Rotary compressor with oleopneumatic delivery control device

Abstract: In a rotary type air compressor comprising a recycling circuit for the air provided with a check valve (29) operatively connected to a fluid actuated control cylinder having a piston member (28) operatively subjected to the vacuum in the intake chamber (13) of the compressor and to the oil pressure in the control cylinder (20) of the intake valve (15) of the compressor, and means (38, 39) connected to said piston member (28) for controlling the air-flow recycled to the intake chamber (13).

Description: This invention relates to oil-injection rotary compressors, and more particularly to an oleopneumatic control device for regulating the air delivery of the compressor. The term rotary compressor is used here to indicate both compressors having an eccentric rotor with sliding vanes, and compressors with a screw-type rotor in which the oil is injected into the air and which is subsequently separated by means of an appropriate oil separation circuit in order to be returned to the oil sump of the compressor. As is known, in the rotary compressors of the type mentioned, the pressurized air, during the idling phase of the compressor, is made to recirculate from the delivery port to the intake chamber through a recycling path comprising a controlled valve, normally referred to as "idling relief valve". Under these conditions, the intake valve provided at the inlet port for the air into the intake chamber is gradually closed by the operative piston of a hydraulic cylinder fed, through a pilot valve or servovalve, with oil under pressure coming from an oil sump, whose pressure vanes according to the delivery pressure of the air of the compressor. As the intake valve closes, in order to reduce the flow rate or delivery of the compressor, a vacuum forms in the intake chamber, which must not exceed a set value so as to avoid instability of the varies or vibrations in the rotor and consequently malfunctioning of the compressor. A system is known which controls the opening of the idling relief valve, exploiting the vacuum which is created in the intake chamber; according to this known system a controlled check valve is used in the recycling duct for the air from delivery to intake, for example in the form of a ball valve member which during closure is pushed against a conical seat by the same pressure of the air. The opening of the valve is controlled by an operating piston, actuated by the vacuum which forms in the intake chamber during idling, and which acts on the ball or closing member to the reaching of a set value of the vacuum, causing compressed air to pass towards the intake. This recycling of compressed air is provided to restrict the values of the vacuum in the intake chamber which would establish after a certain reduction in the flow rate of the compressor and which could render functioning instable. The use of a simple check valve controlled only by the vacuum which is created in the intake chamber is nevertheless the cause of difficulties since the functioning of said valve is not gradual, as would be desirable, but passes directly from the closing position to the opening position causing a sudden change in the vacuum at intake. Therefore under particular choking conditions of the intake, the disadvantage of the immediate reclosure of the idling relief valve may occur, with consequent knocking and instability of the compressor intake valve. The object of this invention is to provide a rotary compressor of the type mentioned, fitted with an idling relief valve suitably controlled to enable it to open gradually, thus eliminating the disadvantage described above. The above is obtained, by the invention as defined in Claim 1, by controlling the operating piston of the idling relief valve, by combining two converging, gradually increasing and interdependent forces, of which one is caused by the pressure of the oil which controls the intake valve of the compressor, the other due to the vacuum which is created in the air intake chamber. The invention is further illustrated hereinunder with reference to the figures of the accompanying drawings in which: Fig. 1 is a longitudinal sectional view of a rotary compressor embodying the improvements according to the invention; Fig. 2 is an enlarged sectional view illustrating the assembly of valves which regulate the compressor, during at full pressure working; Fig. 3 is a sectional view similar to that in Figure 2 in the idling condition of the compressor. By way of an example Fig. 1 shows a rotary compressor, comprising a stator 10 defining a compression chamber 11 in which a rotor 12 having a number of sliding vanes is positioned and rotates eccentrically in relation to the compression chamber. The chamber 11 of the compressor is connected to an air intake chamber 13 having air inlet port 14 provided with an intake valve 15 forming part of the valve assembly 16 shown in Figures 2 and 3 of the drawings. The stator 10 of the compressor is surrounded by an outer casing defining an oil sump 17 in which the lubrication and air cooling oil is collected, at the same delivery pressure; therefore the compression chamber 11, is directly connected both to the oil sump 17 and to the delivery side 18 of the compressor through a labyrinth path 19 defining in a manner which is in itself known a circuit for the separation of the oil from the compressed air. As shown in Figure 2, the intake valve 15 comprises a first hydraulic cylinder 20 whose operating piston member 21 acts on the stem of a disk like closing member 21a biased to close against the seat 22 provided of the air inlet port 14 into the intake chamber 13. The cylinder 20 is connected to the oil chamber 17, through a duct 23 comprising a servovalve 24 whose piston member 25 is biased by a spring 26 to close the passage 23 of the oil under pressure towards the cylinder 20 controlling the intake valve 15. In the condition in Fig. 2 the piston 21 operating the intake valve 15 is biased upwards by the spring 27 and the closing member 21a is raised by its own seat 22 leaving the passage for the air to the compressor intake completely open. The oil under pressure coming from the oil sump 17, through the servovalve 24 and the cylinder 20 of the intake valve 15, is moreover fed to the upper face of the piston member 28 operating a controlled check valve, or idling relief valve 29 positioned in a duct 30 for recycling the pressurised air from the delivery 18 of the compressor towards the intake chamber 13, as shown. As related previously, the side or upper face of the piston 28 of the cylinder controlling the idling relief valve 29, through the duct 31 is subjected to the pressure of the oil in the hydraulic cylinder 20 of the intake valve. Contrarily, the other side of said operating piston member 28 is subjected to the vacuum present in the intake chamber 13, through a branch duct 32; in this way two converging forces act on the two faces of the piston 28 in the same direction, one force being caused by the pressure of the oil and the other by the air, these forces acting in a direction in which the valve 29 opens; the latter in the example shown is in the form of a ball valve 33 kept closed by the action of the pressurised air coming from the recirculation duct 30 and of the spring 34 which pushes the ball 33 against a conical seat 35, in the conditions of the compressor operating at full load. The spring 34 maintains the ball 33 of the valve against its own seat 35 even in the absence of pressure and touching the control stem 36 during opening of the valve. According to the invention, besides controlling the piston 28 operating the idling relief valve, by means of converging oleopneumatic forces, means for controlling the flow of air under pressure recycled to the intake chamber 13 are provided in the recirculation duct 30, 30a. These control means for the air flow comprise an air-flow throttling member for example in the form of a cylindrical end or head 38 of the stem 36, having a diameter slightly smaller than that of the hole in which the same stem 36 slides so as to define a narrow annular passage 39 which opens in the end part 30a of the recirculation duct 30 into the intake chamber 13. The cylindrical head or throttling member 38 acts directly on the ball 33 of the idling relief valve and moves in a perpendicular direction to the hole 30a defining part of the air recirculation circuit towards the chamber 13; in this way the movement of the piston member 28 gradually opens or closes the passage of the air recycled from delivery to the intake chamber of the compressor when changing from the operating conditions at full load in Fig. 2, to the idling conditions of the compressor as shown in Fig. 3. As can be seen in this drawing, when the pressure of the oil coming from the chamber 17, which is at the same pressure as the air delivery of the compressor, reaches a value so as to raise the piston 25 of the servovalve 24, overcoming the thrust of the spring 26 to open a passage 40 towards the duct 23 for feeding the oil under pressure to the cylinder 20 controlling the intake valve, the value of the pressure downstream of the servovalve 24, which acts on the operating piston 21 and consequently on the upper face of the operating piston 28, will vary according to the ratio between the area of said passage 40 opened by the piston 25, and the area of a narrow opening 37 which releases the pressure of the oil into the intake chamber 13. Therefore as said pressure downstream of the servovalve 24 increases, the piston 21 operating the intake valve moves forward against the thrust of the opposing spring 27 pushing the closing member 21a gradually towards its own seat 22. When the intake valve 15 is almost completely closed, as shown in Fig. 3, the vacuum in the intake chamber 13 will take on a value such that, by acting on the lower face of the piston 28 operating the idling relief valve through the hole 32, in combination with the pressure of the oil of the cylinder controlling the intake valve, it will overcome the force which keeps the ball 33 closed against the conical seat 35 of the idling relief valve. The opening of the idling relief valve 29 will thus be gradual since as the pressure of the oil on the upper face of the piston 28 increases, there will be a corresponding decrease in the vacuum of the air acting on the lower face of the same piston, which is regulated by the extent to which the valve itself is open, following the moving forward of the cylindrical head 38 which gradually opens the passage 30a towards the intake chamber 13. In this way the equilibrium is maintained for the forces which otherwise would normally reclose the same idling relief valve. The opening of the idling relief valve 29 in fact allows the passage of a flow of air coming from the delivery of the compressor, around the closing ball 33 and the head 38 of the control stem 36, which also control its amount, and then through the hole 30a which connects said valve 29 with the intake chamber 13. In this way the vacuum at the intake side is controlled by the idling relief valve so that it does not exceed a set value, with a gradual feed of the air under pressure since at each moment said vacuum in the chamber 13 must be such so as to allow the balancing of the forces which would normally open the valve 29 with those which normally would reclose it. From what has been said and shown in the accompanying drawings it is therefore clear that a rotary compressor is provided, having an oleopneumatic device for regulating the flow rate comprising an idling relief valve, whose operating piston is controlled by two converging, progressive and interdependent forces, one caused by the pressure of the oil which controls the intake valve, the other by the vacuum in the air present in the intake chamber 13 of the compressor. Therefore by increasing the delivery pressure of the compressor for a small air demand, the servovalve 24 increases the pressure of the oil which control the closing of the intake valve 15 which gradually closes increasing the vacuum in intake, and viceversa. In this way a system of forces is obtained, at a certain point capable of gradually opening the idling relief valve 29, directly proportional to the closing of the intake valve 15, and sufficiently unaffected by the decrease in the vacuum in intake caused by the feeding of compressed air through the same valve 29. Therefore, as the intake valve 15 is gradually closed by the pressure of the oil coming from the servovalve 24, at a certain point the idling relief valve opens progressively until the vacuum in intake is reduced to an extent sufficient for compensating the increase in pressure of the oil coming from the servovalve, to ensure that the sum of the two forces on the piston 28 remains virtually constant. Due to the gradual functioning of the idling relief valve of the regulation device described, knocking and operational instability of the intake valve 15 can be prevented thus avoiding vibrations in the rotor and consequently malfunctioning of the compressor.

Claims: Rotary compressor of the type comprising a casing including a stator (10) which defines an air compression chamber (11), and an oil sump (17); an eccentric rotor (12) being provided in said chamber (11), said rotor (12) having radially sliding vanes; an intake chamber (13) also being provided having an air inlet valve (15) controlled by a first oil pressure actuated control cylinder (20), said cylinder (20) being controlled by a servovalve (24), said compression chamber (11) being further connected to the oil sump (17) and to the delivery side (18) of the compressor by means of an oil separation circuit, and in which the pressurised air from the delivery side (18) of the compressor is recycled to the intake chamber (13) through a recycling circuit housing a controlled check valve (29), said check valve (29) being operated by a piston (28) of a second control cylinder, characterised in that one side of the piston (28) of the said second control cylinder is operatively connected by a conduit (32) to the air vacuum in the intake chamber (13), and in that the opposite side of the piston (28) of the said second control cylinder is operatively connected by a conduit (31) to the oil pressure in the first control cylinder (20) of the intake valve (15) and in that said recycling circuit comprises valve means (39) for controlling the air flow recycled to the intake chamber (13), said valve means (39) being operatively connected to the piston (28) of the aforementioned second control cylinder of the check valve (29). Rotary compressor as claimed in claim 1, characterised in that said second control cylinder is on one side directly connected to the intake chamber (13), and on the other side is directly connected to said first cylinder (20) controlling the intake valve (15). Rotary compressor as claimed in claim 1, characterised in that said valve means (39) for controlling the recycled air flow comprise a mobile throttling member (38), said throtttling member (38) being operatively connected to the piston member (28) of said second control cylinder. Rotary compressor as claimed in claim 3, characterised in that said throttling member (38) is in the form of a cylindrical element which slides in a narrow annular passage, the said passage forming part of the said recycling circuit and the said throttling member (38) being in direct contact with the closing member (33) of the said check valve (29). Rotary compressor as claimed in claim 3, characterised in that said throttling member (38) is in the form of a cylindrical element moving perpendicularly to an end position of a duct (30a) which duct forms part of the said recycling circuit.

Inventors: BOLDRINI ADOLFO, BOLDRINI, ADOLFO
Applicants: ENEA MATTEI SPA, ING. ENEA MATTEI S.P.A.
Application year: 1990
Main classification: G05D16
IPC codes: G05D16, F04C28