GB2197801A - Improvements in and relating to pressure swing oxygen generation systems - Google Patents

Abstract

In a pressure swing oxygen generation system of the kind in which two exchangeable adsorption beds are coupled cyclically and in alternation to sources of elevated and reduced atmospheric pressure, the instant at which the input to one bed is connected to the source of reduced pressure after having been connected to the source of compressed air is delayed in relation to the instant at which the input to the other bed is switched from the source of reduced pressure to the source of compressed air. In this manner, reduction of the gas pressure prevailing within the reservoir supplied by such a system is partly avoided. <IMAGE>

Description

IMPROVEMENTS IN AND RELATING TO PRESSURE SWING OXYGEN GENERATION SYSTEMS This invention concerns improvements '- #in and relating to pressure swing oxygen generation systems.

So-called pressure swing oxygen generation systems are known. They generally comprise at least two beds of adsorption medium7 means for cyclically coupling to each bed in turn a source of compressed atmospheric air, whereby, by adsorption of unwanted gas or gases within the bed, there is produced at a common outlet from the beds a continuous supply of atmospheric air of which the oxygen content is enriched in comparison with that of the air provided from the original source, each adsorption bed being purged of the waste gas or gases between the periods of time in which it is coupled to the source of compressed air, and means for controlling the duration of each period for which an adsorption bed,is coupled to said source of compressed air in order correspondingly to control the mean concentration of oxygen:#n the air provided at..said common outlet. .They-are-utilised amongst other things for the provision of an oxygen enriched air supply to the air crew of an aircraft when in flight. The generation of an oxygen enriched atmosphere is effected by the separation of nitrogen from atmospheric air by means of a zeolite molecular sieve adsorption bed that has the property of trapping nitrogen molecules by adsorption from atmospheric air pumped through the bed under pressure, whilst allowing oxygen molecules to pass.

Thus, an oxygen enriched atmosphere is generated on the down-stream side of the separator bed and can be supplied, via a suitable reservoir and breathing regulator, to provide a breathable gas supply for the air crew.

Purging of waste gas from the adsorption bed when saturated with nitrogen molecules can be effected by application of reduced atmospheric pressure to the adsorption bed and/or by flushing the bed with a breathable gas derived from an alternative supply. Thus, by utilisation of a plurality of beds in timed relation such that a fresh bed becomes available during the operation of evacuating or purging a saturated bed, a continuously available source of oxygen enriched atmosphere can be provided.

In order to produce a system that is compact and capable of being located in an aircraft in a situation where space is at a premium, a system of the kind described above preferably comprises two beds of adsorption medium serving a common reservoir, and the volume of the reservoir is limited to the minimum that will allow for the required switching between the adsorption beds whilst maintaining a source of oxygen enriched atmosphere available at the maximum anticipated rate of demand by an air crew.

In systems of such limited size, however, there is a tendency for the reservoir pressure to fall during the period immediately following the point of exhaustion of one adsorption bed and the commencement of pressurisation of the replacement bed, as no breathable product gas can be drawn from the latter bed until it reaches the reservoir pressure. A fall in the pressure of the reservoir at this point is undesirable, as it may adversely affect the operation of the air crew breathing regulators and other equipment.

The known system referred to above and the attendant disadvantage is explained in more detail with reference to Fig. 1, wherein Fig. 1A is a diagram of a known pressure swing system, and Fig. 1B is a diagram illustrating the operating cycle of the system. In Fig. 1, the reference numerals 1A and 1B illustrate adsorption beds of the kind referred to above. The numeral 2 illustrates a switching valve which enables compressed air from a source 3 to be applied to one of the beds 1A or 1B whilst a reduced pressure from a waste outlet 4 is applied to the other one of the two beds 1A or 1B. Outlets from the respective adsorption beds 1A and 1B are coupled to a reservoir 5 via non-return ~ valves 6, the outlet from the reservoir 5 providing a breathable air supply.The outlets of the adsorption beds 1A and 1B are connected together, on the upstream side of the non-return valves 6, by means of a conduit 7 incorporating a flow restrictor 8.

Referring to Fig. 1B, the cycle of operation of the arrangement of Fig. 1A is indicated diagramatically. The vertical broken lines T1, T2 and T3 respectively indicate instants in time at which the switching valve 2 is actuated to reverse the connection to adsorption beds 1A and 1B respectively. Thus, at instant Tl, the inlet to the bed 1A is connected to the source of compressed air 3, whilst the inlet to the bed 1B is connected to the waste 4. These connections are reversed at instant T2 and re-established at instant T3. During the period between the instants T1 and T2, the bed 1A is fed with compressed air for a period indicated by the line 10, whereas the bed 1B is evacuated for a period represented by the line 11.

Initially, the bed 1A will be at reduced pressure as a result of having been connected to the waste 4, whereas the pressure within the bed 1B will be reducing following its connection to the waste 4. The pressure at the outlets of both beds will thus be below that within the reservoir 5 and the non-return valve 6 will be closed. As the pressure in the bed 1A rises with the entry of compressed air, the product gas will firstly pass via the restrictor 8 into the bed 1B, to effect purging thereof, and then, when the pressure at the outlet of bed 1A rises above that in the reservoir 5, the non-return valve 6 will open allowing the supply of oxygen enriched atmosphere to the reservoir 5. The corresponding period of delay between actuation of the valve 2 and opening of the relevant non-return valve 6 is indicated by the line 12 in Fig. 1B.

It will therefore be seen that if, during the period represented by the line 12 in Fig. 13, the demand by the air crew for breathable atmosphere exceeds that capable of being supplied from the reservoir 5 the pressure in the latter will drop towards the pressure prevailing within the bed 1A.

It is an object of the present invention to overcome this disadvantage.

In accordance with the invention, the cycle of actuation of a pressure swing oxygen generation system of the kind having two exchangeable adsorption beds is modified so that the instant at which the input to one bed is connected to the source of reduced pressure after having been connected to the source of compressed air is delayed in relation to the instant at which the input to the other bed is switched from the source of reduced pressure to the source of compressed air.

In this manner, reduction of the gas pressure prevailing within the reservoir supplied by such a system is partly avoided in the circumstances described above.

The invention is illustrated by way of example in the accompanying drawings, in which; Fig. 1A is an illustration of a known system as already described above, Fig. 1B is a timing diagram relating to the system of Fig. lA, Fig. 2A is a view corresponding to Fig. 1A illust rating a system in accordance with the invention.

Fig. 2B is a diagram corresponding to Fig. 13, illustrating the operation of the system of Fig. 2A, and Figs. 3A, 3B, 4A and 4B are further, similar views of additional embodiments of the invention.

Referring to Figs.2A and 23, it will be noted that those parts of the drawings which correspond with Figs. 1A and 1E have been indicated with the same reference numerals, and therefore only the differences between the respective drawings will be described in detail.

In Fig. 2A, the valve 2 controlling the cycle of operation of the system is supplemented by further control valves 9A and 9B, the valve 9A being connected in the conduit between the valve 2 and the waste 4, and the valve 9B being connected in the conduit 7.

The valves 9A and 9B are actuated in timed relation to the valve 2 in a manner that will now be described with reference to Fig. '2B. At the instants T1 and T2, as the valve 2 is actuated to reverse the connections to the beds 1A and 13, the valves 9A and 9B are simultaneously. closed. The valves 9A and 9B are then re--opened at instants T'1, T'2 etc., that are relatively displaced in time with respect to the instants T1,T2 etc., by the period corresponding to the initial pressurisation of each adsorption bed, as indicated by the line 12.

As a result of the corresponding actuation of the valves 9A, 9B, the period for which each adsorption bed is vented and purged is reduced in duration, as indicated by the line 13 of Fig. 2B, in comparison with the line 11 of Fig. 1B. This has the result that each adsorption bed continues to supply the reservoir 5 with product gas during the period at which the other adsorption bed is pressurised after having been purged.

Although the cycle of operation illustrated in Fig. 23 results in some reduction in the efficiency of the system in that the resulting concentration of oxygen supplied to the reservoir 5 is reduced, it has the considerable advantage of maintaining pressure in the reservoir 5, which advantage overrides the slight disadvantage of reduced efficiency.

For example, in a typical operating system utilising a compressed air supply of 2 bar, and a part cycle time (e.g. between instant T1 and T2) of 421 seconds and above, the delayed venting cycle involving a period between instants Tl,T'l etc., of betwen 0 and 0.75 seconds was found to decrease the pressure swing in the reservoir 5. Delay times in excess of 0.75 seconds were shown to provide no further advantage.

In one example, with a system operating at a simulated aircraft altitude of 31,000 feet and cabin altitude of 14,050 feet, with an inlet air pressure to the system of 35.0 p.s.i.a. a breaking machine was utilised to simulate aircrew demand. With the cycle time of the system being controlled to hold the composition of the product gas supplied to the reservoir 5 at an average of 53% oxygen, then utilising an operating cycle of the known kind described with reference to Figs. 1A and 1B, the maximum swing of pressure within the reservoir 5 was from 33.2 p.s.i.a.to 23.7 p.s.i.a Utilising a cycle of the kind described with reference to Figs. 2A and 2B, and a delay between instant T1 T'1 etc., of 0.75 seconds, the pressure swing within the reservoir was reduced to a swing from 33.2 p.s.ia to 27.7 p.s.i.a.

It will be appreciated that various alterations may be made to the system described above without departing from the scope of the invention. For example, although the valves 9A and 9B have been described as being closed simultaneously with the changeover of switching valve 2, it may be found preferable to close the valves 9A and 9B shortly before actuation of the valve 2. Also, although the presence of valve 9B is advantageous in preventing the bleed of product gas from the higher pressure bed to the lower pressure bed during the initial pressurisation of the latter, the presence of the valve 9B may be considered as optional.

Also, the valves 2 and 9A may be replaced by any other appropriate valving arrangement, e.g. by providing independent connections between each adsorption bed and the source of compressed air 3 or the outlet to reduce pressure 4.

A further cycle modification on the same theme is to extend the air feed times to cover the vent delay time, Fig. 4. In this way when Bed B is pressurising Bed A is maintained at the feed pressure, as is Bed B when Bed A is pressurising, and the plenum is thus always connected to a fully pressurised bed and the dip of plenum pressure is minimised. The simple switching valve of Figs. 1 and 2 cannot be used but must be replaced by two pairs of feed and vent valves. Also the feed lines must have non-return valves installed to prevent the beds equalising through the feed lines when the feed to the two beds overlaps.

Claims (7)

1. A process for the generation of an oxygen enriched atmosphere, wherein atmospheric air is fed at elevated pressure to each of two exchangeable adsorption beds in alternation whereby waste gases are entrapped therein and there is a correspondingly higher concentration of oxygen in the atmosphere issuing from outputs thereof, and each bed is purged of waste gases between consecutive periods in which it is supplied with atmospheric air, by connection to a source of reduced pressure, wherein the two beds are coupled to a common output and the instant at which each bed is connected to said source of reduced pressure is delayed relatively to the instant at which the other bed is connected to the source of atmospheric air, whereby during initial pressurisation of said other bed the gas pressure at said common outlet is maintained at a higher level than that at the outlet of said other bed.

2. A process as claimed in Claim 1, substantially as described herein.

3. An apparatus for use in the process of Claim 1 or 2, comprising two beds of adsorption medium each having an inlet and an outlet for gas to be passed through the bed, a changeover valve having an inlet port for coupling to a source of compressed air, an exhaust port for coupling to a source of reduced pressure and ports coupled to the inlets of said beds whereby in each position of said valve one bed is coupled to the inlet port and the other is coupled to the exhaust port, and vice versa, a shut-off valve connected on the downstream side of the said exhaust port, and means for actuating said changeover valve and said shut-off valve in timed relation whereby after each actuation of said switching valve said shut-off valve is closed for a predetermined period of time.

4. An apparatus as claimed in Claim 1 or 2 comprising two beds of adsorption medium each having an inlet and an outlet for gas to be passed through the bed; an inlet for coupling to a source of compressed gas; an exhaust outlet for coupling to a source of reduced pressure; a flow control valve connected between the inlet of each said bed and said compressed gas inlet, a flow control valve connected between the inlet of each bed and said exhaust outlet, and means for actuating said valves in timed relation whereby the inlet of each bed is connected in turn to said gas inlet and to said exhaust outlet such that each bed is connected to the exhaust outlet during a period in which the other is connected to the gas inlet and the instant at which the inlet of each bed is connected to said exhaust outlet is delayed with respect to the instant at which the inlet of the other bed is coupled to said gas inlet.

5. An apparatus as claimed in Claim 4, wherein a non-return valve is coupled between said gas inlet and each valve coupling said gas inlet to the inlet of a bed, and said valve actuating means is arranged to close the valve coupling the inlet of a bed to said gas inlet at substantially the same instant as it opens the valve coupling the inlet of the same bed to said exhaust outlet.

6. An apparatus as claimed i11 any one of Claims 3-5, wherein the outlet of each bed is coupled to a common conduit via a non-return valve, a gas bleed by-pass conduit is arranged to connect the outlets of said beds on the upstream side of said non-return valves, a further valve is provided in said by-pass conduit, and said valve actuating means is arranged to close said further valve for a predetermined period of time after the instant at which the inlet of each bed is coupled to said source of compressed gas.

7. An apparatus for carrying out the process of Claim 1, substantially as described herein.