Making industrial waste heat usable for district heating – this was an important goal during the redesign of the sinter dust collection system of a major steel plate production plant. The goal here was to make heat from the 350°C exhaust gas usable as district heating for the benefit of the neighboring households and hospital.

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Quiet operation with high performance required
However, the neighborhood around the apartments and hospital also presented a challenge: in summer, when less heat is required, the excess thermal energy must be dissipated into the air. Due to the proximity of the residential area, the cooler used for this purpose must comply with some very strict requirements and operate extremely quietly – furthermore, the installation space available in the plant is very limited. In other words, the emergency cooler had to have a compact, quiet and highly-efficient design.

A special design to solve a tricky case
The design from Kelvion proved that even seemingly conflicting requirements - efficient, compact and quiet - can be reconciled. The components, a specially designed economizer as well as an individually designed air fin cooler were manufactured and delivered in March 2020. The components will be further fitted out with filters, pipework, valves, etc. by Küttner GmbH & Co. KG, a plant engineering company based in Essen. The dust extraction system with the waste heat utilization system is then scheduled to go into operation later this year.

Durability thanks to a laser-welded heat exchanger
The economizer, which is used to extract the heat from the 350°C waste gas and transfer it to a water circuit, is a special version of a standard heat exchanger. It is designed to extract 46,200 kW of heat from an exhaust gas flow of 500,000 m³/h. Its laser-welded finned tube is very robust so that it can withstand the abrasive particles in the exhaust gas flow over a long service life. In addition, the continuous weld seam between the tube and fin ensures an optimal connection with the most efficient heat transfer. Similar systems operate using an elliptical finned tube, for example, with a zinc surface, which would be more efficient and smaller. These were rejected, however, due to the expected wear.

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Due to its total weight, the economizer is designed for on-site assembly. Ten large heat exchangers that individually weigh over twelve tons must be connected up as one unit on site. Since the heat exchangers are not mounted, but rather inserted on their side into the exhaust gas conduit, rolling side panels are installed so that the ten heat exchangers can be inserted from the side. This also provides a bonus in terms of maintenance as heat exchangers can also be individually disengaged from the exhaust gas conduit.

Air fin cooler with large heat exchangers
River water cooling would certainly have been the quietest alternative to dissipate the heat in summer when the district heating network could not absorb it, but this was rejected due to the lack of a flowing body of water. On the other hand, a cooling tower would be too large and costly, particularly since cooling is not required all year round. It was therefore necessary to design a 36,000 kW, fin-based emergency cooler that operates quietly and can be integrated into the industrial system already installed.

The delivered air fin cooler is manufactured completely from steel in a table-top design because of the high water temperatures. It consists of four modules with fans which produce low noise emissions even at nominal output. The special fan blades contribute to this, along with the slow rotational speeds and air speeds. Accordingly, greater air speeds require larger heat exchangers.

Quieter and more efficient with fans installed below
In order to resolve the conflicting requirements of 'quiet vs compact', the Kelvion designers placed the fans beneath the heat exchanger. This forced draft design offers some small benefits in terms of efficiency compared to top mounted versions as it involves movement of cooler air with a somewhat higher density. In addition, the heat exchangers shield the fans acoustically so that less sound is emitted. A pleasant side effect of bottom-installed, forced draft fans is that the heat exchangers can be cleaned more easily.

Special solutions are the new standard
The sinter dust collection system of the steel plate production plant is just one of many examples where the aim for high energy efficiency and durability while meeting environmental and noise emission requirements cannot be met with standard solutions. That's why Kelvion offers a diversified product range of modular, industrial heat exchangers. They can be adapted to the space available, customized for special requests and/or material selections and adapted for integration into preexisting systems without incurring the costs associated with single-unit production.