Welded heat exchangers show their mettle
25 Jan 2007
All-welded plate heat exchangers are becoming established in an increasing range of process industry applications. Paulo Duarte of Tranter International AB examines why:
Manchester, UK -- In its most basic form, the plate heat exchanger consists of corrugated metal plates compressed in a frame. Hot and cold media, flow on either side of the plate and transfer heat in a fully countercurrent flow arrangement. Each plate is equipped with a double sealing system that keeps the fluids between the channels.
The hallmarks of plate heat exchanger operation are high heat transfer efficiency, compact size, design flexibility, low fouling (as a result of plate pattern turbulence) and low lifecycle cost. A typical heat transfer coefficient for a plate heat exchanger may be four to five times higher than that of a shell-and-tube design, while the compact heat transfer area in a plate heat exchanger results in an efficient design with space requirements as low as 10% of a typical shell and tube design.
The sealing system is selected based on the application and since their introduction in the 1930s, plate heat exchanger designs have evolved to satisfy the changing needs of process industries. When using elastomer gaskets, the upper temperature limit is approximately 160ºC, with specialist Viton materials up to 180ºC. Moreover, some chemicals are not compatible with elastomer gasket materials. For the most demanding applications, welded plate heat exchangers offer optimum performance, with welding on every channel, or on alternating channels, depending on application.
The latest all-welded plate heat exchangers products can offer all the benefits of a plate-type exchanger but overcomes the limitations of gasketed models. They can operate at very high pressures (design pressure 75barg) and at low and high temperatures (-195ºC to 300ºC). It can handle liquids, gases and mixtures of the two.
These products work in the same way as a conventional plate and frame heat exchanger, with alternating channels for hot and cold media and true counter-current flow, which offers full LMTD and allows closer temperature approaches, delivering superior performance compared to cross-flow designed units.
State of the art plate welding technology, meanwhile, can eliminate conventional continuous side welds, enabling it to more efficiently accommodate differential thermal expansion, while also reducing manufacturing cost.
Welded plate heat exchangers are now being used extensively in chemically aggressive processes including oil and gas production and refineries; hydrocarbon processing units such as primaries, intermediates and polymers; pharmaceutical and specialist fine chemicals; and inorganic chemicals production such as chlor-alkali or hydrogen peroxide.
One primary use for welded heat exchangers is as an interchanger for heat recovery or absorption-stripping and distillation unit operations. Welded units are also often used as a condenser or reboiler because they offer large cross-flow area, short flow path (low pressure drop) and flexible connection sizes.
Johnson Matthey’s chemicals plant at Billingham on Teesside uses all-welded PHE equipment to cool solvents in the production of titanium alkoxides. The units, which are in two different material designs, are being employed in the solvent recovery process, improving cost efficiency and reducing emissions of organic solvents to the environment in line with the company¹s stringent environmental policy.
A unit with stainless steel plates is being used as the main condenser cooling solvent vapour from a temperature of almost 70ºC to around 20ºC using circulated cooling water. The second unit, with titanium plates, is employed after the main condenser to cool the solvent vapour further to about -10ºC using brine as the coolant because of its lower freezing point.
Specified for their compact size and low weight compared with traditional shell-and-tube technology, Johnson Matthey considered the all welded, sealed units the most suitable condensers to recover catalyst materials.
According to Rob Peeling, process development manager at the Johnson Matthey Catalysts plant: “While the solvents are not toxic Johnson Matthey has high standards with regards to environmental protection.”
Reliability was also important, added Peeling, as “we just wanted to be able to install the unit and leave it to do its job and that’s what we¹ve been able to do.”
