Condensing theory into practise

One of the biggest exhibits on one of the biggest stands at Achema (held in Frankfurt at the end of May) was Alfa Laval's new AlfaCond. Specifically developed for condensing vapours under low-pressure, low vacuum conditions in evaporation and distillation processes, the plate condenser on show featured a 600mm diameter vapour inlet. The practicalities of building a conventional plate heat exchanger - with four equal sized inlet and outlet ports - to the same specification only served to highlight the ingenious design of the AlfaCond.

Unlike conventional plate exchangers, with ports at each corner of the plate, the AlfaCond has its 600mm vapour inlet and smaller diameter inlet and outlet ports for cooling water, placed one above each other in a vertical line. In this way, the complete width of the plate can be used for the inlet of large volumes of vapour, while the condensate - much smaller in volume - flows out from small outlets placed, conventionally, in the bottom corners of the plate.

According to Tomas Kovaçs, Alfa Laval's marketing manager for evaporators and condensers, 'this smart design means the AlfaCond can handle several times larger capacities than conventional plate exchangers.' But the innovative design goes beyond the geometrical arrangement of inlet and outlet ports on each plate. Again unlike conventional plate exchangers, the plate stack of the AlfaCond consists of a mixture of welded plates and a traditional gasketted arrangement.

The plates themselves, complete with matching inlet and outlet holes, are welded together in pairs to form 'cassettes'. These cassettes are in turn assembled together in the stack with conventional plate gaskets. But the gaps between the assembled stack of plates are asymmetric. The welded plates have a large channel width between them in which the large-volume vapour flow condenses, while the cooling water flows in narrower width channels between the gasketted plates.

'This makes it possible,' says Kovaçs, 'to keep a low pressure drop on the vapour side, while keeping up velocity and turbulence on the cooling water side, so maximising heat transfer efficiency and minimising fouling.' It also means that, in the event of any gasket leak, only cooling water can escape, rather than process fluid.

Compared to shell-and-tube condensers, AlfaCond should have less fouling on the cooling water side because of this high turbulence in the narrow channels. Kovaçs also says that heat transfer coefficients on the vapour side can be as much as three times higher than in an equivalent shell-and-tube condenser. And unlike shell-and-tubes, the capacity of the AlfaCond is easily increased, simply by adding extra cassettes within the length of the frame.

Although the AlfaCond 600 model on show was impressive enough, an 800mm inlet unit is planned for later in the year to complement the 400mm unit already in the range. And if the demand materialises, Alfa Laval has plans for a massive 1200mm inlet diameter model.

Whatever the size, however, all plates can be manufactured in stainless steel (as standard), titanium, Hastelloy, or 'any other pressable material', according to Kovaçs.

Proof of the interest in the new condenser came at the show in the shape of an immediate order for an acetone duty. But the company's target markets range across most of the process industries - inorganic and organic chemicals, hydrocarbon processing, power generation, polyols, food and distilleries, and waste treatment processes.

With its much smaller footprint and flexibility of design, the AlfaCond - like its plate exchanger siblings - is an obvious alternative to shell-and-tube condensers, but Kovaçs points to another rapidly growing market as replacements for direct condensers such as used by many food processors. Here, environmental pressures are likely to bring a halt to the practice of using water as a direct coolant for condensing vapours and then discharging the condensate/coolant mix to drain or the sea.

As Kovaçs puts it: 'Like any really good idea, it all seems perfectly simple now - it's just that nobody thought of it before'.