CONTROL APPARATUS FOR ROTARY AIR COMPRESSORS

Abstract: A control apparatus for an oil-sealed rotary air compressor. The control cylinder (21) for the intake valve (18) of the compressor (10) can be connected to the oil chamber (12) through a first oil path (23, 28) comprising a servovalve (24) operated by the pressurized oil, and also through a branched-off path (32) comprising an idling valve (25) for switching the compressor into the idling working condition, which is held closed by the same pressure of the oil in the compressor (10), said idling valve (25) comprising fluid operated control means (38) connected to the oil separator (13) through a solenoid valve (40) controlled by a pressure switch (42) which senses the pressure of the air delivered by the compressor.

Description: This invention refers to oil-sealed rotary air Compressors, and more particularly to a control apparatus able to switch the working modes of the compressor from an operative condition having modulated adjustment of the delivery, to an intermittent full-load and idle working condition, while at the same time venting the pressure inside the compressor to reduce power consumption. From FR-A-2392258, an oil-sealed rotary air compressor is known, comprising an intake-valve control system, in which a pressure sensing switch senses the delivery pressure of the compressed air and controls the transition from a full-load working condition, in which the compressor operates uninterruptedly, automatically modulating the delivery by means of a suitable servovalve, to an idle-working condition in which, for want of air demand or owing to limited air demand which are satisfied directly by the compressed air stored in tank, the compressor is made to idle, at the same time venting its internal pressure. In the above-mentioned compressor the control of the intake valve is effected by means of a control piston actuated by the pressure of the oil coming from the compression chamber. During modulation of the delivery the pressure of the operative fluid is controlled by a servovalve, through which the oil reaches the control piston of the intake valve. In this way a small variation of the pressure in the oil chamber, causes the pressure on the control piston of the intake valve to assume the necessary values for overcoming the force of the spring restraining the piston, so that it can complete its full stroke. During idling operation, on the other hand, the intake-valve control piston is actuated under the control of an idling valve in such a way that the air pressure of the compressor is applied to the control piston of the intake-valve, through the same servo-valve. The compressor can decompress until the internal pressure assumes the minimum value necessary for keeping the intake valve closed. At this value the intake valve will be kept partially open to compensate the air which is released, so holding the internal pressure constant, the latter being needed also for the circulation of the lubrication oil for the compressor. Therefore in FR-A-2392258, the feeding circuit of the intake-valve control piston is alternatively fed by oil and pressurized air causing pollution when air is discharged to outside; furthermore to have a good seal to pressurized air, it is necessary to provide sealing ring on the control piston, that frictionaly slide inside the control cylinder negatively influencing the position of intake-valve and the working-condition of the compressor. An object of this invention is to provide a new control apparatus allowing the direct passage of the oil from the compression chamber to the intake-valve control cylinder, and which at the same time releases the pressure of the air inside the compressor, said apparatus enabling to control the closing of the intake-valve in complete autonomy from the opening of the servovalve for the modulated working condition of the compressor and therefore from the need to close the air-delivery valve and to reach, the maximum set pressure of the servo valve as required instead with previously known control apparatus. In this way, according to the invention, the control pressure switch, normally present in these types of apparatus, can be adjusted for any operative value of the required pressure and idle working of the compressor can occur without the pressure of the air inside the compression chamber needing to attain the maximum set value of the above-mentioned servovalve. In this way an apparatus of simple design is obtained, wich prevent air pollution and which is able to offer a significant power saving. According to the invention therefore there is provided an apparatus for controlling a rotary air compressor comprising the characterizing features of claim 1 The invention will be described in greater detail below referring to the appended drawings, in which: Fig. 1 shows a general diagram of the control apparatus for a rotary air compressor; Fig. 2 is an enlarged sectional view showing a preferred embodiment of the apparatus of figure 1. Figure 1 shows the diagram of the control apparatus for an oil-sealed rotary air compressor, per sè known, and of which a few essential working parts are presented in figures purely for the purposes of this invention. The compressor 10 comprises a rotor chamber 11, connected to an oil chamber or sump 12, and respectively with a separator or chamber 13 for separating the compressed air from the oil entrained in it; the separator chamber 13 in its turn communicates with an outlet or air discharge duct 14 through a minimum pressure valve 15, whose closing element 16 is pushed in a closed condition against a sealing seat by a spring 17. Reference number 18 in figure 1 indicates the overall assembly of a compressor intake valve through which the rotor chamber 11 can be connected to the outside by means of suitable air passages, not shown, in a fairly conventional way. The intake valve 18 can be of any type and for this purpose can equally be both of the open and closed type. In the diagram in figure 1 the intake valve 18 comprises a closing element 19 which is pushed to close the air intake 20 by a hydraulic cylinder 21 operated by the pressurized oil in the compressor oil chamber 12. In particular, the closing element 19 is mechanically connected to a control piston 21′ pushed in a direction in which the valve opens by a spring 22 acting against the pressurized oil fed on the opposite side through a duct 23; the duct 23 is connectable to the compressor oil chamber 12 through a first path comprising a servovalve 24, and then through a second branched-off path comprising an idling valve 25 for switching the compressor into an idle working condition as explained below. The oil feeding duct 23 for feeding the oil to the intake-valve control cylinder 21 is connected also to the air intake chamber 26 of the compressor through a duct 26′ comprising a restricted orifice 27. The servovalve 24 is connected on one side to the intake-valve control cylinder 21 through the duct 23, as already stated, and on the other it is connected to the oil chamber 12 through a duct 28; the servovalve 24 comprises a closing element 29 for modulating the oil pressure to the cylinder of the intake valve 18, this closing element being caused to open by the pressure of the oil, in opposition to the action of a thrust spring 30. As previously described, the pressurized oil in the oil chamber 12 can be fed directly to the intake-valve control cylinder 21 through the idling valve 25 without interfering with the function of the servovalve 24; in this connection, as shown in the diagram of figure 1, the idling valve 25 comprises a chamber 31 connected directly to the oil chamber 12 or for example through the duct 32 branched-off from the duct 28. In the chamber 31 there is a closing element 33 pushed by the oil pressure to close against a seat formed at the end of an axial hole or passage 34 which opens into the chamber 31, and is also connected to the duct 23 through an annular chamber 35 appropriately provided in the servovalve 24, around the closing element 29. The closing element 33 of the idling valve is connected by a rod 36 to a piston 37 of a control cylinder 38 whose working side is connected via a duct 39 and a solenoid valve 40, to the separator chamber 13. The duct 39 or the working side of cylinder 38 is connected also to the atmosphere through openings 41 in the cylinder chamber 38, which could be fitted with a silencing filter. The solenoid valve 40 is in its turn controlled by a pressure switch 42 activated by the pressure of the compressed air discharged by the compressor; this pressure switch is therefore operatively connected with the delivery 14 through the duct 43. With reference to the diagram of figure 1, a description will now be given both of the full-load and the idle working of the compressor under the control of the apparatus of this invention. During full-load working of the compressor, if the pressure at the outlet 14 does not reach the calibrated value of the pressure switch 42, the compressor governs itself by modulating the delivery through control of the intake valve effected by means of the servovalve 24. In this working condition the solenoid valve 40 is closed to prevent the supply of pressurized air to the idling valve 25; the latter in its turn closes off the direct passage 32, 34 of the pressurized oil from the compressor chamber 12 to the control piston 21′ of the intake valve 18. Accordingly, only the servovalve 24 can deliver oil to the intake valve control cylinder 21, modulating the air pressure. When, through lack of demand for air, the pressure at the output 14 attains the calibrated value of the pressure switch 42, this opens the solenoid valve 40 so allowing the pressure of the air inside the compressor to reach the idling valve 25 through the duct 39. The working piston 37 of this valve 25 under the air pressure, moves the closing element 33 backwards and forces it to open against the pressure of the oil, so making possible a direct connection of the intake-valve cylinder 21, which by-passes or excludes the servovalve 24 through the branched-off passage 32, 34 and 35. At the same time the piston 37, at the completion of its stroke, uncovers venting holes 41 in the cylinder 38 through which the pressure of the air from the separator chamber 13 can be released or vented into the atmosphere. Through the opening of the idling valve 25 a direct connection is established between the oil chamber 12 and the intake valve 18 for any value of the pressure inside the compressor. The intake value 18 will remain closed until the air pressure inside the compressor has fallen to the minimum value necessary for holding the valve completely closed, after which the suction of air will compensate the quantity of air released or vented, thereby maintaining the internal pressure of the compressor at the above-mentioned value. When the pressure at the outlet 14 has fallen, through the demand for air, at the value at which the pressure switch 42 will close the solenoid valve 40, the flow of air to the control piston 37 of the idling valve 25 will be stopped and the piston's face will once again be subject to atmospheric pressure returning through the venting holes 41 in the cylinder. At this point the pressure of the oil from the chamber 12 will return the closing element 33 against the sealing seat and the direct passage of oil to the intake value 18 will be interrupted. Since the piston 29 of the servovalve 24 will be in a fully closed position, as a result of the low pressure in the oil chamber 12, the pressure acting on the piston of the intake valve 18 will drop and will be released through the orifice 27 and the duct 26′, and the full-load working conditions will be re-established. Figure 2 shows a preferred embodiment of an idling valve as described above, forming part of the same assembly comprising the intake valve and the servovalve. The idling valve comprises a screw plug 45 having a tubular extension 46 protruding into a cylindrical hole 48 of the body 10, having a diameter larger than the outer diameter of the extension 46; the fore end portion 47 of extension 46 has a wider diameter, than the rear intermediate portion, equal to that of hole 48, to form a seal, so that the upper part of hole 48, forms annular-shaped passage 50, communicating with one side of the control cylinder of the intake valve 18 through a passage 52. In the case of figure 2, the piston 51 of control valve actuates the closing element 53 of the intake valve overcoming the force of a return spring 54 disposed between the cup-shaped piston 51 and the enlarged portion of a bush member 55, in order to push the closing element 53 normally in a closed condition against a ring-shaped sealing seat 56. Located inside the cylindrical body 46 of the valve 25 in a stem 57 having a mushroom head 58 at one end which forms a seal against a conical seating 59 provided at the end portion 47 of the cylindrical body 46, in respect to the pressurized oil coming from the oil chamber of the compressor, through a duct 60 which feeds the oil both to the servovalve 24 and the idling valve 25, passing through the annular shaped chamber 61 provided by a reduced portion of a sliding member at the bottom of the valve 24; in particular, the chamber or space formed at the bottom of the valve housing or bore 48, into which the duct 60 opens, when the closing element 58 is in open condition, communicates with the annular shaped passage 50 in the axial bore 48 of the cylindrical body 46 at a location where the stem member 57 has a reduced diameter, defining an annular space having radial holes 46′ in the cylindrical body 46 as shown. The head portion of the screw plug 45 is hollow and comprises a cylindrical hole 62 defining a control cylinder within which slides a piston 63 connected to stem 57. The cylinder 62, on the side opposite to stem 57 is connected to the compressor downstream the filtering separator, through the duct 39 and the solenoid valve 40 controlled by the pressure switch 42 which senses the delivery pressure, in the way previously described. The delivery side of the servovalve 24 is in communication with the annular shaped duct 50 through a hole 64 in the body 10 and at the same time, on the opposite side, it communicates with the compressor intake chamber through an annular groove 65 in the piston of valve 24 and a restricted orifice 66. During full-load working it will be the servovalve that operates closure of the intake valve, by feeding it oil at pressure which is modulated according to the passage uncovered by its own piston. When, on the other hand, the pressure switch 42 opens the solenoid valve 40 which supplies the idling valve 25, the piston 63 of the latter, pushed by the pressure, will shift the rod 57 downwards and at the same time will uncover venting holes 67 in the head 45 of the valve 25 so that the pressure of the compressor can be reduced. The pressurized oil from the oil chamber 12 will find an open passage leading to the annular duct 50 and from this it will reach the piston 51 of the intake valve 18 so completely closing the valve. When the pressure switch 42 again closes the solenoid valve 40, and with the thrust of the working piston 63 being released, the pressure of the oil will automatically re-close the element 58. The pressure acting on the intake-valve piston will be released through the orifice 66 towards the compressor intake chamber, the intake valve will re-open and the compressor will revert to full-load working.

Claims: An apparatus for controlling a rotary air compressor (10) having an intake valve (18) operatively connected to the piston (21') of a control cylinder (21), the control side of the control cylinder (21) being connected to the pressurized oil sump (12) of the compressor (10) through a servo valve (24) operated by the pressurized oil, said apparatus comprising an idling valve (25) for switching the compressor into idle working condition by a connection of the intake-valve control cylinder (21) to the oil sump (12), said idling valve (25) being operatively connected to the separator (13) of the compressor (10) through a solenoid valve (40) controlled by a pressure switch (42) which senses the delivery pressure of the air at the out-let (14) of the compressor, the intake-valve control cylinder (21) being connectable to the oil sump (12) through a path comprising the servo valve (24) with the idling valve (25) being normally closed, characterized in that the idling valve (25) comprises an air actuated control cylinder (38) provided with venting holes (41), and that said idling valve (25) is provided in a branched path (23, 32) between the oil sump (12) and the intake-valve control cylinder (21), said idling valve (25) comprising a closing element (33), one side of said closing element (33) being mechanically connected to the piston (37) of said idling-valve control cylinder (38) to oppose the pressurized oil acting on the other side of said closing element (33). An apparatus as claimed in claim 1, characterized by the fact that said idling valve (25) comprises a chamber (31) for the valve closing element (33) said chamber (31) being in communication with the compressor oil sump (12), and respectively with a duct (23) feeding pressurized oil to the intake-valve control cylinder (21) through an axial bore (34) in said idling valve (25) said axial bore (34) being normally closed by the above-mentioned closing element (33). Apparatus as claimed in claim 2, characterized by the fact that the idling valve (25) is connected to the duct (23) for feeding pressurized oil to the intake-valve control cylinder (21) through an annular passage (35) provided around said servo valve (24). An apparatus as claimed in claim 3, in which `aid servovalve (24) comprises a sliding control member (29), characterized by the fact that said passage (35) is made in the form of an annular shaped passage (35) around the sliding member (29) opening within the servovalve (24). An apparatus as claimed in the preceding claims, characterized by the fact that the chamber (31) for the closing element (33) of the idling valve (25) is connected to the oil sump (12) of the compressor through a duct (32) branched-off from the servovalve oil-inlet duct (28). An apparatus as claimed in claim 1, characterized by the fact that said duct (23) for feeding pressurized oil to the intake-valve control cylinder (21) is connected to the compressor intake chamber (13) through a restricted orifice (27). An apparatus as claimed in claim 1, characterized by the fact that said idling valve (25) comprises a screw plug element (45) having a tubular extension (46) protruding into a cylindrical hole (48) of compressor body (10) having a greater diameter, said tubular extension (46) having an enlarged end portion (47) forming a seal against said hole (48) said tubular extension (46) opening at one end into said cylindrical hole (48) and comprising a slidable closing member (58), said tubular extension (46) and said cylindrical hole (48) defining an annular shaped chamber (50) communicating with the intake-valve control cylinder (21), and respectively with the oil sump (12) of the compressor through radial bores (46′) opening into said tubular extension (46) and through said servovalve (24). An apparatus as claimed in claim 7, characterized by the fact that said idling valve (25) communicate with the oil sump (12) through a conduct (60) comprising an annular chamber (61) at one end of a sliding member inside the servovalve (24). An apparatus as claimed in claim 7, characterized by the fact that the annular chamber (50) delimited by said tubular extension (46) of the screw-plug element (45) of the idling valve (25), is connected to the compressor intake chamber through a duct (64) comprising an annular groove (65) in the sliding member of the servovalve (24) and a restricted orifice (66) in the compressor body (10).

Inventors: BOLDRINI, ADOLFO
Applicants: ING. ENEA MATTEI S.P.A.
Application year: 1988
Main classification: F04C29
IPC codes: F04C28