More particularly, the invention concerns a water- injected screw-type compressor of the type which is mainly composed of a compressor element with a housing in which are provided at least two co-operating rotors which are each provided on a rotor shaft which is bearing- mounted in the housing in a rotating manner; an inlet with an inlet valve; an outlet line in which has been incorporated an air receiver which also serves as a water separator, a water circuit with a lubrication pipe which is connected to the above-mentioned air receiver and which opens into the above-mentioned housing of the compressor element and at the above-mentioned bearings.

With water-injected screw-type compressors, the quality of the water in the water circuit is of utmost importance, since this water is injected in the compressor element for the lubrication of the rotors and of the bearings of the compressor element.

While the screw-type compressor is being activated, the water circuit is usually filled with purified water.

A problem occurring with the known devices is that, while the screw-type compressor is in use, the water of the water circuit is polluted by harmful particles, due to various influences, which influences will be discussed hereafter.

Purified water is not always available, so that use is often made of distilled water. During the normal use of the compressor, use is made of a water treatment system which is connected to ordinary tap water in order to filter out harmful particles, such as solved metals, minerals, ions and the like which are present in the tap water and which may pollute the water circuit and form hard layers in places where this is not allowed. That is why this ordinary tap water is not supplied directly to the compressor, but is purified first.

While the screw-type compressor is in use, the water in the water circuit is polluted by dust particles which are sucked in via the inlet and which are separated from the compressed air, together with the injected water, in the above-mentioned air receiver and in this manner end up in the water circuit.

Another source of pollution is due to corrosion of the parts of the screw-type compressor and of the water circuit, as a result of which harmful particles which are affected by corrosion, may be released in the water circuit and may cause damage to the rotors and to the bearings of the compressor element.

Especially purified water usually has a lower conductivity and consequently is very aggressive as far as corrosion is concerned.

Another source of pollution is the growth of micro- organisms in the water and in the compressor system.

Given the importance of the quality of the water and the ensuing possible corrosion, the water should be periodically replaced, at short intervals, in order to remove any possible pollution.

A disadvantage is that such frequent controls and services are expensive and time-consuming, and that the screw-type compressor has to be put out of order each time.

Another problem is that the presence of the above- mentioned pollutions, no matter how frequent the controls and services are carried out, has a negative influence on the life span of the compressor element.

Another disadvantage with the known water- injected screw-type compressors is that the water level in the water circuit has to be checked very regularly, for. example daily, whereby water has to be filled up or drained off during these controls in order to bring the water at the required level. The amount of water in the water circuit may indeed rise under circumstances whereby very damp air is sucked in, or it may drop when the compressor is applied in a rather

dry environment or in case of possible leaks in the water circuit. If there is too little or too much water in the water circuit, the screw-type compressor may be seriously damaged.

The present invention aims to remedy one or several of the above-mentioned and other disadvantages.

To this end, the invention concerns a water-injected screw-type compressor of the above-mentioned type with an improved water supply, whereby the water circuit is connected to an external water supply by means of a device for treating and purifying the externally supplied water, as an absolute water filter is incorporated in the above-mentioned lubrication pipe; whereby means are provided to prevent the growth of micro-organisms in the water and whereby means are provided to automatically keep the water level in the above-mentioned'air receiver at level between a bottom and a top level.

An advantage of the water treatment by means of reverse osmosis is that, no matter to what water the water treatment system is connected, ordinary drinking water can be taken as a basis whose quality, depending on its origin, may be of varying nature. By applying reverse osmosis is in these cases obtained pure water with a reproducible quality from which all harmful metal particles, minerals, ions and the like have been removed, such that a constant water quality is guaranteed.

The absolute water filter is designed to filter out the pollutions that are sucked in via the inlet and the inlet filter, whereby this absolute water filter is preferably provided for filtering out particles larger than 10 micrometer.

The above-mentioned means which fight the growth of micro- organisms, prevent the development and growth of harmful bacteria and they preferably consist of an element which makes contact with the water in the water circuit and which is made of copper or a copper alloy.

As means are provided to automatically keep the water level in the water circuit at level between a bottom and a top level, this level does not have to be checked very , frequently ; they also prevent any damage occurring due to too much or too little water being present in the water circuit, for example as a result of a sudden serious water leak.

It is clear that the invention provides for an improved water supply, whereby all external and bacterial pollutions are filtered out or are prevented, and whereby the water level is moreover automatically checked. In this manner can be guaranteed a long life of the rotors and of the bearings of the compressor element.

The constituent parts of the screw-type compressor are preferably made of a corrosion-resistant material, such

that can be prevented that the water which has been treated by reverse osmosis and which is usually highly corrosive, would be polluted by particles coming from corroded parts.

Depending on the circumstances in which the water-injected screw-type compressor is implemented, an air dryer can be applied in the outlet line of the screw- type compressor in order to dry the compressed air, whereby the water which is separated from the compressed air is put back into the water circuit, which is advantageous in that an extra source of pure water is obtained which does not have to be treated, so that less tap water will have to be used and treated, and as a result of which the device for reverse osmosis will last longer without any maintenance.

When no dryer is applied, a water separator will be mounted instead, which has the same effect. It can also take extra water out of the air, which can also be carried back to the primary water circuit.

In order to better explain the characteristics of the invention, the following preferred embodiment of a water- injected screw-type compressor which is provided with an improved water supply according to the invention is described as an example only without being limitative in any way, with reference to the sole accompanying figure 1, which schematically represents such a water- injected screw-type compressor in perspective.

Figure 1 represents an improved water-injected screw-type compressor 1 which is mainly composed of a compressor element 2 with a housing 3 in which are provided two co-operating screw-shaped rotors 4 whose shaft 5 is bearing-mounted in water-lubricated hydrodynamic and/or hydrostatic bearings 6 and which are driven by means of a motor 7.

The compressor element 2 is provided with an inlet line 8 in which has been provided an inlet valve 9 and which is connected to an inlet filter 10 on its entry.

On the outlet of the compressor element 2 is connected an outlet line 11 which is composed of the parts 11A, 11B, 11C and 11D respectively, whereby an air receiver 12 is provided between the parts 11A and 11B which is provided with a safety valve 13 on its output onto which the part 11B of the outlet line 11 is connected. and whereby, between the part 11B and the part 11C, an air cooler 14 is provided which is equipped with a ventilation screw 15 or which is connected to an external cooling circuit or the like.

Between the part 11C and the part 11D is connected a dryer or another water separator 16 to separate the water present in the compressed air, whereby the water collector of this dryer is connected via a return line 17 to the compressor element and whereby this return line 17 opens in the above-mentioned housing 3, more particularly on the suction side of the compressor element 1, for example between the inlet valve 9 and the

rotor space which is confined by the housing 3.

The dryer 16 is preferably a cool dryer, whereby, as is known, the compressed air is guided through a non- represented heat exchanger which is cooled by a non- represented cooling circuit, whereby the water which is present in the compressed air condenses and is collected at the bottom; a water separator has the same effect, except for the cooling of the air, as for the separation of excess water.

Onto the piping part 11D can be connected compressed air consumers which are not represented in the figures.

The air receiver 12 is provided with a central conductor 18 which makes the compressed air flow through the air receiver 12 in a tangential manner.

At the bottom of the air receiver 12 is connected a lubrication pipe 19 which is composed of the parts 19A, 19B, 19C and 19D, whereby, between the parts 19A and 19B, is provided a non-return valve 20 which is spanned by a branch 19E in which is provided a water pump 21 with a drive 22 whereas, between the part 19B and the part 19C, is incorporated a water cooler 23 which is cooled by means of the above-mentioned ventilation screw 15 of the air cooler 14 or by means of a secondary cooling water circuit.

Between the part 19C and the part 19D is provided an absolute water filter 24 according to the invention with

a filter element 25 which is preferably suitable for filtering out pollutions larger than ten micrometer.

The lubrication pipe 19D opens into the rotor chamber which is confined by the housing 3 of the compressor element 2 on the one hand, and via the branches 19F and 19 G at the above-mentioned bearings 6 of the compressor element 2 on the other hand.

The screw-type compressor 1 is further provided with a device 26 according to the invention for treating water according to the reverse osmosis technique, which device 26 consists of a supply line 27 which can be connected for example to an external water supply 29 for potable water via a valve 28 and which is connected for example to the above-mentioned return line 17 so as to open, either together or if necessary as separately, into the rotor space of the compressor element 2 and in which a number of filters, such as for example a sediment filter 30, a carbon filter 31, a membrane filter 32 and a water reservoir 33 are successively incorporated for the temporary storage of the purified water.

The membrane filter 32 consists, as is known, of a cylindrical casing 34 with a tubular membrane 35 in it which divides the membrane filter 32 in an outer compartment 36 and an inner compartment 37, whereby this outer compartment 36 is connected to the upstream part of the above-mentioned supply line 27 at one far end and is connected to a drain 38 at the other far end,

whereby the inner compartment 35 is connected to the downstream part of the supply line 27.

In the given example, the screw-type compressor 1 is provided with means to keep the water level in the above- mentioned air receiver 12 automatically at level between a bottom level A and a top level B.

These means are in this case formed of two capacitive level sensors 39 and 40 provided on the air receiver 12, at the height of the bottom level A and of the top level B respectively.

The bottom level sensor 39 is thereby electrically connected to a first electrically controlled valve 41, for example via a non-represented control, which valve 41 is provided in the supply line 27 to close or open the water supply 29, whereas the top level sensor 40 is connected in an analogous manner to the second electrically controlled valve 42 to shut a drain pipe 43 for the water which is situated in the air receiver 12.

The working of the water-injected screw-type compressor 1 with an improved water supply according to the invention is very simple and as follows.

While the compressor element 2 is being driven by the motor 7, ambient air is sucked in via the air filter 10 and pressed out to the air receiver 12, and further via the cooler 14 and the dryer 16 to the network onto which consumers can be connected.

An amount of water is present in the air receiver 12 which is injected, due to the pressure in the air receiver 12, via the lubrication pipe 19, into the rotor chamber of the compressor element 2 and at the bearings 6 for the lubrication of these bearings 6.

The injected water is then pushed, together with the compressed air, to the air receiver 12, in which, due to the tangential, thin rotational movement of the air, the water is separated from the air and the water is collected again at the bottom in the air receiver 12, such that the water thus circulates in a predominantly closed water circuit formed of the compressor element 2, the outlet line 11A, the air receiver 12 and the lubrication pipe 19.

Under damp circumstances, extra water is sucked in together with the damp ambient air, such that the water level in the air receiver 12 can be prevented from rising.

Under dry circumstances, however, the water level in the air receiver 12 can then drop again.

It is important that the water level is always situated between the above-mentioned bottom level A and top level B.

Excess water can be discharged via the drain pipe 43, when the level sensor 40 detects water at the top level B.

If, however, the water drops under the bottom level A, the level sensor 39 will open the valve 41 such that water is

supplied to the water circuit, out of the little storage tank in the unit. The water treatment system is constantly connected to the water supply system, whereby this water is filtered by the filters 30 and 31 and whereby, after the membrane filter 32, in the known manner, pure water is obtained which is free of pollutions and which is stored in the reservoir 33 from where it is sucked in via the lines 27 and 17 through the compressor element 2. As'soon as a sufficient amount of water has been stored in the storage tank, the water purification procedure is stopped, until purified water is required again in the compressor.

A second source of pure water is obtained thanks to the water which is separated from the compressed air in the air dryer or in the water separator 16 and which is supplied via the return line 17 to the above-mentioned water circuit.

In this manner, a sufficient supply of pure water is guaranteed, and the water level in the air receiver is automatically kept at level between levels A and B, even if there were an accidental leak in the water circuit, for example.

Via the air filter 10 at the inlet, impurities can be sucked in which may end up in the water circuit. These impurities are filtered out by the absolute water filter 24.

The water which is obtained after the device 26 for

reverse osmosis is usually very corrosive. In order to prevent pollution of the water circuit by corrosion, corrosion-resistant materials are selected for the parts of the screw compressor 1 which make contact with the water or with damp air, such as stainless steel, synthetic materials, copper and copper alloys.

The rotors 4 are preferably made of a synthetic material and the rotor shafts 5 of stainless steel, whereas the housing 3 of the compressor element is made of aluminium bronze.

The lines 11,19, 17, 27 are preferably made of a synthetic material or of rubber, and the connecting coupling of copper or brass.

The air receiver 12, the coolers 14 and 15, the absolute water filter 24 and the dryer 16 are preferably made of stainless steel.

Another reason why elements containing-copper are used in the water circuit is that, in this manner, the growth of micro-organisms is prevented.

It is clear that the invention makes sure that the water which is used to be injected in the compressor element is at all times purified of impurities and of bacteria, so that a long life of the compressor element 2 is guaranteed.

The present invention is by no means limited to the embodiment described as an example and represented in the accompanying drawings ; on the contrary, such a water-injected screw-type compressor with an improved water treatment according to the invention can be made in all sorts of shapes and dimensions while still remaining within the scope of the invention.