The hybrid heat exchanger
Published: 06 March, 2015
Within many sectors of industry, minimising energy consumption and maintaining plant uptime are essential to optimising production costs. Process heat recovery is one significant approach to reducing energy consumption, while improving process performance will contribute to plant efficiency. Heat exchangers when used for heating, cooling, condensing or evaporation play a major role in recovering energy from process gases and liquids, so it follows that their efficiency will influence operating costs. Geoff Mahoney, AxFlow product manager plate heat exchangers reports.
Although a mature and well-proven technology, heat exchangers are having to evolve in order to meet the ever-changing demands of industrial processes and the environments in which they have to work. Plate heat exchangers using the plate corrugation pattern in combination with a relatively narrow gap between the heat transfer plates deliver higher heat transfer efficiency and cost effectiveness combined with low downtime. Their relatively small size and weight allows them to be used where there is a small footprint area. However, they are rather limited to just handling clean liquids because the plates cannot handle solids or other contaminants very efficiently. Furthermore, the gaskets providing the seal between plates, limit the operating pressures and temperatures to around 20 bar and 200°C respectively.
This issue of limited pressures and temperatures can be overcome by heat exchangers incorporating semi-welded plates. In this design, two plates are laser welded together to form a leak-proof or gasket-free channel. The channels between the welded plate pairs are sealed via an elastomeric gasket. However, it too can only accommodate relatively clean fluids. Where more viscous liquids or those containing particles and solids are encountered the shell-and-tube heat exchanger has long been the answer for many applications. The only drawback to this is that its thermal efficiency is limited.
The ideal solution that accommodates the requirements of clean and dirty liquids, high temperatures and pressures and achieves high levels of energy efficiency is a heat exchanger that combines the benefits of both designs. In other words, a hybrid. Making the plate heat exchanger more like a shell-and-tube heat exchanger without losing the inherent thermal efficiency advantages, means that a wider range of applications can benefit from improved heat transfer performance and reduced installed size. The solution comes in the form of welded plate technology. The hybrid design has optimised plate patterns on both the corrugated and tube sides, which delivers higher thermal efficiency than that of shell-and-tube heat exchangers and offer a more compact and efficient solution.
The hybrid
Hybrid fully-welded heat exchanger units offer a broad range of configuration options to optimise performance for a variety of thermal, physical and geometrical conditions. This means that the unit can be used for a wider range of applications than conventional plate and frame heat exchangers. Typical applications include heat recovery units, process heating and cooling, process condensers, steam heaters, vapours, gases highly viscous fluids, fluids containing particles and fouling fluids.
The hybrid heat exchanger delivers many process engineering advantages when compared to shell and tube heat exchangers. Its compact design allows a heat transfer area of up to 1.800m m², while the flow pattern design ensures enhanced gas and liquid heat transfer as well as significant savings on material costs due to enhanced operating efficiency. The extremely efficient use of the plate surface maximises the heat transfer area, thus optimising heat transfer performance. The elimination of dead spots provides for very efficient heat transfer at low pressure drop values, enabling a closer temperature approach.
The plate side flow and the tube side flow are arranged in a crossflow configuration with one or multiple passes over the plates. The plate pattern forms elliptical tube channels on the tube flow side and wave flow passage on the plate side. Larger plate gaps are available to accommodate higher flow rates, reduced pressure drops, and media with larger particulates. This facilitates optimum design for a wide range of process requirements. In the case of the APV hybrid plate heat exchanger, it can be designed for a pressure range from full vacuum to 40 bar.
At the heart of the heat exchanger is a heat exchanger block consisting of one or more plate packs. The dimensions of the plate packs are determined by the length and number of plates included in the plate pack. Plates are welded together to form one or more gas-tight and pressure-resistant blocks. 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 designed to provide good performance. For example, 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 the heating and cooling of liquids, applications involving vapours and gases, which traditionally have been handled by shell-and-tube heat exchangers, can benefit from the efficiency of the welded hybrid plate design.
Applications
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 vapours. 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 and approaches of 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. Without gaskets to limit the temperature and pressure range, hybrid heat exchanger designs can accommodate temperatures from -40 to 350°C and pressures up to 40 bar pressure. The absence of gaskets helps to 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, Hastelloy and nickels to suit almost all corrosive product streams.
Summary
Gasketed plate-and-frame heat exchangers deliver 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 temperatures and pressures and 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 and efficient use of installation space, takes the range of applications beyond the gasketed plate-and-frame configuration into new areas.
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