WO2007118275A1

WO2007118275A1 - Recycling compressed air

Info

Publication number: WO2007118275A1
Application number: PCT/AU2007/000489
Authority: WO
Inventors: Anthony John Kitchener
Original assignee: Ateliers Francois
Priority date: 2006-04-13
Filing date: 2007-04-12
Publication date: 2007-10-25

Abstract

The specification discloses a control arrangement (20) for the supply of compressed air to machinery (5) where some used compressed air is vented to atmosphere and some unused compressed air is available for recycling, the control arrangement (20) including a primary compressor system (1 ) and a booster compressor system (6) each being connected via discharge line means (13, 7) to a pressure vessel (3) from which the machinery (5) draws compressed air at a desired first pressure level, the booster compressor means (6) being connected via a suction line (9) to the machinery (5) and being adapted to receive recycled compressed air at an intermediate pressure less than the first pressure level via the suction line (9), the primary compressor system (1 ) being driven by an electric drive motor or motors (M1) controlled by a variable speed drive device (2), a pressure sensor (10) being provided to sense pressure in the pressure vessel (3) and said variable speed drive device (2) being controlled in response to pressure sensed by the pressure sensor (10).

Description

RECYCLING COMPRESSED AIR

The present invention relates to improvements in control systems for recycling unused compressed air in manufacturing processes.

It has been recognized that compressed air used for blow moulding processes, typically for manufacturing bottles or containers for beverage or other food products, is expensive to produce because the pressure level required is quite high (typically 25 to 40 bars), must be free of contaminants such as lubricant materials, and preferably should also be dry. The compressed air used in such processes, after use, is typically vented to atmosphere, despite still being at quite high pressure levels which provides a substantial inefficiency. It is known in blow moulding processes to reduce compressed air consumption by recycling unused levels of compressed air.

Recycling takes air from the blow moulded article (usually a bottle or closed container) and first vents the high pressure inside the moulding to an intermediate pressure rather than directly to atmosphere. This intermediate pressure air may either be used to pre-blow the bottle (and thereby save air that would otherwise be used for this purpose) or directed to a booster compressor whereby the vented air is raised to the required pressure for blowing a subsequent bottle. It is also known in the art to direct this intermediate pressure air to a multistage compressor and inject this air between stages at the appropriate pressure level. This is disclosed, for example in European Patent Application No. 05450084.8. A major shortcoming of the above mentioned schemes is that there is little flexibility in the selection of the intermediate pressure. In particular the selection of the intermediate pressure either dictates, or is dictated by, the relative displacement volumes of the low pressure compressor and the booster (or in the case of a multistage compressor) the sizing of the cylinders. This sizing problem means that the recycling does not occur at the optimum pressure level and so the expected savings do not always eventuate. The aim of the present invention is to provide a control arrangement for recycling compressed air unused in a manufacturing process (or similar) in a manner allowing flexibility in the selection of the intermediate pressure and at the same time allow the maximum possible power saving to be made. Accordingly, the present invention provides a control arrangement for the supply of compressed air to machinery using said compressed air wherein some compressed air is vented to atmosphere and some compressed air is available for recycling, said control arrangement being characterized by a primary compressor means and a booster compressor means each being connected via discharge line means to a pressure vessel from which said machinery draws compressed air at a first predetermined pressure, said booster compressor means being connected via a suction line to said machinery and being adapted Jto receive recycled compressed air at an intermediate pressure less than said first predetermined pressure via said suction line, said primary compressor means being at least partially driven by an electric motor means controlled by a variable speed drive device, pressure sensor means arranged to sense pressure in said pressure vessel and said variable speed drive device being controlled in response to pressure sensed by said pressure sensor means. In accordance with a further aspect, the present invention also provides a control arrangement for the supply of compressed air to machinery using part of said compressed air and for recycling unused compressed air, said control arrangement being characterized by a primary compressor means delivering compressed air to storage vessel means from which said machinery draws compressed air at a first predetermined pressure, said primary compressor means being driven by a variable speed drive means and a booster compressor means receiving unused compressed air from said machinery and also delivering compressed air compressed by said booster compressor means to said storage vessel means, said booster compressor means being driven by fixed speed drive means.

Preferably the variable speed drive device adjusts the frequency of mains electricity to thereby vary the speed of the electric motor means in response to sensed pressure level changes sensed by said pressure sensor means in said pressure vessel. Conveniently the booster compressor means is driven by electric motor means receiving mains electricity. If the intermediate pressure drops to a second predetermined pressure, said booster compressor means may be shut down and compressed air to said pressure vessel will then be solely supplied by said primary compressor means. The primary compressor means may include a plurality of compression stages that progressively increase the pressure of the air being compressed. The aforesaid compression stages may be provided by reciprocating compressor stages, rotary compressor stages, or a combination of reciprocating and rotary compressor stages. Conveniently the booster compressor means may also include a plurality of compressor stages. Again the compressor stages may be reciprocating compressor stages, rotary compressor stages, or a combination of both reciprocating and rotary compressor stages. Preferably, the number of compressor stages in the booster compressor means is less than the number of stages in the primary compressor means. Preferably the machinery utilizing the compressed air is blow moulding machinery for producing bottles or containers.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawing (Fig 1), showing a preferred control arrangement in schematic block diagram form. In this drawing, a control arrangement 20 is schematically illustrated including variable speed drive device 2 receiving and controlling the frequency of mains electricity 11 and providing same at 12 to electric motor means Mi comprising one or more motors driving a primary compressor system 1 including a number of compressor stages to achieve a desired high compression level. The primary compressor system 1 provides compressed air to a storage pressure vessel 3 via line 13. Compressed air is drawn from the vessel 3, preferably via a drier 4 and lines 14, by machinery 5 utilizing the compressed air. The machinery 5 may be blow moulding machinery for producing bottles or containers from, for example PET, but could be any other machinery utilizing the compressed air. Some of the compressed air is vented to atmosphere at 8 from the machinery 5 but some, as described below, is available for recycling via a suction line 9 through a booster compressor system 6 conveniently formed by a number of compressor stages normally less than the number of stages of the primary compressor system 1 , to the pressure vessel 3 via line 7. The compressor stages for each of the primary and the booster compressor systems 1 , 6 may be reciprocating compressor stages, rotary compressor stages, or a combination of both. The booster compressor system 6 may also be driven by electric motor means Me comprised of one or more motors which is conveniently operated directly by mains electricity 11. The term "mains electricity" used throughout this specification including the annexed claims, should be understood to simply mean a standard fixed supply of electricity available to the machinery whether or not it is modified from an external source. This arrangement enables the use of high pressure air for the blow moulding of PET and similar bottles but other applications are also possible. Typically the high pressure air is 25 to 40 bars pressure. The intermediate pressure available to the suction line 9 leading to the booster compressor system 6 may be from 5 to 15 bars. The optimum intermediate pressure varies depending on bottle blowing rate, bottle size and bottle blowing pressure amongst other factors.

When the rate of recycling of compressed air is greater there is a corresponding reduction in the suction volume of air entering the inlet of the primary compressor system 1 (as some air that normally would have been compressed is replaced by recycled air returning from the venting of the bottle). This reduced suction volume can be accommodated by using conventional compressor unloading methods but these are inefficient and cause power to be wasted. Similarly the volumetric capacity of the booster can be suitably adjusted by conventional unloading means. In the arrangement disclosed in Fig 1 a simpler and more effective control strategy is employed utilizing two compressor systems 1 , 6. The primary compressor system 1 is a 40 bar multistage compressor which compresses air from atmosphere to 40 bars. If no recycling is used this compressor can supply the complete capacity needed by the blow moulding machine. The 40 bar air supplied by the compressor system 1 passes first to the pressure vessel 3 and dryer 4 before entering the blow moulding machine 5. When the bottle is blown some air is vented at 8 and some may be recycled. The recycled air at lower pressure (from 5 to 15 bars) passes via line 9 to the booster compressor system 6. The booster compressor system 6 raises the air pressure back to 40 bars and passes same via line 7 to the pressure vessel 3. Conveniently the 40 bar primary compressor system 1 may be a 3 stage compressor and the booster compressor system 6 may be a two stage compressor although other numbers of stages may be used. The primary compressor system 1 is powered by mains electricity 11 via a Variable Speed Drive (VSD) 2 which adjusts the frequency of the mains electricity so the compressor speed may be adjusted so the air compressed matches the volume needed by the blow moulding machine 5. The volume of air supplied by the 40 bar primary compressor 1 varies depending on the rate of recycling. Accordingly the speed of the primary compressor system 1 is controlled by a pressure sensor 10 which in turn alters the frequency of the electricity supplied to the electric motor or motors Mi driving the primary compressor system 1. This keeps the pressure of vessel 3 at 40 bars, despite the amount of air consumed by the blow moulding machine changing, due to bottle size, speed and finally blowing pressure. The final blowing pressure may be chosen to be less than 40 bars but for the simplicity of this disclosure 40 bars is used as an example, the use of another blowing pressure does not change the nature of the control arrangement.

Variable Speed Drives (VSD) are expensive and although the booster compressor system 6 may be driven via a VSD it is much cheaper to drive the booster compressor system 6 at a fixed speed from the mains electricity. The discharge pressure of the booster compressor system 6 is controlled by the pressure in the pressure vessel 3. A feature of booster compressors is that the mass flow rate through the booster compressor reduces as the suction pressure falls. If a high rate of recycling is desired the following occurs. The speed of the primary compressor system 1 is low. The blow moulding machine 5 returns a large mass of air via line 9 to the booster compressor system 6, which in turn raises the pressure to 40 bars in vessel 3. If a medium rate of recycling is allowed by the blow moulding machine 5, the mass flow to the compressor booster 6 falls and so too does the pressure in line 9. A lower suction pressure at the inlet of the booster compressor system 6 means less mass is pumped back to vessel 3. Accordingly the pressure in vessel 3 would fall slightly and so this is sensed by sensor 10 which in turn speeds up the frequency delivered by the variable speed drive (VSD) 2 so a constant pressure is maintained. If a low rate of recycling is needed the intermediate pressure falls lower again and in turn the mass compressed by the booster compressor system 6 leads to the primary compressor system 1 speeding up again. Finally, if no recycling is allowed by the blowing machine, at a certain level, say <3 or 4 bars in line 9 then the booster is shut down and all air is supplied by the primary compressor system 1. The advantage of this system is that both compressor systems 1 and 6 run at full load so a maximum amount of power is saved. The control of the booster compressor 6 is very simple. The high pressure primary compressor system 1 runs mostly at speeds below full speed which greatly extends ring and valve life.

Claims

CLAIMS:

1. A control arrangement (20) for the supply of compressed air to machinery (5) using said compressed air wherein some compressed air is vented to atmosphere (8) and some compressed air (9) is available for recycling, said control arrangement being characterized by a primary compressor means (1) and a booster compressor means (6) each being connected via discharge line means (13, 7) to a pressure vessel (3) from which said machinery (5) draws compressed air at a first predetermined pressure, said booster compressor means (6) being connected via a suction line (9) to said machinery (5) and being adapted to receive recycled compressed air at an intermediate pressure less than said first predetermined pressure via said suction line (9), said primary compressor means (1) being at least partially driven by an electric motor drive means (Mi) controlled by a variable speed drive device (2), pressure sensor means (10) arranged to sense pressure in said pressure vessel (3) and said variable speed drive device (2) being controlled in response to pressure sensed by said pressure sensor means (10).

2. A control arrangement according to claim 1 characterized in that said variable speed drive device (2) adjusts the frequency of mains electricity to thereby vary the speed of said electric motor means (M-O in response to sensed pressure level changes sensed by said pressure sensor means (10) in said pressure vessel (3).

3. A control arrangement according to claim 1 or claim 2 characterized in that said booster compressor means (6) is driven by electric motor means (MQ) receiving mains electricity.

4. A control arrangement according to claim 3 characterized in that said electric motor means (Me) is a fixed speed drive means.

5. A control arrangement according to any one of claims 1 to 4 characterized in that if said intermediate pressure drops to a second predetermined pressure, said booster compressor means (6) is shut down and compressed air to said pressure vessel (3) is solely supplied by said primary compressor means (1).

6. A control arrangement according to any one of claims 1 to 5 characterized in that said primary compressor means (1) includes a plurality of compression stages.

7. A control arrangement according to claim 6 characterized in that the compression stages are provided by reciprocating compressor stages, rotary compressor stages, or a combination of reciprocating or rotary compressor stages.

8. A control arrangement according to any one of claims 1 to 7 characterized in that said booster compressor means (6) includes a plurality of compressor stages.

9. A control arrangement according to claim 8 characterized in that the compressor stages are provided by reciprocating compressor stages, rotary compressor stages, or a combination of reciprocating or rotary compressor stages.

10. A control arrangement according to claim 8 or claim 9 when appended to claim 5 characterized in that the number of compressor stages in said booster compressor means (6) is less than the number of stages in said primary compressor means (1).

11. A control arrangement according to any one of claims 1 to 10 characterized in that said machinery (5) is blow moulding machinery for producing bottles or containers.

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12. A control arrangement (20) for the supply of compressed air to machinery (5) using part of said compressed air and for recycling unused compressed air, said control arrangement (20) being characterized by a primary compressor means (1 ) delivering compressed air to storage vessel means (3) from which said machinery draws compressed air at a first predetermined pressure, said primary compressor means (1) being driven by a variable speed drive means (2, Mi) and a booster compressor means (6) receiving unused compressed air from said machinery (5) and also delivering compressed air compressed by said booster compressor means (6) to said storage vessel means (3), said booster compressor means being driven by fixed speed drive means (M₆).