GB2045942A - Establishing maximum heat transfer within a fluidised bed - Google Patents

Abstract

A method for establishing a state of maximum heat transfer within a fluidised bed 2, uses a self-heated temperature sensor 1 which is placed in the bed, the and an unheated temperature sensor 3 also placed in the bed at a distance from the first sensor. During fluidisation the temperatures of the sensors are measured and compared at varying gas flow rates through the bed. The point at which the difference between the two temperatures becomes a minimum corresponds to maximum heat transfer within the bed. The gas flow rate can thus be controlled to maintain this minimum differential, either manually or, by using the output of a comparator 6 to control a motorised gas flow valve 11. <IMAGE>

Description

SPECIFICATION Method and apparatus for establishing a state of maximum heat transfer within a fluidised bed The present invention relates to a method and apparatus for establishing a state of maximum heat transfer within a fluidised bed.

Fluidised beds or baths employ the principal of fluidisation of a mass of finely divided inert particles by means of an upward stream of gas passing therethrough, for example air, nitrogen or argon. A state of fluidisation is achieved when the individual parti cles become microscopically separated from each other by the moving gas. This fluidised bed of parti cles has unusual properties which differ markedly from either those of the gas or of the solid particles.

The fluidised bed behaves like a liquid exhibiting characteristics which are generally attributable to the liquid state.

The most commonly used fluidising gas is ordi nary compressed air obtained from a blower or compressor. For situations where a non-oxidising atmosphere is required, nitrogen can be utilised and if a reducing atmosphere is required, cracked gas can be employed with a silicon carbide bed.

The unique characteristic of gas fluidised particles is the relatively high rate of heat transfer which yields highly isothermal conditions, as well as excel lent heat transfer to solid surfaces, and this charac teristic has a wide variety of thermal applications.

For example, a fluidised bath can be used to cali brate thermometers, thermocouples, filled systems, temperature transmitters, and temperature switches.

In addition to instrument and sensor calibration, fluidised baths can be used for thermal testing of temperature sensitive components such as semi conductor devices, transducers, and materials such as wire products, plastics and metal alloys. Thus where reliability, temperature uniformity, and hazard free performance is required fluidised baths are ideally suitable.

The present methods of monitoring and maintain ing maximum heat transfer through thefluidised bed are basically observational.

Thus for example, an operator can use his experi ence from observation of the surface of the bed, to determine when ideal fluidisation has been achieved corresponding to maximum heat transfer through- out the bed. If the operator is of the opinion that ideal fluidisation is not occurring then the gas flow rates can be manually adjusted until the surface appear ance of the bed is judged to be such as to indicate good flow within the bed and therefore ideal temp erature uniformity and heat transfer.

In this way the glas flow rate at any temperature can be measured and to some degree the fluidisa tion rate can be maintained and repeated by ensur ing that a similar gas flow rate is used for further work at that temperature.

To reiterate, this method relies on the operator's experience and repeatability depends upon the gas pressure and the particle size of the fluidising medium remaining constant.

Another method of achieving ideal conditions within a fluidised bed is to plot a series of fluidising gas flow rates against temperature so that the gas flow rate can be adjusted automatically as the temp- erature of the bed is altered.

It is possible in this way to automate the gas flow through a fluidised bed by following this gas flow against temperature curve, by either a step function or a continuous method.

However, this latter method is not self-correcting and does not therefore achieve the maximum heat transfer within the fluidised bed that might otherwise be possible.

It is an object of the invention to overcome the disadvantages of the prior art and provide a method whereby the internal conditions of a fluidised bed can be monitored automatically to establish accurately, and subsequently maintain, a state of maximum heat transfer within the fluidised medium.

In a first aspect of the invention there is provided a method of establishing a state of maximum heat transfer in a fluidised bed comprising positioning a self-heated temperature sensor in the bed, positioning an unheated temperature sensor in the bed spaced from said self-heated sensor, and comparing the temperature of the heated sensor with the temperature of said unheated sensor at varying fluidising gas flow rates through the bed to thereby establish a state of maximum heat transfer through said flu id- ised bed when said differential temperature is a minimum.

In a second aspect the invention provides apparatus for establishing a state of maximum heat transfer in a fluidised bed comprising a first selfheating temperature sensor for positioning in said fluidised bed, a second temperature sensor for mounting in said bed spaced from said first temperature sensor, and comparator means-for comparing the temperatures between said first and second temperature sensors at varying fluidising gas flow rates through the bed to establish when the differential temperature between said first and second heating temperature sensors is a minimum corresponding to the existence of a state of maximum heat transfer within the fluidised bed.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows apparatus for monitoring heat transfer rates within a fluidised bed according to one embodiment of the invention; and Figure 2 shows apparatus for monitoring heat transfer rates within a fluidised bed according to another embodiment of the invention.

The apparatus shown in Figure 1 comprises a first temperature sensor 1 mounted in a fluidised bed 2 and a second temperature sensor 3 also mounted in the fluidised bed 2 and spaced from the first temperature sensor 1.

The first temperature sensor 1 is self-heated, that is it is surrounded by a heating coil 4 connected to a constant power supply unit 5.

The temperature sensors 1 and 3 are coupled to a comparator device 6 which is responsive to signals from the first and second temperature sensors to thereby provide an output representative of the difference between the two measured temperatures.

An indicating meter 7 is coupled to the comparator device 6 to provide a suitable readout of this differential temperature. The indicating meter 7 is a minimumlmaximum indicating meter giving a minimum reading when the differential temperature approaches zero and a maximum reading when the temperature between the temperature sensor 1 and temperature sensor 3 is greatest.

The fluidised bed 2 is contained within a support vessel 8 as shown, and the gas flow necessary to produce fluidisation is supplied to a plenum chamber 9 beneath the bed. It will thus be appreciated that the gas supply flow rate can be adjusted to produce a minimum reading on the indicating meter 7 and this minimum reading can be maintained thereby achieving a state of maximum heat transfer within the fluidised bed.

In the embodiment of Figure 1 the adjustment of the gas supply can be carried out on a manual basis.

However, the embodiment of Figure 2 shows a system whereby this adjustment can be carried out automatically. The system of Figure 2 is provided, instead of the indicating meter 7, with a drive unit 10 connected to the comparator 6, the drive unit being first connected to a motorised valve 11 or variable air blower which controls the flow rate of gas through the valve 12.

In this system, therefore, the output from the comparator is fed into the servo system comprising the drive unit 10 and motorised valve 11 to automatically adjust the rate of fluidising gas to the plenum chamber 9 so that the differential temperature measured by the comparator device 6 is always at a minimum. In this way maximum heat transfer within the fluidised bed is continuously achieved.

The two thermal sensors 1 and 3 may be thermocouples, thermistors, platinum resistance thermometers and the like.

With the two sensors 1 and 3 positioned in the fluidised bed as shown, and with heat supplied to the heating coil 4, the temperature of the self-heated thermal sensor 1 will vary with variation of heat transfer within the fluidised bed.

If the bed is in an unfludised state the fluidising medium will act as an insulator and the temperature of the thermal sensor 1 will rise to a maximum figure. As the bed is increasinglyfluidised the correspondingly increasing heat transfer rate therein will heat-sink energy away from the sensor 1 and its temperature will drop to a minimum value. This minimum value of sensor temperature will correspond to a state of maximum heat transfer within the fluidised bed.

The temperature of the thermal sensor 1 is compared within the comparator 6, with the temperature of the unheated thermal sensor 3. In this way it is possible to look at the heat transfer rate within the fluidised bed irrespective of the temperature thereof.

The differential temperature between the thermal sensors 1 and 3 thus give a true indication of the heat transfer rate at any time within the fluidised bed irrespective of the temperature thereof. When the differential temperature is a minimum then a state of maximum heat transfer within the bed will have been achieved.

Since the differential temperature is not affected by the temperature of the fluidised bed the system herein disclosed will be suitable for indicating heat transfer rates within the bed throughout its maximum temperature range, whether the bed is being controlled at one particular temperature or the bed temperature is increasing or decreasing at that time.

In the em bodiment of Figure 1 the gas supply to the plenum chamber can be manually adjusted to maintain a minimum differential reading on the indicating meter 7 and thus a state of maximum heat transfer within the bed 2, or can be automatically adjusted as shown in Figure 2 to attain the same object. It will be appreciated from the above that the present invention is a considerable advancement over present known methods of monitoring fluidised beds to determine the existence or otherwise of a state of maximum heat transfer.

The present system can be fully automated and obviates the "rough and ready" methods, which have heretofore been employed.

Claims (8)

1. A method of establishing a state of maximum heat transfer in a fluidised bed comprising positioning a self-heated temperature sensor in the bed, positioning an unheated temperature sensor in the bed spaced from said self-heated sensor, and comparing the temperature of the heated sensor with the temperature of said unheated sensor at varying fluidising gas flow rates through the bed to thereby establish a state of maximum heat transfer through said fluidised bed when said differential temperature is a minimum.

2. A method as claimed in Claim 1, wherein the gas flow rate is continuously adjusted to maintain the differential temperature between the self-heated and heated senscrs at said minimum temperature differential.

3. Apparatus for establishing a state of maximum heat transfer in a fluidised bed comprising a first self-heating temperature sensor for positioning in said fluidised bed, a second temperature sensor for mounting in said bed spaced from said first temperature sensor, and comparator means for comparing the temperatures between said first and second temperature sensors at varying fludising gas flow rates through the bed to establish when the differential temperature between said first and second heating temperature sensors is a minimum corresponding to the existence of a state of maximum heat transfer within the fluidised bed.

4. Apparatus as claimed in Claim 3, wherein an indicating meter is coupled to said to said comparator means for indicating when the difference between the temperatures of said first and second sensors deviates from said minimum.

5. Apparatus as claimed in Claim 3, wherein a servo-unit is adapted to adjust the fluidising gas flow rates to the bed in response to signals from said comparator means indicative of deviations from said minimum temperature differential thereby to maintain said state of maximum heat transfer.

6. Apparatus as claimed in any one of Claim 3,4 or 5, wherein said self-heating sensor is a thermocouple provided with a heating element.

7. Apparatus for establishing a state of maximum heat transfer in a fluidised bed substantially as hereinbefore described with reference to the drawings.

8. A method for establishing a state of maximum heat transfer in a fluidised bed substantially as hereinbefore described with reference to the drawings.