Operational velocity

Chaiacteiistics of tfie pads vaiy slighdy witfi mesh density, but void space is typically 97—99% of total volume. Collection is by inertial impaction and direct impingement thus efficiency will be low at low superficial velocities (usually below 2.3 m/s) and for fine particles. The desireable operating velocity is given... 

The heat-transfer quaUties of titanium are characterized by the coefficient of thermal conductivity. Even though the coefficient is low, heat transfer in service approaches that of admiralty brass (thermal conductivity seven times greater) because titanium s greater strength permits thinner-walled equipment, relative absence of corrosion scale, erosion—corrosion resistance that allows higher operating velocities, and the inherently passive film. 

When placing mesh in small diameter vessels it is important to discount the area taken up by the support ring before determining the operating velocity of the unit. For small 6-, 8-, and 12-inch vessels (such as in-line, pipe-with-mesh units) it is usual practice to use 6- or 8-inch thickness of mesh for peak performance. 

The minimum bed diameter is a function of the operating velocity, the particle characteristics and the humidity of the drying gas. The hot gas at the inlet rapidly loses heat and gains moisture as it passes through the bed which it eventually leaves at the bed temperature Tb and with a relative humidity which is approximately equal to the relative humidity which would be in equilibrium with the dried product at the bed temperature. The operating velocity may be taken as twice the minimum fluidising velocity, obtained from the equations in Section 6.1.3, by laboratory tests, or more conveniently from Figure 16.26. 

Figure 16.26. Superficial operating velocity in fluidised bed dryers ...

At low gas velocities, the bed of particles is practically a packed bed, and the pressure drop is proportional to the superficial velocity. As the gas velocity is increased, a point is reached at which the bed behavior changes from fixed particles to suspended particles. The superficial velocity required to first suspend the bed particles is known as minimum fluidization velocity (umf). The minimum fluidization velocity sets the lower limit of possible operating velocities and the approximate pressure drop can be used to approximate pumping energy requirements. For agglomeration process in the fluid-bed processor, air velocity required is normally five to six times the minimum fluidization velocity. 

Finally, it should be noted that in the case of multisized solids, the operating velocity should be higher than the minimum fluidization velocity of the largest particle and smaller that the elutriation velocity of the smallest particles. 

Since, the particle size used is greater than 45 pm, the value of (X45) is zero. The resulting minimum bubbling velocity is less than the operating velocity (1 cm/s). Thus, the operating regime is purely bubbling fluidization. 

Then, for velocities higher than 8.38 cm/s, which is much higher that the operating velocity, it is expected that slugging will occur in the bed. 

Coal slurry pipelines have been constructed in severaJ countries, including a 38-mile (61-kilometer) 12-inch (30.4-centimeter) diameter pipeline m Russia, a 51 -mile (82-kilometer) pipeline in Poland, as well as Olliers in Prance and other locations in Europe. The feasibility of slurry transportation depends upon Ihc resolution of a number of variables, rhe most important of which from a hydraulic standpoint are (I) Size consist (2) velocity and (3) concemrauon. The selection of a proper size eonsisi (gradation) is important in order that homogeneous flow can he achieved at prudent operating velocities. For coal slurry, such a consist is on the order of 8 mesh by 0 (approximately 0.1-inch (2.5-millimeter) particle size to dust) Homogeneous flow (solids evenly distributed across the pipe diameter) is important if excessive wear in the bottom of the pipe is to be avoided and stable operation achieved. 

As follows from Figure 4, for coarse coal transport the power consumption of slurry pipelining reaches from 300 % to 100 % higher values than that in case of hydraulic capsule pipeline transport. The energy reduction increases with reduction of the operational velocity. 

Similar result brings comparison of power consumption reduction for transport of encapsulated viscous liquid (Russian oil) conveyed by water with conventional pipeline transport of the oil. Transport of viscous oil and oil products by means of capsule pipelining may again provide power consumption reduction from 50% to 70%, the reduction increases with operational velocity. Since for low temperature the oil viscosity significantly increases hydraulic capsule pipeline transport of highly viscous oil and oil products for long distances in arctic conditions can be economically attractive. Capsule pipeline transport could be recommended as suitable transport especially for longer distances when power consumption becomes the most important for operational cost. 

In traditional processing devices, increase in relative velocity is limited by various factors. For example, in column equipment the operating velocity must be smaller than that of liquid-flooding the limitation of relative velocity in common gas-solid or liquid-solid suspensions is the terminal velocity, etc. It seems that other approaches must be found in order to raise the relative velocity between phases to higher levels. 

It can be seen that in the arrangement of flow configuration CISD is somewhat similar to that of the batch IS dryer shown in Fig. 6.12, while the others are quite different. The CISD can be considered to be a combination of a spouted bed with central duct and impinging streams. The lower accelerating tube in CISD is equivalent to the central duct in a spouted bed but the operating velocity in the former is usually much higher than in the latter. 

Equations (El 0.13) and (E10.17) can be solved simultaneously to obtain the relationship between the minimum operable gas velocity and solids circulation rate, as given in Fig. E10.2. To illustrate the dependence of operable velocities on system designs, the results for three solids inventory heights in the downcomer are given as shown in Fig. E10.2. It is seen in the figure that for a given solids circulation rate, a higher inventory level yields a lower minimum operable velocity. 

Figure 12.5. The H/v ratio shown as a slope for the two operating velocities u, and "2-...

In Liquid Chromatography. Molecular diffusion, the B term, is seldom of importance in LC because operating velocities are usually well above those at which its effects would be noticed. On the other hand, both C terms are important. As a consequence, plots of the rate equation usually differ somewhat from GC plots, as shown in Figure 2.8. 

As Fig. 5b (Li et ai, 1984) shows, there is no intermediate solids hopper as for type A, and the solids rate device is used only for measurement rather than for control. In operation, solids circulation rate is determined essentially by gas velocity. Therefore, gas velocity and solids circulation rate could not be adjusted independently. Solids circulation rate follows what an operating velocity produces at whatever solids inventory the system was initially filled with. Owing to the absence of an intermediate hopper, the initial solids inventory could influence directly axial voidage profile, that is, the position of the inflection point changes with the initial solids inventory in the system. 

Refs. Oversurfacing of heat exchangers in fouling service results not only in increased equipment costs but also in more cleaning waste, because operating velocities will be lower than the design velocity, and the reduced turbulence results in increased deposits (see Ref. [5]). 

In most cases this critical velocity is well above the maximum operating velocity of the SSHE. 

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