Unfortunately, traditional methods of cleaning tubes within convection banks, whether
finned, bare or studded, leave a great deal to be desired. The majority, if not all, cleaning
medium delivery mechanisms are by way of the cleaning contractor manually inserting a long hollow injection lance through each access door or via a static, in built soot blowing lance whether the heater is on or offline.
They are restricted in that the area of the fouled convection bank that can be physically reached by the cleaning lance is invariably limited. An average sized finned convection bank has a surface area of around 2.5 square kilometres. As can be seen in Figure 3, traditional methods can only target “reachable” areas, of which they barely clean 2% of this, leaving 98% of the total surface area untouched and still fouled.
Figure 3. Limited access means that traditional cleaning methods only ever clean a small
portion of the entire finned convection bank, as the remaining surface area cannot be reached from the available access points
Some fired heaters have zero access to the convection bank, while others have doors
measuring less than 150mm x 200mm. This issue is compounded by the basic inability of
traditional cleaning methods to access between tubes or remove even moderate fouling sitting between the fins or studs.
One popular offline method, water jetting, congeals refractory dust and combustion deposits, turning it and soot into a paste which later dries into a concrete-like deposit between the tubes and finned areas.
Similarly, lance applied abrasive blast media can erode and block fins, creating a “key way” increasing future fouling rates and making fouling impossible to remove. Soot blowers are as the name implies “soot” blowers – they cannot remove deposits that are harder than soot and are statically positioned, moving in and out but not between rows.
Foam cleaning can in theory make “surface contact” with the majority of the convection bank, but this only extends to the surface of the deposit and is not certain given that fouling
characteristics deep inside the convection bank are unknown and foam, as well as chemicals,
always takes the path of least resistance. Due to lack of shear force, foam cleaning relies on the dissolution of oil-based deposits with detergents and solvents and is appropriate for very light dust deposits.
In addition, fouling is more prevalent on the underside of convection banks, but
foam is administered from the top. While on-line cleaning methods do exist, they often struggle with hard, difficult to shift deposits and clean in only one direction using the updraft from the flue gasses and have the limitations associated with manual lance application.