Design advances extend the scope of heat exchangers into new areas

Paul Boughton

Heat exchangers have always been at the heart of industrial heat recovery systems, but the latest advances in their design are making them even more central to manufacturing processes. Sean Ottewell reports.

Following a series of design advances, the Bulk-X-Change bulk materials heat exchanger from German company Coperion Waeschle (Fig.1) is now available for immediate integration into pneumatic conveying systems.

"The direct installation now considerably improves heat transfer, permitting the combination of the two processes - conveying and temperature control - within one single system and thus making the handling of bulk materials even easier," notes the company, which is a specialist in pneumatic conveying and heating/cooling processes.

The design changes have extended the scope of the exchanger's application beyond its traditional handling of free-flowing powders and granules. This new integratable version is also suitable, for example, for fine-particle powders that have a fluidising or cohering tendency during discharge from the silo. Pneumatic conveying systems are frequently used for the handling of such powders. It also ideal for cooling terephthalic acid powder, alumina powder, melamine powder and catalyst powder.

Conveying line

The basic principle of the new process, for which a patent is pending, is the direct installation of the heat exchanger into a pneumatic conveying line.

The pneumatically conveyed powder, together with the conveying gas, enters the vertically arranged heat exchanger from below.

This confers a number of advantages on it, including: high heat transfer rates at low additional pressure loss; short, defined residence times; negligible hold up of product/rapid product changeover; simple retrofitting; easy cleaning; and low capital investment compared to other solutions.

Coperion Waeschle can also offer customers pilot plant facilities with a pneumatic conveying plant that can be equipped with different sizes of heat exchangers and is rated for throughputs of up to 6000kg/h and temperatures of up to 150°C.

Crystallisation and rying

Another recent innovation saw the heat exchanger being used for the first time by a fertiliser manufacturer for an inorganic product. The plant now operates at an increased output of 60t/h, with the exchanger cooling fertiliser product after the crystallisation and drying stages. In particular, this is helping to prevent the formation of lumps during silo storage and so improve product storability and handling.

However, Bulk-X-Change had to undergo a comprehensive series of tests before the manufacturer was satisfied. So Coperion Waeschle made and installed a trial heat exchanger that was operated in parallel to an existing plate cooler.

For several weeks the prospective user tested freshly produced fertiliser under all conceivable operation conditions. "They could not have been more satisfied with the performance given by this bulk materials heat exchanger," says Coperion.

As the final unit in a long process chain, the exchanger makes considerable savings possible not only in respect of capital investment costs but also as regards space requirements and energy and operating costs, especially when compared with conventional fluidised bed and drum coolers.

According to the company, it also contributes considerably to a reduction of the production plant's carbon footprint.

In this application, the fertiliser to be cooled flows downwards through the smooth-walled tubes, while the cooling medium flows upwards on the outsides of the tubes. A specially developed product distribution plate ensures the trouble-free gravity feed of the fertiliser into the tubes by distributing it gently and evenly over the entire nest of tubes. Special technical features, such as a dry air counterflow system, for example, not only increase operating reliability but also reduce maintenance costs.

Because of these operating characteristics, the exchanger is suitable for cooling or heating of numerous other products such as minerals, detergents and foodstuffs.

Design is also key to the new Pharma-Line shell-and-tube exchanger from Alpha Laval. It meets the high hygienic demands of the pharmaceutical industry, with simplified draining and cleaning and a much-reduced risk of the cross-contamination that can plague conventional heat exchangers.

The Pharma-Line heat exchanger is designed to work in the highest hygienic conditions in applications such as water-for-injection (WFI), purified water (PW), and heating/cooling of pharmaceutical products.

Product media flows through a bundle of seamless, electro polished tubes. The service media flows outside the tubes in a cross flow (Fig.2). At the end, the tubes are fastened by two tube-sheets acting as leakage indicators, preventing cross contamination between the product and the service media.It can be installed horizontally or vertically depending on the duty or space. For easy installation, lifting and mounting devices are welded to the unit. Thermal fatigue caused by large temperature changes is managed by a U-bend in two-pass units, and by a welded-in bellow in straight units. In addition, the new Pharma-Line heat exchanger has no dead spots in the tubes, is completely drainable on the product side, and is designed according to FDA and cGMP requirements. Special designs are available on request.

Meanwhile, the designers at Niro have also been busy with a new type of heat exchanger that features a novel plate profile designed to reduce bacterial build-up and to aid cleaning-in-place (CIP).

With ordinary heat exchangers bacterial build-up during operation can easily be returned to the process during shut down when the airflow of the cooling system is reversed.

Avoiding this build-up of bacteria through plate design and frequent CIP cycles helps prevent contamination. This is especially important for sanitary operations.

The heat exchanger is at the heart of Niro's new heat recovery system that cuts heating costs by re-using more of the heat generated within a plant that would otherwise be lost to the atmosphere.

Energy savings

Energy can be saved at various points throughout the production process. For example, hot condensate from the evaporator can be cooled in the pre-heater of the spray dryer. Heat exchangers can take advantage of high-temperature combustion air from the steam heater, hot oil unit or indirect oil heater to heat the air intake to the spray dryer. Also, instead of simply releasing smokestack exhaust, it can be used to heat air in the pre-heater enabling the pre-heater to operate at a lower, energy-saving temperature. Once the paramaters for a particular customer have been established Niro designs an entire package and manages the project with minimal disruption right through to installation and testing.

A heat recovery system can be built into a new plant or retrofitted to an existing plant. With rising fuel costs Niro estimates that the pay back period for a heat recovery system can be as low 2-3 years, depending on the individual plant operating conditions. According to the website, Victrex has managed to enhance Makatec's spiral plate heat exchangers and reduce their overall cost by replacing metal with Aptiv film. Made of PEEK polymer, the film is marketed as a high performing thermoplastic film that meets demands for improved product performance - including durability, reliability, miniaturisation and functionality.

In this design, Aptiv film is spiral bound with high-temperature-resistant glue to provide the right mixture of high-temperature performance and chemical resistance.

It provides an effective barrier against many liquids and gases, is lightweight, halogen-free, and withstands lead-free soldering process temperatures.

Environmentally friendly and 100 per cent recyclable, Makatec heat exchangers achieve heat transfer coefficients in the range of 1kW/m2K (176 BTU/hft2F) comparable to conventional plate heat exchangers. The spiral design allows volume specific heat exchange areas up to 1000m2/m2 (305 ft2/ft2).

The smooth surfaces of Aptiv film reduce the risk of residue build-up inside the heat exchanger, therefore minimising the danger of microbial contamination.

The heat exchangers are well suited for use in potable water systems and in the food sector, as well as in pharmaceutical industry and biotechnology applications.