Convective-section finned tubes

However, the convective-section finned tubes are not intended or designed to withstand such high temperatures, and so the tubes themselves are often made from low-temperature-rated carbon steel, whereas the fins, which are not much cooled by the process flow, are often made from low-chrome steel. As it is much easier to make finned tubes from just one type of metal instead of two, though, the furnace manufacturers will often choose to make the finned convective-section tubes entirely out of low-chrome steel in which case one could expect... 

Mixed with fresh air flowing through convective section leaks, the flue gases may reignite. This phenomenon, called afterburn, results in damage to the convective section finned tubing. Afterburn is promoted by insufficient oxygen in the firebox, excessive draft, and leaks in the convective section exterior walls. 

If the average flue-gas temjjerature in the stack is 300°F, then some of the flue gas will be cooler and some will be hotter. The cooler gas will then begin to precipitate sulfuric acid. This acid reacts with the convective section finned tubes to form iron sulfate. A thick, sticky, moist, acidic, grayish deposit will begin to plug the convective section tube bank. The flow of flue gas will be restricted. A reduced draft and a positive pressure will begin to develop in the firebox. 

Open Tube Sections (Air Cooled) Plain or finned tubes No shell required, only end heaters similar to water units. Condensing, high level heat transfer. Transfer coefficient is low, if natural convection circulation, but is improved with forced air flow across tubes. 0.8-1.8... 

The combustion gases flow across the tube banks in the convection section and the correlations for cross-flow in tube banks can be used to estimate the heat transfer coefficient. The gas side coefficient will be low, and where extended surfaces are used an allowance must be made for the fin efficiency. Procedures are given in the tube vendors literature, and in handbooks, see Section 12.14, and Bergman (1978b). 

Heat transfer to the tubes on the furnace walls is predominantly by radiation. In modern designs this radiant section is surmounted by a smaller section in which the combustion gases flow over banks of tubes and transfer heat by convection. Extended surface tubes, with fins or pins, are used in the convection section to improve the heat transfer from the combustion gases. Plain tubes known as shock tubes are used in the bottom rows of the convection section to act as a heat shield from the hot gases in the radiant section. Heat transfer in the shield section will be by both radiation and convection. The tube sizes used will normally be between 75 and 150 mm diameter. The tube size and number of passes used depend on the application and the process-fluid flow rate. Typical tube velocities will be from 1 to 2 m/s for heaters, with lower rates used for reactors. Carbon steel is used for low temperature duties stainless steel and special alloy steels, for elevated temperatures. For high temperatures, a material that resists creep must be used. 

A conventional typical convection section often has tube bundles for fuel gas or air preheat, feed preheat, boiler feed water preheat, steam generation, and steam superheating. In the convection section, the heat transfer is mainly gas-to-gas heat transfer and the overall heat-transfer coefficients are relatively low. Finned tubes are generally used to improve heat-transfer rates. Material for the convection section tubes varies from carbon steel to a high temperature alloy. Sometimes, high-alloy tubes are positioned in the lower section... 

In-line banks of high-finned tubes are not usually used in air-cooled heat-transfer equipment because the preferential flow path between the fin tips of adjacent tubes allows flow bypassing and reduces the apparent heat-transfer coefficient. This effect tends to disappear in the deeper tube banks used in convection sections of fired heaters. 

The basic rating method presented here is applicable to extended surface equipment in general, but it is designed especially for the finned tube convection section of fired heaters. The rating problem is divided into three parts ... 

Example 2. Using the same basis as the rating example in the radiant section, it is easy to compare plain and finned tubes in the convection section. 

The next problem is the calculation of the flue gas pressure drop as it crosses the selected finned tube convection section. Using the design information for tubes per row, number of rows, tube spacing, fin and tube geometry and fin pitch, compute the net free volume and friction surface of the convection section. 

Finally, the results show an average heat flux of 14,400 Btu/hr./sq.ft. for the convection section. Although this value exceeds the specified maximum for the radiant section, it should not cause overheating of the oil for two reasons (1) the heat input around the tubes is relatively uniform compared with radiant section tubes and (2) the oil temperature is lower than in the radiant section. Whenever a lower heat flux is required to lower either oil or fin temperature, it can be obtained by increasing the spacing between fins or by reducing the fin height. 

This method differs from the convection section method in the previous example in two z xxs (1) it provides for rating finned tube -riins and (2) it includes credit for radiation ge through the shield coil. The radiation... 

Schweppe, J.L. Torrijos, C.Q. How to Rate Finned-Tube Convection Section in Fired Heaters. Hydrocarbon Processing Petroleum Refiner, June 1964, p. 159. 

Addition of more tubes in the available free space in the convection section Installation of better finned tubes Installation of steam... 

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. 

Because the convective section s fins are constructed from high-chrome steel, they can withstand typical maximum flue-gas temperatures of 1,200°F. Should afterburn occur in the convective tube bank, however, the fins may oxidize when exposed to the 2,000°F that results from the localized combustion caused by aflerburn. 

On one particular unit, the fins on the convective section tubes had been reduced to a brittle metal scale. In this condition the fins retarded rather than enhanced heat transfer. The cause of this fin damage was secondary combustion in the convective section. This is an extremely common but frequently unrecognized problem. 

In reality, the heater box was short of air. The unburnt hydrocarbons completed combustion upon mixing with the air drawn into the convective section. The liberation of radiant heat in the midst of the convective tube bank oxidized the steel alloy fins on the tubes. R. D. Reed has discussed this subject in detail. ... 

This fin tube failure could have been prevented by sampling the flue gas just below the convective section inlet. The oxygen concentration at this point represents the true amount of excess air available for combustion in the firebox. 

In some applications, the heating surface or tubes are of the bare-tube type. In other cases, the heating snrface will be of the extended surface or fin-tube type. Utihzing fin tubes allows greater tube surface within the convection section. Greater heat-transfer levels can be achieved with the use of this type of tubing. 

When combustible materials or unburned fuel re-ignite in the convective section, a dramatic increase in flue-gas temperature will occur. The metallurgy of finned carbon steel convective-section tubes is not designed to withstand high temperatures. The fins will become oxidized and, when cool, may become brittle and thicker than before, thus restricting flue-gas flow. The convective-section tubes themselves will become warped and bent and thus restrict flue-gas flow still further. 

In the isoflow stills (g) and (h), finned tubes are used in tke convection section. This greatly reduces the amount of tubing required and resulU in rates of heat transfer in the convection section as large as or sometimes larger than in the radiant section. In still h) the upper ends of the tubes are finned and thus there is no distinct convection section. The Selas still (k) employs ceramic burner cups, spaced at about 30 in., which cover both walls. 

Choose the spacing of the convection tubes so that the mass velocity is G = 0.3-0.4 lb/(sec)(sqft free cross section). Usually this spacing is the same as that of the shield tubes, but the value of G will not be the same if the tubes are finned... 

Numerous fin corrugations have been developed, each with its own special characteristics (Figure 15). Straight fins and straight perforated fins act like parallel tubes with a rectangular cross section. Convective heat exchange occurs due to the friction of the fluid in contact with the surface of the fin. The channels of serrated fins are discontinuous, and the walls of the fins are offset. For air flows,... 

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