GB2080698A - Method and apparatus for low temperature operation of a fluidised bath - Google Patents

Abstract

In a low temperature fluidised bath the fluidising gas is anhydrous, thus avoiding condensation within the bath. The fluidising gas may be nitrogen or helium; it may be boil-off gas from the medium, e.g. liquid nitrogen, cooling the bath, or a combination of boil-off gas and anhydrous gas from an external supply.

Description

SPECIFICATION Method and apparatus tor low temperature operation of a fluidised bath The present invention relates to a method and apparatus for low temperature operation of a fluidised bath, that is at temperatures below zero degrees centigrade.

Fluidised beds or baths employ the principle 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 particles become microscopically separated from each other by the moving gas. This fluidised bed of particles 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 unique characteristic of gas fluidised particles is the relatively high rate of heat transfer which yields highly isothermal conditions, as well as excellent heat transfer to solid surfaces, and this characteristic has a wide variety of thermal applications.

For example, a fluidised bath can be used to calibrate 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 semiconductor 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 most commonly used fluidising gas is ordinary compressed air obtained from a blower or compressor. If the bath is fluidised by compressed air and the bath is to be used below the dew point of the air supplied, condensation forms in the bath and on the fluidised particles which in turn makes the bath difficult if not impossible to fluidise. At very low temperature operation such formation of condensation will cause the bath eventually to freeze.

To avoid these problems the fluidising gas is usually passed through an apparatus for reducing its dew point, that is the gas is dried. However, the quantity of air required to fluidise a bath increases as its temperature decreases and the throughput of the drying apparatus decreases as the dew point is decreased. Hence either a dryer much larger than normally required is fitted or else the drier capacity limits the lowest temperature which can be achieved. Thus there are limits to this solution especially when the bath is to be operated at very low temperatures, for example as low as minus 200 C.

It is an object of the invention to overcome the problems of condensation when operating fluidised baths at low temperatures.

According to the invention there is provided a method of operating a fluidised bath at low temperature comprising cooling the bath during fluidisation, and employing an anhydrous gas to fluidise the bath such as to avoid the formation of condensation therein.

By an an hydros gas is meant a gas which contains no detectable water vapour or the water vapour content of which is so low that no condensation of H2O occurs in the system.

Moreover by a gas is meant a gas consisting of a single atomic or molecular species or a mixture of such gases.

The fluidising gas may be nitrogen. Alternatively it may be helium or a suitable equivalent, the required ment being that the gas is anhydrous so that condensation effects in the fluidised bath are avoided.

The bath may be cooled by a circulatory cooling liquid medium, which evaporates during cooling to produce an anhydrous boil-off gas.

Preferably the medium used to cool the bath is liquid nitrogen. Liquid nitrogen is advantageous since it boils at a temperature of minus 1 96 C at normal atmospheric pressure, and such a refrigerant when evaporated is extremely dry. Thus in its gaseous state after evaporation, it is an ideal agent for condensation free fluidisation of the bath. Undercertain operating conditions and fluidisation required ments therefore, the boil-off gas in this state may be employed as the sole fluidising gas for the bed.

A separate source of pure fluidising gas may be provided. This may be used, depending on the operating conditions and fluidisation requirements, in conjunction with the boil-off gas to fluidise the bath.

The invention will now be described by way of example with reference to the sole accompanying drawing which shows a schematic of a system for a low temperature operation of a fluidised bath.

A fluidising bath 1 is shown in the drawing surrounded by cooling coils 2.

A cooling medium, such as liquid nitrogen, is supplied to the coils 2 through supply line 3 from a liquid nitrogen source 4. Boil-off nitrogen gas, evolved subsequent to evaporate cooling, exits from the coils through line 5.

A fluidising gas line 6 is connected to the bath 1 and is in communication with an external gaseous nitrogen source 7.

A line 8 links the lines 5 and 6 from a point X in waste gas line 5 to a point Z in line 6. A flow control device A is positioned in the line 5 upstream of the point X, and another flow control device B in the line 6 downstream of the point Z.

A pressure control device C is also positioned in line 5 downstream of point X, and a pressure control device D upstream of the point Z. Two non-return valves E and F are positioned in the line 8, and between pointZ and the control device Din line 6, respectively.

The system as just described may beset to operate selectively such that above some equilibrium temperature the dry nitrogen boil-off gas exiting from the cooling coils 2 is used solely to fluidise the bath while below the equilibrium temperature, that is at lower operating temperatures when the demands on the cooling medium are greater, the boil-off gas fluidisation stream is supplemented with gas from the source 7 to maintain fluidisation conditions.

Of course, it will be appreciated that at any temp erature it would be possible to use excessive cooling liquid nitrogen whereby to supply sufficient boil-off gas for fluidising the bath. However, this demands an extra heat input and can lead to reduced bath performance. It is thus preferable to have the either or system as described where the fluidising gas is either boil-off nitrogen from the cooling system or a combination oftheanhydrous boil-off gas and anhydrous gas from an external supply.

Such selectivity is possible with the arrangement as shown in the drawing.

The flow control device A is set to regulate the flow of boil-off nitrogen gas emanating from the cooling coils 2. The pressure control device C sets a known pressure at point X and if the pressure of gaseous flow exceeds the desired value, then the excess is vented to exhaust.

Provided pressure at Xis greaterthan at Z, gas will pass through the norrreturn valve E in line 8, and into the fluidising line 6 to fluidise the bed.

Similarly pressure control device D sets a pressure at Y which is lower or equal to that at X. Again if pressure at Y exceeds that at Z, gas from the source 7 will pass through nor-return valve F.

The flow control device B sets the total fluidising gas flow rate. Normally this will be met by the boiloff gas flowing through valve E. As the temperature in the bed drops the demands on the cooling liquid nitrogen will increase. Thus the pressure at point Z will drop with this decrease in temperature until the pressure at Y is greater than at Z and valve F will open. The boil-off nitrogen gas flowing in line 6 will then be supplemented by nitrogen gas from the source 7 to maintain the required fluidising conditions.

Although the preferred example has been described with reference to liquid nitrogen, other similar mediums such as liquid helium could be employed eitherforthe coolant or fluidising gas.

Moreover the coolant medium and supplementary fluidising gas could be different than the coolant liquid helium and the supplementary fluidising gas nitrogen, the criterion being that the gases fluidising the bed or combinations thereof are anhydrous as hereinbefore described.

The method and apparatus as above described has been shown to have considerable advantages in the operation of fluidised baths at low temperatures.

As well as avoiding troublesome condensation, the use of the boil-off gas from the cooling system as either the main fluidisation source or in conjunction with a supplementary stream, obviates waste and renders the system economically viable comparing favourably with the known use of compressed air as the fluidising gas.

Claims (6)

1. A method of operating a fluiised bath at low temperature comprising cooling the bath during fluidisation, and employing an anhydrous gas to fluidise the bath such as to avoid the forrnationrof condensation therein,

2. A method as claimed in Claim 1, wheintthe fluidising gas is either nitrogen or helium. o

3. A method as claimed in Claim 1, wherein the bath is cooled by circulating liquid nitrogen which evaporates to form an anhydrous boil-off gas, and wherein said boil-off gas is used in performing said step of fluidising the bath.

4. A method as claimed in Claim 3, wherein said boil-off gas is used solelytofluidisethe bath.

5. A method of operating a fluidised bath at low temperature substantially as hereinbefore described with reference to the drawling.

6. Apparatus of operating a fluidlsed batlt sub- stantially as hereinbefore described with reference to the drawing.