Whether for heating, cooling, condensing or evaporating, heat exchangers are a vital component in a variety of industrial processes. Their role in applying or recovering costly thermal energy from process gases or liquids means that the efficiency of heat exchangers impacts directly on energy use and therefore on the operating costs and environmental performance of a plant or process. As energy costs rise and the environmental footprint of industrial processes is increasingly in the spotlight, heat exchanger technology is also evolving to respond to new, more demanding criteria.
For more than 100 years, the classic shell-and-tube heat exchanger has been the solution for many applications, especially those involving solids or contaminants, but its efficiency is limited. Plate-and-frame heat exchangers, first developed by Dr. Richard Seligman of APV in 1923, offer higher heat transfer efficiency and cost effectiveness combined with low downtime and relatively small size and weight, thus reducing installed space requirements. The major contributor to the higher efficiency of plate-and-frame heat exchangers is the plate corrugation pattern in combination with a relatively narrow gap between the heat transfer plates. However, the fluids being handled must be relatively clean because the plates have limitations when solids or other contaminants are involved. In addition, the gaskets, which traditionally provide the seal between plates, limit the operating pressures and temperatures to typically 300 psig (20 bar) and 400°F (200°C), respectively.
Alternatively, semi-welded plate pairs are available for the plate-and-frame heat exchanger. In this design, two plates are laser welded together to form a leakproof or gasketless channel. The channels between the welded plate pairs are sealed via an elastomeric gasket. Typically, this type of heat exchanger is used on higher operating temperatures and pressures, and for aggressive or hazardous fluids such as in ammonia refrigeration. However, it too can only accommodate relatively clean fluids.
Ideally, if the plate-and-frame heat exchanger could be made more like a shell-and-tube heat exchanger without losing the inherent thermal efficiency advantages, then a wider range of applications could benefit from improved heat transfer performance and reduced installed size. Advances in plate-and-frame design do just this in the form of welded plate technology.
By combining the best of the two technologies, the hybrid design has optimized plate patterns on both the corrugated and tube sides, which promotes higher thermal efficiency than that of shell-and-tube heat exchangers.
On one side of the heat exchanger, the hybrid heat exchanger has a plate design and gap that behaves like tube flow. This side can easily handle fluids with solids or can be used for condensing applications requiring low pressure drops. The other passage — the corrugated plate side — has thermal and hydraulic properties similar to that of a traditional plate such as a chevron plate. In this way, the welded hybrid heat exchanger combines the best aspects of both designs.
A hybrid welded plate pack employs advanced pressing and welding technologies, absorbing alternating loads on the welds. The welds are not subjected to mechanical loading during thermal cycling (thermal expansion effects) and therefore are more resistant to fatigue.
Hybrid heat exchanger design features are tailored to provide good performance. For example, in the case of one design’s proprietary profiles, fluids containing solids or contaminants can pass more easily through the tube side because there are no obstructing contact points. Complete accessibility to the plate pack, combined with true mechanical cleanability on the tube side, ensure rapid, effective maintenance when cleaning is required.
In addition to heating and cooling of liquids, applications involving vapors and gases, which traditionally have been handled by shell-and-tube heat exchangers, can benefit from the efficiency of the welded hybrid plate design. In process condensers, one hybrid heat exchanger’s profile is well suited for creating high U-values with a low pressure drop on the condensing side. A low pressure drop means a higher effective MTD and, thus, better recovery of vapors.
The plate profile combined with a flexible connection size also allows gases to be heated as well as cooled. Also, a hybrid plate heat exchanger is suitable for highly viscous fluids that benefit from the low-resistance flow channels combined with high film coefficients offered by both the tube side and corrugated plate pattern on the plate side.
In the case of heat recovery units, the high heat-transfer efficiencies of the hybrid design help achieve close temperature approaches. Approaches as close as 1.8°F (1°C) are possible, thereby recovering more heat to reduce process operating costs and improve carbon footprint.
For applications involving high temperatures and pressures, a welded hybrid also can be used. With no gaskets to limit the temperature and pressure range, hybrid heat exchanger designs can accommodate temperatures from -328 to 752°F (-200 to 400°C) and pressures up to 580 psig (40 bar) pressure. The same absence of gaskets helps avoid compatibility issues and reduces the risk of leaks while making the welded hybrid design suited to hazardous or corrosive fluids. Plates can be produced in materials such as 304L and 316L stainless steels, high-performance austenitic stainless steels, Hastelloys and nickels to suit almost all corrosive product streams, offering cost and heat transfer performance benefits.
Gasketed plate-and-frame heat exchangers have optimum thermal efficiency compared to shell-and-tube designs within a specific set of operating parameters. However, the range of applications for which they are suited is limited. Shell-and-tube units are more accommodating for high fouling or contaminant-laden fluids but are heavier, take up a larger installed area, and lack the heat transfer efficiency of plate technology. A hybrid welded plate heat exchanger — with its tube-like plate profile and true mechanical cleanability — extends the range of applications beyond the gasketed plate-and-frame configuration into new areas.
Carl T. Kozacki is the sales director with industrial heat exchangers for SPX Flow Technology. The Charlotte, N.C.-based company can be reached at 704-752-4400 or visit the website at www.spx.com