GB2469852A

GB2469852A - Multistage gas compressor, eg for blow moulding machines, with gas recycling

Info

Publication number: GB2469852A
Application number: GB0907439A
Authority: GB
Inventors: Julian Oliver Reed; Adrian Graham Alford
Original assignee: Corac Group PLC
Current assignee: Corac Group PLC
Priority date: 2009-04-30
Filing date: 2009-04-30
Publication date: 2010-11-03
Also published as: WO2010125511A1; GB0907439D0

Abstract

A multistage compressor for supplying compressed gas to a machine, eg for supplying compressed air to a blow moulding machine, receives return gas under pressure from the machine for recycling. The compressor comprises at least one rotodynamic (turbocompressor) compressor stage 10,12, eg for low pressure duty, arranged upstream of at least one positive displacement, eg piston, compressor stage 14,16, eg for high pressure duty. A return line 20 is connected between stages of the compressor or directed into the blading of the rotodynamic compressor for receiving the return gas from the machine. In the invention, the return line 20 is connected upstream of the first positive displacement stage 14 of the compressor.

Description

MULTISTAGE COMPRESSOR

Field of the invention

The present invention relates to a multistage compressor that is particularly, though not exclusively, suitable for supplying compressed air to a blow moulding machine.

Background of the invention

Blow mouldinq is used for forming large thin walled articles, such as the two litre bottles commonly used for carbonated beverages. In producing such a bottle, a preform in the shape of a test tube with a screw threaded open end is placed over a core in a mould cavity and heated to soften the plastics material, which is typically PET. Compressed air is introduced through the core into the preform to blow it out until it makes contact with the mould cavity. The mould is then opened and the formed bottle is ejected to allow the cycle to be recommenced.

After the preform has been expanded to the size of the finished bottle, its interior is still filled with air that has been pressurised by a compressor. Releasing such compressed air into the atmosphere is wasteful of energy and it has previously been proposed that the compressed air be recycled by being returned to the compressor while it is still under pressure.

EP 1 600 630 discloses a multistage piston compressor for generating compressed air for a blow moulding machine which accepts return air from the blow moulding machine at an injection point between two piston compressor stages. In operation, such a multistage compressor without substantial cycle modifications would prove unreliable. In particular, the compressor stage lying upstream of the point of injection of the return air would be prone to overheating, which could ultimately result in its failure.

Object of the invention The present invention seeks therefore to provide a multistage compressor that is better suited to the recycling of return air.

Summary of the invention

According to the present invention, there is provided a multistage compressor for supplying compressed gas to a machine and receiving return gas under pressure from the machine for recycling, comprising at least one rotodynamic compressor stage arranged upstream of at least one positive displacement compressor stage and a return line connected between stages receiving the return gas from the machine, characterised in that the return line is connected upstream of the first positive displacement stage of the compressor.

The term vrotodynamic compressor" is used herein to refer to any form of compressor relying on continuous rotation of one or more bladed rotors to move and compress a gas. A common term for this type of compressor is a "turbocompressor".

The term "positive displacement compressor", on the other hand, is used herein to refer to a compressor having a variable volume working chamber. As the volume of the working chamber is enlarged, gas is drawn into the working chamber from the intake conduit and as it is reduced the gas is compressed and discharged into the outlet conduit.

Unlike a rotodynamic compressor, the compressed gas is delivered to the outlet conduit in discrete bursts instead of a continuous stream.

It is possible for the return line to be connected between two rotodynamic stages, both of which are arranged upstream of the first positive displacement stage, but in the preferred embodiment of the invention the return line is connected between the last rotodynamic stage and the first positive displacement stage.

Preferably, a control system is provided to regulate the speed of the rotodynamic compressor stage lying upstream of the return line in such a manner as to maintain a desired pressure at the intake of the first positive displacement compressor stage irrespective of the gas flow rate in the return line.

Because of the different principles on which they operate, rotodynamic and positive displacement compressors are not generally combined with one another and indeed the two are often regarded as falling within totally different engineering disciplines. It is not uncommon for an engineer experienced in one of the two disciplines to have had little or no involvement with other.

The characteristics of positive displacement and rotodynamic compressors are fundamentally different and even within the family of positive displacement compressors, characteristics can be significantly different. Positive displacement compressors can have internal compression, where gas is compressed by the change in the volume of the working chamber. Positive displacement compressors can also have external compression, where gas is compressed by an inrush of reverse flow gas into the working chamber when the outlet port or valve enables communication between the working volume and the downstream region. Many compressors with internal compression such as screw compressors tend to have fixed values of compression ratio, and though they can have high compression efficiencies are less well suited to use with variable speed operation as capacity control for downstream constant-speed positive displacement stages as this fixed compression ratio does not allow for the variance of the screw compressor outlet pressure necessary to control the outlet mass flow of the boosted piston compressor.

Compressors with external compression, though able to vary their compression ratio and mass flow with variable speed, have low isentropic efficiencies. Piston compressors can have high isentropic efficiencies and variable internal compression ratios but are unsuited to the use as capacity control in a compressor with variable speed due to excessive torque ripple and consequent inverter drive problems. In order to achieve a multi-stage oil-free compressor able to be variably capacity controlled and to accept a return flow it is apparent that the use of a mixture of these compressor characteristics will be advantageous.

EP 1 600 630 suggests a solution to this problem by the use of a variable speed, low pressure ratio roots blower followed by piston stages with variable valve timing. Thus the roots blower provides a variable inlet pressure and density to the first piston stage which will allow the outlet mass flow of the compressor to be varied. With passive, pressure actuated valve operation on the piston stages (i.e. variable valve actuation inactivated or not fitted), a reduction in pressure supplied to the first stage of this multi stage piston compressor will result in the increase of the pressure ratio in this and all subsequent stages but with the major part of the adjustment from the final stage, which can increase its pressure ratio to achieve a desired delivery pressure provide the inlet mass flow is suitable. The introduction of a stream of gas between the first and second piston stages will result in an increase in outlet mass flow (which can be attended to by a reduction in speed of the roots blower) and an increase in pressure ratio of the first piston stage (which could cause overheating, lubrication problems and breakdown), which can be attended to by a reduction in swept volume of the first piston stage by actuation of the variable valve timing system. This will cause the roots first stage to increase its compression ratio in order to match its outlet mass flow to its inlet mass flow due to the reduction in volumetric flow caused by the reduction in piston stage swept volume.

Provided the roots compressor is able to provide its nominal outlet pressure at a mass flow of half nominal value without overheating due to a consequent relative increase in reverse flow with relation to ingested mass flow, the machine will be able to accept a recycle flow of up to 50% of supplied mass flow. The disadvantages of this approach accrue due to the relative inefficiency of the roots compressor (which has no internal compression) and the cost of equipping the piston stages with variable valve timing.

The present invention recognises that rotodynamic compressors are better suited for use as the initial stages in a multistage compressor because they are more efficient at generating high volumetric flows at low pressure. They are able to supply a gas stream at varying outlet pressure at high isentropic efficiency (analogous to internal compression in a positive displacement compressor) solely by the use of variable speed operation. They are particularly suited for use as the initial stages of compressors having a return line because they are able to be designed to supply a large dynamic range at constant outlet pressure.

Positive displacement compressors are well suited to producing small volumetric flows at high pressure, making them suitable for the final stages of a multistage compressor. However, when used as the initial stages in a multistage compressor, they must of necessity be large. A first advantage provided by the present invention is therefore that it reduces the physical size of the initial stage(s) of the multistage compressor.

In a rotodynamic compressor operating at any given outlet pressure, the volumetric flow rate through the compressor and its speed of rotation vary predictably with one another over a wide operating range. When feeding a positive displacement compressor, the rotodynamic compressor is required to maintain a given outlet pressure and if a return line is present then the flow rate through the rotodynamic compressor needs to be varied such that the sum of the flow rates through the rotodynamic compressor and the return line remain constant. Because of the characteristics of a rotodynamic compressor, this can readily be achieved by reducing the speed of the compressor as the flow in the return line increases.

In the present invention, if the speed of the rotodynamic stage(s) of the multistage compressor is controlled in dependence upon the output pressure of the multistage compressor, as has already been proposed in respect of the rotodynamic stage in EP 1 600 630, then no further action is required to compensate for variation in the flow rate through the return line.

There comes a point with a rotodynamic compressor, if the flow rate drops too low, when it enters an unstable operating regime. Under such conditions, often referred to as "surge", the compressor vibrates, generates noise and risks being damaged. This vibration is because the back pressure briefly reverses the air flow through the compressor. The reverse flow causes the outlet pressure to drop, which then allows the flow to be re-established in the positive direction. When the outlet pressure again reaches the point at which reverse flow can occur, the cycle is recommenced.

To avoid such damage to the rotodynamic stage(s) of the multistage compressor in the present invention, in the preferred embodiment of the present invention the position of the rotodynamic compressor on its characteristic map is determined and used to control the flow through the return line. This can be effected by throttling of the return line, or dumping some of the return air. As an alternative, it is possible to dump part of the output of the rotodynamic compressor via the pre-existing surge dump silencer.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawing, which is a schematic representation of a multistage compressor in accordance with the present invention.

Detailed description of the preferred embodiment(s) The drawing shows a four stage compressor comprising two rotodynamic compressor stages 10 and 12 and two piston compressor stages 14 and 16. A pressurised line 18 from the last piston compressor stage 16 supplies compressed air to a machine, such as a blow moulding machine (not shown) Within the moulding machine, compressed air is fed to several moulds and at the end of each blow moulding cycle air is present within the blow moulded articles at a pressure higher than atmospheric though less than the compressed air supply pressure. This air is collected and recycled by being fed into a line 20 that leads into the conduit connecting the last rotodynamic stage 12 to the first piston stage 14.

Though return air is delivered to the return line from individual mould cavities in discrete volumes, because of the number of cavities and the resistance in the return line 20, the flow along the return line 20 is more or less constant. If necessary, an accumulator may be included in the return line 20 to ensure a constant flow.

A control system 22 senses the pressure in the supply line 18 and controls the speed of the rotodynamic compressor stages 10 and 12 to maintain a desired supply pressure, regardless of the flow rate of the air in the return line 20.

The illustrated multistage compressor which has rotodynamic compression stages 10 and 12 for the low pressure duty and positive displacement compressor stages 14 and 16 for the high pressure duty, has the ability to accept the introduction of return air through the line 20 while avoiding the need for capacity control of the positive displacement stages. The characteristics of the rotodynamic compressor stages 10 and 12 which make this approach possible derive from their ability to be designed to provide an extremely broad flow range at constant rotational speed with a relatively small change in pressure between minimum and maximum flows. For example, centrifugal compressor stages with a pressure ratio of between 1.5:1 and 3.0:1 can provide a flow between 0.3 and 1.25 times nominal flow capacity at acceptable adiabatic efficiency.

This property of rotodynamic compressors can permit the air flow along the return line 20 to be as high as 50% of the total air flow air entering the positive displacement stage 14.

If it is intended to supply return gas, or gas from some other external source, at a rate that risks causing the rotodynamic compressor to approach its minimum stable flow regime, then surge can be avoided by determining the position of the rotodynamic compressor on its characteristic map and regulating the flow of the return gas accordingly.

Measurement or estimation of the position on the characteristic map can be achieved by a number of methods including measurement of the driving power of the compressor, rotodynamic compressor pressure ratio, gas inlet and outlet temperature, inlet or outlet mass flow rate or mass flow-rate based parameter, vibration signature or any combination of these parameters.

As an alternative to regulating the return flow, a controllable blow-off valve may be used to dump outlet mass flow from the rotodynamic compressor in the event that the minimum stable operating mass flow is approached.

It is common for any return stream of gas from a process to be variable in pressure and this presents a problem for its efficient utilization where interstage pressures of the multistage compressors are predetermined and are not able to be adapted to suit the pressure of the introduced flow stream. Supposing for example that the pressure in the return line 20 is higher than the outlet pressure of the first stage 10 but lower than that of the second stage 12. In such a case, the return line 20 can only be connected to the outlet of the first compression stage 10 with a consequent need for throttling of the return gas stream. Such throttling would inevitably result in energy loss.

In a rotodynamic compressor, the gas pressure at different locations along the rotor increases progressively as the gas moves through the compressor from the inlet to the outlet. The gas velocity also shows an increase between the inlet and the outlet of the impeller. In accordance with a preferred feature of the invention, the return line is connected to a point along the blading of the rotodynamic compressor in such a way that the expansion energy of the introduced gas stream is utilized either to increase compressor pressure ratio or to reduce the shaft power necessary to compress the gas ingested at the compressor stages inlet. Such a connection cannot of course be utilised if the return line is connected between two positive displacement compressor stages.

-10 -In the drawings, atmospheric air to be compressed enters the first of the two centrifugal stages 10 and 12.

The air is compressed and intercooled before passing to the inlet of the second centrifugal stage 12 where it is further compressed and intercooled before passing to the inlet of the two stage piston compressor 14, 16.

Due to its use of variable speed operation, the two stage centrifugal compressor 10, 12 is able to provide air at between 2.5 and 5.0 bar to the piston compressor 14, 16 for subsequent compression to a constant pressure of between and 41 bar, dependent on machine duty.

The variation in inlet density to the fixed speed piston compressor 14 resulting from the variation in speed and outlet pressure of the two stage centrifugal compressor 10, 12 allows the outlet mass flow of the four stage compressor to be varied between 50 and 100% of nominal mass flow.

The centrifugal compressor is designed specifically to be able to provide, at acceptable adiabatic efficiency, a broad range of volumetric flow rate over its entire speed range. This enables the compressor to accept an external flow of compressed air introduced between the outlet of the rotodynamic compressor and the inlet of the piston compressor. The mass flow of air provided by the turbo compressor will reduce by substantially the mass flow of the externally introduced air while any small increase caused by the slight increase in pressure provided by the rotodynamic compressor due to its movement to a lower flow position on its characteristic map can be accommodated by the control system 22.

Claims

-11 -CLAIMS1. A multistage compressor for supplying compressed gas to a machine and receiving return gas under pressure from the machine for recycling, comprising at least one rotodynamic compressor stage arranged upstream of at least one positive displacement compressor stage and a return line connected between stages receiving the return gas from the machine, characterised in that the return line is connected upstream of the first positive displacement stage of the compressor.
2. A multistage compressor as claimed in claim 1, wherein the return line is connected between two rotodynamic stages, both of which are arranged upstream of the first positive displacement stage.
3. A multistage compressor as claimed in claim 1 or 2, further comprising a control system operative to regulate the speed of the rotodynamic compressor stage lying upstream of the return line in such a manner as to maintain a desired pressure at the intake of the first positive displacement compressor stage irrespective of the gas flow rate in the return line.
4. A multistage compressor as claimed in claim 3, wherein the control system comprises means for measuring or estimating when the rotodynamic stage is about to enter an unstable operating regime and for regulating the flow of return gas so as to avoid instability of the rotodynamic stage.
5. A multistage compressor as claimed in claim 3, wherein the control system comprises means for measuring or estimating when the rotodynamic stage is about to enter an unstable operating regime and for operating a valve serving to dump part of the output flow of the rotodynamic stage.-12 -
6. A multistage compressor as claimed in any preceding claim, wherein the return line is connected to a point along the blading of a rotodynamic compressor between the inlet and the outlet of the compressor.