Rotary expansible chamber devices – Heat exchange or non-working fluid lubricating or sealing – With pump for non-working fluid
Reexamination Certificate
2000-11-14
2002-12-24
Vrablik, John J. (Department: 3748)
Rotary expansible chamber devices
Heat exchange or non-working fluid lubricating or sealing
With pump for non-working fluid
C418S091000, C418S201100
Reexamination Certificate
active
06497563
ABSTRACT:
STATE OF THE ART
Ever greater demands made upon the purity of the pumping medium, increasing operating and disposal expenses as well as ever growing obligations set up by environmental control provisions in an increasing extent require vacuum systems to do without operating fluids that get in contact with the pumping medium. These machines, which are running without any sealing or lubricating medium like water or oil in the compression chamber, are generally called dry or dry-compressing vacuum pumps. No concessions to reliability and operational safety can be made to these pumps of course. The manufacturers of vacuum systems met these requirements with different solutions, the successful principles of all of them lying in the mode of operation of the two-shaft positive displacement pumps. To produce a vacuum, these dry-compressing machines run at higher speed because of the compression ratio required, the compression rotors turning without contact in opposite directions in the compression chamber in such a way that they are placed nearest possible relative to each other and to the encompassing pump casing in order to achieve the desired service life. Among the different principles of the dry-compressing vacuum pumps, the system of the screw pump has proved to be particularly advantageous: two cylindrical rotors arranged in parallel and provided on the surface of the cylinder with helical screw-shaped grooves (deepenings) mesh and form in each indentation a compression chamber that is moved from the suction to the pressure side while the two rotors are revolving in opposite directions. On the screw vacuum pump, the high compression ratio wanted for the vacuum pump can be advantageously and simply achieved directly by the number of closed pumping chambers.
The state of the art to which dry-compressing pumps are pertaining is still characterized by some serious drawbacks: today's dry vacuum pumps do not by far equal the current quality values realized by the known sliding vane rotary vacuum pumps and liquid ring pumps. This is particularly true for the uncontested high reliability and solidity of these vacuum pumps, their compactness, and most of all, the low manufacturing costs. The cause of these difficulties lies in the mostly considerable effort today's dry-compressing vacuum pumps still need to furnish to realize the required features of performance like ultimate pressure and pumping capacity.
The object of the present invention is to conceive a dry-compressing vacuum pump that is as simple and robust as possible as well as particularly inexpensive and compact in order to achieve, thanks to the dry mode of operation, considerable improvements in producing vacuum compared to today's state of the art.
According to the invention, the solution of this object is to have the two positive displacement spindles designed in such a way that they are hollow throughout inside and to lead a permanent coolant flow, preferably oil, directly through each of the two compressing cylinders in order to evacuate the heat proceeding from vacuum generation from each of the spindle rotor in a continuous and reliable way.
In this heat transport in the rotor, the better heat transmission coefficient between the material of the positive displacement rotor and the cooling medium with a simultaneously smaller inner surface of the rotor's cylinder compared to a greater heat absorbing outer surface of the positive displacement rotor with a smaller heat transmission coefficient between the rotor material and the pumping medium is advantageously utilized in favour of a well balanced thermal current in the rotor so that, according to a simple thermodynamic layout, the amount of heat that is picked up and the one that is carried off are balanced as desired. The level of temperature may advantageously be adjusted and controlled on purpose for each case anew by controlling the amount of coolant. It is hereby very important to see to it that the amount of coolant is evenly distributed between the two positive displacement rotors by way of appropriate monitoring systems. To improve the cooling effect, the inner bore of the rotor should additionally be advantageously provided with an inner feed screw thread oriented in the direction of rotation in order to improve the inner surface of heat exchange between displacer and coolant as well as the flow of the coolant by way of an appropriate orientation of the thread. The direction of rotation of each positive displacement rotor is unmistakably established according to the lifting direction of the pump so that the orientation of the inner thread of the positive displacement rotor's bore may be designed in precisely such a way that, according to this established direction of rotation of the rotor, the flow of its cooling medium is aided and reinforced.
Another suggestion is to advantageously design the above mentioned inner bores of the rotor with additional option on a thread in such a conical way that the smaller bore diameter is located at the inlet side of the cooling medium and that the somewhat larger bore diameter comes to lie on the outlet side of the cooling medium, so that the conveying effect of the cooling medium is reinforced with the help of the centrifugal force, thus improving even more the cooling of the rotor. It is hence advantageously also possible to operate this vacuum screw pump with the pair of positive displacement rotors either standing upright or orientated in horizontal direction.
For a most effective rotor cooling, the invention additionally suggests designing the surfaces of the rotor's inner bore in the way required by the dissipation of the loss heat of compression. For the output of the compressor and accordingly the occurring power loss as well are not constant in the longitudinal direction of the positive displacement rotor, so that the corresponding surface values are advantageously worked out to be greater in the areas of higher heat loss of the compressor. In general, this particularly concerns the area of the positive displacement rotor located nearer to the outlet and the areas in which the volumes of the work chambers are submitted to greater changes. There is also the possibility to maximize the size of the rotor's inner surface by having the outer curve with the cylindrical grooves followed by the inner hollow curve of this contour by minimizing the overall thickness of wall of the rotor. Besides the mechanical transformation, the technical realization may also be accomplished by explosive forming of an appropriate thin-walled tube or by sheet packing according to EP 0 477 601 A1.
The overall flow of cooling medium is preferably realized in a defined way by means of a pressure generating pump of its own so that this cooling agent (preferably oil) may not only be led in a controlled way through the cavities of the displacer, through storage of special sealing elements as well as through synchromesh and driving gear, but that it may also simultaneously be guided in a controlled way past the housing, if possible with the help of gravity, in order to carry off the heat absorbed. This process, which is permanently repeated in a closed circuit is aided in its task by the well-known additional external possibilities for exchange of heat, starting with a ribbed housing, which is the appropriate material for a casing, and with a simple ventilator and ending with the additional heat exchanger connection through which the flow of coolant directly passes. Alternatively and instead of the pressure generating pump of its own, the kinetic energy of the rotor's rotation may be utilized particularly for smaller-sized machines by connecting an oil pump of its own to the positive displacement rotor according to the well-known principles.
In this way, the temperature distribution in the whole machine may advantageously be much more uniform for dry-compressing vacuum pumps and thus meet the standards usually only met by the well-known sliding vane rotary machines and the liquid ring pumps. These temperature
Akerman & Senterfitt
Steffens Ralf
Vrablik John J.
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