Excess gas velocity

Decrease load to reduce wear Lower-formulation density. Decrease hed-agitation and compaction forces (e.g., mixer impeller speed, fliiid-hed height, bed weight, fluid-hed excess gas velocity, drum rotation speed). 

For group B and D particles, nearly all the excess gas velocity (U — U,nj) flows as bubbles tnrough the bed. The flow of bubbles controls particle mixing, attrition, and elutriation. Therefore, ehitriation and attrition rates are proportional to excess gas velocity. Readers should refer to Sec. 17 for important information and correlations on Gel-dart s powder classification, minimum fluidization velocity, bubble growth and bed expansion, and elutriation. 

Fig. 6 shows the FFT spectrum for calculated bed pressure drop fluctuations at various centrifugal accelerations. The excess gas velocity, defined by (Uo-U ,, was set at 0.5 m/s. Here, 1 G means numerical result of particle fluidization behavior in a conventional fluidized bed. In Fig. 6, the power spectrum density function has typical peak in each centrifugal acceleration. However, as centrifugal acceleration increased, typical peak shifted to high frequency region. Therefore, it is considered that periods of bubble generation and eruption are shorter, and bubble velocity is faster at hi er centrifugal acceleration. 

Decrease load and contact displacement for wear Decrease bed height to effect load and decrease excess gas velocity to lower collision frequency and mixing and therefore contacting. 

Decrease impact velocity for breakage Decrease excess gas velocity, also distributor plate design may be modified. 

It was also shown both theoretically and experimentally (Tardos et al., 1985a,b) that there is a strong correlation between the excess temperature above the minimum sintering point, T-Ts, and the excess gas velocity above minimum fluidization conditions (measured below the sintering point) U-lJm required to maintain fluidization. A general correlation was developed between the excess temperature and the excess gas velocity, which takes the form... 

There is a significant difference in the form of Eq. (46) for amorphous and crystalline materials (Compo et al., 1987 and 1990) for the first group, a slight increase in temperature above minimum sintering results in a significant increase in excess gas velocity as can be seen in both, the value of the... 

Vaux (1978), Ulerich et al. (1980) and Vaux and Schruben (1983) proposed a mechanical model of bubble-induced attrition based on the kinetic energy of particles agitated by the bubble motion. Since the bubble velocity increases with bed height due to bubble coalescence, the collision force between particles increases with bed height as well. The authors conclude that the rate of bubble-induced attrition, Rbub, is then proportional to the product of excess gas velocity and bed mass or bed height, respectively,... 

Based upon these observations, Rowe (1977) derived an expression for the average particle circulation time t around a bed in terms of excess gas velocity and bed height at minimum fluidization... 

Although this expression may not accurately predict circulation time, and in any case particles do not follow a simple predetermined circuit around the bed, it serves to illustrate the significance of the excess gas velocity in determining particle mixing rates. The excess gas flow rate, proportional to the excess gas velocity, is essentially the bubble flow rate. A greater bubble flow generates more bubbles and therefore... 

Table 5.1 Correlation of reported growth mechanism with excess gas velocity.

Maintain constant superficial gas velocity to keep excess gas velocity, and therefore bubbling and mixing conditions, similar. 

Excessive gas velocities due to inadequate absolute pressures in the forward vents lead to foaming of the polymer in the screw channels, which can increase the fill rate to critical levels. A voluminous melt bulge can be observed on the left-hand screw in feed direction which can no longer be fed safely without further corrective action. 

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