GB2404954A

GB2404954A - Piston pump with leak prevention means for a fuel cell system

Info

Publication number: GB2404954A
Application number: GB0319020A
Authority: GB
Inventors: Nigel Paul Schofield; Howard Brian James Stone
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 2003-08-13
Filing date: 2003-08-13
Publication date: 2005-02-16
Also published as: GB0319020D0

Abstract

A vacuum pump comprises at least one pumping chamber (80) defined by a bore (70), a reciprocating piston (68) housed in the bore and a first circumferentially extending sealing device (92). The piston (68) is provided with a second circumferentially extending sealing device (94) spaced apart from the first circumferentially extending sealing device (92) in a lengthways direction of the piston such than an annular chamber is defined in the space between the sealing devices. The pumping chamber (80) has an inlet port communicable with a pump inlet system (84, 85), an exhaust port communicable with a pump exhaust system and a scavenge port (98) communicating with the pump inlet system. The arrangement is such that the scavenge port (98) opens into the annular chamber such that when the piston reciprocates, in use, the first and second sealing devices (92, 94) do not pass the scavenge port (98). Fluid from the pumping chamber (80) that escapes past the first sealing device (90) is returned to the pump inlet system rather than passing the second sealing device (94) to the pump interior. The pump may be used in a fuel cell system comprising a fuel cell, a fuel reformer and a vacuum pump.

Description

Reciurocatine Piston Vacuum Pumps The invention relates to reciprocating

piston vacuum pumps for pumping aggressive or volatile gases or gas/vapour mixtures and particularly, but not exclusively to vacuum pumps for pumping the fuel gas in a fuel cell system.

Fuel cells are electrochemical devices that convert a fuel's chemical energy directly to electrical energy. The electricity is generated by the chemical reaction of a gas, usually hydrogen, across a proton exchange membrane. In one particular system, the hydrogen is formed by the catalytic reforming of natural gas. This produces a hot mixture of hydrogen, nitrogen, carbon dioxide and water vapour, which will often be at 100 to 120 C and may reach temperatures between 240 and 400 C. This mixture has to be pumped through both the fuel reformer and the fuel cell. This can be achieved by the arrangement drawn in Figure 1 in which a vacuum pump 10 is connected between a fuel reformer 12 and fuel cell 14 so as to pull the mixture through the reformer and push it through the fuel cell. Alternatively, as shown in Figure 2, a vacuum pump 16 can be situated downstream of the fuel cell 18 so as to pull the mixture through the fuel cell and the fuel reformer 20.

The nature of the mixture drawn from the fuel cell is such that it is highly desirable that it is contained within the pumping chamber(s) of the vacuum pump and the associated inlet and outlet passages and that it does not penetrate further into the pump. If the mixture were to penetrate further into the pump, there is the potential for corrosion of the pump's bearings by the hot water vapour in the mixture. Furthermore, if the hydrogen in the mixture is able to reach the pump's electric motor, it may be ignited by sparking from the motor.

According to the present invention there is provided a pump comprising a bore, a reciprocating piston housed in the bore for varying the size of a pressure chamber, the pressure chamber having a fluid inlet port and a fluid exhaust port; first and second means spaced apart lengthways of the piston and in sealing engagement with the bore and the piston for defining a second chamber therebetween; and means for communicating with the second chamber to allow fluid entering the second chamber from the pressure chamber to be directed therefrom.

At least one of the first and second means may be a piston ring, a lip seal, a wiper seal or a sealing disc. It may be carried by or mounted on either the piston itself or on the bore. It may be formed integrally with the piston such that a single component fulfils the action of the piston and the chamber defining requirements of the first and second means.

The pump may be configured such that the first and second means do not pass over the communication means as the piston reciprocates. The communication means may be provided by a scavenge port. The fluid port inlet may be communicable with a pump inlet system and the fluid port outlet may be communicable with a pump exhaust system.

The invention also provides a vacuum pump comprising at least one pumping chamber defined by a bore, a reciprocating piston housed in said bore and a first circumferentially extending sealing device carried by said piston, said piston being provided with a second circumferentially extending sealing device spaced apart from said first circumferentially extending sealing device in a lengthways direction of the piston such than a second, annular chamber is defined in the space between said sealing devices and said pumping chamber having an inlet port communicable with a pump inlet system, an exhaust port communicable with a pump exhaust system and a scavenge port communicating with said pump inlet system, the arrangement being such that the scavenge port opens into said annular chamber such that when said piston reciprocates, in use, said first and second sealing devices do not pass the scavenge port and any fluid from said pumping chamber that escapes past said first circumferentially extending sealing device is received in said annular chamber and returned to said pump inlet system via said scavenge port.

The invention further provides a vacuum pump comprising at least one pumping chamber defined by a bore, a reciprocating piston housed in said bore and a first circumferentially extending sealing device carried by said piston, said piston being provided with a second circumferentially extending sealing device spaced apart from said first circumferentially extending sealing device m a lengthways direction of the piston such that an annular chamber is defined in the space between said sealing devices and said pumping chamber having an inlet port communicable with a pump inlet system, an exhaust port communicable with a pump exhaust system and a scavenge port communicating with a pump port separate from said pump inlet system and said pump exhaust system, the arrangement being such that the scavenge port opens into said annular chamber such that when the piston reciprocates, in use, said first and second sealing devices do not pass the scavenge port and any fluid from said pumping chamber that escapes past the first said sealing device is received in said annular chamber and directed out of said pump via said scavenge port and separate pump port.

In order that the invention may be well understood, embodiments thereof, which are given by way of example only, will now be described with reference to the drawings, in which: Figure I is a schematic illustration of a fuel cell system including a vacuum pump; Figure 2 is a schematic illustration of another fuel cell system including a vacuum pump; Figure 3 is a longitudinal cross-section of a vacuum pump suitable for use in a fuel cell system such as those shown in Figures I and 2; Figure 4 is a schematic plan view Illustrating the porting arrangement ofthe vacuum pump shown in Figure 3; Figure 5 is an enlargement of a portion of Figure 3; Figure 6 is a schematic Illustration of a modified version of the vacuum pump shown in Figure 3 in use in a full cell system as shown in Figure 1; Figure 7 is a schematic illustration of a modified version of the vacuum pump of Figure 3 or Figure 6 in use in a fuel cell system as shown in Figure l; and Figure 8 is a schematic illustration of a modified version of the vacuum pump of Figure 3 or Figure 6 in use in a fuel cell system as shown in Figure 2.

Referring to Figures 3 and 4, a vacuum pump 50 comprises a casing 52 housing a pumping mechanism 54 and an electric motor 56 for driving the pumping mechanism.

The electric motor comprises a stator 58 and a rotor 60 that is mounted on a motor shaft 62. The motor shaft 62 is supported at either end by bearings 64, 66. The motor and bearings may be of any suitable type and arranged in any convenient manner for driving the pumpmg mechanism 54. An inverter (not shown) may be provided in the space 57 for controlling the motor speed so as to control the pump throughput.

The pumpmg mechanism 54 composes two pistons 68 mounted in parallel, sideby-side, in respective cylinder bores 70 defined by the pump casing 52. The pistons 68 are connected to respective connecting rods 72 by gudgeon pins 74 and little end bearings 76. In a preferred embodiment, the gudgeon pins 74 are hollow tubes made of stainless steel, the little end bearings 76 are rolling bearings and the connecting rods 72 are made of aluminium. However, it will be appreciated that other suitable materials may be used and the little end bearings need not be rolling bearings. The connecting rods 72 are connected to the motor shaft 62 by respective big end bearings 78. In the embodiment, the big end bearings 78 are rolling ball bearings, but other forms of suitable bearing can be used.

Each piston 68 and cylinder bore 70 provides a pumping chamber 80 located below (as viewed in Figure 3) the piston crown. As shown in Figure 4, each pumping chamber 80 has an inlet port 82, which connects with a pump inlet system comprising a common inlet passage 84 and the pump inlet port 85, which is at the downstream end of the inlet passage. The pumping chamber inlet ports 82 are each provided with a valve (not shown) that operates in a conventional manner to allow the passage of gas/vapour from the inlet passage 84 into the respective pumping chamber 80. Similarly, each pumping chamber 80 has an outlet port 88 provided with a valve (not shown) which operates in a conventional manner to permit the exhaust of gas/vapour from the respective pumping chamber 80 into a pump exhaust system comprising a common exhaust passage 90 and a pump exhaust port 91, which is at the downstream end of the exhaust passage.

Each piston 68 has a first circumferentially extending sealing device 92 and a second circumferentially extending sealing device 94. The sealing devices 92, 94 are spaced apart in the lengthways direction of the piston such than an annular chamber 96 (Figure 5) is defined in the space between them.

It will be appreciated that the width of the annular chamber 96 in Figure 5 has been exaggerated for the purposes of illustration and that in practice the width will be determined by the running clearance provided between the piston 68 and cylinder bore 70.

The sealing devices 92, 94 may take any convenient form suitable for the pumping environment. They may, for example, take the form of conventional piston rings or wiper seals. In one presently contemplated embodiment, the cylinder bore 70 is made of steel, the piston is made of anodised aluminium and the sealing devices are piston rings made of carbon.

One alternative material for the seals would be PTFE loaded with carbon.

A scavenge port 98 is provided in the lengthways extending wall of each cylinder bore 70. The scavenge ports 98 are connected by a common passage 100 that extends between the two bore walls. A connecting passage 102 extends from the common passage 100 to the common inlet passage 84, providing a flow path between the annular chambers 96 and the pump inlet system. The arrangement of the sealing devices 92, 94 and the scavenge ports 98 is such that when the pistons 68 are reciprocating, the sealing devices 92, 94 do not cross the scavenge ports, which are therefore always open to the fluid in the annular chamber 96.

In use, in a fuel cell system such as the fuel cell system shown in Figure 1, the pump inlet port 85 is connected by any suitable conventional means to a pipe leading from the fuel reformer 12. Similarly, the pump exhaust port 91 is connected by any suitable conventional means to a pipe leading to the fuel cell 14. As the pistons 68 reciprocate, a fuel gas mixture is drawn from the fuel reformer 12 into the pumping chambers 80 where it is compressed to increase its pressure to a degree sufficient to push it through the fuel cell 14. If any of the fuel gas mixture escapes past the first sealing device 92, it will be received in the annular chamber defined between the piston skirt, bore wall and sealing devices 92, 94. Because the scavenge port 98 is continually in open flow communication with the pump inlet system, any such fuel gas mixture will be drawn from the annular chamber into the common inlet passage 84. Thus the fuel gas mixture will not escape past the second sealing device 94 and problems with corrosion of the bearings and possible explosions should be avoided.

It will be appreciated that the pressure in the pump inlet system will be at least slightly below atmospheric pressure. It is thus possible that atmospheric air from the pump interior will be drawn past the second sealing device 94 into the annular chamber. However, provided this leakage is relatively small, the overall operation of the system will not be significantly al fected.

It will be understood that the embodiment Is purely illustrative and that the pump may be a single piston pump or have more than two pistons.

Equally, the pumping chambers may have multiple inlet ports and/or multiple exhaust ports and/or scavenge ports. Furthermore, it is not essential that the pumping chamber inlet ports are connected to the pump mlet port by a common inlet passage or that the exhaust ports are connected to the pump exhaust port by a common exhaust passage. Instead, connection could be by way of individual passages to a reservoir or manifold connected to the pump inlet or exhaust port. Similarly, individual passages may be provided for connecting the scavenge ports to pump inlet system.

It is preferred that the scavenge ports 98 are connected with the pump Gimlet system 84, 85 so that any fuel gas mixture that escapes from the pumping chamber into the annular chamber 96 is routed directly to the inlet region of the pump for direct return to the pumping chamber 80. However, as an alternative, it is possible, for example, to connect the scavenge ports 98 to a suitable pressure location external of the pump. For example, as shown schematically in Figure 6, the scavenge ports 98 could be connected by (a) suitable passage(s) 120 to (a) pump outlet port(s) 122 separate from the pump inlet system and pump exhaust system. Piping 124 attached to this outlet port 122 could then lead to an external location at which the pressure is at least slightly below atmospheric so that any fuel gas mixture entering the annular chamber 96 would be drawn through the scavenge port 98, rather than escaping past the second sealing device 94 mto the pump interior. In a fuel cell system such as that shown in Figure 1, the piping 124 could feed into piping 130 that connects the fuel reformer 12 to the pump inlet port 85 (as illustrated in Figure 6), but this is not to be taken as limiting.

In the embodiment, the pump is shown as simply comprising a pumping mechanism for pumping a fluid between the pump inlet 85 and pump exhaust 91. If desired, an air pump could be driven from the motor shaft 62 at the other end of the motor. The construction of the air pump may be the same or similar to that of the pumping mechanism 54, or may be of any suitable conventional construction.

Figure 7 illustrates a fuel cell system similar to that shown in Figure 1 in which a vacuum pump SO as shown in Figure 3 or Figure 6 is connected between a fuel reformer 12 and a fuel cell 14 so as to pull the fuel mixture from the reformer and push it through the fuel cell. The pump shaft 62 is driven by the electric motor 56 and has an air pump 120 driven by an extension 62E of the pump shaft. Air for the fuel cell 14 is taken in by the air pump 120 at 122 and pumped into the fuel cell.

Figure 8 illustrates a fuel cell system similar to that shown in Figure 2 in which a vacuum pump SO as shown in Figure 3 or Figure 6 is connected into the system downstream of a fuel cell 18 so as to pull the fuel mixture through both the fuel reformer 20 and the fuel cell 18. The pump shaft 62 is driven by the electric motor 56 and has an air pump 120 driven by an extension 62E of the pump shaft. Air for the fuel cell 18 is taken into the fuel cell at 124 and drawn through the fuel cell by the air pump 120. It will be appreciated that the connections of the air pumps 120 in Figure 7 and 8 can be made such that the pump 120 in Figure 7 draws air through the fuel cell 14 and the pump 120 in Figure 8 pumps air through the fuel cell 18.

It will be appreciated that although a pump according to the invention is particularly suitable for use in a fuel cell system, it can in principle be used with advantage for pumping any gas or gas/vapour mixture and particularly a gas or gas/vapour mixture that may cause damage If it is allowed to escape past the piston seals into the pump interior.

Claims

Claims 1. A pump comprising a bore, a reciprocating piston housed in the

bore for varying the size of a pressure chamber, the pressure chamber having a fluid inlet port and a fluid exhaust port; first and second means spaced apart lengthways of the piston and in sealing engagement with the bore and the piston for defining a second chamber therebetween; and means for communicating with the second chamber to allow fluid entering the second chamber from the pressure chamber to be directed therefrom.
2. A pump as claimed in claim 1, wherein at least one of the first and second means is one of the group of a piston ring, a lip seal, a wiper seal and a sealing disc.
3. A pump as claimed in claim 1 or claim 2, wherein at least one of the first and second means is carried by the piston.
4. A pump as claimed in any preceding claim, wherein at least one of the first and second means is formed integrally with the piston.
5. A pump as claimed in any preceding claim, wherein at least one of the first and second means is mounted on the bore.
6. A pump as claimed in any preceding claim, wherein the means communicating with the second chamber comprises a scavenge port opening into the second chamber such that when the piston reciprocates, in use, the first and second means do not pass the scavenge port.
7. A pump as claimed in any preceding claim, wherein the Duid port inlet is communicable with a pump inlet system and the fluid port outlet is commumcable with a pump exhaust system.
8. A vacuum pump comprising at least one pumping chamber defined by a bore, a reciprocating piston housed in said bore and a first circumferentially extending sealing device carried by said piston, said piston being provided with a second circumferentially extending sealing device spaced apart from said first crcumferentially extending sealing device In a lengthways direction of the piston such than a second, annular chamber is defined in the space between said sealing devices and said pumping chamber, the pumpmg chamber having an mlet port communicable with a pump inlet system, and an exhaust port communicable with a pump exhaust system, and a scavenge port, the arrangement being such that the scavenge port opens into said annular chamber such that when said piston reciprocates, in use, said first and second sealing devices do not pass the scavenge port and any fluid from said pumping chamber that escapes past said first circumferentially extending sealing device is received in said annular chamber and directed out of the annular chamber via the scavenge port. s
9. A pump as claimed in claim 6 or claim 8, wherein the scavenge port communicates with the pump inlet system so that any fluid directed out of the second, annular chamber via the scavenge port is returned to the pump inlet system
10. A vacuum pump comprising at least one pumping chamber defined by a bore, a reciprocating piston housed in said bore and a first circumferentially extending sealing device carried by said piston, said piston being provided with a second circumferentially extending sealing device spaced apart from said first circumferentially extending sealing device in a lengthways direction of the piston such than an annular chamber is defined in the space between said sealing devices and said pumping chamber, the pumping chamber having an inlet port communicable with a pump inlet system, an exhaust port commumcable with a pump exhaust system and a scavenge port communicating with said pump mlet system, the arrangement being such that the scavenge port opens into said annular chamber such that when said piston reciprocates, in use, said first and second sealing devices do not pass the scavenge port and any fluid from said pumping chamber that escapes past said first circumfercntally extending sealing device is received m said annular chamber and returned to said pump inlet system via said scavenge port.
11. A pump as claimed in claim 9 or 10, wherein said pump inlet system comprises a pump inlet port and an inlet passage connecting said pump inlet port with said pumping chamber inlet port, the pump further comprising a passage extending from said scavenge port and connecting with said inlet passage between said pump inlet port and said pumping chamber inlet port.
12. A pump as claimed in claim 9 or 10, comprising a plurality of said pumping chambers, the pump inlet system comprising a pump inlet port and a common inlet passage that connects the respective inlet ports of said pumping chambers with said pump inlet port, the respective scavenge ports of said pumping chambers being connected with said common inlet passage.
13. A pump as claimed in claim 9 or 10, comprising at least one pair of said pumping chambers disposed side-by-sde, the respective scavenge ports of said pair of pumping chambers communicating with a common scavenge passage that communicates with said pump mlet system.
14. A pump as claimed in claim 13, wherein said pump inlet system comprises a common inlet passage connecting a pump inlet port with the respective inlet ports of said pumping chambers, said common scavenge passage having a downstream end that opens into said common inlet passage.
15. A pump as claimed in claim 9 or IO, comprising a plurality of said pumping chambers, the respective scavenge ports being connected to said pump inlet system by separate scavenge passages.
16. A pump as claimed in claim 6 or 8, wherem the scavenge port communicates with a pump port separate from the pump inlet system and the pump exhaust system so that any fluid directed out of the second, annular chamber via the scavenge port is directed out of the pump via the separate pump port.
17. A vacuum pump comprising at least one pumping chamber defined by a bore, a reciprocating piston housed in said bore and a first circumferentially extending sealing device carried by said piston, said piston being provided with a second circumferentially extending sealing device spaced apart from said first circumferentially extending sealing device in a lengthways direction of the piston such that an annular chamber is defined in the space between said sealing devices and said pumping chamber, the pumping chamber having an inlet port communicable with a pump inlet system, an exhaust port communicable with a pump exhaust system and a scavenge port communicating with a pump port separate from said pump inlet system and said pump exhaust system, the arrangement being such that the scavenge port opens into said annular chamber such that when the piston reciprocates, in use, said first and second sealing devices do not pass the scavenge port and any fluid from said pumping chamber that escapes past the first said sealing device Is received in said annular chamber and directed out of said pump via said scavenge port and separate pump port.
18. A pump as claimed in any preceding claim, wherein the scavenge port is connected to a pressure location whereat, in use, a pressure below that in the annular chamber, preferably below atmospheric, is produced.
19. A pump as claimed in any one of the preceding claims, further comprising a drive motor for the pump and a second pump connected with said drive motor for pumping a fluid separate from said pumping chambers.
20. A fuel cell system comprising a fuel cell, a fuel reformer and a vacuum pump as claimed in any one of the preceding claims, wherein said pump inlet system is connected to said fuel reformer and said pump exhaust system is connected to said fuel cell such that in use, fuel from said fuel reformer is drawn through said fuel cell via said pump.
21. A fuel cell system comprising a fuel cell, a fuel reformer and a pump as claimed in any one of claims I to l9, wherein said fuel reformer is connected to said fuel cell and said fuel cell is connected to said pump inlet system such that in use said pump draws fuel from said fuel reformer via said fuel cell.
22. A fuel cell system as claimed in claim 20 or 21 when dependent on claim 19, wherein said second pump is connected with said fuel cell for pumping air through said fuel cell.
23. A fuel cell system as claimed in claim 20 or 21 when dependent on claim 19, wherein said second pump is connected with said fuel cell for drawing air through said fuel cell.
24. A vacuum pump substantially as herein described with reference to Figures 3 to 5.
25. A fuel cell system substantially as herein described with reference to Figures 1 to 8.