Abstract:
Equipment for continuous regulation of the flow rate of fluid in a reciprocating compressor which has a compression chamber with a piston reciprocally movable therein. The compression chamber has an inlet valve and an outlet valve which delivers fluid to a reservoir. A translation device is movable to open the valve and allow closing of the valve. An actuator engages the translation device and includes a rod. The rod has a magnitizable central element located between solenoids of an electromechanical device. The central element is located in a prefixed position with respect to the solenoids under the resilient loading of a resilient device. Detectors are provided for detecting the position of the piston, the pressure in the reservoir and the position of the actuator.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to reciprocating compressors, and in particular to equipment for continuous regulation of the flow rate in the said compressors. 
     There are various possible methods of regulating the flow rate: devices external to the compressor which may be considered are on/off operation, variation of the speed of the motor driving the compressor, a by-pass between the delivery and inlet, and inlet throttling, while devices forming part of the compressor itself which may be considered are idle/load operation, backflow control and the introduction of an additional dead space which may be constant or variable. 
     Regulation by means of additional dead space is provided by adding a dead space to the cylinder to enable the opening of the pressure valves to be delayed, thus reducing the flow rate; it is possible to carry out either step regulation, by adding various dead spaces of different capacities, or continuous (stepless) regulation, by using an additional dead space of variable capacity, as indicated in U.S. Pat. No. 6,641,371. 
     Idle/load operation, which does not provide continuous regulation of the flow rate, is suitable when a storage reservoir is present in the system and a variation of the delivery pressure is acceptable; the pressure of the reservoir is controlled by a hysteresis regulator. Generally, the flow rate is regulated by actuators composed of pneumatic devices, which, by acting on a body (the pusher) present in each valve, enable the sealing element to be kept in a predetermined position (open), thus making the compressor idle (zero flow rate); when the said devices are inoperative, the compressor operates at maximum capacity. 
     The frequency of actuation of the pneumatic devices which operate the pushers of the inlet valves depends on the amplitude of the hysteresis, the volume of the reservoir and the maximum unbalance between the nominal flow rate and the minimum flow rate of the load; however, the said value must be limited to avoid excessive wear on the pneumatic devices. 
     This type of control of the flow rate of compressors causes a decrease of the global efficiency and of the power factor in the “idle operation” phase; furthermore, the heat generated in the “idle operation” phase is not dissipated, and thus increases the temperature of the sealing elements. Finally, the use of an actuator without position control, its limited response time and rise time, together with the presence of long pipes having limited cross sections and considerable dead space, and the absence of synchronization of the movement with the compressor shaft gives rise to a number of contacts at uncontrolled velocity between the sealing element and the pusher, which reduce the reliability of the valves, causing wear on the pusher and the breakage of the sealing element. 
     Backflow control is provided by delaying the closing of the inlet valve with respect to the closing point in the case of maximum flow rate. The gas which has entered the cylinder flows back into the inlet duct in a quantity proportional to the portion of the compression stroke during which the inlet valves are kept open. 
     The use of continuous regulation permits the use of storage reservoirs of limited capacity, since the pressure variations are practically absent. The actuation methods used up to the present time for controlling the position of the sealing element of the valves are of the pneumatic or oil hydraulic type. 
     Examples of some devices based on continuous backflow regulation are described in the documents U.S. Pat. Nos. 7,331,767 and 5,988,985. These devices use various actuation systems based on fluid which is supplied to a piston. Both systems require a panel for regulating the pressure of the fluid used for the actuation. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is therefore to provide equipment for the continuous regulation of the flow rate in reciprocating compressors, by using essentially simple means which limit the wear of the valve components. 
     The present invention therefore proposes equipment for continuous regulation of the flow rate for a reciprocating compressor, provided with at least one compression chamber in which is slidably inserted a piston means movable with a reciprocating motion, at least one inlet valve for the fluid to be compressed and at least one outlet valve for the compressed fluid being provided in the said chamber, the said outlet valve being connected to a storage reservoir for the compressed fluid, and the said inlet valve being provided with translation means which can act on the obturator of the said valve, the said translation means being movable in a direction perpendicular to the plane of the said obturator, and interacting with actuator means which are movable in the said direction with a reciprocating motion by means of suitable operating means; the said operating means make it possible to control the velocity of displacement of the said actuator means in both directions of their movement; means for detecting the position of the said actuator means, means for detecting the position of the piston in the compression chamber and means for detecting the pressure in the reservoir are provided, the said detection means and the said operating means of the actuator means being connected to a central processing unit. 
     In a preferred embodiment, the operating means of the said actuator means are electromechanical, and in particular they comprise two solenoids. The actuator means comprise a rod provided in its central portion with a radially projecting magnetizable portion, the said portion interacting with the said solenoids and being placed in equilibrium between the solenoids by the use of suitable resilient loading means. One end of the rod is connected to the said translation means of the sealing element, while its opposite end interacts with means for detecting its position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and characteristics will be made clearer by the following detailed description of an embodiment of the present invention, provided, by way of example and without restrictive intent, with reference to the attached sheets of drawings, in which: 
         FIG. 1  is a schematic diagram of a compressor provided with the equipment according to the present invention; 
         FIG. 2  is a view in lateral elevation with parts in section, representing a detail of an inlet valve of the compressor of  FIG. 1 ; 
         FIG. 3  is an enlarged view in longitudinal section of a detail of  FIG. 2 ; 
         FIG. 4  shows a detail in section relating to a variant embodiment of the present invention; 
         FIG. 5  is a graph of the variation of the position of the actuator of the inlet valve during a transition from a closed valve to an open valve state as a function of time; 
         FIG. 6  is a pressure-volume diagram relating to the compressor provided with the equipment according to the invention; and 
         FIG. 7  is a set of diagrams showing the variations of the signals and sealing positions of the valve and of the actuator. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows schematically a compressor provided with the equipment according to the present invention; the compression chamber is indicated by  1 . The said chamber  1  is substantially cylindrical, and into this chamber there is inserted a double-acting piston  101 , connected by a rod  111  to the transmission shaft  20 , which is connected by means of the pulley  21  and the belt  33  to the pulley  31  keyed to the shaft  32  of the geared motor  30 ; the shaft  20  is provided with a sensor  43  for detecting its position, connected to the central processing unit  40 . The chamber  1  is provided with two inlet ports  201  and two outlet ports  301 ; each of the inlet ports is provided with an automatic valve  2 , provided with actuator means  3 , which are described and illustrated more fully below; on the said actuator means  3  there are placed a sensor  42  and control and monitoring means  45 , which in turn are connected to the processing unit  40 . The outlet ports  301  are also provided with automatic valves  4 , through which the compressed fluid is discharged into the storage reservoir  10 , the pressure of which is monitored by means of the sensor  41 , which is also connected to the central processing unit  40 , which also has an operator interface module  44 . 
       FIG. 2  shows the inlet valve assembly  2  more fully. The said valve  2  is placed on the port  201  of the chamber  1 , and is enclosed in a containing body  102  provided at one end with a radial flange  122  which is connected by the fixing means  132  to the outer wall of the chamber  1 , while its opposite end is provided with a bush  142  by which it is connected to the actuator means  3 . Inside the port  201  there is placed a counter-seat  202  of the valve  2 , comprising the passages  212  for the fluid and the resilient loading means  222  for the sealing element  302 , whose passages  312  are coaxial with the passages  212  of the counter-seat  202 . Outside the sealing element  302  there is placed the seat  402 , whose passages  412  are offset with respect to those of the sealing element and of the counter-seat. The prongs  512  of the pusher  502  pass through the said passages, the pusher being axially slidable with respect to the port  201 , and being positioned coaxially with the projecting shaft  322  of the seat  402 . Inside the pusher  502  there is a spring  342 , one end of which bears on a flange  332  projecting from the shaft  322 , while its other end bears on the closing surface  522  of the pusher  502 . 
     The rod  103  extending from the actuator  3  bears axially on the outwardly directed face of the said closing surface  522 , this rod passing substantially through the whole length of the said actuator  3 , and having, substantially in its central portion, the moving element  203 , in the form of a disc of magnetizable material keyed to the said rod  103 , the said moving element being positioned between two solenoids  303  and  403 , and being movable in a reciprocating way over a given path. Resilient loading means  213  and  223 , which interact with the flanges  113  and  123  respectively of the rod  103 , are provided in the actuator  3 . 
       FIG. 3  shows the actuator  3  of the inlet valve  2  in greater detail; identical numerals refer to identical parts. The rod  103  is composed of a plurality of sections interconnected with each other, comprising the end  133  intended to interact with the pusher  502  (see  FIG. 2 ), the portion  143  which carries the flange  113  interacting with the spring  213 , and which is coupled by means of the screw  193  to the portion  153  to support the moving element  203  between the two solenoids  303  and  403 , which are supported on their respective plates  313  and  413  by the fixing means  323  and  423  respectively. The actuator  3  comprises a cylindrical body  803  in which the control and monitoring probe  45  of the solenoids  303 ,  403  is inserted radially, this probe being connected to the central processing unit, indicated by  40  in  FIG. 1 . At the end of the cylindrical body  803  facing the inlet valve  2  there is connected, by the fixing means  813 , the head  703 , which is provided axially with a cavity  723  for housing the spring  213 , and with a threaded shank  713  intended to interact with the bush  142  of the body  102  of the valve  2 . The shank  713  and the cavity  723  are coaxial, and the channel  733 , into which the end  133  of the rod  103  is inserted, passes through both of them. 
     The opposite end of the cylindrical body  803  of the actuator  3  comprises a cap  603  provided with a threaded axial hole  613 , into which is inserted the block  503 , which is also threaded; the said block has a cavity  513  facing towards the inside of the actuator, the spring  223  which interacts with the flange  123  of the rod  103  pressing into this cavity, and a cavity  543  facing the outside of the actuator  3 , this cavity housing the plate  173  connected to the end  163  of the rod  103 , which interacts with the sensor  42 . The two cavities communicate by means of the channel  533 , through which the end  163  of the rod  103  passes. The position of the block  503  can be fixed by means of the locking bolt  523 . 
       FIG. 4  shows a variant embodiment of the present invention; identical numerals refer to identical parts. In the figure, the block  503  is replaced by the block  903 , which is provided with a flange  913 , provided with sealing means  923 , which bears on the cap  603  into which the said block  903  is screwed. The chamber  933  inside the block  903 , into which the end  163  of the rod  103  penetrates, communicates by means of the hole  943  and the pipe  953  with the environment upstream of the valve described above; the chamber  933  is closed by the cap  963 . 
     The operation of the equipment according to the present invention will be made clear by the following text, with particular reference to the figures described above and to the graphs in  FIGS. 5 to 7 . As stated in the introduction, one of the most important problems in the regulation of the flow rate of reciprocating compressors is that of the appropriate control of the means which act on the sealing element of the inlet valve in order to modify its opening and closing times. The response times of these means with respect to a given command and the extent of their impact on the sealing element are crucial factors in achieving the optimal operation of the inlet valve and consequently the optimal regulation of the compressor flow rate. 
     In the equipment according to the present invention, the solution is implemented by providing the sealing element translation means, in this case the pusher  502  of the valve  2  with its prongs  512  which, in a first position acts on the surface of the sealing element  302 , with actuator means operated in such a way as to enable their velocity of displacement to be controlled in both directions of their movement, with markedly reduced reaction times. In a second position, the prongs  512  engage the sealing element, keeping it off of the valve seat and thus keeping the valve open. In this case, the operation is provided by means of the two solenoids  303  and  403  which cause the displacement of the moving element  203  which is fixed to the rod  103 . The processing unit  40  detects the position of the piston  101  by means of the sensor  43  located on the shaft  20 , and then coordinates the movement of the rod  103 . As shown in the graph of  FIG. 5 , the rod  103  of the actuator, in the transition from the closed to the open state of the valve, with the moving element initially attached to the solenoid  403 , as shown in  FIG. 2 , moves fairly rapidly towards the sealing element  302 , which is already opening; its action subsequently becomes markedly slower. 
     The moving part of the pneumatic actuator and consequently the pusher of the inlet valve have a very slow movement, equal to several compression cycles, and therefore a series of impacts occurs between the pusher and the valve obturator. The high transition velocity of the electromechanical actuator makes it possible to complete the whole of the compressor&#39;s loading cycle within a limited portion of the operating cycle, thus controlling the velocity of the impact of the sealing element against the valve seat, and avoiding the series of impacts between the pusher and the sealing element. 
     Thus the regulation of the flow rate of the compressor is achieved while the stress factors causing the deterioration of the sealing element  302  are kept to a minimum; this is because the contact between its surface and the prongs  512  of the pusher  502  always occurs at very low velocities, with a reasonably low degree of impact. Furthermore, the central processing unit always has a precise confirmation of the position of the rod  103 , owing to the sensor  42 , and the signal to the solenoids  303  and  403  can therefore be suitably regulated, by means of the control and monitoring probe  45 . It should be noted that the position of the rod  103  of the actuator  3  can be regulated by means of the block  503 , and similar the distance between the solenoids  303 ,  403  can also be selected conveniently according to the travel required to actuate the pusher  502 . 
       FIG. 4  shows a variant which provides an alternative to the system regulating the position of the rod  103  described above. A chamber  933  maintains an equilibrium between the forces acting on the moving part, when a pressurized fluid is present at the end of the rod  133 ; the said chamber  933 , which is connected by means of a pipe  953  to the environment upstream of the corresponding valve, makes it possible to cancel out the effect of a variation of pressure in the environment upstream of the valve in which is immersed the terminal part of the rod  133  in contact with the pusher. Because there is a difference between the inlet diameter and the outlet diameter, providing a guaranteed cross section equal to that of the rod  133 , the resultant of the forces acting on the rod is zero. 
       FIG. 6  shows the effect of the continuous regulation on the PV diagram of the reciprocating compressor; it should be noted that keeping the inlet valve open at the start of compression reduces the flow rate of the machine (Diagram B) by comparison with the maximum flow rate operation (Diagram A). 
     With reference to the operation of a reciprocating compressor with step regulation of the “idle/load” type,  FIG. 7  shows the variation of the signal (Diagram C) obtained from the sensor  43 , the signal for switching the machine to idle (Diagram D) and the signal indicating the positions of the sealing element of the valve (Diagram E) and of the moving element (Diagram F) of the actuator  3 . 
     The moving part of the actuator starts its positioning not on the rising edge of the signal (D), but on the edge of the signal from the sensor  43  (C), in order to avoid a high contact force caused by the high internal pressure of the cylinder: in this situation, the inlet valve is already open, because the contact pressure due to the impact between the pusher and the sealing element is absent. 
     Similarly, during the return of the actuator rod, a phenomenon found in pneumatic actuators is avoided, owing to the limited return velocity: the moving part of the pneumatic actuator and consequently the pusher of the inlet valve have a very slow movement, equal to several compression cycles, and therefore a series of impacts occurs between the pusher and the sealing element of the valve. The high transition velocity of the electromechanical actuator makes it possible to complete the whole of the compressor&#39;s loading cycle within a limited portion of the operating cycle, thus controlling the speed of the impact of the sealing element against the valve seat, and avoiding the series of impacts between the pusher and the sealing element.