Minimum maintained velocity

TABLE D-57.7—MINIMUM MAINTAINED VELOCITIES INTO SPRAY BOOTHS... 

Minimum Maintained Velocity The velocity of air movement that must be maintained to meet minimum specified requirements for health and safety. 

Size piping system to maintain minimum required velocity to avoid deposition... 

At any instant, pressure is uniform throughout a bubble, while in the surrounding emulsion pressure increases with depth below the surfaee. Thus, there is a pressure gradient external to the bubble which causes gas to flow from the emulsion into the bottom of the bubble, and from the top of the bubble back into the emulsion. This flow is about three times the minimum fluidization velocity across the maximum horizontal cross section of the bubble. It provides a major mass transport mechanism between bubble and emulsion and henee contributes greatly to any reactions which take place in a fluid bed. The flow out through the top of the bubble is also sufficient to maintain a stable arch and prevent solids from dumping into the bubble from above. It is thus responsible for the fact that bubbles can exist in fluid beds, even though there is no surface tension as there is in gas-liquid systems. 

Estimate the number of tubes per pass to maintain minimum water velocity. 

Minimum water velocities should be maintained in the upper floors of high-rise buildings where air may tend to come out of solution because of reduced pressures. High velocities should be used in down-return mains feeding into air-separation units located at a low level in the system. Table 27.10 can be taken as a guide. Water velocities shown in Tables 27.9 and 27.10 are indicative parameters only on the one hand, to limit noise problems and... 

Temperature can also affect fluidized beds if it causes the particles to sinter or become sticky. Siegell (1984) has shown that above a certain critical temperature, the fluidizing gas velocity must be increased above the minimum fluidization velocity in order to maintain fluidization in the bed. Siegell reported that the increase in velocity required to maintain fluidization was linearly proportional to the temperature increase. Increasing bedL/D, and smaller particle sizes, increased the tendency to agglomerate. 

The results of an example calculation for a recirculating fluidized bed coal devolatilizer of 0.51 m in diameter handling coal of average size 1200 pm at 870°C and 1550 kPa are presented in Fig. 11. The calculation is based on operating the fluidized bed above the draft tube at 4 times the minimum fluidization velocity. It is also based on the selection of a distributor plate to maintain the downcomer at the minimum fluidization condition. If the two-phase theory applies, this means that the slip velocity between the gas and the particles in the downcomer must equal to the interstitial minimum fluidizing velocity as shown below. 

The flow required to maintain a complete homogeneous bed of solids in which coarse or heavy particles will not segregate from the fluidized portion is very different from the minimum fluidizing velocity. See... 

Spouting cannot be maintained once the bed height exceeds a certain level, coim monly known as maximum spoutable bed depth, Hm. At Hm, the spout starts to collapse and the bed starts to change from a spouted bed to a fluidized bed. As indicated in Eq. (9.67), the minimum spouting velocity increases with an increase in the bed height. Consequently, (7msp achieves its maximum at Hm. The maximum value of the minimum spouting velocity can be related to the minimum fluidization velocity by... 

A minimum flow velocity was required to maintain a flame. When the flow velocity of the feed was increased, the location of the flame first moved upstream and then downstream again, while the width of the reaction zone decreased and the maximum... 

A suspension of partides in turbulent gas or liquid, or a mixture of both, behaves much like a fluid, hence the terms fluidization and fluidized beds. In the fluidized state, much larger particles can be maintained in suspension than would be possible under static or laminar flow conditions. This provides a means, for example, to make large particles flow like a fluid. The minimum fluidization velocity is the ve-lodty of gas or liquid that is just needed to support the weight of the particles. At... 

We have further studied the problem of impact initiation in two dimensions by varying both the number of layers in the flyer plate and its impact velocity. Results from these simulations are shown in Fig. 13. In this figure the solid squares denote simulations where the detonation was sustained, while the open squares denote simulations where, if exothermic chemistry began at all, it was insufficient to maintain a detonation. The solid line follows the interface between these two regions. Figure 13 demonstrates that as the number of layers in the flyer plate is increased, the minimum impact velocity for initiation decreases, approaching an asymptotic value... 

For operator protection, in the event of a breach in type 2 isolators a minimum breach velocity of 0.7m sec should be maintained. 

In Fig. 3.5-1 a fixed bed of solid particles supported on a grill gate or perforated plate is shown. In the case of a downward flow the fixed bed state is maintained. However, such a bed is converted into a fluidized bed for an upward flow beginning at the minimum fluidizing velocity w (mean superficial velocity based on the... 

The pressure drop in the bed is proportional to the superficial velocity (1-2), whose variation is characteristic of the fixed bed and reaches a maximum value in (3). From this point, fluidization of the particles begins. At this instant, the minimum fluidization velocity u f is identified, maintaining constant pressure drop in the fluidized bed (4-6). Then, particles begin to be dragged out (6) of the catalyst bed, whose pressure drop decreases until it reaches the point (7). 

There are two basic (and interrelated) useful parameters for an open-channel slurry system design the minimum slope (to maintain slurry suspension) of straight lengths of launder, Smin (usually expressed as a percentage), and the velocity head corresponding to the minimum slurry velocity, K (expressed in metres), is often quoted as part of process technology, and may be arrived at directly by practical experience, whereas K is usually derived. The parameters have an approximate theoretical relationship, and the minimum slurry velocity Kmin is essentially the same as the minimum velocity to avoid settlement in full-flow pipes of comparable diameter, in terms of wetted perimeter. 

Type A1 (formerly, Type A) These Class II BSCs maintain a minimum inflow velocity of 75 ft/min, have HEPA-filtered, down-flow air that is a portion of the mixed down-flow and inflow air from a common plenum, may exhaust HEPA-Altered air back into the laboratory or to the environment through an exhaust canopy, and may have positive-pressure contaminated ducts and plenums that are not surrounded by negative-pressure plenums. They are not suitable for use with volatile toxic chemicals and volatile radionucleotides. 

---