WO1992011947A1

WO1992011947A1 - Centrifuge solids deflector

Info

Publication number: WO1992011947A1
Application number: PCT/AU1992/000013
Authority: WO
Inventors: Gordon Francis Eastaughffe
Original assignee: W.D.T. (Engineers) Pty. Ltd.
Priority date: 1991-01-14
Filing date: 1992-01-13
Publication date: 1992-07-23

Abstract

A centrifuge solids deflector is disclosed. The device is a backward inclined radial cascade (11) with an attached radial fan (9) which is affixed to and rotates with the basket (3) in a continuous centrifuge. The device can be used to reduce the velocity of slids discharged from the basket of a continuous centrifuge.

Description

CENTRIFUGE SOLIDS DEFLECTOR

This invention relates to improvements in continuous flow centrifuges used for the separation of solids from solid-liquid mixtures.

In a continuous flow centrifuge, the solids which have been separated from the mixture are discharged from the open end of the rotating bowl or basket which forms the separating or drainage mechanism in the centrifuge.

The solids which are thus discharged are collected in a stationary casing which surrounds the rotating bowl or basket before exiting from the centrifuge.

In many processes which use continuous flow centrifuges, a proportion of the solids which are discharged from the said bowl or basket are damaged by impact with the static portions of the machine, eg. the stationary casing.

This is of particular concern where recovery of an undamaged crystalline solid is the object of the process, for example in the sugar industries.

The invention will be described as applying to the sugar crystals which are being separated from the adhering molasses or syrup by means of a conical perforate basket continuous centrifuge such as is employed for this duty in a sugar mill or refinery. However, it will be understood that the centrifuge solids deflector of this invention may be applied to any continuous centrifuge in which a solid is separated from a surrounding liquid. There have been many proposals to reduce the damage caused in this manner to the sugar crystals after separation in continuous flow centrifuges, including arrangements of either fixed or moving vanes or deflectors to alter the_ direction of flight of the sugar crystals after they have been discharged from the basket.

All of these arrangements depend on friction or collisions between the sugar crystals and the vanes or deflectors to reduce the velocity of the sugar crystals relative to the said stationary casing.

Other proposals include the use of stationary casings which are large in diameter relative to the rotating basket and which depend on the drag of the air to reduce the velocity of the sugar crystals relative to the stationary casing prior to impact of the sugar crystals on the casing.

Other further proposals include the use of air flows directed so as to increase the frictional drag of the air on the sugar crystals after they have been discharged from the basket.

All of these proposals have the disadvantage that they act by influencing the flight of the sugar crystals only after the sugar crystals have been discharged from the rotating basket at the maximum velocity attained by the sugar crystals in relation to the stationary casing. These problems are overcome by the present invention, which provides to reduce the velocity of the sugar crystals relative to the stationary casing prior to the sugar crystals being discharged from the rotating basket.

It is an object of this invention to provide a continuous centrifuge in which the velocity (relative to the stationary casing) of the sugar crystals discharged from the rotating basket is reduced below the velocity at which sugar crystals are damaged by impact with or abrasion on the stationary casing or any adhering coating; this reduction in relative velocity of the sugar crystals occurring within the discharge section of the rotating basket or within the air flow at the exit from this section.

Another object is to improve the drying and cooling of the sugar crystals before discharge from the centrifuge by the evaporation of surface moisture from the sugar crystals under the influence of the air flow through the discharge sections of the rotating basket and stationary casing.

A further object is to provide separation into individual crystals of any agglomeration of sugar crystals (discharged from the separating surfaces of the centrifuge basket) by the action of the air flow through the discharge section of the rotating basket. The present invention comprises:-

1. A backward inclined radial cascade forming the discharge section of a rotating perforate continuous centrifuge basket, and which constrains the discharge of the sugar crystals to take place in a direction generally opposite to the direction of rotation of the basket;

2. A section at the discharge from the separating surface of the basket where the sugar crystals are constrained to enter the said cascade and to be entrained in the air discharged from the radial fan listed in 3. below.

3. A radial fan affixed to and rotating with the basket and which discharges air co-current with the flow of the sugar crystals through the cascade and which acts to increase the velocity of the sugar crystals relative to the said cascade and in an opposite direction to the rotation of the basket.

4. A discharge section of the casing surrounding the basket which receives the discharge of the mixture of the sugar crystals and air from the cascade and directs the mixture in the direction of a separating section of the casing.

5. A section of the casing which is shaped to separate the sugar crystals from the air flow before discharge from the centrifuge casing by an alteration in the flow direction of the air.

6. In one form of the invention, an external fan is used to extract the air from the centrifuge after the separation of the air from the sugar. The volume of air flowing through the fan and cascade may be varied to provide the required acceleration of the sugar crystals through the cascade to suit the peripheral velocity of the basket rim. The optimal condition is for the 'sum of the vectors of the periheral velocity at the basket rim and the maximum discharge velocity of the sugar crystals from the cascade to equal zero (i.e. the discharge velocity is zero relative to the stationary casing) .

In operation the discharge velocity of the sugar crystals may vary from the optimal condition but the operating conditions are satisfied if such discharge velocity (relative to the stationary casing) is less than the velocity at which sugar crystals suffer damage by impact or abrasion.

The temperature and humidity of the air flowing through the fan and cascade may be varied to suit the temperature and dryness conditions required for the sugar crystal.

In order that the invention may be better understood a particular continuous flow centrifuge incorporating the centrifuge solids deflector will be described in more detail with reference to the accompanying drawings in which:-

FIG. 1 shows a partly broken away viev; of a continuous sugar centrifuge incorporating the centrifuge solids deflector.

FIG. 2 shows a plan view in part section of the radial cascade which forms the discharge section of the rotating basket, and also the radial fan which is affixed to and rotates with the basket. FIG. 3 Shows a view of a continuous sugar centrifuge including a fan extracting air from the compartment 6 via opening 13 and outlet 15.

The general operation of a conical perforated basket continuous centrifuge separating massecuite into sugar and molasses (or syrup) will first be described.

The unit consists of a conical perforate basket 3 mounted on axial bearings 5 and rotated at high speed by an electric motor drive 7. A fixed casing forming two compartments 4 and 6 supports the whole rotating assembly.

Inlet for massecuite at 1 leads to an accelerating chamber 2 which is affixed to and rotates with the perforate basket 3. The massecuite flows from the lower end of the chamber 2 and passes up the inner surface of basket 3 under the gravitational effect of the rotating perforate basket 3. This gravitational effect causes the liquid portion of the massecuite (i.e. molasses or syrup) to separate from the solid portion of the massecuite (i.e. sugar) and to flow outwards through the perforations in the basket 3 into a molasses (or syrup) compartment 4. The sugar continues up the inner surface of basket 3 and is discharged from the upper edge of basket 3 to the sugar compartment 6.

In all prior arrangements, the sugar is discharged from the upper edge of the perforate basket 3 at the maximum velocity which it attains during the separating process. In this invention, the crystals of sugar upon reaching the upper edge of the perforate basket 3 enter an annulus 9, where the crystals are entrained with air being discharged from a radial fan 10 which is affixed to and rotates, with the per orate basket 5 3.

The mixture of sugar crystals and air enters a backward inclined radial cascade 11 which forms the discharge section of the rotating perforate basket. The mixture is discharged from the cascade 11 in a direction generally opposite to the direction of -L_Q rotation of the basket 3 into the sugar compartment 6.

The lower boundary of the cascade 11 is formed by the upper surface of the basket 3; the upper boundary of the cascade 11 is formed by the outer edge of the backing plate of the fan 10.

The mixture of air and sugar is separated into its components at 5 the exit from the compartment 6, with the air exiting through opening 13 and the sugar, because of its higher specific gravity, through the opening 14.

In one form of the invention, the opening 13 which forms part of the sugar compartment 6 is connected with an annular duct 16 by a 0 series of tubes as shown typically by tube 17 which are open to allow the passage of air. Each tube (as is shown in Fig. 1) also acts as a structural member to support membrane 18 which forms the outer support for bearing assembly 19. A flanged connection 15 exhausts air from the duct 16.

An external fan 20 may be used to exhaust the said air via connection 15 from duct 16.

A drain pipe 21 σonnneσts molasses or syrup compartments 4 to an external outlet point.

The perforate basket 3 is normally internally lined with a renewable perforate sheet metal filtering surface.

In one form of the invention, a damper 22 is used to control the quantity of air discharged from fan 10.

The air supplied to fan 10 may in all cases have the temperature and humidity controlled if so required by the process. The controlling would be external to the centrifuge.

The components of the centrifuge solids deflector may be made of any suitable material, such as metal, or may be of moulded plastic or sintered metallic powders.

Claims

The claims defining the invention are as follows:

1. A centrifuge solids deflector including; a backward inclined radial cascade securely affixed to the discharge ares of the rotatable basket of a perforate conical continuous centrifuge and through which the solids discharged from said basket pass; a radial fan affixed to the said basket and discharging air through the said cascade; a chamber formed at the entrance to the said cascade and in which the said solids are entrained in the said air; a stationary chamber surrounding the said cascade into which the said solids and air are discharged generally in the opposite direction to the rotation of the said basket and fan and cascade; a stationary chamber in which the solids are separated from the said air by a reversal of direction of the said air.

2. A centrifuge solids deflector as claimed in claim 1 in which the said backward inclined radial cascade forms a part of the said rotatable basket.

3. A centrifuge solids deflector as claimed in prior claims in which either or both the temperature and humidity of the air flowing through the said radial cascade is controlled.

4. A centrifuge solids deflector as claimed in claims 1 and 2 in which the volume of air flowing through the said radial cascade is controlled.

5. A centrifuge solids deflector as claimed in prior claims in which a separate fan is employed to extract the said air from the discharge of the said centrifuge solids deflector or the subsequent said sugar compartment.

6. A continuous centrifuge in which a backward inclined radial cascade affixed to and rotating with the basket is used to reduce the velocity of the solid material discharged from the said basket relative to the stationary portions of the said centrifuge.

7. A continuous centrifuge in which a fan affixed to and rotating with the rotatable basket is used to pass air or other gases through the solids discharged from the said basket for the purpose of altering the temperature or moisture content of said solids.

8. A continuous centrifuge substantially as described with reference to Fig. 1, 2 and 3 of the accompanying drawings.

9. A continuous centrifuge incorporating a centrifuge solids deflector as claimed in prior claims in which the basket is of imperforate (i.e. solid) construction.