Surface non-concentricity results in mechanical vibration due to nonsymmetrical buoyancy effects, so high precision is needed in geometry. Air flow and degassing of the feed stream become more complicated with submerged hubs. Some new designs avoid these problems by permitting small hubs designed with high stiffness and high natural frequency . However, within the last decade, immersed hubs have been designed to float on the pond, considerably reducing potential vibration and enabling higher speeds [ 2 ].
Decanter Design 31 2. Naturally the helix profile has to be tapered to suit the beach section. Each section is welded in turn to the conveyor hub and then welded to the adjacent section. Double welding both sides with grinding afterwards is essential where hygiene is of importance. However double welding is c o m m o n practice, even w h e n hygiene is not required. The flights will be normally perpendicular to the decanter axis or bowl wall. It is not always appreciated w h a t a complex shape the surface of a flight is.
The pitch angle is the angle the tip of the flight subtends to a right circle of the bowl. If the flights are not to be protected from wear, then their tips will be ground smooth and perhaps chamfered, to provide a m i n i m u m of area in contact with the heel, to minimise torque. The feed enters the feed zone c h a m b e r from the feed tube. Once in the II Figure 2. A conveyor flight section before welding to the conveyor hub. A typical feed zone. To assist the feed up to speed, a c c e l e r a t o r v a n e s will sometimes be found on the " t a r g e t " , the plate opposite the feed tube end.
These vanes could be radial, at an angle to the radii, or curved.
In extreme cases of wear, parts inside the feed zone are hard surfaced or specific erosion resistant c o m p o n e n t s are used. Hard s u r f a c i n g is often used on the accelerator plate, p a r t i c u l a r l y on leading edges and the tips of the vanes. Some shaped accelerators have been m a d e completely of u r e t h a n e rubber. W h e n the feed leaves the feed tube, in most cases it is at a high axial velocity.
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W h e n it hits the r o t a t i n g target, some splashing inevitably occurs. In fact a dense aerosol mist is often produced. At the back of the feed zone a tube is sometimes built in, to s u r r o u n d the end of the feed tube. On the outside of this tube, small accelerator v a n e s are welded to accelerate and condense the mist and also to accelerate liquor up to speed, should the feed zone become flooded. Ideally, air is allowed to enter the feed zone from a r o u n d the feed tube.
Decanter Centrifuge Handbook
It will be d r a w n in by the fan effect of the feed zone and t h r o w n out of the feed zone exit ports. The air would t h e n pass along the bowl to exit over the centrate. This d r a u g h t helps to prevent splash back of feed from the feed zone. The exit ports from the feed zone are themselves subject to a considerable range of designs and i n n o v a t i o n s. It is not usual to h a v e just one exit port.
For s y m m e t r y and balance an even n u m b e r of ports is usual, two, four, six or eight. The basic design has these ports fitted with t u b u l a r nozzles, often lined with a ceramic w e a r protection. New feed zones have been introduced recently to reduce feed particle attrition, by slowing and e x t e n d i n g the acceleration time to bring the feed up to speed, and to reduce the inlet t u r b u l e n c e in the s e p a r a t i o n zone. Decanter Design 33 2. Flocculant c a n be relatively expensive, and on an effluent application c o n t r i b u t e s a large p e r c e n t a g e to the total t r e a t m e n t costs.
An additive chamber. Floc Addition. A seal will be used to retain the grease and a second outer seal will be used to prevent ingress of process fluids and solids into the bearing. Thus two seals are fitted back to back at each end.
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Grease channels have to be Decanter Design Areas of Protection , Areas of wear protection. A typical conveyor bearing assembly with seals. The differential rotation between the two races of each bearing is low and thus the life of these bearings should be good when adequately greased and sealed. There are two main types of gearboxes used on decanters. These are the epicyclic gearbox and the Cyclo gearbox, made by Sumitomo of Japan. However there are a number of decanters which have eliminated the gearbox by using a hydraulic system called a Rotodiff manufactured by the Swiss company Viscotherm.
The Rotodiff and the Cyclo gearbox will be covered in more detail in Sections 2. The epicyclic system consists of a pinion shaft and gear, which engages three planetary gears mounted on carrier plates which in turn engage a ring gear fixed to the gearbox casing. For the decanter the epicyclic gearbox involves two stages, although recently three stages have been in use.
The carrier plate of the first stage holds a second pinion shaft carrying the sun gear for the second stage. The ratio of the gearbox is the product of the ratios for each stage. A two stage epic! Decanter Design 37 m a x i m u m ratio for a two-stage epicyclic gearbox of to Three stage epicyclic gearboxes with ratios over have been used on decanters. If the central pinion shaft is held stationary, the differential speed between conveyor and bowl will be the bowl speed divided by the gearbox ratio.
If the pinion shaft is allowed to rotate at some speed below the bowl speed, then the differential between bowl and conveyor will be the difference between bowl speed and pinion speed, divided by gearbox ratio. If the pinion speed is controlled by using a brake, or a variable speed motor, differential speed m a y be varied from close to m a x i m u m , w h e n the brake is at its slowest speed, to nearly zero, w h e n the brake is almost at bowl speed. Reducing the pinion speed below zero, i.
Using an epicyclic gearbox causes the conveyor to rotate slower than the bowl, whereas it is normally faster w h e n using a Cyclo gearbox. Generally the conveyor flight helix is "left h a n d e d " with an epicyclic gearbox and right handed with a Cyclo gearbox. More usually it is fabricated from steel channel or box sections. The flame needs to be a rigid support for the rotating assembly. The surfaces for the main bearing pillow blocks are accurately machined in the same plane, and in line, to ensure no end-to-end misalignment of the rotating assembly, which would cause p r e m a t u r e bearing failure.
Some m a n u f a c t u r e r s fill part of the main flames of their larger machines with concrete effectively to form an inertia J Figure 2. Some flames have been used as a reservoir for the lubricating oil for the main bearings.
hukusyuu.com/profile/2020-09-16/spy-camera-recorder-with-night-vision.php The flame and casing see Section 2. When oil lubricated, the pillow blocks will be piped to an oil system, which will include a circulating pump, an oil reservoir and cooler with associated pressure, flow, and t e m p e r a t u r e i n s t r u m e n t a t i o n see Section 2. A clamp or flange holds it on a support extension from the main flame. It extends to the feed zone and within a few centimetres of the accelerator in the feed zone. A main hearing and pillow block assemblH.
Decanter Design 39 Figure 2. Feed tube. The geometry of m a n y decanters is such t h a t there is a risk of r e s o n a n t vibration of the feed tube at frequencies a r o u n d the bowl speed. To c o u n t e r this, feed tubes have been made slightly tapered, and made of lighter materials such as glass fibre and even carbon fibre, and sometimes composite material. Entering the conveyor with the feed stream is a flow of leakage air, w h i c h passes through the clearance between the conveyor and the feed tube.
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