A paper manufacturer requires high-quality heat exchangers for paper drying. The customer installed five modular hairpin heat exchangers with smooth tubes (Type: Air Dryer VT D5LD) from Kelvion for optimal energy recovery in the critical drying process.
These Kelvion high-performance air dryers can recover 12 MW of thermal energy per second. On an annual basis, this corresponds to the energy requirement of 6000 households. These 8.4 x 4 x 1.5 meter heat exchangers have an air-side mounting frame with an airtight design. They are installed in the dryer section at the end of the cardboard production process before smoothing takes place. The paper is dried using hot air. The air extracts the moisture from the material. This is a critical process: If the humidity is too high, the water drips off in an uncontrolled manner. This would result in droplets soaking through the paper again, which would decrease the quality.
In order to prevent this, the air is cooled down from 70°C to 50°C during the dehumidification process in the Kelvion Air Dryer and then maintained at a constant relative humidity of 60 %. Here the air flows around the heat exchanger’s stainless steel smooth tubes, which are bent in the shape of a hairpin, and cools down. The water then condenses on the tubes and rains down in a controlled manner bringing about a reduction in the absolute humidity. A glycol-water mixture ensures efficient heat exchange. The energy recovered in this way is then fed to another location in the production process. This preserves the primary energy to a large extent while promoting resource-saving manufacturing.
Kelvion air dryers do not require an additional elongation module. A significant advantage. This is because the thermally-derived expansion is equalized between the tube bundle and mounting frame and thus compensated for in the heat exchanger itself. The hairpin design makes hassle-free installation possible. The pressure-carrying and bearing parts of the heat exchangers are made from stainless steel and welded together into a compact unit. All weld connections have been pickled. This protects against corrosion. Following heat recovery, the cooled air is routed through a chimney into the atmosphere.
Along with the Kelvion air dryers, the customer has also installed ten heat exchangers at various locations and in various temperature zones of the drying process as air heaters. The Kelvion type ET-D7-FE air dryers pre-heat the continuous flow of incoming fresh air to approx. 90°C. They also have an air-side mounting frame with an airtight design that can withstand a maximum pressure of 1.45 psi (10,000 Pa). The thermally-derived expansion takes place in the heat exchanger, just like with the air dryers. The heat exchanger surface consists of exterior hot dip galvanized steel finned tubes with elliptically-shaped core tubes and rectangular fins that ensure a uniform air flow.
This tube design results in low pressure loss and high heat transfer coefficients. Because of this, the heat exchanger surfaces can be significantly smaller than tubes with a circular design while providing the same capacity. This reduces the installation size as well as production costs. The pressure-carrying and bearing parts of the heat exchangers are made from carbon steel and welded together. Once they are assembled, they receive a two-part corrosion protection coating with aerosol, Zink-Hell, P565-4019, silicon-free that is resistant up to 752°F (400°C).
According to the manufacturer, this cardboard machine is the most advanced of its kind in the world – as well as being the most effective and the largest – and was successfully put into operation on June 28, 2019. It sets performance standards for production lines for the most varied grades of cardboard: It has an annual capacity of 8,305,000 tons (7,550,000 tonnes) with a construction speed of 0.75 mi/min (1,200 m/min) and a track width of 28.9 ft (8,800 mm). Commissioning took just seven days from material on strainer to having paper on the roller.