Rotary expansible chamber devices – Interengaging rotating members – Helical or herringbone
Reexamination Certificate
1999-04-15
2001-05-08
Vrablik, John J. (Department: 3748)
Rotary expansible chamber devices
Interengaging rotating members
Helical or herringbone
Reexamination Certificate
active
06227834
ABSTRACT:
The present invention relates to a screw compressor having slide means for adjustment of capacity and compression, respectively. The capacity is adjusted at the intake end in that the slide means are caused to change the axial location of the area, from which the two co-operating screw rotors “cut off” the intake gas, such that the cut off gas can only be housed inside the remaining axial working length of the two rotors, while the degree of compression is determined by way of such a slide adjustment which affects the effective degree of opening of the discharge port of the compressor. The gas discharge takes place both radially and to a smaller degree axially, and it is thus possible to adjust it by a more or less pronounced projection of a wedge body into a fixed discharge port in the compressor housing at the discharge end of the screw rotors.
In practice, the said slide means are arranged at the “meeting side” of the counter rotating screw rotors, with the discharge port located at the end of this area. The slide means should fit very closely to the surface configuration of the rotors, though out of touch therewith, whereby the slides comprise an elongated sealing part with a roof ridge like cross section and an associated guide body part mounted in a guiding groove in the wall of the compressor housing. In practice it is preferred to make use of a circular cylindrical shape of the said body part, even though this may seem unsuitable in view of the fact that the “roof ridge part” should be stabilised in the lateral direction in such a manner that with its extremely close proximity to the rotor surfaces it should be effectively prevented form being laterally displaced or rotated into touch with these surfaces. A cylindrical body part is not effective for this purpose, but for economical reasons it is necessary to use reasonably simple working techniques, also for the shaping of the guiding groove of the compressor housing. It should then in some other way be ensured that the slides as respective wholes cannot rotate in the part cylindrical guiding groove.
The are known more different solutions of this problem, e.g. the use of a cut axial guiding track in the slides diametrically opposite to the active sealing part, whereby a fixed pin or roller of the compressor housing may intrude into this track and thus prevent a rotation of the slide. According to another known solution the ‘roof ridge part’ is anchored in the transverse direction in being in a direct, sliding guiding engagement with the side walls of the said discharge port and intake port, respectively. However, these guiding principles suffer from certain drawbacks, which should be seen in connection with the remainder of the slide construction, confer remarks below.
If it was or is desirable to be able to effect an adjustment solely of the degree of compression, then this is achievable by means of a single slide, which can be axially displaced such that its pointed front end can fill out the discharge port to a higher or lesser degree, while at the intake port it will seal against the rotors irrespective of its axial position. The problem underlying the invention is that a natural desire of an adjustability even of the capacity of the screw compressor will require the effective length of the slide to be variable, such that at the intake area the slide can provide for a more or less pronounced rotor sealing or, respectively, a cancellation of such a sealing along a certain axial length, whereby the rotors cannot build up neither a suction nor a compression effect along this length.
Traditionally, this has resulted in the slide means being constituted by two coaxially arranged slides which, by suitable moving means, are axially displaceable in such a manner that the foremost slide is controllable to a graduated projection into the discharge port, while a rear slide is separately axially displaceable such that at the intake end it may define a more or less wide gap between the two slide elements for adjusting the intake capacity. The two slide elements should be axially coupled together in a sort of telescopic system, and this is the reason for the above indicated problems to exist:
In response to various external forces, e.g. originating already from thermal influences, the two slide elements may behave in an almost inpredictable manner, whereby the said telescopic connection may be subjected to uncontrolled and highly damaging flexing in view of the extreme tolerance requirements.
On this background and in connection with the invention it has been realised that it is indeed possible to provide a slide, which can act as a unitary, stiff member, viz. in the from of a large-slide which, itself, has a length sufficiently for it to carry out a full adjustment displacement at the discharge port, without thereby being displaced so as to expose the rotors at their intake ends, said large slide over a length at the intake end being shaped with a guiding groove in the very ‘roof ridge area’ of the slide, in which groove there is slidably received a mini slide, which has a portion projecting from the groove so as to complete the large slide in forming the said roof ridge portion, said mini slide being axially guidable between an advanced position, in which it fills out the guiding groove, and a retracted position, in which the groove is open towards the rotors at least along a substantial partial length of the groove. In this manner it will be possible to still effect a capacity adjustment, even though the width of the minislide is noticeably smaller than the total width of the roof ridge area of the large slide; even a relatively narrow shorting between the rotors will give rise to such a drop of compression that this can be used directly for a capacity adjustment.
The invention presents more different constructional and functional aspects. The reference to a large slide should not only apply to the slide length, which, in total, will be greater than the length of the rotors, but also to the cross sectional size of the slide, which should preferably be relatively large, viz. of the same magnitude as that of the rotors, while conventionally the slide size has been considerable smaller. According to the invention it is even a preferred feature that the guiding groove for the large slide is made with exactly the same diameter as the rotor bores of the compressor housing, as this will favour a rational working of the precast compressor housing.
However, irrespectively of the more detailed design of the large slide there are circumstances worth being mentioned on the background already discussed:
1) The large slide will have the character of a very stiff beam, this being unaffected by the presence of the minislide. The large slide is influenced crosswise by the compression pressure, with a maximum adjacent the discharge port. Inasfar as the pressure is countered at the area between the opposite ends of the slide, the latter as a whole will be forced against its guiding base in a well defined manner, such that there will be no remaining uncertainty with respect to force phenomena as in the known telescopic connections.
2) Due to the fact that the ‘roof ridge faces’ of the large slide will engage with the rotors over an increased peripheral portion thereof, it is possible to design the discharge port with generally increased dimensions, this being highly advantageous in particular in connection with gasses of a low specific volume and at high suction pressures. It has previously been theoretically realised that the port areas used so far have been smaller than an optimal size, but it has been necessary to choose a compromise between ideal conditions at the intake port and the discharge port, respectively; this necessity, however, is eliminated by the present invention.
3) At the intake end it is correspondingly ideal that the large slide permanently forms a prolongation of the walls of the fixed rotor bores, only leaving space for the relatively narrow minislide adjacent to the very roof ridge area. In traditional systems with doubl
Nixon & Peabody LLP
Sabroe Refrigeration A/S
Safran David S.
Vrablik John J.
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