GB2163216A

GB2163216A - Gas mixing assembly

Info

Publication number: GB2163216A
Application number: GB08520412A
Authority: GB
Inventors: Alan James Collings
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1984-08-15
Filing date: 1985-08-14
Publication date: 1986-02-19
Also published as: GB8520412D0

Abstract

Gas mixing assembly in which a pressurised gas is passed through an inspirator nozzle 4 in a vessel so as to inspirate further gas from upstream of the nozzle. The flow rate of the pressurised gas is small enough to prevent the onset of critical orifice conditions and great enough to prevent gas mixing solely by diffusion processes. At pressures close to ambient pressure, a flow regulator is used to provide pre-determined gas flows upstream of the inspirator nozzle. The assembly may be used for various purposes including preparation of gas mixtures for analysis or calibration, administration of medicine by inhalation and generation of gas mixtures for burners. <IMAGE>

Description

SPECIFICATION Gas mixing assembly The present invention relates to gas mixing and more particularly relates to an assembly and a process for gas mixing.

There are many applications where it is desired to mix or dilute one or more gases in a controlled and reproducible manner. Examples of these applications include generation of gas mixtures for burners, oxygen enrichment of air, administration of medicines by inhalation and preparation of test gas mixtures for analysis or calibration.

In conventional gas dilution or mixing techniques, a pump is used to pass the gas to be diluted through a flow meter, the gas then being subsequently diluted with a compressed gas such as air. This procedure has the disadvantages of being dependent on the smoothness of the pump, the consistency of compressed gas flow and gas expansion due to pump warm up during operation. Also if various flow rates are required then the pump flow rate and diluent gas flow rate need to be varied in proportion so as to maintain a constant final dilution.

The present invention relates to gas mixing assemblies and methods of gas mixing which may alleviate or remove these disadvantages. Thus according to the present invention there is provided a gas mixing assembly comprising a vessel having an inspirator nozzle located therein there being one or more inlets to the vessel upstream of the inspirator, the inlets having means for controlling the gas flow into the vessel.

Preferably the inspirator comprises a tube within the vessel having its longitudinal axis co-axial with the axis of the vessel, the tube having an outlet nozzle which is supplied from a source of a pressurised gas.

According to a further aspect of the invention there is provided a method for mixing two or more gases comprising the steps of: (a) passing a pressurised first gas through an inspirator nozzle of a gas mixing assembly as herein described so as to inspirate further gas from upstream of the nozzle; (b) the further gas passing through an orifice upstream of the nozzle; and (c) the flow rate of the pressurised first gas being small enough to prevent the onset of critical orifice conditions and great enough to prevent mixing of further gas and first gas solely by diffusion processes.

At pressures close to that of the ambient atmosphere especially where low flow rates of the gases or vapours are to be mixed, it is desirable to have very short gas flow paths otherwise adsorption on the walls of the gas flow tubes may occur. Under these conditions it is less simple to dilute gas/vapour mixtures at low flow rates.

Thus it is preferred to use a flow regulator in the gas flow lines which gives pre-determined gas flows upstream of the inspirator nozzle.

Thus according to a further aspect of the invention there is provided gas mixing assembly having a flow regulator comprising a moveable member having a plurality of orifices therethrough, the cross section of the orifices being of a predetermined area, whereby, during use, change of the orifices presented to the gas or vapour flow causes a change of flow rate. The gas/vapour flows to be mixed are each caused to pass through the orifice appropriate to the desired flow rate for the particular gas or vapour, the separate gas or vapour flows being combined downstream of the regulator and upstream of the inspirator.

The preferred configuration for the regulator is a circular plate carrying orifices around its periphery, the orifice sizes preferably being arranged in one ascending and one decreasing series. Advantageously the plate has means for rotation so that the inlet pipes for gas or vapour may be connected to pre-selected orifices by rotating the plate.

An alternative configuration of regulator is an elongate strip having a number of orifices of predetermined cross sectional area along its length. The strip is capable of linear movement whereby the inlet pipes for gas or vapour may be connected to pre-selected orifices.

The invention will now be described by way of example only and with reference to the accompanying drawings.

Figure 1 shows a gas mixing assembly in which a vessel 1 is connected through an orifice 2 to an inspirator via a connecting tube 3. The inspirator comprises a nozzle 4 supplied with compressed air which lies concentrically within the connecting tube 3. A further inlet 5 is open to the atmosphere (air).

During use, the vessel 1 contained an acetone/air mixture (370 parts acetone per million of air). Compressed air was passed through nozzle 4 to thereby create a negative pressure and entrain the acetone/air mixture from vessel 1 and air through the inlet 5.

The flow of compressed air through the inspirator was raised and the output concentration of acetone was measured. The results for compressed air flow rate and acetone concentration are shown in the table.

Table

Compressed air flow rate Acetone concentration through the inspirator in the output gas (litreslminute) (parts of acetone per million parts of air) 50 58 40 59 30 61 20 64 15 64 10 64 5 72 The results illustrate that the output concentration of the acetone/air mixture remains substantially constant for a wide variation of compressed air flow rates provided that the compressed air flow is not sufficiently great to approach critical orifice conditions anywhere in the system and is not so low that the inspirated flow is overcome by diffusion from the vessel.

Thus it is possibie to dilute a gas with air by a controlled amount using an inspirator nozzle, the degree of dilution being independent of the flow rate of inspirator gas over a wide range of inspirator gas flow rates.

Also the technique may be extended to the mixing of a plurality of gases by use of additional connecting tubes and/or valves. The inspirator gas used in the example is air but clearly other gases may also be used instead of air if desired.

Applications envisaged for the above technique include generation of gas mixtures for burners, measurement of flame velocity, oxygen enrichment of air, preparation of gas mixtures for analysis or calibration and administration of medicines by inhalation.

Figure 2 shows a perspective and a plan view of the flow regulator for regulating inlet gas or vapour flow upstream of the inspirator nozzle of the gas mixing assembly.

Figure 3 shows a cross section of the flow regulator in working relationship with the inspirator nozzle.

The flow regulator comprises an inlet plate 7 carrying a pair of inlet pipes 8, 9 and an outlet plate to carrying a pair of outlet pipes 11, 1 2. The plates are locked together by a rectangular cross section pin 1 3 on the outlet plate 10 locating with a complementary hole 14 in the inlet plate 7. The inlet and outlet plates are spaced apart by a circular plate 1 5 having eight orifices 1 6 in its periphery, the orifices 1 6 being spaced so that diametrically opposite pairs of orifices can be sequentially connected to the inlet and outlet pipes.

The cross sectional area of the orifices around the plates are arranged in the order 8:7:6:5:1:2:3:4 units. It is preferred that the orifice having an area of 8 units is of the same diameter as the inlet and outlet pipes or of a smaller diameter.

During use, two gases to be mixed are passed separately along inlet pipes. The desired relative flow rate of the gases is determined by rotating the plate so that the desired orifices are located in the gas flow paths. The gases then exit through the outlet pipes and are combined into a single flow upstream of the inspirator nozzle.

Clearly by modifying the plate any number of gas or vapours may be combined together upstream of the plate, the plate being rotated to present the required orifice to the gas flows.

Claims

1. Gas mixing assembly comprising a vessel having an inspirator nozzle located therein there being one or more inlets to the vessel upstream of the inspirator, the inlets having means for controlling the gas flow into the vessel.

2. Gas mixing assembly according to claim 1 in which the inspirator comprises a tube within the vessel having its longitudinal axis co-axial with the axis of the vessel, the tube having an outlet nozzle which is supplied from a source of pressurised gas.

3. Gas mixing assembly according to claim 1 or claim 2 having a flow regulator comprising a moveable member having a plurality or orifices therethrough, the cross section of the orifices being of a predetermined area, whereby, during use, change of the orifices presented to the gas or vapour flow causes a change of flow rate.

4. Gas mixing assembly according to claim 3 in which the moveable member comprises a circular plate having orifices around its periphery.

5. Gas mixing assembly according to claim 4 in which the orifice sizes are arranged in one series of ascending sizes and one series of descending sizes.

6. Gas mixing assembly according to claim 4 or claim 5 in which the plate has means for rotation that the inlet pipes for gas or vapour may be connected to pre-selected orifices by rotation of the plate.

7. Gas mixing assembly according to claim 3 in which the moveable member comprises an elongate strip having a number of orifices of predetermined cross sectional area along its length, the strip being capable of linear movement whereby the inlet pipes for gas or vapour may be connected to preselected orifices.

8. Gas mixing assembly as hereinbefore described and with reference to the accompanying drawings.

9. A method for mixing two or more gases comprising the steps of: (a) passing a pressurised first gas through an inspirator nozzle of a gas mixing assembly according to any one of the preceding claims so as to inspirate further gas from upstream of the nozzle; (b) the further gas passing through an orifice upstream of the nozzle; and (c) the flow rate of the pressurised first gas being small enough to prevent the onset of critical orifice conditions and great enough to prevent mixing of further gas and first gas solely by diffusion processes.