Low coefficient of expansion rotors for blowers

A blower assembly includes, but is not limited to, a blower housing defining a blower chamber and including a gas inlet and a gas outlet; a first rotor positioned within the blower chamber and adapted for rotation therein, the first rotor including a first shaft and at least two lobes defining a first lobe profile; and a second rotor positioned within the blower chamber and adapted for rotation therein, the second rotor including a second shaft and at least two lobes defining a second lobe profile, wherein the first and second rotors are formed from a metal having a coefficient of thermal expansion from about 1 (10−6 in/in*K) to about 13 (10−6 in/in*K), and wherein at least one of the outer surface of the first rotor, the outer surface of the second rotor, or the blower chamber includes a coating.

BACKGROUND

Positive displacement (PD) blowers utilize rotors that rotate in opposite directions to compress a gas. One type of blower is the roots-type blower. Roots-type blowers utilize two rotors that are positioned within a blower housing. The rotors include lobes that intermesh with each other during rotation. The rotors rotate within the blower housing and can create a positive pressure to provide a pressurized gas for various applications. Another type of blower is the screw type blower. Screw type blowers can include two or more screw rotors that are positioned within a blower housing. The rotors include helical flights that intermesh with each other during rotation.

SUMMARY

In an aspect, a blower assembly includes, but is not limited to, a blower housing defining a blower chamber, the blower housing formed to include a gas inlet for allowing gas to enter the blower chamber and a gas outlet to allow gas to exit the blower chamber; a first rotor positioned within the blower chamber and adapted for rotation therein, the first rotor including a first shaft and at least two lobes having an outer surface that defines a first lobe profile; a second rotor positioned within the blower chamber and adapted for rotation therein, the second rotor including a second shaft and at least two lobes having an outer surface that defines a second lobe profile; and a coating positioned on at least one of an inner surface of the blower chamber or the outer surface of each of the first rotor and the second rotor, the coating including at least one of an abradable coating or a formable coating, wherein the first and second rotors are formed from a metal having a coefficient of thermal expansion from about 1 (10−6in/in*K) to about 13 (10−6in/in*K).

In an aspect, a blower assembly includes, but is not limited to, a blower housing defining a blower chamber, the blower housing formed to include a gas inlet for allowing gas to enter the blower chamber and a gas outlet to allow gas to exit the blower chamber; a first screw rotor positioned within the blower chamber and adapted for rotation therein, the first screw rotor including a first shaft and a first helical flight around the first shaft, the first helical flight having an outer surface that defines a first screw profile; a second screw rotor positioned within the blower chamber and adapted for rotation therein, the second screw rotor including a second shaft and a second helical flight around the second shaft, the second helical flight having an outer surface that defines a second screw profile; and a coating positioned on at least one of an inner surface of the blower chamber or the outer surface of each of the first screw rotor and the second screw rotor, the coating including at least one of an abradable coating or a formable coating, wherein the first screw rotor and the second screw rotor are formed from metal having a coefficient of thermal expansion from about 1 (10−6in/in*K) to about 13 (10−6in/in*K).

In an aspect, a method for forming a blower assembly includes, but is not limited to, forming a blower housing from a metal via investment casting, the blower housing formed to include an interior chamber, a gas inlet for allowing gas to enter the blower chamber, and a gas outlet to allow gas to exit the blower chamber; forming a first rotor from a metal having a coefficient of thermal expansion from about 1 (10−6in/in*K) to about 13 (10−6in/in*K) via investment casting, the first rotor having an outer surface; machining a portion of the outer surface of the first rotor to remove a portion of the metal to define a first rotor profile; forming a second rotor from a metal having a coefficient of thermal expansion from about 1 (10−6in/in*K) to about 13 (10−6in/in*K) via investment casting, the second rotor having an outer surface; machining a portion of the outer surface of the second rotor to remove a portion of the metal to define a second rotor profile; applying a coating including at least one of an abradable coating or a formable coating to one or more of the outer surface of the first rotor, the outer surface of the second rotor, or a surface of the interior chamber of the blower housing; and positioning the first rotor and the second rotor within the interior chamber for rotation therein.

DETAILED DESCRIPTION

Overview

Blowers have rotational components that intermesh during operation to compress gas received from an inlet to drive a pressurized gas through an outlet of the blower. During operation, the rotational components dimensionally expand as operating temperatures and pressures increase. Dimensional variation in rotational components limits operating efficiencies over various operating conditions and can result in damage at higher temperatures and pressures. Moreover, the rotational components can include smooth surface textures that permit gas to slip past the surfaces of the rotational components during operation, which can decrease blower efficiency and can increase operating temperatures of the blower. At slower rotational speeds of the rotational components, leakage of swept volume back towards the inlet (sometimes referred to as “slip”) can be significant. This leakage or slip can significantly lower operational efficiencies of blower units, whereas recirculation of the compressed gas can result in significant heating of the gas, which in turn can cause expansion and damage to the rotors (e.g., through rotor to rotor contact, rotor to housing contact, etc.).

Accordingly, the present disclosure is directed, at least in part, to systems and methods for providing rotors for blower units that have increased operating efficiencies over a wide range of operating temperatures and pressures, which can facilitate use of variable speed drives. The rotors described herein support blower operation at slower speeds, higher temperatures, greater pressures, and with improved mass flows than traditional blowers, resulting in higher thermal and volumetric efficiencies. In an aspect, the blower assemblies described herein utilize rotational components that reduce incidences of “rotor clash,” where due to thermal expansion, the rotor expands and contacts the blower housing, resulting in the rotor friction-welding itself to the housing causing a catastrophic failure of the blower.

In an aspect, the rotors are formed from materials having low coefficients of thermal expansion within a blower housing and are provided with a coating to prevent gas slippage past the rotors during operation. In an aspect, the rotors are formed from an investment casting process and machined to include a precise outer profile to ensure strict tolerances between the rotors and between a given rotor and the housing. The rotor profiles and the coating can facilitate low dimensional variation in the rotational components, which can facilitate greater bearing life, higher speeds of rotation, and improved operating efficiencies and ranges.

EXAMPLE IMPLEMENTATIONS

A roots type blower100is shown inFIGS.1and2in accordance with example embodiments of the present disclosure. Blower100is adapted to provide vacuum for various industrial applications. Blower100includes a blower chamber101that is formed by a plurality of components. Blower100includes a blower housing102and first and second end plates104that together form a blower chamber101. The blower housing102is formed to include a gas inlet128for allowing gas to enter the blower chamber101and a gas outlet130to allow gas to exit the blower chamber101.

Blower100includes a first rotor103positioned within the blower chamber101that is adapted for rotation about a first axis of rotation. For example, the first axis of rotation can extend through ends145,147of the first rotor103(e.g., as shown inFIG.3). The first rotor103includes a first shaft108and at least two lobes118and120. The lobes118,120include an outer surface123that defines a first lobe profile125.

Blower100also includes a second rotor105positioned within the blower chamber101that is adapted for rotation about a second axis of rotation. For example, the second axis of rotation can extend through ends141,143of the second rotor105(e.g., as shown inFIG.3). In implementations, the second axis of rotation is substantially parallel to the first axis of rotation (e.g., as shown inFIG.2). The second rotor105includes a second shaft110and at least two lobes122,124. The lobes122,124include an outer surface127that defines a second lobe profile129. In implementations, the first and second rotors103,105are formed from metal having a coefficient of thermal expansion (CTE) from about 1 (10−6in/in*K) to about 13 (10−6in/in*K), for example from about 6 (10−6in/in*K) to about 11 (10−6in/in*K), to limit expansion of the rotors103,105during operation of blower100where temperatures can affect rotors103,105. Such structural integrity limits unwanted metal to metal contact between the rotors103,105and the blower housing102when the blower100is run at higher temperatures and pressures.

First and second rotors103and105can include surface treatments, textures, or materials to facilitate operation of the blower100during a wide range of operating conditions while maintaining tolerances between the rotors103,105and the blower housing102. For example, the first rotor is shown inFIGS.3-5including a coating131on the outer surface123. Coating131can include, but is not limited to, an abradable coating, a formable coating, or combinations thereof. In implementations, the coating131is applied to the first and second rotors103,105in a thickness from about 0.001 inches to about 0.025 inches. For example, coating131can be applied to the first and second rotors103,105at a thickness from about 0.001 inches to about 0.006 inches. All or portions of the first and second rotors103,105, including the ends of the rotors, can be covered with the coating131. In implementations, the coating131is sprayed onto the first and second rotors103,105, the blower housing102, or combinations thereof, but the coating131can be applied by other coating methods. First and second rotors103and105can include the coating131on the outer surfaces123,127, onto ends of the respective rotors (e.g., ends145,147of the first rotor103, ends141,143of the second rotor105), or combinations thereof.

In implementations, the coating131applied to outer surface123,127of the first and second rotors103,105has a surface roughness from about 125 Ra to about 1000 Ra. Surface roughness of rotors103,105is important as testing indicates that a surface roughness in the range of about 125 Ra to about 1000 Ra limits the amount of gas that slips past the rotor lobes (e.g.,118and120,122and124) of first and second rotors103,105during operation of the blower100. Reduction in the amount of gas that slips past the rotor lobes increases blower100efficiency and reduces operating temperatures. In implementations, testing the blower100at the lower range of operating temperatures during manufacture permits the coating131to form and abrade at end use operating and processing conditions (e.g., temperatures, pressures, flows), which can minimize slip or leakage, resulting in higher operating efficiencies at the end use process conditions.

In implementations, the coating131is applied in multiple layers. For example, the coating131can be applied in two coating layers, three coating layers, or greater than three coating layers. In implementations, the coating131is applied in multiple layers and the layers are formed from two or more different coating materials. In implementations, a surface of the blower housing102(e.g., forming a boundary of the blower chamber101) includes an abradable and formable coating. Depending upon manufacturing tolerances between the rotors103,105and the blower housing102, rotor to rotor contact or rotor to housing contact can cause a portion of the coating131from the first and second rotors103,105to partially transfer onto a portion of the blower housing102during operation of the blower100. The coating131applied to the rotors103,105preferably can include a coefficient of friction from about 0.04μ to about 0.2μ. In implementations, the coating131includes a lubricant including, but not limited to, polytetrafluoroethylene (PTFE), graphite, molybdenum disulfide, or combinations thereof, to provide lubricity between the rotors103,105. In various operating scenarios, the use of a lubricant in the coating131allows for tighter tolerances between the rotors103,105and the blower housing102than if no lubricant is included. In implementations, the blower100is manufactured so that the operating clearances between the first and second rotors103,105when assembled into blower housing102is from about 0.003 inches to about 0.032 inches and the operating clearances between the rotors103,105and the blower housing102is from about 0.002 inches to about 0.025 inches.

Rotors103,105used in the blower100are manufactured from a low CTE material, which limits thermal expansion of the rotors103,105during operating the blower100at higher temperatures and pressures. In implementations, the first and second rotors103,105are formed from a metal that includes from about 50% to about 100% iron. The first and second rotors103,105can also include nickel, for example, nickel in an amount from about 20% to about 35% nickel. The first and second rotors103,105can also include cobalt, for example, cobalt in an amount from about 10% to about 25% cobalt.

Blower100is shown with the blower housing102and two transverse end plates104. The end plates104include apertures106through which two rotor shafts108,110extend. Shafts108,110are supported at each end by bearings112. In implementations, a motor114drives rotation of one shaft108and a gear mechanism116transmits the rotational power to the other shaft110. The gear mechanism causes the shafts108,110to rotate in synchronization in opposite directions. The first rotor103with rotor lobes112,120is mounted to the shaft108, which provides rotation to the first rotor103during operation of the motor114. The second rotor105with rotor lobes122,124is mounted to the shaft110, which provides rotation to the second rotor105during operation of the motor114(e.g., via the gear mechanism116). As the shafts108,110rotate, the lobes118,120and122,124sweep past an internal surface126of the blower chamber101thereby moving gas from a chamber inlet128to a chamber outlet130(e.g., shown inFIGS.1and2). The tolerances between the rotor lobes118,120and122,124and the internal surface126are controlled to avoid gaps between the rotor lobes118,120and122,124and the internal surface126through which gas can pass, which would decrease the efficiency of the blower100. Similarly, the tolerances between the first and second rotors103,105are controlled to avoid gaps between the portions of the first and second rotors103,105that interact during rotation through which gas can pass, which would decrease the efficiency of the blower100.

Referring toFIGS.2-5, the first rotor103is shown including the first lobe118and opposed second lobe120. First and second lobes118,120are interconnected by a base132. While a double lobe rotor arrangement is shown for the first and second rotors103,105, it is contemplated that a triple or butterfly type lobe arrangement could also be used to form the first and second rotors103,105. In implementations, the first and second rotors103,105are formed using machining, solid casting, investment casting, precision casting, or combinations thereof. Investment casting is an industrial process based on lost-wax casting.

The lobes118,120and122,124of the first and second rotors103,105can include structural features that provide structural stability of the lobes118,120and122,124under high operating temperatures, pressures, and speeds. For example, the lobe118of the first rotor103can be formed with a first sidewall segment134and a second side wall segment136(e.g., as shown inFIGS.3-5), where the first and second sidewall segments134,136interconnect at an apex138of the lobe118. In implementations, the first and second sidewall segments134,136are convex-shaped to form the lobe118and to include an interior cavity140that is defined by the first and second sidewall segments134,136.

Lobe118of the first rotor103may also include a tensile bar142, examples of which are shown inFIGS.5and7. Tensile bar142extends from a base144of the lobe118to the apex138. In implementations, the tensile bar142divides the interior cavity140into a first chamber146and a second chamber148, where the first and second sidewall segments134,136define a boundary of a portion of the first chamber146and the second chamber148. In implementations, the tensile bar142is formed as a singled piece with the lobe118. Alternatively or additionally, the tensile bar142or portions thereof may be manufactured as a separate piece having the same or different CTE from the lobe118and affixed to the lobe118and the base144. Tensile bar142, in combination with first and second chambers146,148provides a support structure that maintains stability of the lobe118under high operating temperatures, pressures, and speeds. For example, the tensile bar142allows for minimal deflection of the apex138and first and second sidewall segments134,136of the lobe118during operating conditions, as shown inFIG.7. In implementations, the second lobe120of the first rotor103has substantially the same structure of the first lobe118to provide a substantially symmetrical rotor shape, to provide substantially identical lobes shapes, or combinations thereof.

Base132of the first rotor103interconnects the first and second lobes118,120. Base132includes a first concave side wall157and an opposed second concave side wall150. First concave side wall157interconnects the first sidewall segment134of first lobe118with a first sidewall segment152of the second lobe120. Similarly, the second concave sidewall150interconnects the second sidewall segment136of the first lobe118with a second sidewall segment154of the second lobe120. In implementations, the base132of the first rotor103is formed to include a cylindrical bore156that extends at least partially through the first rotor103. Cylindrical bore156of the base132of the first rotor103is adapted to accept first and second rotor shaft segments108a,108b, as shown, for example, inFIG.4. First and second rotor shaft segments108a,108bare adapted to be press fit or otherwise inserted into the cylindrical bore156in directions158,160to form a completed rotor assembly, as shown inFIG.6. Alternatively or additionally, one or more of the shaft segments108a,108bcan be cast into the first rotor103. Alternatively, a continuous shaft can be used in place of the first and second rotor shaft segments108a,108b, with the cylindrical bore156extending through the base132. The combined first and second rotors103,105and shaft portions can be then installed inside of the blower chamber101.

In implementations, the first and second rotors103,105are investment cast from a material having a low coefficient of thermal expansion (CTE). Use of a low CTE material to form the first and second rotors103,105reduces the thermal growth of the first and second rotors103,105during operation, allowing for a higher temperature and pressure operation. Low CTE materials that can be used for investment casting the first and second rotors103,105include cast iron, which has a CTE of about 11 (10−6in/in*K). Materials with lower CTE can also be used to investment cast rotors such as the material KOVAR™, which has a CTE of about 6 (10−6in/in*K), INVAR™ which has a CTE of about 4 (10−6in/in*K), and SUPER INVAR™, which has a CTE of about 1.5 (10−6in/in*K). Materials with a high CTE, such as aluminum, are generally avoided as the thermal expansion of the aluminum metal is too great to gain the desired efficiencies.

The blower100can include other rotor configurations to facilitate generating a vacuum for industrial applications. For example, referring toFIG.8, a blower200is shown including a screw-type rotor mechanism. Blower200includes a blower housing202having first and second end plates203,204that together form a blower chamber201. The blower housing202includes a gas inlet for allowing gas to enter the blower chamber201and a gas outlet to allow gas to exit the blower chamber. The blower housing202includes a first screw rotor205positioned within the blower chamber201. The first screw rotor205is adapted for rotation in the blower housing202and includes a first shaft206and a helical flight208around the first shaft. The helical flight208includes an outer surface that defines a first screw profile. Blower200also includes a second screw rotor207positioned within the blower housing202. The second screw rotor207is adapted for rotation in the blower housing202and includes a second shaft and a helical flight around the second shaft. The helical flight of the second shaft includes an outer surface that defines a second screw profile. First and second screw rotors205,207are formed from metal having a coefficient of thermal expansion from about 1 (10−6in/in*K) to about 13 (10−6in/in*K). The flights of the first and second screw rotors205,207are coated with an abradable coating, a formable coating, or a combination of an abradable and formable coating.

Low CTE rotors have more dimensional stability than high CTE rotors across a broader range of temperatures and pressures. The dimensional stability allows the low CTE rotors to be used in combination with abradable and formable (A/F) coatings. Under extreme operating conditions of pressure and high temperatures, A/F coated traditionally-structured rotors would thermally grow in dimension and so abrade the coatings, creating larger coating gaps when the rotors return (and shrink) to normal operating conditions of temperature and pressure. The more thermally stable A/F coated low CTE rotors described herein have smaller gaps between the coated rotors and the housing under a range of operating temperatures and pressures, improving overall efficiencies and lower operating temperatures due to less slip between the rotors and housing. In implementations, the A/F coating is an ultra-thin closed cell polymer coating that includes polyamide resin, wear resistant particles (e.g., nanometer-scale particles), and a solid lubricant (e.g., PTFE). One example A/F coating is DB L-908 by Orion Industries. The coating can be applied to the rotors using spraying, powder coating, or other coating techniques. In implementations, the coating is applied to the rotors (103,105,205,207), the blower housing (102,202), or combinations thereof as a permanent application.

Reducing clearance between the rotors for a blower or screws or cylinder for a vacuum pump reduces the slip and blowby of the blower to improve efficiency. The A/F coating can be applied to one or more of the rotors, the housing, or the end plates to improve blower efficiency. A zero clearance in the blower is created by having a line-on-line contact or slight interference between the first and second rotors103,105. During an initial run-in of the blower100, the first and second rotors103,105are rotated, which abrades and forms the A/F coating to a near zero clearance condition. Using an A/F coating on the CTE rotors reduces the tolerances required in manufacturing the rotors, making the manufacturing of the rotors more cost effective. Additionally, having dimensionally stable, material-optimized rotors can facilitate greater bearing life and higher speeds of rotation.