Convective tube fouling

Fouled convective section. For oil-fired healers with finned convective tubes, fouling may reduce draft. A gradual loss of draft probably means the convective section needs to be water washed. Deposits from the burnt fuel oil have built up around the lubes thick enough to restrict the passage of flue gas. The increased pressure drop through the convective section reduces the vacuum in the firebox. Permanently installed soot blowers are one answer to this problem. 

The inside of the convection tubes rarely foul, but occasionally the Hquid unsaturates in feedstocks tend to polymerize and stick to the walls and thus reduce the heat transfer. This soft coke is normally removed by mechanical means. In limited cases, the coke can also be burnt off with air and steam. Normally, the outside surface of the convection section fouls due to dust and particles in the flue gas. Periodically (6 to 36 months), the outside surface is cleaned by steam lancing. With Hquid fuel firing, the surface may require more frequent cleaning. 

Convection tube spacing is important when the fuel is residual oil or coal, especially co with low asn-fusion or high ash-fouling tendencies. The amount of the ash and, even more important, the characteristics of the ash must be specified for design. 

A laboratory scale (nominally 50 kW) combustion system was used to conduct the combustion tests. The over stoker combustor is mounted in a rig comprising a convective tube bank, fouling probe, stack, cyclone and associated sampling, and monitoring equipment. Detailed descriptions of the experimental apparatus are provided elsewhere (9). It should be noted that no secondary air was used in the combustion process. 

The key scientific principles associated with the operation of a heat exchanger system include Temperature—preheat, condenser, reboiler, conversions Heat transfer—conductive, convective Tube growth—expansion Pressure—delta, inlet, outlet Fouling Boiling points pH of water... 

Flame impingement. This is often caused by dirty burner tips, lack of combustion air, poorly designed burners, high burner tip pressure, improper adjustment of the burner, or improper draft. I have seen a heater in Cartagena, Colombia, with the flames being forced outward against the upper radiant wall tubes. The problem was an extreme positive pressure in the firebox, due to excessive pressure drop of the flue gas in the fouled convective section. 

Consider the typical case of heat transfer between one fluid inside a tube and another fluid outside the tube, shown in cross section in Fig. 11. Heat is transferred by convection from the hot fluid (taken arbitrarily to be the fluid inside the tube) to the fouling deposit (if any) on the inside surface, through the fouling deposits and tube wall by conduction, and then by convection to the fluid outside the tube. At the point where the inside fluid temperature is T and the outside is t. the local heat flux inside the tube is... 

FIGURE 11 Cross section of a heat exchanger tube, with convective heat transfer in the fluids and fouling deposits on the surfaces. 

Gs = shell-side mass velocity across tubes based on the minimum free area between baffles across the shell axis, lb/(hXft2) h = film coefficient of heat transfer, Btu/(hXft2X°F) subscript c indicates convection subscript d represents dirt or fouling subscript co indicates conduction... 

Ash fouling appears to be initiated by the formation of a layer of sodium sulfate on the boiler tube. It is thought that thermal decomposition of sodium salts of carboxylic functional groups in the coal is the start of a sequence of reactions leading ultimately to the formation of sodium sulfate in the flame or flue gas. The convective mass transfer diffusion of the sodium-containing species through a boundary layer around the tube results in deposition of sodium sulfate on the tube surface. 

A double pipe (shell-and-tube) heat exchanger is constructed of a stainless steel [k = 15.1 W/m O inner lube of inner diameter O/ = 1.5 cm and outer diameter 1.9 cm and an outer shell of inner diameter 3,2 cm. The convection heat transfer coefficient is given to be h,- = 800 W/m °C on the inner surface of the tube and h = 1200 W/m °C on the outer surface. For a fouling factor of f f, - 0.0004 m °C/W on the tube side and Ri =- 0.0001 m °C/W on the shell side, determine (a) the thermal resistance of the heat exchanger per unit iength,and (6) the overall heat transfer coefficients, Ujand U based on the inner and puter surface areas 0) the tube, respectively. 

Fouling is the accumulation of mineral-derived ash on the superheater and reheater tubes in the convective (heat exchanger) section downstream from the furnace. Fouling restricts the flow of exhaust gases and impedes heat transfer through superheater tube walls and thereby reduces the amount of steam generated. 

Impedance to gas flow is the result of heavy fouling on tubes in the convective section. Problems of this type are most likely to occur with coals having high sodium contents, such as those found in low rank coal deposits in Western U.S. seams. Hard, bonded deposits can occur which are resistant to removal by the retractable sootblowers. 

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