In heating plants and saw-mills, a cost-efficient and compact way to recuperate the energy in the hot flue gas, is by means of a smoke tube boiler. As the name suggests in a smoke tube boiler, the flue gas passes inside tubes, surrounded by a water mantle cooling the smoke tubes. The boilers are often mounted vertically and have a few tube passes with several hundreds of tubes inside the boiler.
On-line cleaning of smoke tube boilers is challenging and must often be combined with manual cleaning as well. Traditional cleaning methods include compressed air blowing from the top of the boiler as well as manual cleaning of each individual tube. For the manual cleaning, the boiler must often be taken offline and an oil-fired boiler can be necessary to compensate the production loss. The manual cleaning is a dangerous work that can take much time and cost both man hours and oil consumption.
Most commonly, the fouling of a smoke tube boiler starts in the bottom turning chamber, where there is no automatic cleaning and where the compressed air blowing from the top does not reach. The fouling mechanism includes accumulation of so called “beard” at the inlet of the smoke tubes. As the beard grows, it is sucked into the tubes and eventually clog them. Since each tube constitutes for 0.5 % of the heat transfer area of the pass, heat transfer is quickly deteriorated resulting in losses in thermal efficiency. Using long range infrasound, the smoke tubes are kept open in the bottom, avoiding said losses and prolonging the time between outages for manual cleaning.