Rotary expansible chamber devices – Interengaging rotating members – Helical or herringbone
Reexamination Certificate
1999-12-09
2001-08-14
Denion, Thomas (Department: 3748)
Rotary expansible chamber devices
Interengaging rotating members
Helical or herringbone
C418S015000
Reexamination Certificate
active
06273696
ABSTRACT:
BACKGROUND OF THE INVENTION
The temperature of the gas conveyed by a compressor rises according to the compression pressure ratio. In screw-type compressors which depend on the least possible play both between the two rotors and between the rotors and the casing, the thermal expansion caused on the parts of the compressor may lead to problems. It is known (DE-A-195 22 559) U.S. Pat. No. 5,924,855, by means of pre-admission, to lower the Temperature of the gas contained in the feed cells of the machine. By this is meant admitting cooler feed medium into the feed cells from a point of higher pressure. The pre-admission quantity supplied in each case to the chambers is small, as far as the efficiency of the machine is concerned. Thus, for example, when a screw-spindle machine is operated as a compressor (U.S. Pat. No. 4,812,110; U.S. Pat. No. 5,082,427), it is sufficient for only some of the conveyed gas to be recirculated for pre-admission. Also, when a screw-spindle machine is operated as a vacuum pump, it is necessary to comply with different preconditions from those occurring when it is operated as a compressor. Firstly, the pressure ratio is disproportionately higher in the vacuum mode than in the compressor mode, in particular typically well above 100. Secondly, in accordance with this pressure ratio, the temperature reached in the conveyed gas is substantially higher. Finally, it is necessary to ensure that the achievable vacuum is not impaired by pre-admission backflow.
SUMMARY OF THE INVENTION
The object on which the invention is based is to provide a screw-spindle vacuum pump and a method for operating it which, by means of pre-admission, allow effective cooling, with the efficiency and achievable vacuum being only slightly impaired.
The solution according to the invention is found in the features set forth herein. These presuppose a screw-spindle vacuum pump which, along each rotor, has at least three feed chambers located one behind the other. The latter are in each case closed off, with the exception of the play which is unavoidable in the case of dry conveyance. In such a machine, there is provision, according to the invention, for the chamber which is last on the delivery side to be brought virtually or completely to the compression limit pressure by means of pre-admission, shortly before it opens towards the delivery side, by admitting a pre-admission stream of cool gas which is at least five times greater than the intake mass stream. In this case, an operating point is presupposed, at which the ratio of external compression to internal compression is at least five. On he one hand, effective cooling in the region of the rotors which is the most critical for temperature control is thereby achieved. On the other hand, this cooling also has an effect on the penultimate chamber, since some of the cooler gas in the last chamber, the said gas being under substantially higher pressure, flows back to the penultimate chamber. Finally, the advantage of this arrangement is that there is a considerable reduction in noise being generated, because, when the last chamber opens towards the delivery side, pressure equalization has already been essentially completed. This means that at least 75% of the limit pressure, preferably 90%, is reached by means of pre-admission before the last chamber opens on the delivery side.
In known machines having a smaller number of chambers, such high pre-admission is not possible, because, due to pronounced leakage losses, the pressure in the chamber has already risen relatively sharply when the outlet is opened, and, consequently, a lower pressure difference is available for pre-admission.
Also, in this respect, the considerable difference in the pressure ratio between compressors and vacuum pumps once again plays a part; owing to the lower pressure ratio, a relatively higher pressure prevails in the chamber opening towards the outlet in the case of compressors than in the case of vacuum pumps.
By internal compression is to be meant the ratio of the volumes of the chamber nearest to the suction side, when this chamber closes, and of the chamber nearest to the delivery side, when this chamber opens. If the cross-sectional shape of the screw-spindles is constant over their length, internal compression is equal to 1.
Another possibility for defining the pre-admission according to the invention is that the pre-admission volume stream supplied to the chamber which is last on the delivery side, before the latter opens towards the delivery side, is to be greater than 75% of the theoretical suction capacity of this chamber at the time of pre-admission, divided by the internal compression ratio. If pre-admission extends over a timespan of appreciable length, the time at which pre-admission ends is to be taken as a basis. Instead, the mid-point in time between the opening and closing of pre-admission may also be taken as a basis. The volume stream must be related to the outlet pressure and to the temperature of the gas to be admitted. The theoretical suction capacity is the volume of the chamber at the critical time, multiplied by the rotational speed.
The hitherto conventional small pre-admission orifices, in which a considerable throttle effect is inherent, are not sufficient for this purpose. According to a rule of thumb, the cross-section of the pre-admission orifice in mm
2
should be at least as great as the theoretical suction capacity of the associated chamber in m
3
/h, but preferably twice, furthermore preferably three times as great. This, of course, presupposes that the pre-admission orifice, that is to say the wall orifice which introduces the gas into the chamber, is not preceded by any narrower cross-sections which once again impair the effect of the orifice width. In this respect, the theoretical suction capacity of the chamber is the product of the volume of this feed chamber, the number of screw flights and the rotational speed, the maximum rotational speed to be expected in continuous operation being taken as a basis.
This definition of the theoretical suction capacity, in contrast to the definition given above contains the number of screw flights as a factor. This is explained by the fact that, here, all hose admission orifices are referred to which may be assigned simultaneously to a plurality of chambers in the case of a multi-flight screw spindle, whereas only a single chamber is considered above.
The strong pre-admission according to the invention in the last stage is particularly effective when the screw-flight pitch of the rotors is constant, that is to say compression theoretically takes place isochorically. However, in the case of a decreasing pitch, the invention proves appropriate, since, as a rule, the pitch is never reduced to such an extent that, even without pre-admission in the last stage, the limit pressure is reached when the pump is at the normal operating point. Moreover, the invention does not rule out also providing weak pre-admission in earlier stages in addition to the strong pre-admission in the last stage, although this is unnecessary or even undesirable in most instances of use.
Since the pre-admission according to the invention takes place only in the last stage and at least three successive feed chambers are provided, the impairment of the suction capacity of the vacuum pump is negligible, provided that the rotational speed is not too low.
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Dahmlos Christian
Kösters Heiner
Alix Yale & Ristas, LLP
Denion Thomas
Sterling Fluid Systems (Germany) GmbH
Trieu Theresa
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