The technology of plate heat exchangers has advanced rapidly in recent years, which is leading to their widespread adoption for process heating applications. Compared with shell-and-tube designs, today’s steam plate exchangers are more compact, offer greater reliability, are highly efficient and are easy to maintain. Their lower overall life cycle cost makes them ideal for chemical processing lines where low and medium pressure steam is the heating medium.

A few years ago, Alfa Laval, arguably the world leader in heat exchange technology, developed stainless steel plates featuring an innovative corrugated pattern specifically for steam applications. The geometry of the corrugations makes maximum use of the heat transfer area, optimises the turbulence and pressure drop on the primary and secondary sides for more efficient heating and offers higher pressure capabilities.

The turbulent flow also reduces the build up of any lime scale on the secondary side, considerably reducing maintenance costs in many steam-to-water applications. Shell-and-tube exchangers have smoother flows and suffer greater fouling, leading to more frequent dismantling for cleaning.

Alfa Laval then turned its attention to materials. The company developed a new gasket material called HeatsealF that tolerates temperatures up to 180°C, enabling plate heat exchangers to be used on most industrial steam applications. These flexible gaskets also help to prevent thermal fatigue of the plates by allowing expansion and contraction with temperature changes. In shell-and-tube designs, thermal fatigue can cause cracked components and unwanted system downtime for repairs unless special thermal expansion arrangements are used. U-tube designs are often used with steam to allow for thermal expansion, but with most of the tube horizontal, achieving condensate drainage can be a problem.

The benefits do not stop there. Plate heat exchangers are only about a fifth of the size of shell-and-tube heat exchangers. Their compactness and light weight make them easier and less costly to install. They take up less plant room space and are easy to integrate with existing installations (Fig.1).

It is also easy to add or remove plates to change the capacity of a plate heat exchanger should demand change.

Furthermore, the high heat transfer coefficient allows a much lower steam pressure in the heat exchanger, resulting in a lower metal temperature and less scaling, fouling or product deterioration on the secondary side. The low steam pressure reduces or eliminates any condensate flash steam losses for significantly increased efficiency of steam use and higher energy efficiency.

Control is another key feature. The use of modern controls makes the most of the plate heat exchanger's inherent fast response and close temperature control capabilities, which makes them great for applications such as supplying instantaneous hot water.

Reducing energy costs

It is this ability to provide instantaneous hot water that helped Spirax Sarco reduce energy costs at the Manchester facility of sugar processor Tate & Lyle. Spirax Sarco supplied the sugar processor with a skid-mounted EasiHeat engineered system, which incorporates an Alfa Laval plate heat exchanger, together with all the necessary ancillaries and controls.

The EasiHeat was installed as part of a bigger project that included a new boiler. When installed, the whole project was expected to pay for itself within about two years, with early estimates from Tate & Lyle engineers suggesting that the EasiHeat itself will achieve a payback of less than 12months (Fig.2).

The EasiHeat steam-to-hot water package also enables Tate & Lyle to cope with peaks in demand for liquid sugar that it could not previously meet.

The facility in Trafford Park, Manchester, produces 15-tonne batches of liquid sugar for delivery by road tanker. The company used three electric boilers to fill a 60-tonne storage tank with hot water overnight, ready to supply the next day’s production process.

The site had 60tonnes for use each day, but if a customer rang up and asked for an extra delivery, the plant could not deliver because it took too long to heat the water up.

Electricity was a convenient but expensive option for heating the water. The original design dated back to an era when energy usage was not a major design factor.

The sugar company’s initial idea was to carry on heating the water overnight using a new stainless steel heating coil fitted inside the hot water storage tank and supplied with steam from a new boiler. But this proposal fell foul of health and safety legislation. The company was not allowed to run the boiler unmanned overnight, so it could not use that time to heat the water. That is when engineers called in Spirax Sarco.

Spirax Sarco’s solution was the EasiHeat system, which supplies up to five tonnes of instantaneous hot water per hour, eliminating the need to run the boiler to heat water overnight. The new steam boiler is switched on at the start of each day and in just 20minutes there is enough hot water to begin processing sugar. The plant is no longer limited by the capacity of the hot water tank and can expand production to meet demand.

The process dissolves solid sugar in 90°C water and cools the resulting liquid sugar solution to 30°C prior to shipping. During cooling, fresh water is heated to 40°C, which is then fed to the EasiHeat system where the system’s internal plate heat exchanger heats it to 90°C to supply the process.

The Liverpool-based Tate & Lyle engineering team who carried out the project did so as part of their service to internal and external clients.

Clearly, the modern plate heat exchanger offers several advantages for many processing applications. Chief among these are its higher efficiency, better controllability and lower overall costs.

Rick Plummer is UK Engineering Manager with Spirax Sarco, Cheltenham, UK. www.spiraxsarco.com