Soot blowing is often resorted to in coal-fired or heavy oil-fired boilers. In packaged boilers, both steam and air have been used as the blowing media, and both have been effective with heavy oil firing. Rotary blowers are sometimes used with distillate oil firing. Steam-blowing systems must have a minimum blowing pressure of 170-200 psig to be effective. The steam system must be warmed up prior to blowing to minimize condensation. The steam must be dry. Increasing the capacity of a steam system is easier than increasing that of an air system. With an air system, the additional capacity of the compressor must be considered. Also, because steam has a higher heat transfer coefficient than air, more air is required for cooling the lances in high gas temperature regions compared to steam. Moisture droplets in steam can cause erosion of tubes, and often tube shields are required to protect the tubes. The intensity of the retractable blower jet is more than that of the rotary blower jet, and its blowing radius is larger, thus cleaning more surface area. However, one must be concerned about the erosion or wear on the tubes.
Sonic cleaning has been tried on a few boilers. In this system, low frequency high energy sound waves are produced when compressed air enters a sound generator and forces a diaphragm to flex. The resulting sound waves cause particulate deposits to resonate and dislodge from the surfaces. Once dislodged, they are removed by gravity or by the flowing gases. Typical frequencies range from 75 to 33 Hz. Sticky particles are difficult to clean. The nondirectional nature of the sound waves minimizes accumulation in blind spots where soot blowers are ineffective. Piping work is minimal. Sonic blowers operate on plant air at 40-90 psi and sound off for 10 s every 10-20 min.