon as a leak is detected, the exchanger is taken out of service, the leaking tube is plugged on both ends and the exchanger is quickly restarted. When all of the extra tubes reserve has been used and the exchanger efficiency begins to decrease, it is taken out of service and the plugged tubes are replaced. This, of course, takes a longer time.
The percentage of extra tubes depends on the service they are subjected to. The designer experience will indicate the most suitable amount.
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Giovanni S. Crisi
Sao Paulo - Brazil
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