AU3093299A - Static electricity dissipation in air compressors - Google Patents

Description

WO 99/47211 PCT/US99/05744 I STATIC ELECTRICITY DISSIPATION IN AIR COMPRESSORS 2 BACKGROUND OF THE INVENTION 3 1. Field of the Invention 4 This invention relates to compressor fluids, such as those used in air 5 compressor systems. and in particular to the dissipation of static electricity in such 6 fluid. 7 2. Description of the Prior Art 8 Air compressors and other similar compressors, such as vacuum compressors 9 and refrigerant compressors, use a liquid fluid for cooling, sealing and lubrication. 10 Although this fluid is commonly referred to as "oil," it is more properly a specially 11 selected organic liquid chosen primarily for its heat exchange characteristics, 12 viscosity and lubricity. Examples of fluids used as oil in air compressor systems 13 include polyalphaolefin (PAO), polypropylene glycol, polyolester (POE), diester 14 based oil, combinations of PAO and diester fluid, petroleum-based fluid, silicon 15 based oil and severely hydro-treated paraffinic oil. Many of these fluids have found 16 use in compressor systems after being developed for other applications, such as 17 hydraulic fluid for hydraulic systems. 18 Some of these fluids, particularly the PAO fluids, were originally used as 19 coolants in electric transformers. The heat exchange characteristics of PAO fluids 20 exhibited as a transformer coolant made these fluids a popular choice for adoption as WO 99/47211 PCT/US99/05744 -2 1 oil in air compressors. where they), are commonly used today. Because of their 2 development for use in electrical transformers, these fluids are dielectric fluids, that 3 is, fluids which act as an electrical insulator and prevent any transfer of electricity. 4 PAO fluids are sold for use in air compressors under various brand names, including: 5 Sullube 32. sold by Sullair Corporation ofMichigan City. Indiana: AEON 9000. sold 6 by Gardner Denver. Inc. of Quincy. Illinois: Quin-Syn series, sold by Quincy 7 Compressor Division of Coltec Industries of Quincy. Illinois: and Roto Inject fluid. 8 sold by Atlas Copco Air Power of Wilrijk. Belgium. 9 The compressor fluid or oil is used not only for sealing and cooling but also 10 for lubrication, and for this purpose some of the oil becomes suspended in the air 11 stream. Air-oil separators are typically used to remove suspended oil mist from the 12 air stream before the compressed air is discharged. The separator allows the 13 discharged air to be used without the contamination of oil and provides for the 14 recovery of the oil so that it can be reused. The air-oil separator is typically mounted 15 in a housing or tank having a separation chamber through which the air flows above 16 an oil reservoir. The separator includes coalescing media through which the 17 discharge air passes while the oil is separated from the air flow. The coalescing media 18 is cylindrically shaped and is typically mounted vertically, that is, in which the axis 19 of the cylindrical coalescing media extends in a vertical direction. The oil-laden air 20 usually enters the separation chamber from outside the air-oil separator and flows into 21 the center of the separator where it then flows axially out of the separation chamber. 22 As the air flows radially through the layers of the separator, the oil coalesces and 23 collects in the interior of the separator where it can be syphoned off or drained into 24 the reservoir, typically by means of a scavenging system. so that it can be reused. The 25 flow directions may also be reversed in which the oil-laden air is introduced into the 26 center of the air-oil separator and flows radially outwardly through the separator with 27 the oil coalescing and collecting on the outside of the separator where it drains into 28 a reservoir.

WO 99/47211 PCT/US99/05744 -3 1 Since fluids such as PAO are electrically nonconductive, static electric charges 2 are prone to build up on the bulk oil as well as the atomized oil that is entrained in air 3 flow. The fluid is subjected to extreme high shear in the compressor chamber, 4 causing this build-up of static electricity. Because the fluid is dielectric, this static 5 charge will remain in the fluid, even if the walls and other metal components of the 6 compressor in contact with the fluid are grounded. The combination of a static 7 electricity build-up along with the potential high temperatures and readily supply of 8 combustion air creates a situation in which the discharge air may become highly 9 flammable. The flammability of the mixture is particularly evident in and around the 10 air-oil separator and the reservoir tank. 11 SUMMARY OF THE INVENTION 12 The present invention provides for addressing the problem of static electricity 13 build-up in the oil in air streams of air compressor systems by providing for the 14 dissipation of static electricity in the compressor fluid. In accordance with the 15 present invention, the compressor fluid is treated with an electrically static dissipative 16 compound. making the compressor fluid less susceptible to static electricity build-up, 17 and thus reducing or dissipating the potential static charge in the compressor air 18 stream before it reaches a potentially flammable and dangerous condition. 19 According to the present invention, the electrically static dissipative 20 composition may be added to the compressor fluid in several ways. The static 21 dissipative composition may be added directly to the compressor fluid or oil already 22 in the compressor, increasing the electrical conductivity of the suspended mist of oil 23 in the air stream and dissipating any static charge in the air stream before it reaches 24 potentially dangerous levels. Alternatively, the air-oil separator may be coated or 25 impregnated with the electrically static dissipative composition. This composition WO 99/47211 PCT/US99/05744 -4 1 would then leach out into the compressor fluid as the fluid is being separated from 2 the air steam and returned to the reservoir, thereby treating the compressor fluid with 3 the static dissipative compound. In addition, the coating or impregnation of the air 4 oil separator with the composition would render the air-oil separator more conductive 5 itself, and, if the separator is properly grounded, provide added safety in the reservoir 6 tank. As a further alternative, a compressor fluid which is dielectric, such as those 7 comprising primarily PAO, could be treated with the static dissipative compound 8 during its manufacture, so that when the compressor fluid is replaced by the user, the 9 new fluid has increased conductivity. 10 These and other advantages are provided by the present invention of a method 11 of operating an air compressor, which comprises the steps of providing a discharge 12 air stream; using an electrically insulative compressor fluid for cooling and 13 lubrication, droplets of such fluid being suspended in the air stream: and treating the 14 compressor fluid by adding a static dissipative compound to the fluid to increase the 15 electrical conductivity of the fluid and prevent excessive static charge build-up, the 16 addition of the compound changing the droplets in the air stream from insulative to 17 static dissipative. 18 BRIEF DESCRIPTION OF THE DRAWINGS 19 FIG. 1 is a side elevational view, partially in section, of an oil reservoir tank 20 assembly with an air-oil separator which may be used as part ofthe present invention. 21 FIG. 2 is side elevational view of an oil filter for an air compressor system 22 which may be used as part of the present invention. 23 FIG. 3 is an end elevational view of the oil filter of FIG. 2.

WO 99/47211 PCT/US99/05744 -5 1 FIG. 4 is a graph showing the conductivity effect of various concentrations of 2 an anti-static agents in air compressor fluid. 3 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 4 Referring more particularly to the drawings and initially to FIG. 1, there is 5 shown an oil tank assembly 10 for use in an air compressor. The tank assembly 6 shown and described herein is typical. but it is only one of many arrangements which 7 may be used. The tank assembly 10 comprises a body 11 having a reservoir 12 8 formed at the bottom for collection of the compressor fluid or oil removed by in the 9 oil separation process. The upper portion of the body 11 forms a separation chamber 10 13. The top of the separation chamber 13 is enclosed by a tank cover 14 which is 11 attached to the body 11 by a plurality of bolts or other suitable fastening devices with 12 a tank seal or gasket provided between the body and the tank cover. An air inlet 15 13 is provided on one side ofthe body 11 for air to enter the separation chamber 13. The 14 air flows from the separation chamber through a passage (not shown) in the tank 15 cover 14 and through an air outlet 16 provided in the tank cover. 16 Within the separation chamber 13 may be a pre-separation configuration, such 17 as a generally cylindrical shroud 18 which diverts the incoming air flow from the air 18 inlet 15 and causes the air to flow down and around the shroud. This provides a first 19 stage air-oil separation, in that, large droplets of oil are separated by the abrupt 20 change in air flow and these oil droplets fall into the reservoir 12. Other known pre 21 separation configurations may be used in place of the shroud 18. A safety valve 17 22 is also provided in the body 11 extending through the shroud 18. The safety valve 23 17 is a pressure relief valve which opens in the event that air pressure inside the 24 shroud 18 increases above a predetermined level. The air flow then passes upwardly WO 99/47211 PCT/US99/05744 -6 1 and axially inwardly, through an air-oil separator 19. The separator 19 typically 2 comprises two or more coaxially arranged layers, including an upstream coalescing 3 stage layer and a downstream drain stage layer, each comprised of any suitable 4 combination of materials used in air-oil separation, such as fiberglass, polyester, 5 polypropylene or metal, some of which may be pleated in a conventional manner, 6 or which may be molded, formed, wrapped or otherwise shaped. The air-oil 7 separator 19 also preferably includes an outer wrap layer on the exterior of the 8 separator. and a support member along the interior surface. Each end of the layers 9 are set in a hardenable sealing or potting material, such as urethane, epoxy or o10 plastisol. to make generally circular end caps. usually with metal backing, in 11 accordance with conventional air-oil separator design. 12 A scavenging tube 20 extends downwardly from the tank cover 14 into the 13 separation chamber inside the separator 19. Oil draining from the separator 19 can 14 be withdrawn therefrom using the scavenging tube 20. 15 The compressor fluid or oil which is stored in the reservoir 12 and used in the 16 air compressor is a liquid which may consist primarily of polyalphaolefin (PAO), a 17 substance which is dielectric, that is, nonconductive or insulative. and which was 18 developed for use in cooling electrical power transformers. As used herein, the terms 19 "conductive," "static dissipative" and "insulative" have generally the same meaning 20 as defined by the Electrostatic Discharge Association (ESD Association) of Rome. 21 New York. A material or substance which is considered to be "conductive" has a 22 conductivity of less than 105 ohms per square unit of surface area. A material or 23 substance which is considered to be "static dissipative" has a conductivity of 105 to 24 1012 ohms per square unit of surface area. A material or substance which is 25 considered to be "insulative" has a conductivity of greater than 1012 ohms per square 26 unit of surface area. Compressor fluids such as those which are PAO-based fluids 27 fall within this "insulative" range. Therefore. the small droplets or mist of the WO 99/47211 PCT/US99/05744 -7 1 compressor fluid or oil suspended in the compressor air stream is prone to the build 2 up of a static charge. The insulative properties of the oil keep this static charge from 3 dissipating. As the air is acted upon by the compressor, the static charge builds up 4 along with the temperature of the air, and the air stream becomes highly combustible. 5 In accordance with the present invention, this compressor fluid or oil is treated 6 with a liquid which is an electrically static dissipative compound, or anti-static agent, 7 bringing the compressor fluid, including the fluid suspended in the air stream, from 8 the "insulative" range to the "static dissipative" range, and dissipating static 9 electricity charges which might otherwise build-up. An example of such a compound 10 is a product sold under the trademark Staticide and available from ACL Incorporated 11 of Elk Grove, Illinois. This product is an anti-static polymeric composition. Other 12 suitable static dissipative compounds or anti-static agents may be used. 13 The treatment of the oil may be accomplished in several different ways. 14 During its manufacture or thereafter, one or more of the layers of the air-oil 15 separator 19 may be coated or impregnated with the electrically static dissipative 16 compound. If a sufficient amount of the static dissipative compound is impregnated 17 into the separator 19, it will slowly leach out into the fluid. Since the separator 19 is 18 changed at regular intervals in most compressor systems, each new separator will 19 bring a new supply of the static dissipative compound which will continue to treat the 20 compressor fluid. The separator would thus act as a dispenser, dispensing an 21 electrically dissipative compound into the compressor fluid over a period of time to 22 constantly treat the compressor fluid and make the fluid more electrically dissipative. 23 By the time that the compound has fully leached from the separator, the separator 24 would be ready for replacement, so that a new supply of the compound would be 25 available to leach into the compressor fluid supply. 26 Coating or impregnation of the static dissipative compound may also make the 27 air/oil separator itself more electrically dissipative. Thus, any remaining static 28 charges which may build up on the treated oil droplets will be dissipated when the oil WO 99/47211 PCT/US99/05744 -8 1 encounters the electrically conductive separator 19. In order to take advantage of this 2 effect, the separator itself must be electrically conductively mounted in the tank 3 assembly. In other words. the separator must be grounded. It has been known to 4 electrically ground air-oil separators by providing metal staples in the rubber gaskets. 5 However, this process may adversely effect the effectiveness of the gaskets. It is 6 preferred to coat the rubber seal or urethane potting compound which is used to hold 7 the ends of the separator layers with the static dissipative material. In this manner, 8 the entire separator 19 may be grounded to the body of the tank assembly, which is 9 itself grounded. The static dissipative material could be coated onto the gasket or 10 urethane potting layer, or it could be mixed with the urethane prior to the curing of 11 the urethane, making the urethane static dissipative, by reducing the resistance of the 12 urethane to, for example. 10 5 to 10 9 ohms. 13 Instead of the air-oil separator, the oil filter can also be used as a dispenser for 14 the electrically static dissipative material. An example of an oil filter used in an air 15 compressor is shown in the oil filter 21 of FIG. 2. The oil filter 21 is a spin-on filter 16 having a rugged external casing 22 and an internal thread 23 at one end separating 17 two concentric channels 24 and 25 used for the oil inlet and outlet. The filter 21 is 18 mounted by its threaded connection 23 to the oil supply on or near the reservoir 12. 19 Inside the casing 22 is one or more layers of filter media 26, each comprised of any 20 suitable combination of materials used in oil filtering, such as fiberglass, polyester, 21 polypropylene or metal, some of which may be pleated in a conventional manner, 22 or which may be molded, formed, wrapped or otherwise shaped. One or more these 23 layers may be coated or impregnated with the static dissipative compound. If a 24 sufficient amount of the static dissipative compound is impregnated into the oil filter 25 21. it will slowly leach out into the oil. Since the oil filter 21, like the separator 19, 26 is changed at regular intervals in most compressor systems. each new separator will 27 bring a new supply of the static dissipative compound which will continue to treat the WO 99/47211 PCT/US99/05744 -9 1 oil. The oil filter would thus also act as a dispenser for the electrically dissipative 2 compound. By the time that the compound has fully leached from the oil filter, the 3 filter would be ready for replacement, so that a new supply of the compound would 4 be available to leach into the oil supply. 5 In addition to treating existing fluid, either by adding the electrically 6 dissipative compound to the fluid directly or though leaching from the air-oil 7 separator, the compressor fluid may be treated with the additive initially during its 8 manufacture, so that the amount of anti-static additive will not be dependent upon the 9 amount of material added to the fluid in use. Treating the compressor fluid initially 10 may be preferred in new systems or when the compressor fluid is completely replaced 11 in an existing system. 12 The result of adding the electrically static dissipative compound or anti-static 13 agent to compressor fluid is to increase significantly the electrical conductivity of the 14 fluid. Tests have been conducted using a commonly used commercial PAO-based 15 compressor fluid, and adding various levels of a static dissipative agent to the fluid. 16 The electrical conductivity of the fluid was then measured using the standard test 17 method ASTM D 4308, which applies to the determination of the electrical 18 conductivity of aviation fuels and other similar low-conductivity hydrocarbon liquids 19 in the range of 0.1 to 2000 picosiemens per meter (pS/mn). Picosiemens per meter 20 (pS/m) is the common unit of electric conductivity, with a siemen being the reciprocal 21 of an ohm. 1 pS/m = lx 1012 Q- 1 m-1 22 Various concentrations of three different static dissipative additives, one of which 23 was Staticide, were added to the commonly used commercially available PAO-based 24 compressor fluid, and the conductivity of the fluid was measured according to the 25 ASTM D 4308 test standard. The results of these tests, with the concentration of the WO 99/47211 PCT/US99/05744 -10 1 static dissipative agents shown in parts per million (ppm), are shown in the following 2 table and in FIG. 4. Fluid Conductivity (pS/m) 3 Concentration 4 of Additive Additive Additive Additive 5 (ppm) A B C 6 10 6 83 17 7 100 12 331 53 8 500 25 862 96 9 1.000 49 1997 121 10 The test results show that the typical PAO-based compressor fluid by itself is 11 insulative, having very low measurements of conductivity. The addition of an static 12 dissipative agent, such as Staticide, significantly increases the conductivity of the 13 compressor fluid. The addition of relative small amounts of the additive can change 14 the compressor fluid from "insulative" to "static dissipative" as defined above. The 15 increased conductivity of the compressor fluid with the added static dissipative 16 additive can be compared to the desired electrical conductivity for aviation turbine 17 fuels which should be 50 to 450 pS/m to prevent static charge flammability problems 18 in fuel tanks. By raising the conductivity of the fluid to 1 pS/m, the fluid becomes 19 "static dissipative." Preferably, the conductivity of the fluid is raised to 50 pS/m or 20 higher by the addition of the additive in order for the fluid to have sufficient static 21 dissipative properties that dangerous levels of static charge build-up are avoided. It 22 can be seen that such levels of electrical conductivity can be achieved with the 23 addition of small levels of an anti-static agent to compressor fluids. 24 Other variations and modifications of the specific embodiments herein shown 25 and described will be apparent to those skilled in the art, all within the intended spirit 26 and scope of the invention. While the invention has been shown and described with WO 99/47211 PCT/US99/05744 -11 1 respect to particular embodiments thereof, these are for the purpose of illustration 2 rather than limitation. Accordingly, the patent is not to be limited in scope and effect 3 to the specific embodiments herein shown and described nor in any other way that is 4 inconsistent with the extent to which the progress in the art has been advanced by the 5 invention.

Claims (10)

-12-CLAIMSWhat is claimed is:

1. A method of operating an air compressor, which comprises the steps of: providing a discharge air stream; using an electrically insulative compressor fluid for cooling and lubrication, droplets of such fluid being suspended in the air stream; and treating the compressor fluid by adding a static dissipative compound to the fluid to increase the electrical conductivity of the fluid and prevent excessive static charge build-up, the addition of the compound changing the droplets in the air stream from electrically insulative to static dissipative.

2. A method of operating an air compressor as in claim 1, wherein the addition of the static dissipative compound changes the compressor fluid to a conductivity of at least 1 pS/m.

3. A method of operating an air compressor as in claim 2, wherein the addition of the static dissipative compound changes the compressor fluid to a conductivity of at least 50 pS/m. -13-

4. A method of operating an air compressor as in claim 1. wherein the compressor fluid has a conductivity of less than 1 pS/m prior to treating the fluid with the static dissipative compound.

5. A method of operating an air compressor as in claim 1. wherein the compressor fluid which is provided is a polyalphaolefin based fluid.

6. A method of operating an air compressor as in claim 1. wherein the treating step comprises the addition of Staticide to the fluid.

7. A method of operating an air compressor as in claim 1. wherein the fluid is treated by impregnating an air-oil separator with the static dissipative compound and allowing the compound to leach out into the fluid as the fluid is separated from the air stream by the separator.

8. A method of operating an air compressor as in claim 1. wherein the fluid is treated by impregnating an oil filter with the static dissipative compound and allowing the compound to leach out into the fluid as the fluid is filtered. ^Γûá 14-

1 9. A method of operating an air compressor, which comprises the steps

2 of:

3 providing a discharge air stream;

4 using an electrically insulative compressor fluid for cooling and lubrication,

5 droplets of such fluid being suspended in the air stream;

6 using an air-oil separator to remove the suspended droplets from the air

7 stream for reuse: and

8 treating the compressor fluid by adding a static dissipative compound to the

9 fluid to increase the electrical conductivity of the fluid and prevent 10 excessive static charge build-up, the addition of the compound l i changing the droplets in the air stream from insulative to static 12 dissipative.

1 10. A method of operating an air compressor as in claim 9. wherein the

2 fluid is treated by impregnating the air-oil separator with the static dissipative

3 compound and allowing the compound to leach out into the fluid as the fluid is

4 separated from the air stream by the separator.