GB2121112A

GB2121112A - Rotary positive-displacement fluid-machine

Info

Publication number: GB2121112A
Application number: GB08314581A
Authority: GB
Inventors: Karl Bammert
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-06-01
Filing date: 1983-05-26
Publication date: 1983-12-14
Also published as: JPS58214601A; GB8314581D0; IT8321359A0; DE3220516A1; IT1169432B; BE896887A; FR2527701A1

Abstract

In a gas compressor or expander of the externally-meshing screw type having a pair of male and female screw rotors the latter are in direct dry contact with each other without gear synchronization. Preferably, a wear- resistant coating of e.g. a carbide material, is applied to at least one of the rotors in at least the contact regions. Alternatively, the contact regions of the rotors may be made of dissimilar materials e.g. steel and bronze, there being other examples mentioned.

Description

SPECIFICATION Apparatus for compression or expansion of a gaseous medium The present invention relates to apparatus for the compression or expansion of a gaseous medium, and has particular reference to a screw machine with meshing engagement between a main rotor having helical ribs and a secondary rotor having helical grooves, one of the rotors being connected with a power drive or take-off and a part of the supplied or extracted power being transmitted to or from the other rotor.

Screw machines of this kind, which are also designated external axle rotary piston machines, are employed as compressors, in which case drive power is supplied in order to compress a gas, or as expansion motors, in which case a gas expanding in a working space turns both rotors and drive power can be derived from one of the rotors.

Two basic kinds of construction of screw machines are known, namely dry-running screw machines and screw machines with injection. In the case of dry-running machines, only the gaseous medium to be compressed or expanded is introduced into the working space. The shafts of the two rotors are basically connected one with the other through a synchronising gear which so synchronises the rotation of the rotors that direct contact of the flanks of the rotors is avoided. The power transmission between main rotor and secondary rotor thus takes place exclusively through the synchronising gear.This was considered to be absolutely necessary, since a direct, power-transmitting contact between rotating, unlubricated parts, according to all experience of machine construction, would lead to premature scuffing and breakdown of the parts and therefore was not permissible for the practical employment in machines. The disadvantages resulting from use of a synchronising gear, namely higher constructional cost, increased space requirement, power loss and noise development through the synchronising gear itself, and lower compression performance due to the constantly present gap between the contactfree rotors, were accepted.

These disadvantages are not present in the mode of construction of screw machines with injection, which has become established especially in the last two decades. In this mode of construction, oil or another lubricant-coolant liquid is injected into the working space so as to effect lubrication, cooling and gas sealing of the rotors, the lubricant being injected in appreciable quantities that can amount to a multiple of the mass flow of the gaseous medium. The rotors, which in this manner are intensively lubricated and cooled, can contact each other directly and can transmit power therebetween, a synchronising gear not being employed. Compared with dryrunning machines, this mode of construction has the advantage of lower constructional cost and a higher attainable compression or expansion pressure by reason of the improved cooling and mutual sealing of the rotors.An appreciable disadvantage of the screw machines with injection, however, is that the working medium is mixed with and polluted by the oil or the coolantlubricant liquid and that expensive, and frequently not very effective, measures for subsequent separation of the liquid from the working medium are necessary.

There is therefore a need for a screw machine which is simpler and less expensive than the previous dry-running machines with synchronising gear, but does not have the disadvantages connected with oil injection.

According to the present invention there is provided apparatus for compression or expansion of a gaseous medium, comprising a main rotor provided with helical rib means and a secondary rotor provided with helical groove means receiving the rib means for transmission of power between the rotors directly by way of dry contact, without addition of lubricant, of the flanks of the rib means with the flanks of the groove means, the rotors being arranged to operate without gear synchronisation with each other, and means being connected to one of the rotors for supply or removal of part of the power.

Thus, in apparatus of this kind, namely a dryrunning screw machine, power transmission from one to the other rotor can take place directly by way of the flanks, which are contacting dryly without introduction of a lubricant into the working space, of helical ribs and helical grooves of the rotors, which are not synchronised through a synchronising gear. It has now been established that the rule of operating a dry-running machine with a synchronising gear preventing direct flank contact of the rotors, is unnecessary and that a dry-running machine with direct, powertransmitting flank contact between the rotors has, contrary to all expectations, a long operational life without occurrence of wear phenomena and the like.

The advantage of simplification achieved by the invention compared with the previous dry-running machines with synchronising gear and with oilinjected screw machines, is readily appreciated.

Moreover, a screw machine embodying the present invention may also be more reliable operationally than machines with synchronising gear. In the case of the latter, slight displacement in the synchronising gear invariably leads to faulty synchronisation between the rotors, which results in flank contact of the rotors and thereby to conflicting forces between the mutual contact zones and the rotor flanks on the one hand and in the synchronising gear on the other hand. The forces arising in this case can be substantially geater than the forces need for normal power transmission and can lead to scuffing, wear and overheating and to damage of the machine. The difficulties resulting from phase errors in the synchronisation have hitherto strengthened the view that an exact, contact-free synchronisation of the rotors is absolutely necessary.In the case of a machine embodying the present invention, however, such disadvantages and damage caused through synchronisation errors cannot arise and the forces to be transmitted through direct flank contact of the rotors are merely those that are required for the transmission of part of the power from one rotor to the other. It has been found that these forces are small and moreover are distributed over a large part of the length of the rotors, so that the pressure loading at the contact zones is sufficiently smali and excessive wear and scuffing phenomena do not arise, even in prolonged operation with dry contact.

A further advantage over the known dryrunning machines with synchronising gear is that the machine is less liable to torsion oscillation of the rotor shafts. In the known machines, the system of rotor-shaft-gear forms an oscillatory system, in which undesirable torsion oscillations can arise. Yet another advantage is that a higher compression performance can be achieved than in the case of machines with synchronising gear, since a gap, maintained by the synchronising gear, between the rotors is no longer present.

Compared with the screw machines with injection, a machine embodying the present invention has all advantages of the dry-running machines, particularly in respect of the working medium being kept clean and the elimination of all equipment required for the supply, separation and recirculation of the lubricating and cooling liquid.

According to an advantageous development of the invention, any possible risk of wear at the directly contacting rotor flanks can be further reduced by providing either or both of the rotors with a wear-reducing layer at least in the engagement region of their flanks. It is particularly advantageous if the flank surface of the two rotors consist of different material providing a material pairing with particularly favourable friction conditions. Examples for such material pairings are steel and cast iron, steel and bronze, steel and brass, cast iron and bronze,and cast iron and brass.

Another possibility consists in the choice of a particularly hard surface layer composed entirely or to a large part of silicon carbide or other carbides.

Since the machine is a dry-running machine, the cooling of the rotor flanks, which are in contact in power transmission, takes place substantially through the gaseous working medium. It has been established that adequate cooling occurs in the normal case, bearing in mind that each rotor flank has contact with the other rotor for only a fraction, for example 600, of each revolution and is freely washed around by the gaseous working medium during the remaining part of the revolution. According to the compression or expansion of the gaseous medium produced in the machine, an appreciable temperature difference of the medium can exist over the length of the working space, so that one end of the machine, namely the entry end in the case of compressor operation, constitutes a "cold end" of the machine.In this cold region, the cooling effect exerted by the working medium on the rotors is particularly good. It can therefore be provided that the power-transmitting direct contact between the rotor flanks takes place exclusively or predominantly in this cold region of the working space. This can be achieved through appropriate distribution, varying over the length of each rotor, of the play between the rotors. For example, the rotor flanks can be constructed to have increasingly larger tolerances from the cold to the warm end. A desired unequal play distribution can also be provided through a slight divergence of the rotor axes.

In an embodiment of the invention, the apparatus can be operated as compressor, wherein the main rotor is driven and about 10 to 1 5% of the supplied drive power must be transmitted to the secondary rotor. In the previous mode of construction, this would take place 'through an oil-lubricated synchronising gear with an efficiency of about 98%. There thus arises a loss component of 1 5% x 2% = 0.3% of the total machine power. This loss component must be conducted away with the lubricating oil of the synchronising gear, In the machine embodying the invention, the synchronising gear is redundant and the transmission of 1 5% of the power to the secondary rotor takes place through direct rotor contact. The efficiency of this power transmission is less than in an oil-lubricated synchronising gear and amounts to 80% by way of estimate, with improvements of this efficiency possibly being available through suitable flank profiles and/or selection of the material of the rotor flanks. A loss component of 1 5% x 20% = 3% of the total power, which is caused by the power transmission, thus arises in the working space in addition to the thermodynamic losses. This loss component can, however, be readily absorbed and conducted away by the working medium and does not lead to overheating of the rotors.

Claims

1. Apparatus for compression of expansion of a gaseous medium, comprising a main rotor provided with helical rib means and a secondary rotor provided with helical groove means receiving the rib means for transmission of power between the rotors directly by way of dry contact, without addition of lubricant, of the flanks of the rib means with the flanks of the groove means, the rotors being arranged to operate without gear synchronisation with each other, and means being connected to one of the rotors for supply or removal of part of the power.

2. Apparatus as claimed in claim 1 , wherein at least one of the rotors is provided with a wearresistant coating at least in the contact region of the associated flanks.

3. Apparatus as claimed in claim 1, wherein the flank surfaces of the rib means consist of a material different from that of the flank surfaces of the groove means, the respectively different constituent materials providing a low friction couple in the flank contact region.

4. Apparatus as claimed in claim 1, wherein at least one of the rotors is provided with a hard surface at least in the contact region of the associated flanks, the surface comprising a carbide material.

5. Apparatus as claimed in claim 4, wherein the carbide material is silicon carbide.

6. Apparatus as claimed in any one of the preceding claims, wherein the rotors are operable to produce a temperature difference in the gaseous medium between one end of each rotor and the other, the flank play in the region of the ends of the rotors at which, in use, the lower temperature of the medium prevails being arranged to be smaller than in the region of the other ends of the rotors in such a manner that the major part of the power transmission between the rotors takes place in the first-mentioned region.

7. Apparatus as claimed in claim 1, the apparatus being substantially as hereinbefore described.