Convection section piping

Pipe StiU furnaces vary greatly and, in contrast to the early units where capacity was usuaUy 31.8—79.5 m /d (200—500 bbl//d), can now accommodate 3975 m (25,000 bbl) or more of cmde oU per day. The waUs and ceiling are insulated with firebrick and the interior of the furnace is partiaUy divided into two sections a smaller convection section where the oU first enters the furnace and a larger section fitted with heaters where the oU reaches its highest temperature. 

C. C. Monrad, Heat transmission in the convection section of pipe stills, Ind. Eng. Chem. 24, 505 (1932). 

There is one point to watch in regard to the crossover temperature. The fluid temperature determined as previously described corresponds to the transition from true convection tubes to shield tubes. In many heaters the shield tubes are part of the convection section, and the crossover piping is actually between the shield and radiant tubes. The temperature in the crossover piping is then higher than that calculated in the rating by an amount corresponding to heat absorption in the shield rubes. 

It should be understood that there are many variables to this desCTipiion. Burners may be located In the side walls or roof of the radiant section. Insulation may be reftactory brick, ceramic fibers, or a mineral wool blanket. The produa may flow into the convection section tubes, exit that seaion through crossover piping, and flow through the radiant seaion. Multiple furnaces may be tied to one common sack by breeching. 

A fixed soot blower, shown in Exhibit 7-26, is mounted directly on the convection section wall and is hard-piped as shown. A retractable soot blower, illustrated in Exhibit 7-27, allows the lance to be removed from the convection. seaion during operation. Some of the principal components are the support channel, the gear-driven carriage, the poppet valve (used to control the flow of the cleaning medium), and the lance with nozzles. Exhibit 7-28 depicts a soot blower in operation. As the lance enters the heater, the blowing medium cuts a path through the deposits until the lance reaches its apex. The lance then reverses rotation and is indexed so that on the retraction path it cleans surfaces not covered on insertion. The reversed rotation and indexing allow the soot blower to peel and strip all deposits efficiently and with less chance of heater tube erosion. 

In the conveaion section, three stationary soot blowers are fed by 1.5-in leads from the fire steam header located below them. This piping is kept close to the convection section wall to maximize the available work area for plant personnel. The four 3-in product inlet lines have manual control valves and local flow indicators that must be visible when personnel are operating the valves. They are located at the... 

EXHIBIT 7-44 Convection Section Steam Piping with Miscellaneous Structural Details... 

Exhibit 7-44 illustrates the convection section steam coil piping along with some miscellaneous structural details. The steam inlet line can be run horizontally to the inlet nozzles because the elevation is above head-room. The superheated steam outlet piping can be routed below the platform so that it will not pose a... 

Obtain by dimensional analysis a functional relationship for the wall heat transfer coefficient for a fluid flowing through a straight pipe of circular cross-section. Assume that the effects of natural convection can be neglected in comparison with those of forced convection. 

The heat transfer coefficient to the vessel wall can be estimated using the correlations for forced convection in conduits, such as equation 12.11. The fluid velocity and the path length can be calculated from the geometry of the jacket arrangement. The hydraulic mean diameter (equivalent diameter, de) of the channel or half-pipe should be used as the characteristic dimension in the Reynolds and Nusselt numbers see Section 12.8.1. 

The heat transfer coefficient at the inside wall and pressure drop through the coil can be estimated using the correlations for flow through pipes see Section 12.8 and Volume 1, Chapters 3 and 9. Correlations for forced convection in coiled pipes are also given in the Engineering Sciences Data Unit Design Guide, ESDU 78031 (2001). 

In order to understand these boundary conditions, let us consider that the inlet pipe in which ideal plug flow occurs has the same diameter (shown by broken lines) as the reactor itself (Fig. 2.21). Inside the reactor, across any section perpendicular to the z-direction, the flux of the reactant, i.e. the rate of transfer is made up of two contributions, the convective flow uC and the diffusion-like dispersive flow... 

For heat transfer for a fluid flowing through a circular pipe, the dimensional analysis is detailed in Section 9.4.2 and, for forced convection, the heat transfer coefficient at the wall is given by equations 9.64 and 9.58 which may be written as ... 

With mixed convective flow in a horizontal pipe the buoyancy forces act at right angles to the direction of forced flow leading to the generation of a secondary motion as discussed earlier. The equations governing this type of flow will be briefly discussed in this section. 

Natural convective flows in porous media occur in a number of important practical situations, e.g., in air-saturated fibrous insulation material surrounding a heated body and about pipes buried in water-saturated soils. To illustrate how such flows can be analyzed, e.g., see [20] to [22], attention will be given in this section to flow over the outer surface of a body in a porous medium, the flow being caused purely by the buoyancy forces resulting from the temperature differences in the flow. The simplest such situation is two-dimensional flow over an isothermal vertical flat surface imbedded in a porous medium, this situation being shown schematically in Fig. 10.25. 

The plumbing system of a house involves a 0.5-m section of a plastic pipe (fc = 0.16 VV/m °C) of inner diameter 2 cm and outer diameter 2.4 cm exposed to tl e ambient air. During a cold and windy niglit, the ambient air temperature remains at about —5°C for a period of 14 h. The combined convection and radiation heat transfer coefficient on the outer surface of the pipe is estimated to be 40 W/ni C, and the heat of fusion of water is 333.7 kJ/kg. Assuming the pipe to contain stationary water initially at 0°C, deienninc if the water in that section of the pipe will completely freeze that night. 

Analysis The thermal resistance network for this problem involves four resistances in series and is given in Fig. 7-40. The inner radius of the pipe is r-j = 0.8 cm and the outer radius of the pipe and thus the inner radius of the insulation is rj = 1.0 cm. Letting ra represent the outer radius of the insulation, the areas of the surfaces exposed to convection for an L = 1-m-long section of the pipe become... 

Hot water at 90°C enters a 15-m section ofa cast iron pipe (k = 52 W/m °C) whose inner and outer diameleis arc 4 and 4,6 cm, respectively, at an average velocity of 1.2 m/s.Tlie outer snrface of the pipe, whose emissivily is 0.7, is exposed to the cold air al 10°C in a basement, with a convection heal transfer coefficient of 12 W/ni C. Taking the walls of Ihe basement to be at 10°C also, determine (a) the rate of heat loss from tile water and (f>) the temperature at which the water leaves Uie basement. 

A 6-m-long section of an 8-cm-diameter horizonlal hot-water pipe shown in Fig. 9-13 passes through a large room whose temperature is 20 C. If the outer surface temperature of the pipe is 70 C, determine the rate of heat loss from the pipe by natural convection. ... 

A 10-m-long section of a 6-cm diameter horizontal hot-water pipe passes through a large room whose temperature is 2TC. If the temperature and the emissivity of Ihe outer surface of the pipe are 73°C and 0,8, respectively, determine the rale of heal loss from the pipe by (a) natural convection and (b) radiation. 

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