Patent Application
20100150766
A “sliding” rotary vane compressor is a positive displacement machine that uses a rotor, which may be, but is not necessarily, eccentric, placed within a cylindrical chamber that is located within a rotor housing and is used to compress compressible fluids such as gases. The rotor has slots along its length, and each slot contains a blade, i.e. a vane. The vanes are thrown outwards (radially) by centrifugal force when the compressor is running and the vanes move in and out of the slot. The outer (radial) edge of the blades will follow the contour of the inner chamber wall, and the two side (axial) edges of the blades will each be adjacent to a side head plate of the compressor. The vanes thereby create individual cells or pockets of gas which, because of the vanes' movement, are compressed as the rotor turns. The vanes sweep the cylinder, sucking air in on one side and ejecting it on the other. As each cell approaches the discharge port, its volume is reduced and the compressed fluid is discharged. The compressor can be utilized to compress any gas, including air. (The use of the terms “radial” and “axial” and derivatives thereof as locations on a compressor blade of the invention are in reference to the blade's placement within a compressor.)
A major concern with sliding vane compressors is discharge temperature, which must be controlled within reasonable limits to avoid serious mechanical damage to the compressor. Uncontrolled discharge temperature can lead to thermal growth of internal components causing jamming, internal components degrading or melting and lubrication failure.
One cause of increased discharge temperature is the leakage of air from pocket to pocket within the compressor. As gas is compressed, its temperature will increase, so that gas within a compressor will be at its greatest temperature at the point of discharge from the compressor. If air from a compressor pocket leaks “backwards” to preceding pockets opposite the direction of movement of the rotor, the air temperature in a preceding pocket will increase, with the subsequent increase in the air discharge temperature as the pocket moves to the discharge point. With this cycle being continuously repeated, this will result in a steady increase in discharge temperature, causing the problems discussed above.
Poorly sealing blades are the main causes for leakage from a pocket that is at a higher pressure to an adjacent pocket that is at a lower pressure. Leakage across each blade becomes significantly greater when the compressor is designed to operate at higher discharge pressures, in which case the differential pressure across each blade increases. Numerous conditions may cause excessive leakage across a blade. The major sources for leakage are intermittent sealing contact of the blade tip at the cylinder wall, little or no axial sealing contact at the head plates located at both ends of the cylinder housing, and poor sealing contact in the rotor slot.
Leakage paths across the blade tip at the cylinder wall and within the rotor slot can be corrected by precision machining of the blade, cylinder, and rotor. This invention addresses the major leakage paths that are created by poor sealing at the head plates.
A significant leakage path is for air to travel from one pocket to an adjacent pocket in the area between an axial edge of a vane and an adjacent side head plate of the compressor. This leakage condition can exist at each head plate at both ends of the cylindrical cylinder. It is therefore an object of this invention to have a rotary vane compressor that provides a tight seal between the vanes and both compressor head plates.
In order to create a tight axial seal at head plates a multi-sectioned blade has been developed, in which the axial edges of the end sections are forced against the compressor's end plate. However, using a multi-sectioned blade may create its own disadvantage in that another significant leakage path is for air to travel from one pocket to an adjacent pocket at the joints between the multiple piece blade sections. It is therefore a further objective of this invention to have a joint at adjoining blade sections that provides a tight seal while automatically compensating for wear.
An oil flooded rotary vane compressor utilizes oil as both a lubricant and a sealant. An oil free compressor will typically utilize a self-lubricating blade. Such blades, however, are much more subject to wear then conventional compressor blades. If a worn blade breaks it can cause great harm to the compressor, and therefore it a further object of this invention to have a means of detecting advanced blade wear in a self-lubricating blade before a blade wears to the extent that it breaks and causes damage to the compressor.
The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawings. The drawings are not necessarily drawn to scale.
The above and other objects are realized by the present invention which provides for a multiple piece blade for use in a rotary sliding vane compressor. Each blade section is designed to provide ongoing sealing as the blade wears. The blade may be used in both oil lubricated and oil free compressors.
The blade utilized in the present application is made from multiple radially cut sections. It is a feature of the present invention that an axially directed force originating in the area in which the sections are joined is incorporated within the blade and is utilized, as the blade wears, to force adjacent blade sections apart from each other to thereby create a tight seal of the axial outer edges of the multi-sectioned blade against the head plates of the radial compressor. The force is applied automatically to adjust axially and compensate for thermal growth and blade wear while providing a seal of the blade against the head plate.
The blade is made from two or more radially cut sections that can accommodate a spring loaded feature. In one example, compression springs may be used to spread the blade sections axially to create a tight seal where the ends of the blade rub on the head plates. This spring loaded action automatically adjusts axially and compensates for thermal growth and blade wear. Manufacturing the blade out of two or more pieces is also a feature of the invention, as it is difficult to economically manufacture blades for large compressors in one single piece. Furthermore, long single piece blades are prone to warping, which will cause the blade to jam in the rotor slot. The blade will break if it can not move freely up and down within the rotor slot. Thus, it is preferable to manufacture a long blade from two or more shorter blade sections cut width-wise (radially) and not length-wise (axially).
Referring to the drawings by characters of reference, in
During rotation in the direction shown by arrow R of the rotor each vane 104 is thrown outwards by centrifugal force so that its outer edge sweeps the internal cylindrical surface 112 of chamber 102. The free space between adjacent vanes is thus divided into closed cells (such as 105, 106, 107). Inlet 108 and outlet 109 extend through housing 102. Air or other fluid at atmospheric pressure is taken in at stationary fluid inlet 108 in the direction of arrow A and is thus compressed as the free space in each cell diminishes as the rotor turns and the compressed air exits at stationary fluid outlet 109 in the direction of arrow B. Accordingly in the operation of a rotary vane compressor the closed cells to either side of any particular vane are at different pressures as the vane passes from the inlet port to the outlet port.
The present invention can be advantageously utilized on essentially any prior rotary vane compressor. Therefore, it can be used on rotary vane compressors having a rotor mounted in an elongated cavity which may be cylindrical with, for example, an essentially circular, elliptic, or epitrochoidal cross section formed therein. In certain prior art compressors the bore of the cavity can have an undercut in which the rotor sits lower in the housing in which case the cross section of the cavity would not be, for example, a perfect circle.
Another feature of angled slots is a portion of the centrifugal force acting on the blade is absorbed into the rotor slot. The blade tip does not rub on the same plane as the resulting centrifugal force acting on the center of gravity of the blade as it rotates about the axis of the rotor. This reduces rubbing pressure of the blade tip on the cylinder wall and the result is less wear at the blade tip, less frictional losses from the blade rubbing on the cylinder wall, reduced discharge temperature due to lower frictional heat, and less shaft power to operate the compressor.
If the blades of the present invention are utilized in an oil free compressor it is preferred that they be self lubricating. A self lubricating blade must wear to provide lubrication, and therefore will require a greater slot depth than a conventional compressor blade, since it is important to maintain sufficient engagement of the blade in the rotor slot or the blade might be thrown out of the slot, which can jam the compressor and cause breakage. Although the question of increasing slot depth without weakening the core of a rotor may be addressed by increasing the size of the rotor, this goes against the ideal of using the smallest compressor block to produce the greatest amount of compressed air. In view of the above, an angled slot may be preferred in an oil free compressor to provide significantly greater slot depth while providing an ample core area.
Typically, the angled slot will be at an angle of from about 1° to about 30° from a line that extends from a point on the center of the blade tip as the blade tip contacts the cylinder wall, the point of the maximum blade extension out of the rotor slot, with respect to the center of the rotor. Although increasing the slot angle provides the benefits of increasing the allowable slot depth, increasing the size of the undisturbed center core, and reducing frictional rubbing at the tip, it also increases the maximum distance the blade must extend out of the rotor slot. This requires the blade to be much stronger and larger, which greatly increases the mass of the blade and resulting centrifugal forces acting on the blade. As a result of diminishing returns, the preferred angle is from about 20° to about 25° and preferably about 22°.
With the rotation of the rotor, the blades are thrown outward toward the inner wall or surface (112, 212, with reference to
Referring to
It is a feature of the present invention that there is a force applying means at the junction 411 of any adjacent blade sections that constantly acts upon the adjacent blade section pieces to force them apart from each other in an axial direction. For example, interior to the blade there can be at least one compression spring 407 which is biased to constantly force the two blade pieces 403 and 404 axially apart to thereby accordingly force each blade edge 408 into a sealing engagement with its adjacent head plate. As the edges wear the compression spring will continue to force the blade pieces further apart to thereby compensate for the wear and to keep the edges in a constant sealing relationship with the head plate.
In a preferred embodiment the upper portion 405 of the joint of the two blade pieces 403 and 404 comprises an overlapping lap joint in combination with a butt joint. Such a combination joint provides a seal in all potential planes of air leakage flow. For example—the vertical upper portion 405 of the lap joint provides a seal across the thickness (
Alternatively, portions of the two (or more) blade pieces can be joined together by other type of edge joints such as tongue and groove joints, sliding slot joints, V-joints, dovetail joints, lap joints or a combination thereof.
a shows one side edge of blade 400 as seen from the perspective of arrows C. Depicted is trailing surface 408a and leading surface 408b. With reference to blade tip 401a, when a self-lubricating blade is utilized, the geometry of the machine (cylinder size, shape, rotor diameter, slot angle, etc) will cause the blade tip to eventually wear to the optimum profile. However, this will not happen in the short term with non-lubricating blades, as they will have better wear properties. In either scenario, if new blades are installed and the tip profile is not optimum there may be undesirable leakage over the blade tip (that is, the tip makes a poor seal). Thus, it is desirable to machine the optimum tip profile (as close as possible) into a new set of blades to minimize the break in period.
The material of construction of the blade will be dependent upon whether it is desired that the blade be self-lubricating. If the blade is not self-lubricating it can be constructed of, for example, a carbon-fiber resin composite, magnesium, aluminum or various reinforced resin bonded materials. If the blade is self-lubricating, materials of choice include solid graphite, composite graphite, or any other binder material with additives that provide lubrication. Lubrication additives include any dry lubricant that lowers friction such as PTFE, graphite, hexagonal boron nitride, molybdenum disulfide, and various plastic resins.
During operation the sections of the compressor blade remain forced apart, such as in the depicted example by the compressive spring force, in an axial direction as shown by arrows ←D→ on
Numerous means can be used to spread the blade sections. These include one or more compression springs, leaf springs, wave springs, air springs, or any other means that uses a mechanical means or differential pressure to create a spreading force. The spreading force can vary (such as with a compression spring) or be constant (as with an air spring) with respect to the amount of joint displacement.
The blade can also be constructed to be self spreading at the joint without the need for a spring or any other separate spreading device. The joint can be offset from the center of the blade (in the thickness direction) to provide unequal lateral areas within the joint. The larger area faces the higher pressure (leading side) of the blade and the smaller area faces the lower pressure (trailing side) of the blade. The pressure in the leading pocket is significantly higher than in the trailing pocket when sealing is required. The higher pressure in the leading pocket acts on the larger lateral area in the joint. The lower pressure on the trailing side of the blade acts on the smaller lateral area in the joint. This creates a spreading force at the joint that overcomes the opposing force at the blade ends that rub at the head plates. The result is a natural spreading force that automatically increases with differential pressure across the blade, providing optimum sealing at the head plates when needed most.
The combination of a lap joint and a butt joint has a further advantage. As the blade's upper surface becomes worn by contact with the internal cylindrical surface of the compressor chamber, the blade will gradually start to withdraw from the rotor slot. Eventually lower area 406 will extend out from the slot. Contemporaneously with the wearing of the blades upper surface 401, side edges 408 of the blade will also begin to be worn through contact with end plates 302. Adjoining surfaces 506a and 506b (
It is a feature of the preferred embodiment of the invention that when the self-lubricating blades of the present invention wear they are designed to cause leakage at a point before the wear becomes so excessive as to cause possible breakage; in effect at a predefined point of wear of said at least one blade there is a controlled leakage of air across the blade from one pocket to an adjacent pocket. This feature, coupled with the fact that high leakage across a blade will cause the discharge temperature of the compressor to rise significantly, results in a system of monitoring blade wear. Therefore, a preferred embodiment of the invention employs a compressor that utilizes multi-piece blades having a S shaped, combination lap and butt, joint coupled with a temperature sensor at the discharge of the compressor and a processor means that serves to automatically shut down the compressor when a specified temperature is reached. This feature can prevent significant mechanical damage to the compressor. Alternatively or in addition, the blade can be designed with one or more holes through its thickness that, when the blade is not worn or lighten worn, will always be within the rotor slot. As the blade is worn to a predetermined point these holes will become exposed to the air pockets and consequently air will leak therethrough from a high pressure pocket to a low pressure pocket. Thus, the blade of the present invention will not only automatically compensate for wear in the radial direction (by continuing to move further out of the slot with wear) but also in the axial direction. In addition to compensating for wear in the axial direction, the blade of the present invention automatically compensates for differences in thermal growth in the axial direction so that it will provide a seal at the head plates independent of the operating temperatures of the compressor. Further, the force of the axial blade side rubbing (due to the spreading force from the axially directed force) on the head plates mechanically pushes the blade into the rotor slot, opposing to a certain extent the centrifugal force throwing the blade out of the slot and lessening the rubbing pressure of the blade tip on the cylinder wall.
Preferably, multi piece blade segments may be made interchangeable for all size compressors, assuming the blades have the same blade width and thickness. In such a case the end segments of the blades of
It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims.
