Corrosion failures in plate heat exchangers arc discussed with reference to equipment design, service conditions and materials of construction. Included are case histories that illustrate service experience.
Plate heat exchangers are chosen over shell-end-tube exchangers for applications requiring superior heat transfer efficiency and compactness, and lower weight. Efforts to maximize their inherent advantages drive plate heat exchanger design toward the use of thin plate sections that require highly corrosion-resistant materials, and narrow flow passages that are conducive to fouling.
Plate heat exchangers in general require extensive scaling along the edges of the plate. Consequently, crevice corrosion may occur under gaskets or adjacent to seal welds. Localized corrosion maybe either initiated or aggravated by the leaching of harmful ionic species into crevices from polymer gasket materials. Stress-corrosion failures are also encountered, particularly at cold formed corrugations incorporated into some designs to contain gaskets or to improve heat transfer coefficients.
Increased energy costs end conservation needs in recent decades have enhanced the attractiveness of efficient heat exchanger designs. Plate heat exchangers have more heat transfer area per unit volume then shell-end-tube heat exchangers, providing more efficient performance along with space end weight savings. The superior heat-transfer efficiency of plate heat exchangers reduces water requirements in cooler and condensers and permits waste heat to be recovered economically with smaller temperature differences.
The effects of such variables as component geometry, heat flux and service fluid contaminants are important in promoting localized attack in plate heat exchangers, Corrosion considerations are complex in this equipment, making past service experience especially valuable in anticipating possible failure mechanisms so that appropriate corrosion control measures may be implemented.
Examples of a few plate heat exchanger configurations are illustrated in Figures 1 through 3. In the design in Figure 1, the heat exchange elements are spiral-formed plates that conduct the opposing streams between the cylindrical outer wall and the central axis. The plates are sealed at the edges to prevent the streams from mixing. One of the streams enters the heat exchanger at the center and leaves radially at the outer shell, while the second stream enters at the outer shell, flows counter-current to the opposing stream and exits axially at the center.
Figure 2 shows a multiple-pass plate-and-fin heat exchanger. Flow passages between plates are directed by sets of corrugated fin as illustrated in the inset drawing. The opposing streams in this example flow across one another, with the different passes separated by baffles. The flow streams are sealed at lined panels by means of gaskets.
A popular plate-and-frame heat exchanger configuration, shown in Figure 3(a), employs a stack of plates mounted on carrying bars and held between end covers by means of impression bolts. Each plate is gasketed about its periphery to contain the flow. Gaskets also control the paths of the opposing streams which flow through ports cut into the comers of the plates. Cold-formed corrugations act to stiffen the plates and to induce controlled turbulence for improved heat transfer.
