Return leg

In circulating fluidized beds two main attrition sources, namely the riser and the return leg, may be distinguished. Although a lot of information is available about solids flow patterns and flow structures inside the circulating fluidized bed risers, no systematic investigations have been found in the open literature on the influence of riser geometry and flow conditions inside the riser on attrition. With respect to attrition occurring in the return leg, the work of Zenz and Kelleher (1980) on attrition due to free fall may be mentioned (cf. Sec. 4.3). 

Figure 11. Pressure distribution in solids recycle system of a circulating fluidized bed (a) fluidized bed (b) return leg.

Fig. 2.6. Hypothetical localized variants on the proton circuit, a, fully delocalized circuit b, proton current flows along surface of membranes (note that circuit is still in equilibrium with the bulk phases) c, one leg of the proton circuit is conducted through a lateral channel insulated from the bulk phase d, both legs of the proton circuit are conducted through lateral channel insulated from the bulk phases R, respiratory chain A, ATP synthase. Note the necessity for both outward and return legs in all models.

The dried and crushed wood fuel is pressurised in a lock-hopper system to a level which basically is determined by the pressure ratio of the gas turbine, and is fed by screw feeders into the gasifier a few meters above the bottom. The operating ten erature of the gasifier is 950 - 1000 C and the pressure is approximately 17 bar, The gasifier is of a circulating fluidized bed type and consists of the gasifier itself, cyclone and cyclone return leg. The three parts are totally refractory lined. 

The fuel is dried, gasified and pyrolized immediately on entering the gasifier. The gas transports the bed material and the remaining char towards the cyclone. In the cyclone, most of the solids are separated from the gas and are returned to the bottom of the gasifier through the return leg. The recirculated solids contain some char, which... 

Skrifvars B.-J, et al. (1997) Mechanisms of bed material agglomeration in the cyclone and the return leg of a petroleum coke fired circulating fluidized bed boiler. In Proceedings in I4th International Conference on Fluidized Bed Combustion, Vancouver, Canada, 819-829. 

LONG-TUBE EVAPORATORS WITH UPWARD FLOW. A typical long-tube vertical evaporator with upward flow of the liquid is shown in Fig. 16.1. The essential parts are (1) a tubular exchanger with steam in the shell and liquid to be concentrated in the tubes, (2) a separator or vapor space for removing entrained liquid from the vapor, and (3) when operated as a circulation unit, a return leg for the liquid from the separator to the bottom of the exchanger. Inlets are provided for feed liquid and steam, and outlets are provided for vapor, thick liquor, steam condensate, and noncondensable gases from the steam. 

CONVEYORS. Pneumatic conveyors for solids are described in Chap. 7. Other common devices for transportation include belt conveyors and bucket elevators, closed-belt conveyors with zipperlike fasteners, and various kinds of drag and flight conveyors. These all include a return leg that carries the empty belt or chain back from the discharge to the loading point. Vibrating conveyors and screw conveyors have no return leg but only operate over relatively short distances. Pneumatic conveyors also have no return leg and are not so limited as to distance of travel. Conveyors for solids are discussed in Ref. 9a. 

Fig. 3.14. Heat treating furnace with radiant U-tubes on the roof and back wall. The return legs (2nd and 4th from the hearth) are less radiant than the burner legs (1 st and 3rd from the hearth). Tumbling around the bends completes gas-air mixing so the renewed delayed-mixing flame (type F, fig. 6.2) causes a glow in the second leg. Courtesy of Rolled Alloys, Temperance, Ml.

Figure 3 shows that operating regimes from the onset of turbulent fluidization (beyond the transition velocity, U ) follow different paths depending upon the two general CFB designs defined by Kobro and Brereton [62] and Kunii and Levenspiel [63]. In the first, Fixed Inventory System (FIS), solids inventory in the return leg or standpipe is not controlled. Setting gas velocity and system inventory establishes the riser suspension density profile and solids circulation rate. In the second, Variable Inventory System (VIS), a vessel external to the riser acts as a... 

Study on application of closing valve in the return leg, observation of an identifiable particle, using a device to record the force imparted by returning solids form the cyclone, measuring the pressure drop across the constriction in the return loop, and estimating solids mass flow from the heat balance on a calorimetric section in the standpipe... 

Primary system temperatures are sensed by thermocouples located in the supply and return legs of the heat exchanger and displayed as traces on the recorder at the console. 

Circulating fluidized beds (CFBs) are high velocity fluidized beds operating well above the terminal velocity of all the particles or clusters of particles. A very large cyclone and seal leg return system are needed to recycle sohds in order to maintain a bed inventory. There is a gradual transition from turbulent fluidization to a truly circulating, or fast-fluidized bed, as the gas velocity is increased (Fig. 6), and the exact transition point is rather arbitrary. The sohds are returned to the bed through a conduit called a standpipe. The return of the sohds can be controUed by either a mechanical or a nonmechanical valve. 

There are two types of input power buses. DC power buses are single-wire power connections such as found in automobiles and aircraft. The ground connection forms the other leg of the power system. The other form of input connection is the ac, or two or three-wire feed systems as found in ac power systems. The design of the EMI filter for dc systems is covered in Section 3.12 and takes the form of a simple L-C filter. All the noise is common-mode between the single power wire and the ground return. The dc filter is much more complicated, because of the parasitic behavior of the components involved. 

Orifice Mixing Column An orifice mixing column consists of a series of orifice plates contained in a pipe. The pipe normally is fabricated of two vertical legs connected by a return bend at the bottom with the orifice plates installed between flanges in the vertical legs. Typical use is for cocurrent contacting in caustic and water washing operations. 

Air vents are most effective when they are fitted at the end of a length of 300 mm or 450 mm of uninsulated pipe that can act as a collecting/cooling leg. Air is an excellent insulating material, having a thermal conductivity about 2200 times less than that of iron. The last place where it can be allowed to collect is in the steam space of heat exchangers. Further, as it contains oxygen or carbon dioxide, which dissolve readily in any subcooled condensate that may be present, the presence of air initiates corrosion of the plant and the condensate return system. 

Baroreceptors are sensitive to changes in MAP. As VR, CO, and MAP decrease, baroreceptor excitation is diminished. Consequently, the frequency of nerve impulses transmitted from these receptors to the vasomotor center in the brainstem is reduced. This elicits a reflex that will increase HR, increase contractility of the heart, and cause vasoconstriction of arterioles and veins. The increase in CO and TPR effectively increases MAP and therefore cerebral blood flow. Constriction of the veins assists in forcing blood toward the heart and enhances venous return. Skeletal muscle activity associated with simply walking decreases venous pressure in the lower extremities significantly. Contraction of the skeletal muscles in the legs compresses the veins and blood is forced toward the heart. 

---