GB2117660A - Fluidisation apparatus - Google Patents

Abstract

Fluidisation apparatus suitable to produce an isothermal temperature enclosure suitable e.g. for thermometric calibration purposes is provided with a gas supply tube in the fluidised particles to preheat the fluidising gas to the same temperature as the particles, a calibrated restriction in said tube to regulate the gas flow to that required at various operating temperatures, an inner container to control and direct the fluidisation and flow of the particles, and a floating layer of insulation material to reduce surface heat loss and particles loss from the fluidised mass. <IMAGE>

Description

SPECIFICATION Fluidisation apparatus The present invention relates to a fluidisation apparatus in which gas flows through loosely connected particles to motivate them into a state having fluid properties.

Fluidisation is the process by which a gas is passed through a mass of particles at a sufficient velocity to lift and separate the individual particles thus giving the whole mass some of the properties of a fluid.

When fluidised, a mass of particles resembles a fluid in that it flows under gravity, displays buoyancy effects and transports heat well; it is dissimilar in that it is compressible and has no true melting or boiling points.

The properties of efficient heat transport plus the absence of melting or boiling points make fluidised particles a good method of producing an isothermal temperature bath for calibrating thermometers, thermostats or heat treating processes. The benefits of a fluidised system over conventional liquid baths are a wide operating temperature range, an inert non-penetrating medium, and its safe operation, having no toxic or inflammable fumes. The disadvantages between fluidised particles and liquid baths have been poorer temperature stability, uniformity and heat transfer, contamination of surrounding areas by escaped medium and the increased complexity of the system.

It is the object of the invention to provide an improved fluidisation apparatus which obviates those problems of temperature stability and uniformity, heat transfer, contamination and complexity previously associated with equipment in the field of fluidised powder temperature baths.

Temperature stability and uniformity in a fluidised mass of particles relies on there being established a controlled, balanced heat transfer condition between the heat source (heaters) and the heat sink (work pieces). The factors affecting the balance are: 1. The heat flow between the fluidised medium and its container.

2. The heat flow from any exposed surfaces of the fluidised medium.

3. The heat flow between the medium and the fluidising gas.

4. The heat flow between the heat source and the fluidised medium.

5. The heat flow between thefluidised medium and the workpieces.

Temperature stability and uniformity relies on controlling each of the above to a constant level and this was achieved by the following means 1. Heat flow between the fiuidised medium and container needs to be kept to a minimum level. This is achieved by insulating the container's outer surface against heat flow in a conventional manner.

2. Heat flow from the exposed top surface of the fluidised medium must also be minimised to a constant level and past systems have done little to achieve this. It is an object of the invention to overcome this by using the fluidised medium's fluid properties. It has been discovered that by using insulating materials in the form of fibre, particles or balls which float on the surface of the fluidised medium, heat flow from that surface is reduced, and less medium is exhausted with the gas because of the floating layer's filtering effects.

3. Heat flow between the medium and the fluidising gas in the past has either been uncontrolled or at best partially controlled. In the uncontrolled situation the fluidising gas passes to the medium at an unspecified temperature. As the gas passes through, two things occur; firstly the temperature of the gas and the medium becomes the same, and secondly any alteration in the gas temperature causes a corresponding volume change in the gas. The result is instability both in the temperature and in the fluidisation through the medium. In the prior art partial control has been achieved by first passing the gas around the outside of the container to scavenge waste heat. This brings the gas temperature near to that of the medium before it comes into contact with it. It is an object of the invention to obtain control of the heat flow to the gas and therefore control of its temperature.This is achieved by passing the gas through thefluidised medium via a tube before it comes into free contact with the medium. In this way the gas is brought into thermal equilibrium with the fluidised medium with minimum disturbance to the medium.

To obtain optimum fluidisation over a wide temperature range; the air flow from the supply to the fluidised medium must be adjusted to compensate for volume changes in the gas as it comes into equilibrium with the fluidised medium. Previous methods of achieving this have either been by manual adjustment, based on observation, or by electromechanical automation. It is an object of the invention to control the air flow at the required level automatically without using electro-mechanical means. This is accomplished by regulating the air flow to the required value after it reaches thermal equilibrium with the fluidised medium but before it comes into free contact with it. This is unlike the previous methods which regulate the gas flow before thermal equilibrium takes place. The invention is a flow restriction which is calibrated to restrict the gas to the correct flow rate.The restriction can be present in any part of the system which carries gas in thermal equilibrium with the fluidised medium. Therefore it can form part of the gas supply system or the exhaust system. In this application it was more convenient to incorporate the restriction in the gas supply line. It took the form of a reduction in the internal diameter of the gas supply tube which passed through the fluidised medium.

4 & 5. The degree of fluidisation and the flow pattern in the fluidised medium have not been strictly controlled in the past to give optimum conditions at the heat source, the sink, and the area between them. At the heat source the fluidisation needs to be vigorous to promote efficient heat transfer to the medium, a rapid flow of the heated medium away from the source, and a mixing action to encourage thermal uniformity. At the heat sink where isothermal conditions are required the fluidisation should be smooth and unvarying. In between the source and the sink, rapid transport of the fluidised medium between the source and the sink and back again should take place combined with further mixing of the fluidised medium. It is an object of the invention to obtain the optimum fluidisation and flow pattern throughout the fluidised medium.

This is achieved by using an open-ended inner container to house the heat sink and affect both the fluidisation and the flow. The container is totally immersed in the fluidised medium and is specifically designed to inhibit the fluidisation around the heat sink by minimising the inner space between the container and the sink. The outer space between the heat source and the container is made purposely spacious to promote vigorous fluidisation. The difference in the degree of fluidisation on either side of the inner container, and the vigorous nature of the fluidisation around the heat source, both combine to produce a rapid upward flow offluidised medium.

The inner container also prevents any fluidised medium travelling directly across from the heat source to the sink. This is unlike other systems where the flow, although encouraged, is not controlled. The fluidised medium, after moving upwards past the heat source, passes into a space between the source and the sink, which is above the inner container. In this space further mixing occurs to achieve thermal uniformity in the medium before it passes to the heat sink. The isothermal fluidised medium passes by the heat sink in a uniform manner transferring its heat efficiently. After this the medium moves out of the bottom of the inner container and back up past the heat source as it begins the cycle again.

The existing fluidised systems tend to leakflui dised particles primarily because of the open nature of the containers. It is an object of the invention not to allow the particles to escape. This is achieved by using an efficiently sealed container. The waste fluidising gas is cleaned of entrained particles by a filter assembly. The filter is designed to be self cleaning through having smooth vertical sides which do not hold significant numbers of particles.

This is unlike the present fluidised systems which use forced extraction systems to handle the waste gas. These are more complicated, expensive to run, deplete the fluidised mass, and usually require frequent maintenance.

It is an object of the invention not to have complex systems involved with its operation and so enhance its reliability and operating life. This has been achieved by not using any electro-mechanical moving parts in its fundamental operation unlike existing systems.

Figure 1 is a schematic sectional view of fluidisation apparatus in accordance with the specifications of the improved design.

An embodiment according to the invention is shown in Figure 1.An outer layer of insulation (1) fully encapsulates a container (2) to reduce heat loss to a minimum. Inside the container (2) is a mass of particles (9), normally Aluminium Oxide powder, to be fluidised.

In operation, gas at a constant pressure is directed, as shown by arrow A, into the pre-heat tube (3) for pre-heating the gas in a controlled manner to the same temperature as the mass of particles. The gas is then regulated to the required flow rate by a calibrated restriction (4) before it flows into the plenum chamber (12) below the porous plate (5). The porous plate (5) distributes gas over the whole base area of the mass of particles (9). The gas flows upward and fluidises the mass of particles (9). The inner container (7) allows vigorous fluidisation around the heaters and smooth fluidisation around the re-entrant tubes (8). The inner container also directs the flow of fluidised particles upwards, past the heaters and then back down past the re-entrant tubes (8), as shown by the arrows C.

By controlling the temperature of the heaters (6), the temperature of the flow of fluidised particles, and thus that of the re-entrant tubes, can be controlled over a wide temperature range with good heat transfer and temperature stability.

A floating layer of insulation (10) prevents heat loss from the surface of the fluidised particles (9) but allows the waste gas to pass out. The gas, after being partially filtered of entrained particles by the layeroffloating insulation (10), passesthrough a filter (11). The filter removes the remainder of the particles before the gas passes out to the surrounds, as shown by arrow B.

Claims (10)

1. Fluidisation apparatus comprising container -a means for containing a mass of particles, such as powder, to be fluidised, a means of distributing a gas into the base of said mass of particles characte rised by positioning a gas supply tube in the fluidised particles to preheat the gas to the same temperature as the particles, and a calibrated restriction in said tube to regulate the gas flow to that required at various operating temperatures. An inner container to control and direct the fluidisation and flow of the particles. A floating layer of separate insulation material to reduce surface heat loss from the fluidised particles.

2. A method, as claimed in 1, where the required gas flow rate is controlled, at any operating temperature, by regulating the gas when it is at the operating temperature using a flow restriction.

3. Apparatus for establishing the required flow rate at any operating temperature in fluidised particles comprising a means of heating the gas to the operating temperature and a flow restriction to regulate the gas to the required flow rate.

4. A method for controlling the gas flow rate in a fluidised mass of particles substantially as hereinbefore described with reference to the drawings.

5. Apparatus for controlling the gas flow rate in a fluidised mass of particles substantially as hereinbefore described with reference to the drawings.

6. A method is claimed in 1, where surface heat loss from the top of a fluidising mass of particles is reduced by a floating layer of insulation material on the mass of particles.

7. Apparatus for reducing surface heat loss from the top of a fluidising mass of particles comprising a floating layer of insulation material on top of the mass of particles.

8. A method for reducing surface heat loss substantially herein before described with reference to the drawings.

9. Apparatus for reducing surface heat loss substantially hereinbefore described with reference to the drawings.

10. An apparatus for providing isothermal heat transfer conditions substantially hereinbefore described with reference to the drawings.