GB2165160A - Vapourisers - Google Patents

Abstract

A vapouriser for entraining a source vapour in a carrier gas stream has a flask-shaped bubbler vessel 2,3 with an air jacket 12 round the neck portion 3 to provide thermal insulation. The bubbler vessel is connected to a reservoir 1 that lies at the same level as the thermally insulated neck. This provides stability of temperature in the bubbler, and hence stability of rate of vapour take-up under conditions of constant carrier gas flow rate, while the liquid level falls through the height of the reservoir. <IMAGE>

Description

SPECIFICATION Vapourisers This invention relates to a vapouriser for entraining vapour from a source liquid into a carrier gas stream.

In a conventional vapouriser the source liquid is contained in a chamber, and the carrier gas is introduced into the chamber below the surface of the liquid so that it bubbles up to the surface entraining source vapour as the bubbles rise. Typically the bubbles become saturated with vapour after rising only a few centimetres, and hence the rate at which source vapour is removed from the chamber depends primarily upon the carrier gas flow rate and the temperature of the source liquid in the vessel. Usually of lesser significance is the dependence upon pressure, though the pressure within the bubbler can in some circumstances also be a significant factor. This is particularly liable to be the case when the temperature of the source liquid is so high that the partial pressure of the vapour begins to assume a significant proportion of the total pressure above the liquid surface.For applications for which the rate of vapour transport is critical, such as for instance in the manufacture of optical fibres by methods involving vapour deposition processing, the chamber is typically immersed in an oil-bath whose temperature is maintained at a constant value. It is found in practice however, that a uniform flow rate of carrier gas of typically between 5 and 1000cm3per minute does not provide a uniform rate of vapour transport, but rather the rate of vapour transport tends to decrease as the liquid level in the chamber drops. This effect is observed even when the height through which the bubbles have to rise is at all times well in excess of 5 centimetres, the value by which saturation is believed to have been achieved.

The present invention is concerned with providing a reduced dependence of vapour transport upon the height of liquid in the chamber.

According to the present invention there is provided a vapouriser including a bubbler vessel provided with a gas inlet for introducing carrier gas into the vessel beneath the level of source liquid contained therein, and also provided with a gas outlet for removing source vapour-laden carrier gas from above the source liquid level in the vessel, characterised in that the vapouriser is provided with a source liquid reservoir communicating with the vessel, and in that the vessel is preferentially thermally insulated over the region through which the source liquid level falls as the liquid in the reservoir drains into the vessel.

There follows a description of a vapouriser embodying the invention in a preferred form. The description refers to the accompanying drawing which depicts a schematic sectional view of the vapouriser.

The principle components of the vapouriser depicted in the accompanying drawing are a reservoir 1 and a generally flask-shaped bubbler vessel having a chamber portion 2 surmounted by a tubular portion 3. This particular vapouriser has been designed for entraining vapours for use in the manufacture of optical fibres. An inlet pipe 4 for introducing carrier gas into the vapouriser protrudes through one wall of the chamber 2 to terminate centrally beneath the tube 3.

Also communicating with the chamber 2 is a delivery pipe 5 from the reservoir 1. The top of the tube 3 is connected to an outlet pipe 6 for removing source-vapour-laden carrier gas from the vapouriser to where it is to be used in a vapour deposition process. Teeing off this outlet pipe 6 is a further pipe 7 connected with the top of the reservoir 1 which has been provided for pressure equalisation between the reservoir and the bubbler vessel. The reservoir is filled with source liquid 8 by means of a filling pipe 9 provided with a valve 10. The whole assembly is immersed in oil 11 maintained at constant temperature.

The tubular portion 3 of the bubbler vessel is thermally insulated from the oil by means of an air jacket 12 which extends from a point approximately level with the bottom of the reservoir upwardly at least as far as a point level with the top of the reservoir. At least the majority, if not the entire surface of the chamber portion 2 of the bubbler vessel is not thermally insulated.

When carrier gas is bubbled through the source liquid in the bubbler vessel at a constant flow rate heat is lost to the gas stream from the source liquid by virtue of the phenomenon of the heat of vapourisation. This loss of heat is made good by heat transfer through the walls of the bubbler vessel from the oil 11 to the source liquid 8. The source liquid in the bubbler vessel is thus at a slightly lower temperature than that of the oil. The amount of this temperature dfference depends in part upon the efficiency of heat transfer across the walls of the bubbler vessel. in the absence of the thermal insulation provided by the air jacket 12, this efficiency would fall as the liquid level in the bubbler vessel falls because the surface area of the bubbler vessel walls in contact with the source liquid is reduced with the fall in source liquid level.Such a fall in temperature reduces the rate of vapour taken up by the carrier gas because the saturation vapour pressure of the source liquid is a function of temperature. This effect is significantly diminished by the presence of the air jacket because the thermal insulation that it provides means that virtually all the heat transfer from the oil to the source liquid takes place across the uninsulated part of the bubbler vessel walls, that is the chamber portion of the bubbler vessel. This lies beneath the level of the bottom of the reservoir, and hence the reservoir can drain from top to bottom without changing the area of contact beween the source liquid and that portion of the bubbler vessel to which heat transfer is effectively confined.

In one particular example of a vapouriser constructed substantially as illustrated the reservoir 1 was approximately 130mm in diameter and 150mm in height. The chamber portion of the bubbler was approximately 80mm in diameter at its base and 80mm in height to the bottom of the tu bular portion 3. The tubular portion 3 was approximately 15mm in internal diameter and 200mm in height. The air gap 12 was approximately 10mm from inner wall to outer wall.

Claims (4)

1. A vapouriser including a bubbler vessel provided with a gas inlet for introducing carrier gas into the vessel beneath the level of source liquid contained therein, and also provided with a gas outlet for removing source vapour-laden carrier gas from above the source liquid level in the vessel, characterised in that the vapouriser is provided with a source liquid reservoir communicating with the vessel, and in that the vessel is preferentially thermally insulated over the region through which the source liquid level falls as the liquid in the reservoir drains into the vessel.

2. A vapouriser as claimed in claim 1, wherein the preferential thermal insulation is provided by an air jacket.

3. A vapouriser as claimed in claim 1 or 2, wherein the vessel is flask-shaped having a prefer entiaily thermally insulated columnular portion surmounting a chamber portion.

4. A vapouriser substantially as hereinbefore described with reference to the accompanying drawing.