EP0330555B1 - Air compression arrangement with an oilless rotary compressor, and relative process - Google Patents

Description

The present invention relates to an air compression assembly, comprising a rotary compressor operating "without oil". By "oil-free" is meant that the oil is not used as a means for sealing the compression chamber (s).

The present invention also relates to a method for compressing air by means of a rotary compressor.

It is known to make oil-free compressors of the rotary type, using twin screw compressors, the two rotors of which are synchronized by external gears, so that they do not come into contact with one another.

However, in order to obtain sufficient efficiency, and more particularly to limit the relative flow rate of leaks, while there is no seal against leakage between the rotors, it is necessary to rotate the rotors at great speed. speed, of the order of 10,000 revolutions / minute, which makes these compressors very noisy; the flow is simultaneously increased and, consequently, if it is a question of supplying air at a discharge pressure of 7 to 8 bars, it is difficult to manufacture compressors of this type, whose power either less than 50 or 100 horsepower (35 or 70 kW). These compressors are also very expensive because the rotor synchronization gears must be immersed in oil and isolated from the rotors by seals.

One solution to overcome this difficulty and to obtain smaller but efficient compressors is to inject water into the compression chamber; the compressors are then made silent and the leaks sealed, which allows much lower speeds.

Certain twin screw compressors and even single-screw compressors - as described for example in the French invention patents FR-A-1 331 998, FR-A-1 586 832 - and GB-A-846 390 have been sold and are sold with such a water injection replacing the oil injection. The advantage of water injection is that it does not add any additional product to the air, oil for example, water existing in the form of moisture in the air drawn in; therefore, compressed air can be referred to as oil-free air.

However, such compressors remain very expensive, having to be protected against corrosion; this implies that, firstly, the materials used for the foundry, the screws, etc., must be stainless - like bronze - or protected against corrosion - for example by nickel-plating. Secondly, the bearings must be protected from water and lubricated with grease or oil if they are standard anti-friction bearings such as ball or roller bearings.

Such compressors thus end up not being cheaper than standard oil-free compressors such as piston compressors with polytetrafluoroethylene segments, i.e. they cost at least 50 to 80% more than compressors with oil injection.

This additional cost has greatly limited the use of such oil-free compressors, whose market share for air compressors does not exceed 10-15%. If the cost were the same, oil-free compressors would hold some 60% of this market, since this is already the percentage of the market occupied by devices associating a dryer with a compressor, a combination where, obviously, users are looking for clean air.

The object of the present invention is thus to propose a rotary compressor and a method making it possible to compress air without using oil for sealing or resorting to solutions whose cost is prohibitive.

According to a first aspect of the invention, the air compression assembly, comprising a rotary compressor whose delivery is connected to a tank partially filled with water, the lower part of this tank being connected by a pipe to at least an injection hole, located on the rotary compressor, this pipe being provided with a means for cooling the water, is characterized in that the water contains in solution borate, at a concentration sufficient to avoid corrosion of the metals ferrous by oxygen.

• injecter un liquide dans le compresseur;
• séparer ce liquide de l'air comprimé fourni par le compresseur;
• utiliser le liquide séparé pour poursuivre l'injection de liquide dans le compresseur, caractérisé en ce qu'on utilise comme liquide de l'eau additionnée de borate en solution.

According to a second aspect of the invention, the method for compressing air in a rotary compressor comprises the steps consisting in:

• inject a liquid into the compressor;
• separate this liquid from the compressed air supplied by the compressor;
• use the separated liquid to continue injecting liquid into the compressor, characterized in that water added with borate in solution is used as liquid.

It has indeed been found that by simply adding borate and in particular borate of sodium or potassium borate in water, the same compressor used for air compression with oil injection, can also be used for air compression with water injection; in other words, this oil can be replaced by water containing borate and it is not necessary to replace the foundry or screw-rotor materials, usually cast iron, with stainless materials, or to change the material of the conduits, the pressure tank, etc. to protect them from corrosion. As a result, the cost of oil-free compressors is made close to that of oil-injected compressors, thus allowing much more widespread use of oil-free air.

It has further been discovered that due to the very low vapor pressure of the borate crystal, the flow of borate leaving the compressor with the compressed air is insignificant and therefore it was possible to operate very for a long time the compressor according to the invention without having to add crystal. This is an important advantage over liquid corrosion inhibitors, such as soluble oils or products called 5-95; these liquids have a significant vapor pressure; during the tests carried out, it was impossible to operate for more than 100 hours a compressor using such a liquid without having to add inhibitor; this renders these liquids unusable, firstly, because of the cost of the liquid which must be frequently added, secondly, because of the technical problems posed by the timely refilling to avoid any corrosion.

It has also been discovered that liquid inhibitors foam water and that it is difficult to avoid this foaming: use expensive centrifugal systems or coalescing elements; moreover, when the compressor is stopped, the tank pressure drops to atmospheric pressure; the air dissolved in the water produces bubbles and with such liquid inhibitors there is instantly obtained a very significant formation of foam which leaves in the compressed air line unless, to avoid this departure, a large tank is provided and expensive. On the contrary, the aqueous solutions of borate do not produce foam and behave, in this respect, like water.

It has also been discovered that microscopic droplets containing borate which escape from the water tank, whatever their size, go into the dryer and therefore end up in the condensed water normally discharged into the sewer; the concentration of borate in this condensed water is very low, of the order of a fraction of a percent; once diluted in the sewer water, this concentration should not significantly increase the borate content, the borate already existing in water in its natural state; thus, dumping this amount of borate down the drain poses no threat to the environment.

It has also been discovered that, unlike other inhibitory crystals, such as chromates, nitrites, phosphates or silicates, borates are not unstable in the presence of oxygen and up to certain thresholds are not dangerous for the health.

On the contrary, it has been found that since aqueous solutions containing borate are slightly basic with a pH around 9, they prevent the growth of bacteria or destroy them. One of the problems so far having strongly limited use of water injection compressors is precisely that the water, lukewarm or hot when the compressor is running, promotes the rapid development of bacteria; this means constantly changing the water and removing polluted water. This is costly and creates other problems such as scale formation. With the borate in solution, the analysis reveals a total absence of bacteria after thousands of hours of operation.

In addition, the bacteria sucked in with air by the compressor are, in large part, killed by the passage through the compressor. During a test, it was found that air containing 60 bacteria per cubic meter on suction contained less than six bacteria per cubic meter after compression accompanied by injection of an aqueous borate solution.

The present invention will be better understood on reading the description below and the attached drawing, given by way of nonlimiting example and where the figure represents the structure of an air compression assembly, in accordance with the invention .

The figure shows a compressor 1, driven by a motor not shown, sucking air through a line 2 and a non-return valve 3 which closes when the compressor is not in use.

The compressor is of the rotary type and preferably with a single screw, as described in patents FR-A-1 331 998 or FR-A-1 586 832.

The compressor discharges into line 4, connected to the reservoir 5, which contains a liquid shown in 6. When the reservoir 5 is pressurized by the compressor, the liquid 6 exits through line 7 and passes through a filter 8 and through a heat exchanger 9 where it is cooled, in this example, by air conveyed by a fan 10 (but it could be cooled by other means such as water). The liquid is then injected into the compressor through at least one orifice 11, where it is recompressed with the gas, then separated by gravity in the reservoir 5. The orifice 11 is formed through the compressor casing so as to communicate with the or each compression chamber when it is subjected to a pressure lower than the discharge pressure. Thus, the injection takes place due to the discharge pressure which prevails in the reservoir 5.

The compressed air leaves the reservoir 5 via the line 12, connected to an after-cooler 13, followed by a reservoir 14. In this reservoir 14, the steam condensates can be separated from the air flow, and, in a conventional manner such as by a float valve 15, can be returned to the compressor suction. The cooled gas then takes the line 16 and then joins the dryer 17, consisting, for example, of a coil 18, cooled by a refrigerant circulating in a coil 19. Part of the calories of the gas is transferred from a coil 20 to the first part 21 of the coil 18. A separator 22, with a float valve 23 returns all the condensate to a three-way valve 24, actuated by the level 30 of the tank 5. When the level is below a first threshold, or low threshold , the three-way valve 24 is positioned to return the condensates to the suction of the compressor; otherwise, they are sent to the sewer via line 25. When the level of the tank exceeds a second threshold, or high threshold, a sensor, via line 26, controls the opening of valve 27, so that the whole or part of the exchanger 9 is short-circuited by the line 28. Such a system has been described in patent n ° 2 171 653.

With the exception of the differences mentioned more far, this system is very traditional, and has been used for years with oil, and even water with several variants: one can for example use a pump to convey water, instead of the pressure d delivery air, install or not in the tank a coalescing element to rid the compressed air of small droplets left in the gas, etc. All these variants can be used without changing the nature of the invention.

• il est prévu le court-circuit 28, pour maintenir la concentration de borate dans l'eau, conformément au brevet FR-A-2 171 653;
• l'échangeur de chaleur 9 est plus grand pour limiter la température de l'eau et diminuer la formation de vapeur quand l'eau est injectée dans le compresseur, car une telle vapeur diminue le volume utile aspiré et en conséquence le rendement. Le borate étant basique, et attaquant par conséquent l'aluminium, il faut utiliser un échangeur de chaleur en cuivre ou en acier mais il s'est avéré que le coût des échangeurs de chaleur en aluminium, en cuivre ou en acier est pratiquement identique;
• l'élément coalesceur, habituellement disposé dans le réservoir 5 pour éliminer le "brouillard d'huile", peut être, soit éliminé (la tension de surface de l'eau étant plus élevée, les gouttelettes sont plus grosses et peuvent être collectées plus facilement), soit remplacé par un système de chicanes très simple;
• les paliers à roulement sont remplacés par des paliers lisses, en particulier des paliers au carbone, capables de fonctionner dans l'eau et dont le coût, au moins pour les compresseurs à vis unique qui ne produisent que de très faibles charges sur les paliers, est comparable à celui des paliers à roulement lubrifiés à l'huile.

The main differences in particular between the system described and a conventional system in which the oil would be the liquid represented in 6 are essentially:

• short-circuit 28 is provided to maintain the concentration of borate in the water, in accordance with patent FR-A-2 171 653;
• the heat exchanger 9 is larger to limit the temperature of the water and reduce the formation of vapor when water is injected into the compressor, since such vapor decreases the useful volume sucked in and consequently the yield. Since borate is basic, and therefore attacking aluminum, a copper or steel heat exchanger must be used, but it has been found that the cost of aluminum, copper or steel heat exchangers is practically identical;
• the coalescer element, usually placed in the tank 5 to eliminate the "oil mist", can be either eliminated (the surface tension of the water being higher, the droplets are larger and can be collected more easily ), or replaced by a very simple baffle system;
• rolling bearings are replaced by plain bearings, in particular carbon bearings, capable of operating in water and whose cost, at least for single screw compressors which produce only very low loads on the bearings, is comparable to that of oil-lubricated rolling bearings.

Furthermore, the two sets are identical, both being able to use the same ferrous materials for the foundry, the bearings, the pipes, the tank, neither of them requiring any particular protection.

Since borate is an inexpensive crystal and since it is generally used with a percentage of less than 10% and although the water needs to be deionized (otherwise the borate would precipitate the calcium carbonate), the liquid remains less expensive than the required oil refill in oil compressors.

Overall, the cost of the two systems is identical to within a few percent, with the added advantage that the parts are largely the same, which reduces stocks and allows us to consider launching a production of oil-free machines for a moderate study and investment cost.

If the air compression assembly is intended to be used with temperatures below 0 ° C, special protection must be used such as an electric heater placed at the bottom of the assembly, which is triggered when the temperature becomes lower than a determined value, in order to avoid ice formation in the circuit.

It has been found that sodium borate, commonly known as borax, at a concentration of 4 to 5%, is sufficient to prevent corrosion. However, it is preferred according to the invention to use potassium borate and in particular dipotassium tetraborate tetrahydrate K₂O · 2B₂O₃ · 4H₂O., Because of its high solubility in water, so that it can have some losses without running the risk of any corrosion. With sodium borate, which has a lower solubility, such a high concentration is possible when the water is hot, but crystals appear on cooling; such crystals, carried with the liquid, could damage the compressor at start-up. So sodium borate has the disadvantage that its maximum concentration to avoid the crystals is little higher than the minimum concentration to avoid corrosion, which leaves only a small margin.

Also, when the compressor is stopped, if the suction was not closed by a means 3, for example non-return valve or a pneumatically or electrically controlled valve, the compressor would dry when stopped and the crystals of borate would cause the screw to stick to the housing, resulting in difficult starting and mechanical problems.

It is possible to eliminate the water left in the compressor, for example by letting the air from the reservoir 5 escape through the compressor 1 after stopping. But closing the suction when the compressor is stopped, as proposed above, is much simpler; whatever liquid remains in the compressor, it cannot dry out.

During the tests, it was found that with potassium borate concentrations between 5 and 10%, no trace of rust appeared in the compressor or the pipes; the water did not foam more than without borate, hence a behavior very different from that observed with liquid type corrosion inhibitors; for an air compressor of 20 horsepower (approximately 15 kW) delivering at 7 bars relative, the quantities of borate entrained with compressed air to the dryer proved to be very low: the concentration of borate condensate in the water produced by dryer 17 was less than 0.5%.

None of these results have been achieved with other inhibitors; all the liquid inhibitors tested, or which one can think of, exhibit a vapor pressure and a vapor entrainment in compressed air such that they deprive the water of its inhibitor in a few hours; the compressor then becomes subject to corrosion; or it is necessary to carry out expensive refills and add a system for controlling the minimum inhibitor content.

Other solid inhibitors, such as sodium chromate, represent a health hazard, or are not stable over time, in the presence of oxygen.

Thus, borate and more particularly potassium borate appears to be the only solution solving the problem.

In addition, it has the unexpected advantage of killing bacteria, avoiding bacteriological growth in the tank, and making it possible to deliver compressed air hygienically cleaner than the air drawn.

Claims (8)

1. An air-compression assembly comprising a rotary compressor (1), the discharge of said compressor being connected to a tank (5) partly filled with water, a lower part of said tank being connected via a conduit (7) to at least one injection hole (11) provided on said rotary compressor, said conduit being provided with a means (10) for cooling the water, characterized in that the water contains dissolved borate at a concentration sufficient to avoid the corrosion of ferrous metals.

2. An air-compression assembly according to claim 1, characterized in that the borate is a potassium borate.

3. An air-compression assembly according to claim 1, characterized in that the borate is a dipotassium tetrahydrate tetraborate.

4. An air-compression assembly according to claim 1, characterized in that closing means (3) are located on the intake line (2) of the compressor, in order to close the intake when the compressor is at rest.

5. A method of compressing air in a rotary compressor, comprising the steps of:

- injecting a liquid into the compressor (1);

- separating said liquid from the compressed air discharged by the compressor (1);

- using the separated liquid for continuing injection of liquid into the compressor, characterized by using, as said liquid, water containing dissolved borate.

6. A method according to claim 5, characterized in that potassium borate is used as said borate.

7. A method according to claim 5, characterized in that dipotassium tetrahydrate tetraborate is used as said borate.

8. A method according to any of claims 5-7, characterized by the step of closing the intake (2) of said compressor (1) when said compressor is at rest.

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