Screw radial clearance

Helical screws operate in the laminar range at normally high impeller to vessel diameter ratio (DA/DT) with a radial clearance equal to 0.0375 Da. The impeller usually occupies one-third to one-half of the vessel diameter. They function by pumping liquid from the bottom of a tank to the liquid surface. The liquid returns to the bottom of the... 

Burkhardt, G. J. (1967) Effect of pitch, radial clearance, hopper exposure and head, on performance of screw feeders. Trans. ASME, J. Engng for Industry, 685-690. 

In simple terms, the screw is a cylindrical rod of varying diameter with a helical flight(s) wrapped around it. The outside diameter of the screw, from flight tip to flight tip, is constant on most extruders. The clearance between screw and barrel is usually small. Generally, the ratio of radial clearance to screw diameter is around 0.001, with a range of about 0.0005 to 0.0020. 

It is clear that the unrestrained sag starts to exceed the standard radial clearance (= 0.2 mm) when the L/D ratio exceeds 10. At normal L/D ratios of 20 to 30, the unrestrained sag is about one to two orders of magnitude larger than the standard radial clearance. From these simple considerations, it is obvious that the polymer between the screw and barrel must play a considerable support function to prevent contact between screw and barrel. The supporting force necessary to counteract the sagging by the weight of the screw has to increase very strongly when the L/D is increased. 

The final portion of the screw has a deep channel section following a decompression section. The channel depth is constant over the last screw section. Again, the deeper channel in the final screw section will reduce the pressure generating capability of the screw. A more effective power reduction can be obtained by not only changing the channel depth, but the channel depth, helix angle, flight width, and radial clearance in an optimum fashion as discussed in Section 8.3. 

The annular blister ring is simply a smooth cylindrical screw section with a small radial clearance. All the material has to pass through this clearance to exit from the extruder. Since no positive drag transport takes place over the barrier clearance, the... 

When a screw is installed in an extruder, the typical radial clearance between the screw and the barrel is 0.001 D, where D is the diameter of the extruder. This is the clearance at room temperature. When the machine is in operation the actual clearance between the screw and the barrel can be quite different. There are two main reasons for the change in clearance under actual processing conditions. One reason is temperature the other is compressive load on the screw. When the processing temperature is much greater than room temperature, the clearance can change when a) the coefficient of thermal expansion (GTE) of the screw and barrel is different and b) the temperature of the screw is different from the barrel. 

For a 25.40 mm barrel running at 333.3°C above room temperature, the increase in I.D. is 9.652E-2 mm when the GTE is 11.34/°C. For a 25.3492 mm Monei screw with a GTE of 13.86E-6/°C running at 333.3°C above room temperature, the increase in screw diameter will be 0.1168 mm. Thus, the difference between the thermal expansion of the screw and barrel diameter is about 0.02 mm or 0.01 mm based on the radius. If the radial clearance is 0.0254 mm, the clearance will reduce to 0.01524 mm due to the differential thermal expansion. Thus, the clearance is reduced but still greater than zero provided both the screw and barrel are at the same temperature. 

The average temperature rise is directly proportional to the consistency index m and the tangential flight width w/sincp. The temperature rise is strongly dependent on the radial clearance S, the power law index of the polymer melt n, and the screw speed N. Figure 11.24 shows the effect of flight clearance 6 and the power law index n for a 114-mm (4.5-in) extruder running at 100 rpm the specific heat is 2250 J/kg°C, the melt density is 900 kg/m, and the consistency index is 10 Pa - s . 

The thermal development length is directly proportional to the barrel velocity Vb (and thus the screw speed) and to the radial clearance squared. With thermal diffu-sivity values of about 10 mys, the thermal entrance length will be the same order of magnitude as the tangential flight width when the clearance has the normal design value (8 s 0.001 D). Thus, the temperature profile at the exit of the flight clearance will be very close to the fully developed temperature profile. The fully developed temperature profile for the isothermal case can be written as [67] ... 

The liquid film seal uses metallic sealing rings and is liquid buffered to maintain a fluid film in the clearance area and thereby preclude gas leakage. It is not unusual in the screw compressor to find the radial bearing and seal combined. 

A mechanical clearance between the top of the screw flight and the barrel wall helix angle at the barrel 6c helix angle at the screw core 6 r) helix angle at radial position r... 

The screw is placed within a barrel of diameter Df, Ds I 25f, where 6y is the radial flight clearance. This is shown schematically in Fig. 9.2. The figure shows a pelletizing extruder, but the discussion that follows is valid for any melt extruder equipped with any kind of die, and for the melt region in a plasticating extruder as well. 

Barrel internal diameter D Centerline distance A Radial screw clearance 8 Reciprocal screw clearance s 57.350 mm 48.000 mm 0.170 mm 0.525 mm... 

Material carried within the cross-section of the feed screw is deterred from rotating with the hlade by the boundary conditions at the outside radius of the screw flight. Frictional resistance to rotation is offered over the area of the screw periphery that is not exposed to the inlet flow from the hopper, hy material in contact with the outer casing surface. This restraining surface is radially displaced from the tip of the screw flight, by the working clearance of the screw in the casing. Products which shear thin can tend to log in the screw due to the low restraint offered to... 

The channel depth H is, in most cases, much larger than the radial flight clearance 6. Therefore, the shear rate in the clearance will be much higher than the shear rate in the screw channel. A typical value of D/H is 20 and a typical value of D/8 is 1000. Thus, the shear rate in the flight clearance will be approximately 50 times higher than the shear rate in the screw channel. This has important implications for the operation of the extruder, as will be discussed in more detail in the next two chapters. 

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