In operations in which materials are combusted and heat recovered from the flue gas stream from such combustion, heat-exchange surfaces, usually in the form of banks of tubes, are provided in the flow path of the flue gas. The heat exchange surfaces remove heat from the flue gas stream to a cooler fluid medium flowing through the tubes.
Such operations include combustion furnaces of varying types, including coal-fired boilers, oil-fired boilers and pulp mill recovery furnaces, and generally result in the presence of particulates in the flue gas stream. Some of such particles deposit on and adhere to the exposed tube surfaces. These deposits build up on the tube surfaces and decrease the efficiency of heat transfer from the flue gas stream to the heat-exchanger tubes.
From time-to-time, these deposits are removed by jets of high pressure steam or other suitable gas from a so-called sootblower. A sootblower generally consists of an elongate support rod or lance which reciprocates between the bank of tubes and has a spray head having two opposing convergent-divergent nozzles from which high speed steam jets emanate and are aimed at the heat transfer tubes.
In studying the operation of existing sootblower nozzles, we found that sonic conditions exist at the throat of the nozzle with supersonic flow at the exit and that, a normal shock wave (i.e. a shock wave normal to the direction of gas flow) forms a short distance from the nozzle exit. This shock wave causes a considerable reduction in the stagnation pressure of the steam jet. As a result, there is a substantial reduction of available energy for removal of the deposits on the tube bank.