The present invention relates to a rotary screw compressor for air provided with inlet channel means and outlet channel means and having at least one compression stage, each stage including at least one rotor with at least one shaft journal mounted in bearing means, which shaft journal is provided with seal means between the rotor and the bearing means, which seal means include at least three frictionless seal means surrounding the shaft journal and a plurality of annular chambers surrounding the shaft journal, which frictionless seal means and annular chambers are arranged in an alternating sequence along the shaft journal.

In such compressors handling oil-free air an effective seal between the working space and the bearings is necessary in order to avoid that oil-contaminated air from the bearing housing mixes with the compressed air which can be dry or contain water. The use of direct contact mechanical seals for that purpose results in high friction losses. This is avoided with the type of seal arrangement mentioned above where frictionless seals are used together with a blocking gas, e.g. air.

Seals of this type are disclosed in US-A-3 975 123 and FR-A-2 569 780.

In US-A-3 975 123 the seal arrangement has four frictionless seals with three annular chambers between them. Blocking air is supplied to the intermediate annular chamber of the seal and flows outward towards the outermost annular chamber, from which it is withdrawn. The innermost annular chamber is inactive at full load and connected to inlet pressure at part load for supply of air.

In FR-A-2 569 780 a similar seal device is applied to a two stage air compressor, but in this case only two annular chambers are present. Also in this disclosure the supply and withdrawal of blocking air is such that the blocking air flows in the outward direction.

The relative localisation of the supply and withdrawal of the blocking air in the known devices has the consequence that oil-contaminated air from the roller bearings leaks to the withdrawal channel where it is mixed with the blocking air and leaking air from the

compression stage. This causes a problem in that the withdrawn air has to be purified if it is led to the compressor inlet or to the ambient atmosphere

The object of the present invention is to attain a seal arrangement of the kind in question in which the above described problem is overcome.

According to the invention this has been achieved in that supply channel means connect the outermost annular chamber to a source of pressurized air and withdrawal channel means are connected to another of the annular chambers. With outermost is meant being most remote from the rotor.

By supplying the blocking air to the outermost annular chamber, the oil-contaminated air in the bearing housing is prevented from leaking along the shaft journal to the withdrawal channel. The air withdrawn from the seal means thus is free from oil

The invention is particularly, but not exclusively intended for a multistage compressor, in which case each of the withdrawal channel means are connected to a common collecting channel communicating with the compressor inlet channel downstream variable throttling means in the inlet channel.

In an alternative embodiment when applied to a multistage compressor the withdrawal channel means at the high pressure end of the end stage are directly connected to atmosphere, whereas all the other withdrawal channel means are selectively connected to atmospheric air via shut-off valve means.

Further advantageous embodiments of the invention are specified in the dependent claims.

The invention will be further explained through the following detailed description of preferred embodiments thereof and with reference to the accompanying drawings.

Fig. 1 is a schematic section through a part of a compressor according to the invention.

Fig. 2 is a diagrammatic illustration of a two stage compressor according to the invention.

Fig. 3 is an illustration of a modification of the compressor in fig. 2.

Fig. 1 shows a part of one of the screw rotors 10 in a twin screw compressor. The shaft journal 1 1 at the high pressure end of the rotor 10 is mounted in roller bearings 12, which are

oil-lubricated. Between the rotor 10 and the roller bearings 12 there is a seal arrangement consisting of first 13, second 14 and third 15 labyrinth seals. Other kinds of frictionless seals of course can be used, e.g. of the floating bushing type as disclosed in US-A-5 009 583. And any of the three seals can be composed of a plurality of seal units.

Between the first and second labyrinth seal there is an inner annular chamber 16 communicating with a withdrawal channel 18 and between the second and third labyrinth seals there is an outer annular chamber 17 communicating with a supply channel 1 . The supply channel 19 is connected to the outlet side of the compressor through a main supply channel 20. In the main supply channel 20 there is provided a valve 21 for reducing the pressure to a range within 1, 1 to 2,0 bars, preferably within 1,3 to 1,5 bars. The withdrawal channel 18 is connected to the compressor inlet.

In operation air of about 1,4 bars is supplied to the outer annular chamber 17. A fraction of the supplied air leaks outward towards the bearings 12 thereby preventing any oil- contaminated air to leak inward from the bearings 12. The rest of the blocking air leaks inward to the inner annular chamber 16, from where it is drained through the withdrawal channel 18 to the compressor inlet.

Fig. 2 illustrates an embodiment of the invention applied to a two-stage screw compressor. In each stage a pair of screw rotors cooperate to form compression chambers, but only one of the rotors 10, 1 10 in each pair can be seen in the figure. Screw rotor 10 operates in the first stage and screw rotor 1 10 in the second stage. The first stage has an inlet or low pressure channel 24 and an outlet channel 25, which forms an intermediate pressure channel and is connected to the inlet channel 26 of the second stage. The compressed air leaves the compressor through the outlet or high pressure channel 27 of the second stage.

Each rotor 10, 1 10 is provided with a shaft journal 1 1a, l ib, I l ia, 11 lb at each end, which are mounted in roller bearing means 12a, 12b, 1 12a, 112b. Between each rotor 10, 110 and each roller bearing means 12a, 12b, 1 12a, 1 12b there is provided a seal arrangement. The seal around the shaft journal 1 lb at the low pressure end of the first stage has only two labyrinth seals separated by one single annular chamber 17b, whereas the seal around the shaft journal 1 la at the high pressure end of the first stage and those around the shaft journals 1 1a, 1 1 lb of the second stage are similar to the seal illustrated in fig. 1. Each of them thus has three labyrinth seals or the like separated by an inner 16a, 116a, 1 16b and an outer 17a, 117a, 117b annular chamber. Each of the inner annular chambers 16a, 116a, 116b communicates with a withdrawal channel 18a, 1 18a, 1 18b, respectively, which channels are connected to the compressor inlet channel 24 through a common collecting channel 23, having cooling 30 and

liquid separating 31 devices. Each of the outer annular chambers 17, 1 17a, 117b and the annular chamber 17b around the shaft journal 1 lb at the low pressure end of the first stage communicate with a supply channel 19a, 1 19a, 1 19b, 19b, respectively, which channels through a main supply channel 20 are connected to the compressor outlet channel 27 downstream cooling 28 and liquid separating 29 devices provided in the outlet channel 27. In the main supply channel 20 there is an adjustable valve 21 for reducing the pressure from the compressor outlet channel 27. The pressure reducing valve 21 is controlled by a governing unit for maintaining a pressure of about 1,3 to 1,5 bar in the supply channels 19a, 19b, 1 19a, 1 19b.

The compressor inlet channel 24 is provided with a variable throttling valve 32 for regulating the compressor capacity. The connection between the collecting channel 23 and the compressor inlet channel 24 is downstream that throttling valve 32.

The rotors not shown in the figure, which cooperate with the rotors 10 and 110 also have similar seals around their shaft journals, and their supply and withdrawal channels are also connected to the main supply channel 20 and the collecting channel 23, respectively.

In operation blocking air is supplied from the compressor outlet channel 27 through the main supply channel 20 and the individual supply channels 19a, 19b, 119a, 119b to the annular chamber 17b around the shaft journal 1 lb at the low pressure end of the first stage and to the outer annular chambers 17a, 1 17a, 1 17b around all the other shaft journals 11a, I l ia, 11 1b. From annular chamber 17b the blocking air leaks to the low pressure side of the compressor space, and from the annular chambers 17a, 117a, 117b the air leaks to the corresponding inne annular chamber 16a, 1 16a, 1 16b, from where it is drained through the individual withdrawal channels 18a, 1 18a, 1 18b and the collecting channel 23 to the compressor inlet channel 24. Since the connection of the collecting channel 23 to the compressor inlet channel 24 is located downstream the inlet throttle 32, the pressure on the withdrawal side will always be low enough to secure an effective drainage, also at part load.

An alternative embodiment of a two-stage compressor according to the invention is illustrate in fig. 3, which embodiment differs from the above described one only in respect of the withdrawal system. In this embodiment the withdrawn blocking air reaches the compressor inlet channel 24 upstream the inlet throttle 32. The withdrawal channel 118a from the shaft journal 1 1 la at the high pressure end of the second stage is directly connected to the compressor inlet channel 24. In a collecting channel 223 communicating the other withdrawal channels 18a, 1 18b to the compressor inlet channel 24 a shut-off valve 33 is provided, which at full load is kept open. If the compressor is throttled the shut-off valve will be closed in

order to avoid a back flow in these withdrawal channels 18a, 1 18b due to the low pressure which under such conditions prevails in the first compressor stage and at the low pressure end of the second stage.