WO2021064340A1

WO2021064340A1 - Reciprocating pump or compressor comprising lubricator

Info

Publication number: WO2021064340A1
Application number: PCT/GB2020/000081
Authority: WO
Inventors: Benjamin Luke KARDOOSH
Original assignee: Ernest H. Hill, Limited
Priority date: 2019-10-04
Filing date: 2020-10-02
Publication date: 2021-04-08
Also published as: GB2587663B; GB2587663A; GB201914390D0

Abstract

A device for compressing or pumping fluid comprises a reciprocating piston (101) riding inside a cylinder (102) providing two or more expanding and contracting chambers (103, 104) of which at least one of the chambers (103) is utilised to facilitate compression or pumping of a fluid, while at least one or more other chambers (104) act instead upon a fixed volume of gas in communication with a lubricator (105), the lubricator comprising a cavity configurable to contain both liquid lubricant (106) and gas (107) and the lubricator comprising a vessel internally linked to one or more of the chambers and otherwise sealed, a venturi (108) through which gas is drawn back and forth in a reciprocating manner by pressure differential, and an uptake channel (109) such as a hose connected at one end to the venturi with its other open end configurable to be submerged in a liquid lubricant in said cavity such that the relative partial vacuum produced at the choke of the venturi draws lubricant up the uptake channel (109) and into the gas flow whereupon a mixing effect occurs creating a lubricating mist suspended in the fixed volume of gas which travels back and forth exchanging repeatedly between the cavity of the lubricator and the varying volume of the connected chamber or chambers thereby delivering lubricant to the internal surfaces of the chamber or chambers and in turn to one or more dynamic seals (110,111) located between the piston and the cylinder

Description

RECIPROCATING PUMP OR COMPRESSOR COMPRISING LUBRICATOR

Field of the Invention

[0001] The present invention relates to the field of pumps and compressors. More particularly, but not exclusively, the present invention relates to air compressors and pumps for generating high pressures of the order of 20,000 kPa (200 bar) and above.

Background of the Invention

[0002] As those skilled in the art will understand there are various types of compressors and pumps which may be categorised into two basic groups: (a) regular air compressors/pumps that are high volume/low pressure devices which operate up to about 1000 kPa (10 bar, approximately 150 psi) and (b) low volume/high pressure devices that operate in the range typically from about 1000 kPa (10 bar) to 30,000 kPa (300 bar). Thus, for example, various types of pneumatic (PCP) devices, such as, for example air rifles, air guns and paint ball guns typically utilize such high pressures in order to provide the required propulsion. These kinds of devices are frequently referred to as pre-charged pneumatic devices that comprise small cylinders which require a means of filling or charging with high pressure air that is both clean and dry. There are two basic ways of achieving this: a hand operated pump or a relatively larger scuba tank. A pump is advantageous in that it provides self-sufficiency as regards the supply of required high pressure air whereas a tank comprises a finite amount of such air that will run out. Nevertheless, pumps are associated with a disadvantage in that they are arduous to use: they are easy as a cylinder is started to be filled, but become more and more arduous/difficult as the pressure increases during the process of a given cylinder being filled.

[0003] When a reciprocating piston in a cylinder is used to create a sealed chamber for moving or pressurizing fluid such as in compressors, some part of the piston typically makes contact with the wall of the cylinder to form a seal which must perform while enduring some amount of friction when moving against the cylinder wall. Since excessive friction would in most cases result in excessive heat and abrasive wear of the piston seal, a lubricant is often used to reduce the friction. In addition the presence of lubricant can help to cool the seal and surrounding components by increasing the rate of heat transfer, and also improve the effectiveness of the seal by viscously filling microscopic gaps between the sealing surfaces. [0004] In compressors which feature a typical crank assembly, the lubricant used to lubricate the crank components is often composed largely of hydrocarbon chains either extracted from crude oil or produced synthetically. To make further use of this lubricant, many compressors are designed with cylinders which have an open end in communication with the crank case such that the same lubricant can be splashed or fed onto the cylinder walls to lubricate the piston seals. However since the piston seals ride on a thin layer of this lubricant, a thin residue will tend to remain on the cylinder wall and the working fluid will inevitably suffer from some level of contamination with the lubricant, which in some cases may not be satisfactory. Such cases may include applications in which the working fluid is for human or animal consumption, for medical uses, or where the working fluid, or materials it later interacts with, may react with traces of the lubricant. If the working fluid contains oxygen and is to be compressed to high pressures and temperatures, the presence of contaminating hydrocarbons may result in ignition and explosive combustion.

[0005] Various advancements have already been made to prevent, or negate the effects of, lubricant-contaminated fluids in pumps and compressors. One popular solution is the invention of oil-less compressors in which the piston has no liquid lubrication at all. This can be achieved via a variety of approaches, including the use of self-lubricating piston rings made from materials such as filled PTFE, and the use of alternative sealing technology such as Labyrinth seals, which effectively ride on a cushion of the working fluid itself. Both these approaches work well for systems required to develop relatively low pressures, since neither can produce a perfect seal, and thus a small amount of fluid will tend to escape past the piston with each stroke. In the case of the labyrinth seal oil-less compressor, the maximum pressure obtainable tends to be limited by the speed at which the compressor can run, and the aspect ratio of the piston to its clearance from the cylinder wall. For this reason the use of labyrinth seals to generate high pressures is limited to larger compressors where the tolerances required to achieve a high aspect ratio are realistic in production. Oil-less compressors featuring PTFE seals suffer from a similar limitation since PTFE is prone to scratching and abrasive wear, and therefore over time such seals become less effective. While high pressures are obtainable on a small scale, the PTFE seals would likely need to be replaced regularly, and would also require highly polished cylinder walls.

[0006] Yet another approach to clean lubrication is to use thick silicone grease which remains in place for a long period of time without needing to be toped up. This method lends itself to hand pumps with exposed piston tubes on which grease can be applied directly without disassembly. Although this does not necessarily prevent lubricant contamination, the levels of contamination are relatively small and the grease is non-toxic and non-flammable. The quantity of lubricant present however is not controlled and will diminish over time until the next re-application. Having excessive lubricant present may cause build-ups in the cylinder of the device, which may hydraulically obstruct the piston towards the end of its stroke. This is not a problem for hand-operated compressors since the user will sense this obstruction and not attempt to push the piston any further. The obstructing lubricant may in fact not even significantly impact the functionality of a hand operated device, but simply reduce its effective swept volume. A typical motor driven compressor however, must push the piston all the way to end of its sweep in order for the crank to fully rotate. Therefore an obstruction of excess lubricant would likely cause the entire machine to seize or produce enough hydraulic pressure to cause catastrophic failure. [0007] UK patent publication number GB951837 is entitled ‘A Machine Capable of Operating as a Compressor or Pump’ : proprietor ‘Pablo August’ ; filing date JanuarylO, 1962 In this disclosure, a rotary compressor is provided with an integrated lubrication system, featuring a reservoir of lubricant, and a displacement chamber dedicated to moving air past the reservoir in a circuit, which then carries lubricant (presumably either as particles of mist or a slowly flowing film along the walls of the circuit) around the circuit and to the lubrication displacement chamber, where some small amount is allowed to escape into the rotary mechanism. The lubricant is encouraged to move from the reservoir into the airflow by a wick or tube via capillary action. The machine disclosed also features a ‘venturi’ /nozzle, whose function is to encourage the air to flow in a consistent direction to form a circuit.

Summary of the Invention [0008] An object of the present invention is to provide an automatic air compressor or pump that is suitable for use in high pressure applications such as in the filling of pre-charged pneumatic devices.

[0009] According to a first aspect of the present invention there is provided a device for compressing or pumping fluid comprising a reciprocating piston (101 ) riding inside a cylinder (102) providing two or more expanding and contracting chambers (103, 104) of which at least one of said chambers (103) is utilised to facilitate compression or pumping of a fluid, while at least one or more other chambers (104) act instead upon a fixed volume of gas in communication with a lubricator (105), said lubricator comprising a cavity configurable to contain both liquid lubricant (106) and gas (107) and said lubricator comprising a vessel internally linked to one or more of said chambers and otherwise sealed, a venturi (108) through which gas is drawn back and forth in a reciprocating manner by pressure differential, and an uptake channel (109) such as a hose connected at one end to the venturi with its other open end configurable to be submerged in a liquid lubricant in said cavity such that the relative partial vacuum produced at the choke of the venturi draws lubricant up the uptake channel (109) and into the gas flow whereupon a mixing effect occurs creating a lubricating mist suspended in said fixed volume of gas which travels back and forth exchanging repeatedly between the cavity of the lubricator and the varying volume of the connected chamber or chambers thereby delivering lubricant to the internal surfaces of the chamber or chambers and in turn to one or more dynamic seals (110,111) located between said piston and said cylinder.

[0010] Preferably, the reciprocating piston is driven by a separately lubricated crank assembly comprising a con-rod (112) connected at one end to an eccentric crank shaft (113) and at its other to a surrogate piston (114) which rides inside a surrogate cylinder (115) and acts mainly to constrain the motion of the piston to be linear by substantially opposing any lateral component of the forces transmitted by the con rod, the surrogate piston transmitting the same linear motion to the piston via a linkage.

[0011] Preferably, the surrogate piston (114) features a valve-ring retained loosely in a groove (601) and lightly energised against the bore of the surrogate cylinder, the groove having one or more holes (602) connecting it to the cavity of the crank case, such that when the surrogate piston moves in a direction away from the crank shaft, the valve ring sits at the bottom of its retaining groove and a passage is opened between the cavity above the surrogate piston and the crank case cavity below, encouraging gas and residual crank case lubricant to flow back into the crank case cavity, while when the surrogate piston moves in a direction towards the crank shaft, the valve-ring forms a seal against both the cylinder wall and the top of the retaining groove effectively preventing the flow of air and lubricant past the surrogate piston.

[0012] Preferably, said piston is driven by an engine such as a heat engine or an electric motor.

[0013] Preferably said lubricator comprises a pressure relief valve to limit pressure build-up within said lubricator. Brief Description of the Drawings

[0014] Fora better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which:

[0015] Fig. 1 schematically illustrates a cross sectional view in order to demonstrate the underlying principle of a single stage compressor or pump as is configured in accordance with the present invention;

[0016] Fig. 2 schematically illustrates, in perspective view, the exterior of a preferred embodiment of a multistage compressor as is configured in accordance with the present invention;

[0017] Fig. 3 schematically illustrates a cross sectional view of the preferred embodiment of a multistage compressor shown in Fig. 2, through a plane which intersects the centroid of both the lubricator and the cylinder; [0018] Fig. 4 schematically illustrates a cross sectional view of the preferred embodiment of a multistage compressor shown in Fig. 2, through a plane perpendicular to the axis of rotation of the crank and intersecting the centroid of the cylinder; [0019] Fig. 5 schematically illustrates a cross sectional view of the preferred embodiment of a multistage compressor shown in Fig. 2, through a plane which intersects both the axis of the rotation of the crank and the centroid of the cylinder; and

[0020] Fig. 6 schematically illustrates a detailed cross- sectional view of the surrogate piston and surrogate cylinder shown in Fig. 2, through a plane which intersects the centroid of the cylinder and shows the geometry of the valve-ring. Detailed Description of the Embodiments

[0021] There will now be described by way of example a specific mode contemplated by the inventors. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the description.

[0022] To overcome the problem of lubricant cross-contamination, the present invention comprises a compressor with a simple secondary lubrication system to lubricate piston seals in isolation from any other lubricants in the machine. This secondary lubrication system uses a liquid lubricant chosen specifically for characteristics which make it advantageous within the context of operation. The seal material may then be chosen for its compatibility with the chosen lubricant. The fact that this secondary system works in isolation gives way to vastly more choice in both seal materials and lubricant used, since neither is required to be compatible with materials and lubricants of any crank assembly or other running gear. Alternative lubricants may include, but are not limited to: polydimethylsiloxane- based oils, Krytox ™ oils, and glycerine-based compounds. Although the use of multiple lubricants in a compressor is known, the present invention provides a simple mechanism by which the chosen liquid lubricant can be delivered to a compatible piston arrangement in the form of a mist suspended in gas. The invention provides both cost and performance advantages over other lubrication systems in specific cases.

Single stage compressor or pump [0023] An aspect of the invention as is illustrated schematically in Fig.1 relates to a device for pumping or compressing fluid, featuring a lubricated piston arrangement. The lubricated piston arrangement includes a reciprocating piston (101) a cylinder (102) and a lubricator (105). The piston and cylinder are arranged such that during operation the reciprocation of the piston relative to the cylinder provides two or more expanding and contracting chambers of which one or more (103) are utilised to facilitate compression of a working gaseous fluid, while one or more others (104) act instead upon a fixed volume of gas in communication with the lubricator (105). The lubricator further comprises a vessel designed to contain a reservoir of liquid lubricant and a volume of gas, internally linked to one or more said chambers and otherwise sealed, a venturi (108) through which gas is drawn back and forth in a reciprocating manner by pressure differential, and an uptake channel (109) such as a tube connected at one end to the venturi with its other open end submerged in the liquid lubricant reservoir such that the partial vacuum produced at the choke of the venturi draws lubricant through the uptake channel and into the gas flow whereupon a mixing effect occurs creating a lubricating mist suspended in said fixed volume of gas which travels back and forth repeatedly between the cavity of the lubricator and the connected chamber or chambers thereby delivering lubricant to the internal surfaces of the chamber or chambers and in turn to the dynamic seals (110,111 ) between the piston and cylinder.

[0024] In a preferred embodiment of the invention as depicted schematically in fig.1 , the reciprocating piston may be driven by a separately lubricated crank assembly comprising a con-rod (112) connected at one end to an eccentric crank shaft (113) and at its other to a surrogate piston (114) which rides inside a surrogate cylinder (115) and acts mainly to constrain the motion of the piston to be linear by opposing any lateral component of the forces transmitted by the con rod. The piston may then be connected to the surrogate piston by a flexible joint (116) and driven in a linear reciprocating motion without being acted upon by any significant lateral forces, allowing a clearance gap to be maintained between the piston and cylinder while only the seals between the two make dynamic contact. These seals are such that they are preferably made from materials including but not limited to Polyurethane so as to allow the use of a wide range of lubricants in the mist lubrication system while separately using a chemically different lubricant (117) in the crank assembly. In addition the lubricants used in the crank case may include, but are not limited to, hydrocarbon based mineral or synthetic oil, while lubricants used in the mist lubrication system may include but are not limited to polydimethylsiloxane based oils which may be resistant to high temperatures while also being non-toxic and bio-inert.

Multistage compressor or pump

[0025] In a further preferred embodiment shown in Figs. 2 to 6, a single reciprocating piston (301) interfaces with a cylinder (201) to provide three compression chambers (303, 312, 313) and one lubrication chamber (304). As previously described for the case of the single stage compressor embodiment of Fig. 1, the lubrication chamber (304), being in communication with the lubricator (202), similarly provides direct lubrication of the piston seals (310, 311 ) in both neighbouring chambers (303, 312). During operation a small amount of lubricant slowly escapes past these seals and enters the airflow, this then feeds the final piston seal (314) with sufficient lubrication. In this embodiment the multiple compression chambers are used to compress gas in several stages between a series of intercoolers. This allows the pressure of the gas to be steadily increased while limiting thermal stress on the piston seals which are made from a synthetic polymer. The seals are chosen to be compatible with a polydimethylsiloxane based lubricant (306) which is non-toxic and non-flammable. The uptake channel (309) in this case comprises a thin plastic tube, and the venturi (308) is integrated into the body of the lubricator. The surrogate piston (315) features a valve-ring (319) which encourages gentle airflow into the crank case cavity (320), while scraping residual oil from the walls of the cylinder. This airflow enters through an annular passage which opens during the up-stroke of the surrogate piston, and closes during the down-stroke. As the air enters through the annular passage, any oil which has been scraped from the walls of the surrogate cylinder by the valve-ring, is carried back into the crank case cavity. This prevents crank case oil (318) from escaping past the surrogate piston and accumulating to contaminate the shaft (321) of the main piston. To achieve this valve-effect the valve-ring sits loosely in a retaining groove (601 ) in the surrogate piston. This groove is significantly deeper and taller in cross- section than the valve ring, providing scope for the valve-ring to move within the groove. The groove has an array of radial holes (602) at its base which allow communication with the crank case cavity. The valve-ring is slightly oversized compared to the bore of the surrogate cylinder before installation, and therefore naturally energised causing it to lightly grip and seal against the wall of the cylinder. By virtue of the resulting friction between the valve-ring and the cylinder wall, on the down-stroke of the surrogate piston, the valve-ring pushes and seals against the top of its retaining groove preventing the flow of air and oil from the crank case cavity past the surrogate piston. On the upstroke of the surrogate piston, the valve- ring pushes instead against the bottom of its retaining groove leaving a narrow gap above it. This gap forms an annular passage which connects to the array of holes and allows air and oil to flow from above the surrogate piston into the crank case. Since this method of retaining the crank case oil requires a small but constant flow of air into the crank case cavity, to prevent pressure build-up and allow the process to be sustained, a lightly restrictive breather is provided. This breather is connected to the main crank case cavity by a series of baffles which isolate it from splashes of oil. Due to the slightly restrictive nature of the breather, a partial vacuum is formed on the upstroke of the surrogate piston which encourages air and oil flow in past the valve ring.

[0026] In the best mode contemplated of either a single stage or a multistage compressor as configured in accordance with the present invention, the lubricator suitably comprises a pressure relief valve to limit pressure build-up within the lubricator. Those skilled in the art are familiar with appropriate pressure relief valves of the kind that may readily be used for this function.

[0027] The present invention provides numerous important advantages as compared with prior art pumps and compressors. For example, by isolating the lubrication system of the piston from that of the crank assembly, longevity of the crank assembly can be assured while allowing alternative lubricants to be used for the piston seals. Also, by using non-toxic and non-flammable lubricants such as polydimethylsiloxane or Krytox ™ oils to lubricate the piston seals, the working gas can be safely raised to higher temperatures and pressures without the risk of ignition, while remaining uncontaminated by toxic substances. As will be understood by those skilled in the art, the lubrication system disclosed in accordance with the present invention is unlike conventional forms of mist-lubrication, being mechanically simple while offering a high degree of control of lubricant dosage, reducing the chance of both insufficient lubrication and obstruction due to excess lubrication. During operation the density of the mist increases only to a finite point whereupon the system reaches a dynamic equilibrium. The density of the mist can be tuned at the manufacturing stage by altering either the length of the uptake tube, the internal bore of the uptake tube, or the aperture of the throat of the venturi. In conjunction with Polyurethane seals, experimentation has shown that the disclosed method of lubrication can allow even relatively small pistons of less than 6mm diameter to sustainably generate relatively high pressures of above 30, 000 kPa (300 bar), with component tolerances that are aligned with typical machining standards.

Claims

1. A device for compressing or pumping fluid comprising a reciprocating piston (101) riding inside a cylinder (102) providing two or more expanding and contracting chambers (103, 104) of which at least one of said chambers (103) is utilised to facilitate compression or pumping of a fluid, while at least one or more other chambers (104) act instead upon a fixed volume of gas in communication with a lubricator (105), said lubricator comprising a cavity configurable to contain both liquid lubricant (106) and gas (107) and said lubricator comprising a vessel internally linked to one or more of said chambers and otherwise sealed, a venturi (108) through which gas is drawn back and forth in a reciprocating manner by pressure differential, and an uptake channel (109) such as a hose connected at one end to the venturi with its other open end configurable to be submerged in a liquid lubricant in said cavity such that the relative partial vacuum produced at the choke of the venturi draws lubricant up the uptake channel (109) and into the gas flow whereupon a mixing effect occurs creating a lubricating mist suspended in said fixed volume of gas which travels back and forth exchanging repeatedly between the cavity of the lubricator and the varying volume of the connected chamber or chambers thereby delivering lubricant to the internal surfaces of the chamber or chambers and in turn to one or more dynamic seals (110,111) located between said piston and said cylinder.

2. A device as described in claim 1 , wherein the reciprocating piston is driven by a separately lubricated crank assembly comprising a con-rod (112) connected at one end to an eccentric crank shaft (113) and at its other to a surrogate piston (114) which rides inside a surrogate cylinder (115) and acts mainly to constrain the motion of the piston to be linear by substantially opposing any lateral component of the forces transmitted by the con rod, the surrogate piston transmitting the same linear motion to the piston via a linkage.

3. A device as described in claim 2, wherein the surrogate piston (114) features a valve-ring retained loosely in a groove (601 ) and lightly energised against the bore of the surrogate cylinder, the groove having one or more holes (602) connecting it to the cavity of the crank case, such that when the surrogate piston moves in a direction away from the crank shaft, the valve ring sits at the bottom of its retaining groove and a passage is opened between the cavity above the surrogate piston and the crank case cavity below, encouraging gas and residual crank case lubricant to flow back into the crank case cavity, while when the surrogate piston moves in a direction towards the crank shaft, the valve-ring forms a seal against both the cylinder wall and the top of the retaining groove effectively preventing the flow of air and lubricant past the surrogate piston.

4. A device for compressing or pumping fluid as claimed in any preceding claim, wherein said piston (101) is driven by an engine such as a heat engine or an electric motor.

5. A device for compressing or pumping fluid as claimed in any preceding claim, wherein said lubricator (105) comprises a pressure relief valve to limit pressure build-up within said lubricator.