Effect of Clearance

Another source of error that can become quite important is the leakage flow through the clearance between the flight and the barrel. A normal design clearance (radial) is 0.001 D, where D is the diameter of the screw. When the clearance is normal, the flow through the clearance will be quite small. However, if the screw and/or barrel is subject to wear, the actual clearance can increase substantially beyond the normal design clearance. This can cause a considerable reduction in output and it is important to know how to evaluate the effect of clearance flow. 

The clearance reduces the drag flow rate. The drag flow rate is reduced by a factor 5/H. The corrected drag flow becomes  

The proper derivation of the pressure induced leakage flow is rather involved. For details of the derivation, the reader is referred to the publications of Mohr and Mal-louk [228], Tadmor [103], or Rauwendaal [271]. The total volumetric output including the effect of leakage can be written as  

The correction factor for pressure induced leakage through the flight clearance can be written as  

The viscosity in the clearance ij. is differentiated from the viscosity in the channel j, because, in reality, the viscosity in the clearance will be substantially different from the viscosity in the channel as a result of differences in local temperature and shear rate. When the flight clearance is close to the normal design clearance, the value of 4 will be very close to zero and can be neglected unless extreme accuracy is required. However, when the radial clearance is considerably larger than the normal design clearance, for instance, as a result of wear, the actual value of fL should be used in the expression for the total volumetric output, Eq. 7.224. 

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Figure 12-17B. Effects of clearance volume on performance efficiency of reciprocating compressor cylinder (valve design effect). (Used by permission Livingston, E. H. Chemical Engineering Progress, V. 89, No. 2, 1993. American Institute of Chemical Engineers, Inc. All rights reserved.)...

The first three factors are present in compressors, but they are small and on the whole can be neglected. The clearance volume problem, however, is unique to reciprocating piston compressors. The volumetric efficiency e estimates the effect of clearance. The volumetric efficiency can be approximated as... 

The effect of clearance volume may now be taken into account. If the clearances in the successive cylinders are C], c2, cj,. .the theoretical volumetric efficiency of the first cylinder... 

Effectiveness of clearance. Where the general level of air flow at the workstation is poor, as in a comer, the less effective is the general ventilation. 

Fig. 2.3 Effect of clearance and volume of distribution on half-life for a simple single compartment pharmacokinetic model.

If the rate of elimination decreases in Scheme 7.2, then what happens to clearance Clearance is unchanged. For each 4.0-second pass, the liver clears 50 mL (CLh = 12.5 mL/s) out of the total 100 mL of blood that flows through the organ. Literally, 50% of the blood volume is cleared, so the actual impact is a decrease in Cp by 50%. While clearance is constant, the effect of clearance on Cp varies with Cp. Clearance depends on the action of metabolic enzymes on the drug and, at very high drug concentrations, the enzymes can become saturated with substrate. Under these conditions, which are rare, clearance is not constant. Therapeutic concentrations of modem drugs are normally well below the concentrations required to saturate liver enzymes. The tubular secretion and reabsorption processes in the kidneys can also be saturated and affect renal clearance. As with hepatic clearance, variable renal clearance is rare. 

Efficiencies.—The effect of clearance upon capacity is usually expressed in terms of volumetric efficiency. 

If considerable intensification of heat transfer is required for Re < 100, a stirrer with a very small wall clearance (d/D 1.00) is recommended. Zlokamik [614] found no effect of clearance in the whole Re range for anchor stirrers with d/D = 0.91-0.98, although there was an extremely large increase in heat flow for d/D 1.0 (without wiper blades ). For cooling this is approximately a factor of 4 higher, whereas for heating it is only approximately a factor of 2 higher. These results can be described by the relationship... 

FIGURE 9.6 Effect of clearance on power. (From Bates et al., 1963). 

Figure 5 shows the effect of clearance between the lower lip of the channel and the moving floor. These measurements demonstrate that the general effect of clearance is to render the apparent depth-width ratio greater than the actual magnitude. These observations have two implications that a small gap at the contact between channel and floor will not produce a disproportionate effect on the readings, and that the apparent flow behavior with a large gap is very much like that without a gap. 

The effects of clearance on the stress distribution in the laminate around the hole boundary are presented first for a model in which homogeneous anisotropic material properties for the laminates were used. This is the approach often used in previous smdies, and gives an overview of the effects of clearance without the complexity... 

In this section, the effects of clearance on the damage response of the C1 C1 C1 double-lap joint (i.e. a control case) and the C3 C3 C1 double-lap joint (i.e. a worst case scenario in the sense that all the load is initially carried by one bolt) are investigated. The progression of damage in the joints is shown at applied load levels of 10,30 and 50 kN for the four different failure modes considered, i.e. tensile matrix failure compressive matrix failure tensile fibre failure and compressive fibre failure. These load levels were chosen so that the damage could be tracked fiom initiafion up to the point of extensive damage. 

McCarthy MA, McCarthy CT. Finite element analysis of the effects of clearance on single-shear, composite holted joints. Plastics, Ruhher Compos 2002 32(2) 65—70. 

Goggin PM, Marrero JM, Spychal RT, Jackson PA, Corbishley CM, Northfield TC (1992) Surface hydrophobicity of gastric mucosa in Helicobacter pylori infection effect of clearance and eradication. Gasterolology 103 1486-1490... 

Mineralogical analysis only provides a reflection of the particle burden retained in the sample at the time of sampling, and the effect of clearance should not be neglected. This may be especially relevant in cases with very old exposures or exposures to particles or elements having low biopersistence in the lung, such as chrysotile asbestos, cobalt or some man-made vitreous fibers. 

Table 13.4 Experimental conditions for examining the effect of clearance between membrane and catalyst bed...

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