Title of Invention	METHOD FOR CONTROLLING THE OIL RECIRCULATION IN AN OIL-INJECTED SCREW-TYPE COMPRESSOR AND COMPRESSOR USING THIS METHOD
Abstract	This invention relates to a method for controlling the oil recirculation in an oil-injected screw-type compressor which, between the oil separator (10) and the compressor element (1), comprises an oil recirculation conduit (17) in which an oil cooler (18) is installed which is bridged-over by a passage or bypass (30), said control taking place by means of a thermostatic valve (24) having a valve element (26) which can be moved by means of a temperature-sensitive element (34). During the transition of the screw compressor from the unloaded to the loaded condition, the effect of the temperature-sensitive element (34) temporarily is switched off at least partially, such that the valve element (26) takes a position in which, regardless of the temperature of the oil, at least the bypass (30) is open and thus the recirculation of oil from the oil separator (10) to the compressor element (1) takes place through this bypass (30).

Title of Invention

METHOD FOR CONTROLLING THE OIL RECIRCULATION IN AN OIL-INJECTED SCREW-TYPE COMPRESSOR AND COMPRESSOR USING THIS METHOD

Abstract

This invention relates to a method for controlling the oil recirculation in an oil-injected screw-type compressor which, between the oil separator (10) and the compressor element (1), comprises an oil recirculation conduit (17) in which an oil cooler (18) is installed which is bridged-over by a passage or bypass (30), said control taking place by means of a thermostatic valve (24) having a valve element (26) which can be moved by means of a temperature-sensitive element (34). During the transition of the screw compressor from the unloaded to the loaded condition, the effect of the temperature-sensitive element (34) temporarily is switched off at least partially, such that the valve element (26) takes a position in which, regardless of the temperature of the oil, at least the bypass (30) is open and thus the recirculation of oil from the oil separator (10) to the compressor element (1) takes place through this bypass (30).

Full Text	METHOD FOR CONTROLLING THE OIL RECIRCULATION IN AN OIL-INJECTED SCREW-TYPE COMPRESSOR AND COMPRESSOR USING THIS METHOD This invention relates to a method for controlling the oil recirculation in an oil-injected screw-type compressor comprising a compressor element, connected thereto an inlet conduit and a pressure conduit, an oil separator in said pressure conduit, an oil recirculation conduit between said oil separator and the compressor element, in which recirculation conduit an oil cooler is installed, and a bypass bridging-over the oil cooler in the recirculation conduit, which controlling is performed by means of a thermostatic valve having a valve element which can be moved by means of a temperature-sensitive element, whereby the temperature-sensitive element measures the temperature of the recirculating oil and the valve element, if this temperature is below a certain value, opens the bypass, such that the separated oil from the oil separator can flow directly towards the compressor element without having to flow over the oil cooler and, if the temperature of the oil is above a certain value, which is higher than or egual to the aforementioned value, the valve element closes off the bypass. According to the known methods, the valve element of the thermostatic valve is in that position in which it opens the bypass, when the oil is cold, and this when the compressor is without load as well as when the compressor changes from the unloaded to the loaded status. When the oil is warmer than a well-defined temperature, then the valve element is in that position in which it closes the bypass, as a consequence of which the oil from the oil separator is forced to flow over the oil cooler before being injected back into the compressor element. When the compressor is running without load and thus no air is suctioned into the compressor element, the pressure in the oil separator, which latter also serves as a pressure vessel, is kept as-low as possible in order to limit the unloaded power consumption. When transiting into the loaded working condition, and thus when opening the inlet valve, the screw-type compressor element maximally suctions air which then is compressed. Due to the low pressure in the oil separator, the oil pressure at the beginning of the transition also is low. When the oil temperature is high, the bypass thus is closed, such that the oil flows over the oil cooler, which moreover causes a pressure drop, such that the oil injection pressure temporarily is particularly low. As a consequence, with these known methods high tempera- ture peaks may be created at the outlet of the compressor element. The pressure in the oil separator during the unlpaded operation of the compressor element and, thus, the consumed input, can not be chosen optimally low in order to prevent the occurrence of said temperature peaks. The invention aims at a method for controlling the recirculation of the oil, whereby the pressure in the oil separator, when the compressor element is working without load, can be kept lower, without the risk of temperature peaks at the outlet of this compressor element during the transition from unloaded to loaded operation. According to the invention, to this aim, during the transition from the unloaded to the loaded condition of the screw-type compressor, the influence of the temperature-sensitive element temporarily is switched off at least partially, such that the valve element temporarily takes a position in which, regardless of the temperature of the oil, at least the bypass is open and thus the recirculation of oil from the oil separator towards the compressor element temporarily takes place at least by means of this bypass. Thus, the additional pressure drop in the oil cooler temporarily is switched off, such that, notwithstanding the low pressure of the oil, there still is a sufficient injection pressure in order to avoid temperature peaks at the outlet of the compressor element. This switching-off of the influence of the temperature- sensitive element is solely of a short duration, in consideration of the fact that, under load, the pressure in the oil separator rapidly increases. When transiting from unloaded to loaded, the valve element, preferably takes a position whereby the bypass as well as the recirculation conduit are open, such that the oil temporarily can flow back to the compressor element through the bypass as well as through the oil cooler, regardless of the temperature of the oil. The temporarily, at least partially, switching-off the effect of the temperature-sensitive element can take place by realizing a part of the wall of the thermostatic valve, against which the temperature-sensitive element normally is situated, as a piston of a pneumatically controllable piston mechanism, whereby the temperature- sensitive element can expand without moving the valve element, for example, by pushing away this piston, and whereby, for example, the pressure in the oil separator and the control pressure for operating a controlled inlet valve in the inlet conduit are used as control pressures. The invention also relates to an oil-injected screw-type compressor which is suitable for being controlled according to the method described in the aforegoing. Thus, the invention also relates to an oil-injected screw-type compressor comprising a screw-type compressor element, connected thereto an inlet conduit and a pressure conduit, an oil separator in said pressure conduit, an oil recirculation conduit between said oil separator and the compressor element, in which recirculation conduit an oil cooler is arranged, and a bypass bridging-over the oil cooler in the recirculation conduit and which can be closed off by means of a valve element of a thermostatic valve with a valve element that can be moved by a temperature-sensitive element situated in the recirculation conduit, and with as a characteris- tic that the screw-type compressor comprises a control system which, when transiting from the unloaded to the loaded condition, temporarily switches off the effect of the temperature-sensitive element onto the valve element of the thermostatic valve at least partially, such that during this transition, the valve element is in a position whereby at least the bypass is open, regardless of the temperature of the oil. The bypass can be limited to a passage between a part of the recirculation conduit situated between the oil separator and the oil cooler, and a part of the recirculation conduit situated between the oil cooler and the compressor element. In a particular form of embodiment of the invention, the valve element of the thermostatic valve is situated in the bypass as well as in the recirculation conduit, upstream from the bypass, such that, in one position, it simultaneously opens the bypass and closes off the part of the recirculation conduit situated between the outlet of the oil cooler and the bypass, in another position simultaneously closes off the bypass and further opens the aforementioned part of the recirculation conduit, and preferably in the first-mentioned position and/or in an intermediate position opens the bypass as well as opens the aforementioned part of the recirculation conduit. The valve element takes up the first-mentioned position, amongst others, when, during the transition from the unloaded to the loaded condition, the working of the thermostatic valve is switched off at least partially. The aforementioned control system may comprise a piston mechanism, the piston of which, in a well-defined posi- tion, forms a stop for the temperature-sensitive element. When this piston is freely movable, then the temperature- sensitive element of the thermostatic valve can freely change its length, and the effect of this thermostatic valve thus is switched off at least partially. With the intention of better showing the characteristics of the invention, hereafter, as an example without any limitative character, a preferred form of embodiment of a method for controlling the oil recirculation in an oil- injected screw—type compressor and screw-type compressor controlled in this manner, according to the invention, is described, with reference to the accompanying drawings, wherein: Figure 1 schematically represents a screw-type compressor according to the invention, during cold starting; figure 2, in cross-section and at a larger scale, represents a practical embodiment of the part indicated by F2 in figure 1; figure 3 represents the screw-type compressor of fi- gure 1, however, during the normal regime operation, either loaded or unloaded, when the oil is warm; figure 4, in cross-section and at a larger scale, represents a practical embodiment analogous to that from figure 2, of the part indicated by F4 in figure 3; figure 5 represents the screw-type compressor during the transition from unloaded to loaded operation, when the oil still is warm; figure 6, in cross-section and at a larger scale, represents a practical embodiment analogous to that of figures 2 and 4, of the part indicated by F6 in figure 5; figure 7 represents a cross-section analogous to that from figures 2, 4 and 6, however, relating to another status of the screw-type compressor. The screw-type compressor represented in the figures comprises a compressor element 1 comprising a housing 2 surrounding a rotor chamber 3 in which two mutually cooperating screw-shaped rotors 4 are installed. The compressor element 1 is driven by a motor, not represented in the figures. At the inlet side, an inlet conduit 5 gives out in the rotor chamber 3, in which conduit 5 air filters 6 and a controlled inlet valve 7 are provided, whereas at the outlet side, a pressure conduit 8, by means of an outlet valve 9 which, for example, is a return valve, connects to the rotor chamber 3. In the pressure conduit 8, successively an oil separator 10, an air cooler 11, and a water separator 12 are arranged. In the oil separator 10, there is a vessel 13 which is provided with an outlet 14 at the top. Opposite to outlet 14, a filter 15 is installed in the vessel 13, and a minimum pressure valve 16 is installed in the outlet 14. The major part of the oil is collected in the lower part of the vessel 13, and the underside of vessel 13 is connected to an injection point of the compressor element 1 by means of a recirculation conduit 17. In this recirculation conduit 17 for the oil, successive- ly an oil cooler 18, an oil filter 19 and a controlled oil valve 20 are provided. For control, the oil valve 20, by means of a control conduit 21, is in connection with the outlet of the compressor element 1. By means of a conduit 22, the interior of the filter 15 is in connection with the interior of the rotor chamber 3 for recirculating the oil collected at the bottom of the filter 15. The oil cooler 18 and the air cooler 11 are cooled by a common fan and have radiators which are united to one single block. The oil filter 19 is provided on the housing 23 of a thermostatic valve 24. This valve 24 comprises a space 25 in which a valve element 26 is situated and a space 28 separated therefrom by a partition 27. The space 25 is in connection with the inlet of an oil filter 19 placed on the housing 23 and thus is situated in the recirculation conduit 17. This space 25 forms the connection between said oil filter 19 and the part 17B of the recirculation conduit 17 situated between the outlet of the oil cooler 18 and the housing 23. The connection of the £>art 17B tio the space 25 forms a passage 29 which can be closed off by the valve element 26. A bypass having the form of a passage 30 from the part 17C of the recirculation conduit 17, situated between the oil separator 10 and the inlet of the oil cooler 18, to the space 25 gives out into the space 25. This passage 30, too, can be closed off by the valve element 26. The bypass for the oil bridges-over the oil cooler 18, and through this bypass or passage 30, oil can flow directly from the oil separator 10 to the oil filter 19 and further to the compressor element 1 without passing through oil cooler 18. When the valve element 26 closes off the passage 30 and thus the bypass, it opens the passage 29, and reverse, when the valve element 26 opens the passage 30, it closes off the passage 29. In an intermediate position, the valve element 26 leaves open both passages 29 and 30. The space 28 is in connection with, on one hand, the outlet of the filter element of the oil filter 19 and, on the other hand, the part 17A of the recirculation conduit 17 situated between the oil filter 19 and the oil valve 20. As is represented more detailed in figures 2, 4, 6 and 7, the thermostatic valve 24 can be composed as follows: The valve element 26 is a bush which is axially movable in a bore 25A which forms part of the space 25 and into which ring-shaped chambers 31 and 32 give out, which respectively form part of the passages 29 and 30 to which the conduit parts 17B and 17C connect. The valve element 26 is provided with a slot 33 extending over a part of the circumference parallel to the chambers 31 and 32 and being smaller than the width of the chambers 31 and 32 in axial direction. A temperature-sensitive element 34 is axially installed in the valve element 26, said element 34 having a base 35 and a finger 36 moving out of it when the temperature increases. Normally, the finger 36 cooperates with a stop which is movable and which, in the represented example, is formed by a piston 37 which is situated in the prolongation of the bore 25A. This piston 37 forms part of a control system 38 which shall be described in the following. The base 35 is attached to the valve element 26 by the intermediary of a disk ring 39. A spring 40, which is provided between said disk ring 39 and a collar 25B of the wall of the bore 25A, pushes the valve element 26 into the direction of the housing 41 of the control system 38. Said piston 37 consists of a plunger 37A fitting into an opening 42 in the housing 41, and a head 37B with larger diameter situated in a chamber 43 in the housing 41. At the plunger side of the head 37B, the chamber 43, by means of a duct 44, is in connection with the atmosphere. At the other side of the head 37B, the chamber 43, by means of a duct 45, connects to a conduit 46 ending up in the vessel 13. This duct 45 can be put into connection with the atmosphere by means of an auxiliary control, formed by a relief valve 47. Said relief valve 47 comprises a valve body 48 having a hollow part provided with radial openings 49 in its wall, which, for one position of this valve body 48, connects the duct 45, through the interior of this last-mentioned valve body 48, to the atmosphere. A part of the duct 45 forms a ring-shaped duct 45A around, the bore 50 for this valve body 48, and for said position of the valve body 48, the openings 49 give out onto this ring-shaped duct 45A. Whereas the interior of the valve body 48 at one extremity, by means of a chamber 51 and a duct 52 in the housing 41, is in connection with the atmosphere, the hollow valve body 48 is closed off at the other extremity and has a piston-forming part 48A which is movable in a cylinder-forming chamber 53. The most outwardly situated extremity of this chamber 53 connects, by means of a duct 54, to a control conduit 55 which is in connection with the control conduit 55A for supplying the control pressure PI to the inlet valve 7. By means of a not represented duct, the other extremity of the chamber 53 is in connection with the atmosphere. In the chamber 51, two springs 56 and 57 are arranged which counteract the movement of the valve bodv 48 under the influence of this control pressure P1, to wit a relatively weak spring 56 between this valve body 48 and the end of a tubular element 58, and a stronger spring 47 which is provided around the tubular element 58 between a collar of the tubular element 58 and the extremity of the chamber 51. The control of the recirculation of oil from the vessel 13 to the compressor element 1 takes place as follows: When the screw-type compressor is at rest, the inlet valve 7 is closed and there is no control pressure P1. The part 48A of the valve body 48 is situated against the extremity of the chamber 53, and the openings 49 are. closed off by the housing 41. The pressure P2 in the oil separator 10 is situated minimum 0,6 bar above atmospheric pressure, such that the piston 37 is pushed into withdrawn position, whereby its end surface forming a stop for the finger 36 is situated in the plane of the end of the bore 25A, as represented in figures 2 and 4. When the oil flowing from the oil separator 10 back to the compressor element 1 has a temperature which is lower than a well-defined value, as, for example, with a first start before the compressor is put under load, then the finger 36 is slid maximally into the base 35, this is until the widened extremity of the finger 36 is situated against the base 35, as represented in figure 2. Hereby, the valve element 26 is in the position in which the passage 29 is closed off and the passage 30 is open. The oil flows from the oil separator 10, through the passage 30 and thus without being cooled in the oil cooler 18, to the compressor element 1, as represented by arrows in figures 1 and 2. When the temperature of the oil increases, then the temperature-sensitive element 34 becomes longer and the finger 36 is pushed out of the base 35, which means that, considered that the piston 37 does not change its position by the pressure P1, the base 35 is moved away from the piston 37. By means of the disk ring 39, the base 36 takes along the valve element 26, against the effect of spring 40. At a well-defined moment, this valve element 26 will leave open both passages 29 and 30. Once the oil has reached its normal operation temperature, then the finger 36 is slid out maximally, and the condition represented in figures 3 and 4 is obtained. The valve element 26 closes off the passage 30 entirely, whereas the passage 29 is maximally open. All of the oil flows back through oil cooler 18, as represented by arrows in figures 3 and 4. At the moment that the control of the compressor gives a signal for the transition from unloaded to loaded condition, in other words, when compressed air has to be delivered, the pressure P2 prevailing in the oil separator 10, by means of control conduit 55A, is immediately used as control pressure PI of the inlet valve 7. In the chamber 53, thus a control pressure Pl, prevails which is equal to the pressure P2 in the oil separator 10. This control pressure PI is sufficiently high in order to move the valve body 48 against the force, of the weakest spring 56, however, is insufficient in order to equally compress the stronger spring 57. Thereby, the valve body 48 takes a position as represented in figure 6, whereby the openings 49 give out onto the duct 45. Consequently, the chamber 43 temporarily is in connection with the atmosphere and the piston 37 in fact is free,, and the temperature-sensitive element 34 can push the piston 37 away. Under the influence of the spring 40, the valve element 26, as represented in figure 6, will be pushed against the end of bore 25A, whereby the passage 29 as well as the passage 30 are open and the oil thus can flow through the oil cooler 18 as well as through the bypass or passage 30. At that moment, the inlet valve 7 still is closed. From figure 6. it is obvious that the valve element 26 takes said position regardless whether the oil is cold or warm. When the temperature-sensitive element 34, as a result of the warm oil, has a maximum length, it simply pushes the piston 37 further into the chamber 43, as represented in figure 6. The pressure P2 in the oil separator 10 increases continuously until it is high enough to open the inlet valve 7. At this stage, the risk is the largest that temperature peaks occur in the compressor element 1 because of insufficient oil lubrication as a result of too low an oil pressure P2. Due to the fact that the oil, as represented by arrows in figure 6, can flow through the passage 30 and the chamber 25 directly to the compressor element 1, the pressure drop in the oil cooler 18 is avoided, as a result of which a higher pressure is obtained at the inlet of the oil valve 20 and whereby thus a better oil lubrication is obtained during said transition stage from unloaded to loaded operation of the screw-type compressor. After opening the inlet valve 7, the pressure P2 in the oil separator 10 and thus also the control pressure P2 increases more rapidly. When the control pressure P1 is sufficiently high, the valve body 48, against the effect of the stronger spring 57, is moved further up into the position represented in figure 7. The passages 49 then are closed off by the housing 41. The part of the chamber 43 onto which the duct 45 gives out, then no longer is in connection with the atmosphere, but is at the pressure P2. Thereby, the piston 37 is pushed into its position represented in figure 7, Whereby the plunger 37A fills the opening 42 and forms a stop in the plane of the end of the bore 25A. The pressure of the oil in the chamber 25, however, also is approximately equal to P2, however, this pressure is exerted onto a smaller surface, to wit that of the plunger 37A, than the surface of the head 37B. As the oil is at operation temperature, the finger 36 of the temperature-sensitive element 34 is maximally pushed out, as a result of which the valve element 26, against the effect of the spring 40, is brought into the position represented in figure 7. This valve element 26 then closes off the passage 30, whereas the passage 29 is open. The oil flows as is represented by arrows in figures 3 and 7, this is through the part 17C of the conduit 17 to the oil cooler 18 and from there through the part 17B and through the passage 29 to the filter 29. When the load of the compressor, which by now is warmed up, stops, then first the inlet valve 7 is closed and the control pressure PI drops below said minimum value, as a result of which the valve body 48, by the springs 56 and 57, is pushed back to the position represented in figures 3 and 4. The pressure P2 in the oil separator and thus also the pressure of the oil effecting on the piston 37, drops to a minimum value, which nevertheless still is sufficient for keeping the piston 37 pushed in, such that the condition from figure 4 is obtained and the warm oil, as represented in figure 3, must flow through oil cooler 18. When the compressor again changes from the unloaded to the loaded condition, the process described heretofore in connection with such transition is repeated. Thus, this means that with each transition from the unloaded to the loaded condition of the compressor, when the oil pressure is low, the passage 30, as represented in figure 6, is temporarily opened and thus the oil substantially can flow through the bypass formed by this passage 30 directly from the oil separator 10 to the filter 19 and from there to the oil valve 20, whereby an additional pressure drop over the oil cooler 18 is avoided. In that during the transition, as also represented in figure 6, passage 29 is open, too, the oil also will partially, however, to a lesser extent, flow through the oil cooler 18, as a result of which, at the end of said transition phase, when the passage 30 suddenly is closed off and the maximum oil flow rate must flow through the oil cooler 18, the oil flow rate through this oil cooler 18 will increase less sudden and the transition thus will take place at a steadier pace. As with each transition from an unloaded to a loaded condition, each time the oil cooler 18 is bypassed, the pressure drop in the oil is smaller, as a result of which the oil is injected into the compressor element 1 at a higher pressure and consequently a better lubrication is obtained, such that the risk of temperature peaks at the outlet of the compressor element 1 diminishes. According- to the same argumentation, it can be stated that during unloaded operation, the oil pressure in the oil separator 10 may drop lower than in a classical compressor without control system 38 according to the invention, without the risk of such damaging temperature peaks. The invention is in no way limited to the form of embodiment described in the aforegoing and represented in the accompanying drawings, however, such method for controlling the oil recirculation in an oil-injected screw-type compressor and such controlled screw—type compressor can be realized in various variants, without leaving the scope of the invention, as determined by the accompanying claims. WE CLAIM: 1. Method for controlling the oil recirculation in an oil-injected screw-type compressor wherein controlling is preformed by means of a thermostatic valve (24) having a valve element (26) which can be moved by means of a temperature-sensitive element (34), whereby the temperature-sensitive element (34) measures the temperature of the recirculating oil and the valve element (26), if this temperature is below a certain value, opens a bypass (30), such that the separated oil from an oil separator (10) can flow directly towards a compressor element (1) without having to flow over an oil cooler (18) and, if the temperature of the oil is above a certain value, which is higher than or equal to the aforementioned value, the valve element (26) closes off the bypass (30), characterized in that during the transition of the screw compressor from the unloaded to the loaded condition, the effect of the temperature-sensitive element (34) temporarily is switched off at lest partially, such that the valve element (26) temporarily takes a position in which, regardless of the temperature of the oil, at least the bypass (30) is open and thus the recirculation of oil from the oil separator (10) to the compressor element (1) temporarily takes place at least through this bypass (30). 2. Method as claimed in claim 1, wherein during the transition from unloaded to loaded, the valve element (26) temporarily takes a position whereby the bypass (30) as well as the recirculation conduit (17) are open, such that the oil temporarily can flow through the bypass (30) as well as through the oil cooler (18) back to the compressor element (1), regardless of the temperature of the oil. 3. Method as claimed in claim 1 or 2, wherein the temporarily switching off of the effect of the temperature-sensitive element (34) takes place by realizing a part of the wall of the thermostatic valve (24), against which the temperature-sensitive element (34) normally is situated, as a piston (37) of a pneumatically controllable piston mechanism. 4. Method as claimed in any of the preceding claims, wherein as control pressures, the pressure (P2) in the oil separator (10) and the control pressure (P1) for operating a controlled inlet valve (7) in the inlet conduit (5) are used. 5. Method as claimed in claims 3 and 4, wherein the pressure (P2) prevailing in the oil separator (10) is exerted onto a head (37B) of the piston (37), whereas the oil pressure itself is exerted onto an end of the piston (37) with a smaller surface, said end forming a plunger (37A), and the side along which the first- mentioned pressure (P2) is exerted, can be put into connection with the atmosphere by means of an outlet (52) which is controlled by a valve body (48) which in its turn is controlled by the controlled by the control pressure (PI) of a controlled inlet valve (7). 6. Oil-injected screw-type compressor for controlling the oil recirculation in an oil-injected screw-type compressor according to the method as claimed in previous claims comprising a screw-type compressor element (1), connected thereto an inlet conduit (5) and a pressure conduit (8), an oil separator (10) in said pressure conduit (8), an oil recirculation conduit (17) between said oil separator (10) and the compressor element (1), in which recirculation conduit (17) an oil cooler (18) is installed, and a bypass (30) bridging-over the oil cooler (18) in the recirculation conduit (17) and which can be closed off by the valve element (26) of the thermostatic valve (24) with a valve element (26) which can be moved by means of a temperature- sensitive element (34) situated in the oil recirculation conduit (17), wherein the screw-type compressor comprises a control system (38) which, during the transition from the unloaded to the loaded condition, temporarily switches off the effect of the temperature-sensitive element (34) onto the valve element (26) of the thermostatic valve (24) at least partially, such that during this transition, the valve element (26) is in a position in which at least the bypass (30) is open, regardless of the temperature of the oil. 7. Screw-type compressor as claimed in claim 6 wherein the bypass (30) is limited to a passage (30) between a part (17C) of the recirculation conduit (17) situated between the oil separator (10) and the oil cooler (18), and a part (17B) of the recirculation conduit (17) situated between the oil cooler (18) and the compressor element (1). 8. Screw-type compressor as claimed in claim 6 or 7, wherein the valve element (26) of the thermostatic valve (24) is situated in the bypass (30) as well as in the recirculation conduit (17) upstream from the bypass (30), such that, in one position, it simultaneously opens the bypass (30) and closes off the part (17B) of the recirculation conduit (17) situated between the outlet of the oil cooler (18) and the bypass (30), and in another position simultaneously closes off the bypass (30) and opens said part (17B) of the recirculation conduit (17). 9. Screw-type compressor as claimed in claim 8, wherein the valve element (26) in the first-mentioned position and/or in an intermediary position opens the bypass (30) as well as said part (17B) of the recirculation conduit (17). 10. Screw-type compressor as claimed in claim 7, wherein the thermostatic valve (24) comprises a housing (23) with a space a (25) inside, in which space a valve element (26) is movable, and that the passage (30) is an opening giving out onto this space (25). 11. Screw-type compressor as claimed in claim 10, wherein it comprises an oil filter (19) which is installed in the recirculation conduit (17), between the bypass (30) and the compressor element (1), and the space (25) is in connection with the inlet of the oil filter (19). 12. Screw-type compressor as claimed in any of the claims 6 to 11, wherein the control system (38) comprises a piston (37) which is movable in a chamber (43) and in one position forms a stop for a temperature-sensitive element (34) of the thermostatic valve (24). 13. Screw-type compressor as claimed in claim 12, wherein the chamber (43), at one side of the piston (37), is in connection with the oil separator (10), such that the piston (37) can be maintained in said position by the pressure (P2) in this oil separator (10), and the control system (38) comprises an auxiliary control in the form of a relief valve (47) putting the chamber (43) at said side in connection with the atmosphere when a control pressure (P1) is situated between two well- defined values. 14. Screw-type compressor as claimed in claim 9, wherein the relief valve (47) is controlled by the control pressure (PI) of the inlet valve (7). 15; Screw-type compressor as claimed in claim 13 or 14, wherein the relief valve (47) comprises a valve body (48) with a hollow part giving out to the atmosphere and which in its wall is provided with at least one opening (49) which, for a certain position of the valve body (48), gives out onto a duct (45) by which the chamber (43) is in connection with the oil separator (10). 16. Screw-type compressor as claimed in claim 14 and 15, wherein at one extremity, the valve body (48) has a piston-forming part (48A) which is movable in a chamber (53) which is in connection with a part of the compressor where the control pressure (PI) for opening the inlet valve (7) is prevailing. 17. Screw-type compressor as claimed in claim 16, wherein the other extremity of the valve body (48) cooperates with two springs (56, 57), whereby the one (57) thereof is stronger than the other and only can be compressed by the valve body (48) after the other (56) has been partially compressed. This invention relates to a method for controlling the oil recirculation in an oil-injected screw-type compressor which, between the oil separator (10) and the compressor element (1), comprises an oil recirculation conduit (17) in which an oil cooler (18) is installed which is bridged-over by a passage or bypass (30), said control taking place by means of a thermostatic valve (24) having a valve element (26) which can be moved by means of a temperature-sensitive element (34). During the transition of the screw compressor from the unloaded to the loaded condition, the effect of the temperature-sensitive element (34) temporarily is switched off at least partially, such that the valve element (26) takes a position in which, regardless of the temperature of the oil, at least the bypass (30) is open and thus the recirculation of oil from the oil separator (10) to the compressor element (1) takes place through this bypass (30).

Full Text

METHOD FOR CONTROLLING THE OIL RECIRCULATION IN AN
OIL-INJECTED SCREW-TYPE COMPRESSOR AND COMPRESSOR USING
THIS METHOD
This invention relates to a method for controlling the
oil recirculation in an oil-injected screw-type
compressor comprising a compressor element, connected
thereto an inlet conduit and a pressure conduit, an oil
separator in said pressure conduit, an oil recirculation
conduit between said oil separator and the compressor
element, in which recirculation conduit an oil cooler is
installed, and a bypass bridging-over the oil cooler in
the recirculation conduit, which controlling is performed
by means of a thermostatic valve having a valve element
which can be moved by means of a temperature-sensitive
element, whereby the temperature-sensitive element
measures the temperature of the recirculating oil and the
valve element, if this temperature is below a certain
value, opens the bypass, such that the separated oil from
the oil separator can flow directly towards the
compressor element without having to flow over the oil
cooler and, if the temperature of the oil is above a
certain value, which is higher than or egual to the
aforementioned value, the valve element closes off the
bypass.
According to the known methods, the valve element of the
thermostatic valve is in that position in which it opens
the bypass, when the oil is cold, and this when the
compressor is without load as well as when the compressor
changes from the unloaded to the loaded status.
When the oil is warmer than a well-defined temperature,
then the valve element is in that position in which it
closes the bypass, as a consequence of which the oil from

the oil separator is forced to flow over the oil cooler
before being injected back into the compressor element.
When the compressor is running without load and thus no
air is suctioned into the compressor element, the
pressure in the oil separator, which latter also serves
as a pressure vessel, is kept as-low as possible in order
to limit the unloaded power consumption.
When transiting into the loaded working condition, and
thus when opening the inlet valve, the screw-type
compressor element maximally suctions air which then is
compressed. Due to the low pressure in the oil separator,
the oil pressure at the beginning of the transition also
is low.
When the oil temperature is high, the bypass thus is
closed, such that the oil flows over the oil cooler,
which moreover causes a pressure drop, such that the oil
injection pressure temporarily is particularly low.
As a consequence, with these known methods high tempera-
ture peaks may be created at the outlet of the compressor
element.
The pressure in the oil separator during the unlpaded
operation of the compressor element and, thus, the
consumed input, can not be chosen optimally low in order
to prevent the occurrence of said temperature peaks.
The invention aims at a method for controlling the
recirculation of the oil, whereby the pressure in the oil
separator, when the compressor element is working without
load, can be kept lower, without the risk of temperature
peaks at the outlet of this compressor element during the
transition from unloaded to loaded operation.

According to the invention, to this aim, during the
transition from the unloaded to the loaded condition of
the screw-type compressor, the influence of the
temperature-sensitive element temporarily is switched off
at least partially, such that the valve element
temporarily takes a position in which, regardless of the
temperature of the oil, at least the bypass is open and
thus the recirculation of oil from the oil separator
towards the compressor element temporarily takes place at
least by means of this bypass.
Thus, the additional pressure drop in the oil cooler
temporarily is switched off, such that, notwithstanding
the low pressure of the oil, there still is a sufficient
injection pressure in order to avoid temperature peaks at
the outlet of the compressor element.
This switching-off of the influence of the temperature-
sensitive element is solely of a short duration, in
consideration of the fact that, under load, the pressure
in the oil separator rapidly increases.
When transiting from unloaded to loaded, the valve
element, preferably takes a position whereby the bypass as
well as the recirculation conduit are open, such that the
oil temporarily can flow back to the compressor element
through the bypass as well as through the oil cooler,
regardless of the temperature of the oil.
The temporarily, at least partially, switching-off the
effect of the temperature-sensitive element can take
place by realizing a part of the wall of the thermostatic
valve, against which the temperature-sensitive element
normally is situated, as a piston of a pneumatically
controllable piston mechanism, whereby the temperature-
sensitive element can expand without moving the valve

element, for example, by pushing away this piston, and
whereby, for example, the pressure in the oil separator
and the control pressure for operating a controlled inlet
valve in the inlet conduit are used as control pressures.
The invention also relates to an oil-injected screw-type
compressor which is suitable for being controlled
according to the method described in the aforegoing.
Thus, the invention also relates to an oil-injected
screw-type compressor comprising a screw-type compressor
element, connected thereto an inlet conduit and a
pressure conduit, an oil separator in said pressure
conduit, an oil recirculation conduit between said oil
separator and the compressor element, in which
recirculation conduit an oil cooler is arranged, and a
bypass bridging-over the oil cooler in the recirculation
conduit and which can be closed off by means of a valve
element of a thermostatic valve with a valve element that
can be moved by a temperature-sensitive element situated
in the recirculation conduit, and with as a characteris-
tic that the screw-type compressor comprises a control
system which, when transiting from the unloaded to the
loaded condition, temporarily switches off the effect of
the temperature-sensitive element onto the valve element
of the thermostatic valve at least partially, such that
during this transition, the valve element is in a
position whereby at least the bypass is open, regardless
of the temperature of the oil.
The bypass can be limited to a passage between a part of
the recirculation conduit situated between the oil
separator and the oil cooler, and a part of the
recirculation conduit situated between the oil cooler and
the compressor element.

In a particular form of embodiment of the invention, the
valve element of the thermostatic valve is situated in
the bypass as well as in the recirculation conduit,
upstream from the bypass, such that, in one position, it
simultaneously opens the bypass and closes off the part
of the recirculation conduit situated between the outlet
of the oil cooler and the bypass, in another position
simultaneously closes off the bypass and further opens
the aforementioned part of the recirculation conduit, and
preferably in the first-mentioned position and/or in an
intermediate position opens the bypass as well as opens
the aforementioned part of the recirculation conduit.
The valve element takes up the first-mentioned position,
amongst others, when, during the transition from the
unloaded to the loaded condition, the working of the
thermostatic valve is switched off at least partially.
The aforementioned control system may comprise a piston
mechanism, the piston of which, in a well-defined posi-
tion, forms a stop for the temperature-sensitive element.
When this piston is freely movable, then the temperature-
sensitive element of the thermostatic valve can freely
change its length, and the effect of this thermostatic
valve thus is switched off at least partially.
With the intention of better showing the characteristics
of the invention, hereafter, as an example without any
limitative character, a preferred form of embodiment of a
method for controlling the oil recirculation in an oil-
injected screw—type compressor and screw-type compressor
controlled in this manner, according to the invention, is
described, with reference to the accompanying drawings,
wherein:
Figure 1 schematically represents a screw-type

compressor according to the invention, during cold
starting;
figure 2, in cross-section and at a larger scale,
represents a practical embodiment of the part
indicated by F2 in figure 1;
figure 3 represents the screw-type compressor of fi-
gure 1, however, during the normal regime operation,
either loaded or unloaded, when the oil is warm;
figure 4, in cross-section and at a larger scale,
represents a practical embodiment analogous to that
from figure 2, of the part indicated by F4 in figure
3;
figure 5 represents the screw-type compressor during
the transition from unloaded to loaded operation,
when the oil still is warm;
figure 6, in cross-section and at a larger scale,
represents a practical embodiment analogous to that
of figures 2 and 4, of the part indicated by F6 in
figure 5;
figure 7 represents a cross-section analogous to that
from figures 2, 4 and 6, however, relating to another
status of the screw-type compressor.
The screw-type compressor represented in the figures
comprises a compressor element 1 comprising a housing 2
surrounding a rotor chamber 3 in which two mutually
cooperating screw-shaped rotors 4 are installed. The
compressor element 1 is driven by a motor, not
represented in the figures.
At the inlet side, an inlet conduit 5 gives out in the
rotor chamber 3, in which conduit 5 air filters 6 and a
controlled inlet valve 7 are provided, whereas at the
outlet side, a pressure conduit 8, by means of an outlet
valve 9 which, for example, is a return valve, connects
to the rotor chamber 3.

In the pressure conduit 8, successively an oil separator
10, an air cooler 11, and a water separator 12 are
arranged.
In the oil separator 10, there is a vessel 13 which is
provided with an outlet 14 at the top. Opposite to outlet
14, a filter 15 is installed in the vessel 13, and a
minimum pressure valve 16 is installed in the outlet 14.
The major part of the oil is collected in the lower part
of the vessel 13, and the underside of vessel 13 is
connected to an injection point of the compressor element
1 by means of a recirculation conduit 17.
In this recirculation conduit 17 for the oil, successive-
ly an oil cooler 18, an oil filter 19 and a controlled
oil valve 20 are provided.
For control, the oil valve 20, by means of a control
conduit 21, is in connection with the outlet of the
compressor element 1.
By means of a conduit 22, the interior of the filter 15
is in connection with the interior of the rotor chamber 3
for recirculating the oil collected at the bottom of the
filter 15.
The oil cooler 18 and the air cooler 11 are cooled by a
common fan and have radiators which are united to one
single block.
The oil filter 19 is provided on the housing 23 of a
thermostatic valve 24. This valve 24 comprises a space 25
in which a valve element 26 is situated and a space 28
separated therefrom by a partition 27.

The space 25 is in connection with the inlet of an oil
filter 19 placed on the housing 23 and thus is situated
in the recirculation conduit 17. This space 25 forms the
connection between said oil filter 19 and the part 17B of
the recirculation conduit 17 situated between the outlet
of the oil cooler 18 and the housing 23. The connection
of the £>art 17B tio the space 25 forms a passage 29 which
can be closed off by the valve element 26.
A bypass having the form of a passage 30 from the part
17C of the recirculation conduit 17, situated between the
oil separator 10 and the inlet of the oil cooler 18, to
the space 25 gives out into the space 25. This passage
30, too, can be closed off by the valve element 26.
The bypass for the oil bridges-over the oil cooler 18,
and through this bypass or passage 30, oil can flow
directly from the oil separator 10 to the oil filter 19
and further to the compressor element 1 without passing
through oil cooler 18.
When the valve element 26 closes off the passage 30 and
thus the bypass, it opens the passage 29, and reverse,
when the valve element 26 opens the passage 30, it
closes off the passage 29. In an intermediate position,
the valve element 26 leaves open both passages 29 and 30.
The space 28 is in connection with, on one hand, the
outlet of the filter element of the oil filter 19 and, on
the other hand, the part 17A of the recirculation conduit
17 situated between the oil filter 19 and the oil valve
20.
As is represented more detailed in figures 2, 4, 6 and 7,
the thermostatic valve 24 can be composed as follows:

The valve element 26 is a bush which is axially movable
in a bore 25A which forms part of the space 25 and into
which ring-shaped chambers 31 and 32 give out, which
respectively form part of the passages 29 and 30 to which
the conduit parts 17B and 17C connect.
The valve element 26 is provided with a slot 33 extending
over a part of the circumference parallel to the chambers
31 and 32 and being smaller than the width of the
chambers 31 and 32 in axial direction.
A temperature-sensitive element 34 is axially installed
in the valve element 26, said element 34 having a base 35
and a finger 36 moving out of it when the temperature
increases.
Normally, the finger 36 cooperates with a stop which is
movable and which, in the represented example, is formed
by a piston 37 which is situated in the prolongation of
the bore 25A.
This piston 37 forms part of a control system 38 which
shall be described in the following.
The base 35 is attached to the valve element 26 by the
intermediary of a disk ring 39.
A spring 40, which is provided between said disk ring 39
and a collar 25B of the wall of the bore 25A, pushes the
valve element 26 into the direction of the housing 41 of
the control system 38.
Said piston 37 consists of a plunger 37A fitting into an
opening 42 in the housing 41, and a head 37B with larger
diameter situated in a chamber 43 in the housing 41.

At the plunger side of the head 37B, the chamber 43, by
means of a duct 44, is in connection with the atmosphere.
At the other side of the head 37B, the chamber 43, by
means of a duct 45, connects to a conduit 46 ending up in
the vessel 13.
This duct 45 can be put into connection with the
atmosphere by means of an auxiliary control, formed by a
relief valve 47. Said relief valve 47 comprises a valve
body 48 having a hollow part provided with radial
openings 49 in its wall, which, for one position of this
valve body 48, connects the duct 45, through the interior
of this last-mentioned valve body 48, to the atmosphere.
A part of the duct 45 forms a ring-shaped duct 45A around,
the bore 50 for this valve body 48, and for said position
of the valve body 48, the openings 49 give out onto this
ring-shaped duct 45A.
Whereas the interior of the valve body 48 at one
extremity, by means of a chamber 51 and a duct 52 in the
housing 41, is in connection with the atmosphere, the
hollow valve body 48 is closed off at the other extremity
and has a piston-forming part 48A which is movable in a
cylinder-forming chamber 53.
The most outwardly situated extremity of this chamber 53
connects, by means of a duct 54, to a control conduit 55
which is in connection with the control conduit 55A for
supplying the control pressure PI to the inlet valve 7.
By means of a not represented duct, the other extremity
of the chamber 53 is in connection with the atmosphere.
In the chamber 51, two springs 56 and 57 are arranged
which counteract the movement of the valve bodv 48 under

the influence of this control pressure P1, to wit a
relatively weak spring 56 between this valve body 48 and
the end of a tubular element 58, and a stronger spring 47
which is provided around the tubular element 58 between a
collar of the tubular element 58 and the extremity of the
chamber 51.
The control of the recirculation of oil from the vessel
13 to the compressor element 1 takes place as follows:
When the screw-type compressor is at rest, the inlet
valve 7 is closed and there is no control pressure P1.
The part 48A of the valve body 48 is situated against the
extremity of the chamber 53, and the openings 49 are.
closed off by the housing 41.
The pressure P2 in the oil separator 10 is situated
minimum 0,6 bar above atmospheric pressure, such that the
piston 37 is pushed into withdrawn position, whereby its
end surface forming a stop for the finger 36 is situated
in the plane of the end of the bore 25A, as represented
in figures 2 and 4.
When the oil flowing from the oil separator 10 back to
the compressor element 1 has a temperature which is lower
than a well-defined value, as, for example, with a first
start before the compressor is put under load, then the
finger 36 is slid maximally into the base 35, this is
until the widened extremity of the finger 36 is situated
against the base 35, as represented in figure 2. Hereby,
the valve element 26 is in the position in which the
passage 29 is closed off and the passage 30 is open.
The oil flows from the oil separator 10, through the
passage 30 and thus without being cooled in the oil
cooler 18, to the compressor element 1, as represented by

arrows in figures 1 and 2.
When the temperature of the oil increases, then the
temperature-sensitive element 34 becomes longer and the
finger 36 is pushed out of the base 35, which means that,
considered that the piston 37 does not change its
position by the pressure P1, the base 35 is moved away
from the piston 37. By means of the disk ring 39, the
base 36 takes along the valve element 26, against the
effect of spring 40. At a well-defined moment, this valve
element 26 will leave open both passages 29 and 30.
Once the oil has reached its normal operation
temperature, then the finger 36 is slid out maximally,
and the condition represented in figures 3 and 4 is
obtained. The valve element 26 closes off the passage 30
entirely, whereas the passage 29 is maximally open. All
of the oil flows back through oil cooler 18, as
represented by arrows in figures 3 and 4.
At the moment that the control of the compressor gives a
signal for the transition from unloaded to loaded
condition, in other words, when compressed air has to be
delivered, the pressure P2 prevailing in the oil
separator 10, by means of control conduit 55A, is
immediately used as control pressure PI of the inlet
valve 7. In the chamber 53, thus a control pressure Pl,
prevails which is equal to the pressure P2 in the oil
separator 10. This control pressure PI is sufficiently
high in order to move the valve body 48 against the force,
of the weakest spring 56, however, is insufficient in
order to equally compress the stronger spring 57.
Thereby, the valve body 48 takes a position as
represented in figure 6, whereby the openings 49 give out
onto the duct 45.

Consequently, the chamber 43 temporarily is in connection
with the atmosphere and the piston 37 in fact is free,,
and the temperature-sensitive element 34 can push the
piston 37 away. Under the influence of the spring 40, the
valve element 26, as represented in figure 6, will be
pushed against the end of bore 25A, whereby the passage
29 as well as the passage 30 are open and the oil thus
can flow through the oil cooler 18 as well as through the
bypass or passage 30. At that moment, the inlet valve 7
still is closed.
From figure 6. it is obvious that the valve element 26
takes said position regardless whether the oil is cold or
warm. When the temperature-sensitive element 34, as a
result of the warm oil, has a maximum length, it simply
pushes the piston 37 further into the chamber 43, as
represented in figure 6.
The pressure P2 in the oil separator 10 increases
continuously until it is high enough to open the inlet
valve 7. At this stage, the risk is the largest that
temperature peaks occur in the compressor element 1
because of insufficient oil lubrication as a result of
too low an oil pressure P2. Due to the fact that the oil,
as represented by arrows in figure 6, can flow through
the passage 30 and the chamber 25 directly to the
compressor element 1, the pressure drop in the oil cooler
18 is avoided, as a result of which a higher pressure is
obtained at the inlet of the oil valve 20 and whereby
thus a better oil lubrication is obtained during said
transition stage from unloaded to loaded operation of the
screw-type compressor.
After opening the inlet valve 7, the pressure P2 in the
oil separator 10 and thus also the control pressure P2
increases more rapidly. When the control pressure P1 is

sufficiently high, the valve body 48, against the effect
of the stronger spring 57, is moved further up into the
position represented in figure 7. The passages 49 then
are closed off by the housing 41.
The part of the chamber 43 onto which the duct 45 gives
out, then no longer is in connection with the atmosphere,
but is at the pressure P2.
Thereby, the piston 37 is pushed into its position
represented in figure 7, Whereby the plunger 37A fills
the opening 42 and forms a stop in the plane of the end
of the bore 25A.
The pressure of the oil in the chamber 25, however, also
is approximately equal to P2, however, this pressure is
exerted onto a smaller surface, to wit that of the
plunger 37A, than the surface of the head 37B.
As the oil is at operation temperature, the finger 36 of
the temperature-sensitive element 34 is maximally pushed
out, as a result of which the valve element 26, against
the effect of the spring 40, is brought into the position
represented in figure 7.
This valve element 26 then closes off the passage 30,
whereas the passage 29 is open. The oil flows as is
represented by arrows in figures 3 and 7, this is through
the part 17C of the conduit 17 to the oil cooler 18 and
from there through the part 17B and through the passage
29 to the filter 29.
When the load of the compressor, which by now is warmed
up, stops, then first the inlet valve 7 is closed and the
control pressure PI drops below said minimum value, as a
result of which the valve body 48, by the springs 56 and

57, is pushed back to the position represented in figures
3 and 4.
The pressure P2 in the oil separator and thus also the
pressure of the oil effecting on the piston 37, drops to
a minimum value, which nevertheless still is sufficient
for keeping the piston 37 pushed in, such that the
condition from figure 4 is obtained and the warm oil, as
represented in figure 3, must flow through oil cooler 18.
When the compressor again changes from the unloaded to
the loaded condition, the process described heretofore in
connection with such transition is repeated.
Thus, this means that with each transition from the
unloaded to the loaded condition of the compressor, when
the oil pressure is low, the passage 30, as represented
in figure 6, is temporarily opened and thus the oil
substantially can flow through the bypass formed by this
passage 30 directly from the oil separator 10 to the
filter 19 and from there to the oil valve 20, whereby an
additional pressure drop over the oil cooler 18 is
avoided.
In that during the transition, as also represented in
figure 6, passage 29 is open, too, the oil also will
partially, however, to a lesser extent, flow through the
oil cooler 18, as a result of which, at the end of said
transition phase, when the passage 30 suddenly is closed
off and the maximum oil flow rate must flow through the
oil cooler 18, the oil flow rate through this oil cooler
18 will increase less sudden and the transition thus will
take place at a steadier pace.
As with each transition from an unloaded to a loaded
condition, each time the oil cooler 18 is bypassed, the

pressure drop in the oil is smaller, as a result of which
the oil is injected into the compressor element 1 at a
higher pressure and consequently a better lubrication is
obtained, such that the risk of temperature peaks at the
outlet of the compressor element 1 diminishes.
According- to the same argumentation, it can be stated
that during unloaded operation, the oil pressure in the
oil separator 10 may drop lower than in a classical
compressor without control system 38 according to the
invention, without the risk of such damaging temperature
peaks.
The invention is in no way limited to the form of
embodiment described in the aforegoing and represented in
the accompanying drawings, however, such method for
controlling the oil recirculation in an oil-injected
screw-type compressor and such controlled screw—type
compressor can be realized in various variants, without
leaving the scope of the invention, as determined by the
accompanying claims.

WE CLAIM:
1. Method for controlling the oil recirculation in an oil-injected
screw-type compressor wherein controlling is preformed by
means of a thermostatic valve (24) having a valve element (26)
which can be moved by means of a temperature-sensitive
element (34), whereby the temperature-sensitive element (34)
measures the temperature of the recirculating oil and the valve
element (26), if this temperature is below a certain value, opens
a bypass (30), such that the separated oil from an oil separator
(10) can flow directly towards a compressor element (1) without
having to flow over an oil cooler (18) and, if the temperature of
the oil is above a certain value, which is higher than or equal to
the aforementioned value, the valve element (26) closes off the
bypass (30), characterized in that during the transition of the
screw compressor from the unloaded to the loaded condition,
the effect of the temperature-sensitive element (34) temporarily
is switched off at lest partially, such that the valve element (26)
temporarily takes a position in which, regardless of the
temperature of the oil, at least the bypass (30) is open and thus
the recirculation of oil from the oil separator (10) to the
compressor element (1) temporarily takes place at least through
this bypass (30).
2. Method as claimed in claim 1, wherein during the transition
from unloaded to loaded, the valve element (26) temporarily
takes a position whereby the bypass (30) as well as the
recirculation conduit (17) are open, such that the oil
temporarily can flow through the bypass (30) as well as through
the oil cooler (18) back to the compressor element (1),
regardless of the temperature of the oil.

3. Method as claimed in claim 1 or 2, wherein the temporarily
switching off of the effect of the temperature-sensitive element
(34) takes place by realizing a part of the wall of the
thermostatic valve (24), against which the temperature-sensitive
element (34) normally is situated, as a piston (37) of a
pneumatically controllable piston mechanism.
4. Method as claimed in any of the preceding claims, wherein as
control pressures, the pressure (P2) in the oil separator (10) and
the control pressure (P1) for operating a controlled inlet valve (7)
in the inlet conduit (5) are used.
5. Method as claimed in claims 3 and 4, wherein the pressure (P2)
prevailing in the oil separator (10) is exerted onto a head (37B)
of the piston (37), whereas the oil pressure itself is exerted onto
an end of the piston (37) with a smaller surface, said end
forming a plunger (37A), and the side along which the first-
mentioned pressure (P2) is exerted, can be put into connection
with the atmosphere by means of an outlet (52) which is
controlled by a valve body (48) which in its turn is controlled by
the controlled by the control pressure (PI) of a controlled inlet
valve (7).
6. Oil-injected screw-type compressor for controlling the oil
recirculation in an oil-injected screw-type compressor according
to the method as claimed in previous claims comprising a
screw-type compressor element (1), connected thereto an inlet
conduit (5) and a pressure conduit (8), an oil separator (10) in
said pressure conduit (8), an oil recirculation conduit (17)
between said oil separator (10) and the compressor element (1),

in which recirculation conduit (17) an oil cooler (18) is installed,
and a bypass (30) bridging-over the oil cooler (18) in the
recirculation conduit (17) and which can be closed off by the
valve element (26) of the thermostatic valve (24) with a valve
element (26) which can be moved by means of a temperature-
sensitive element (34) situated in the oil recirculation conduit
(17), wherein the screw-type compressor comprises a control
system (38) which, during the transition from the unloaded to
the loaded condition, temporarily switches off the effect of the
temperature-sensitive element (34) onto the valve element (26)
of the thermostatic valve (24) at least partially, such that during
this transition, the valve element (26) is in a position in which
at least the bypass (30) is open, regardless of the temperature of
the oil.
7. Screw-type compressor as claimed in claim 6 wherein the
bypass (30) is limited to a passage (30) between a part (17C) of
the recirculation conduit (17) situated between the oil separator
(10) and the oil cooler (18), and a part (17B) of the recirculation
conduit (17) situated between the oil cooler (18) and the
compressor element (1).
8. Screw-type compressor as claimed in claim 6 or 7, wherein the
valve element (26) of the thermostatic valve (24) is situated in
the bypass (30) as well as in the recirculation conduit (17)
upstream from the bypass (30), such that, in one position, it
simultaneously opens the bypass (30) and closes off the part
(17B) of the recirculation conduit (17) situated between the
outlet of the oil cooler (18) and the bypass (30), and in another
position simultaneously closes off the bypass (30) and opens
said part (17B) of the recirculation conduit (17).

9. Screw-type compressor as claimed in claim 8, wherein the valve
element (26) in the first-mentioned position and/or in an
intermediary position opens the bypass (30) as well as said part
(17B) of the recirculation conduit (17).
10. Screw-type compressor as claimed in claim 7, wherein the
thermostatic valve (24) comprises a housing (23) with a space a
(25) inside, in which space a valve element (26) is movable, and
that the passage (30) is an opening giving out onto this space
(25).
11. Screw-type compressor as claimed in claim 10, wherein it
comprises an oil filter (19) which is installed in the recirculation
conduit (17), between the bypass (30) and the compressor
element (1), and the space (25) is in connection with the inlet of
the oil filter (19).
12. Screw-type compressor as claimed in any of the claims 6 to 11,
wherein the control system (38) comprises a piston (37) which is
movable in a chamber (43) and in one position forms a stop for
a temperature-sensitive element (34) of the thermostatic valve
(24).
13. Screw-type compressor as claimed in claim 12, wherein the
chamber (43), at one side of the piston (37), is in connection
with the oil separator (10), such that the piston (37) can be
maintained in said position by the pressure (P2) in this oil
separator (10), and the control system (38) comprises an
auxiliary control in the form of a relief valve (47) putting the

chamber (43) at said side in connection with the atmosphere
when a control pressure (P1) is situated between two well-
defined values.
14. Screw-type compressor as claimed in claim 9, wherein the relief
valve (47) is controlled by the control pressure (PI) of the inlet
valve (7).
15; Screw-type compressor as claimed in claim 13 or 14, wherein
the relief valve (47) comprises a valve body (48) with a hollow
part giving out to the atmosphere and which in its wall is
provided with at least one opening (49) which, for a certain
position of the valve body (48), gives out onto a duct (45) by
which the chamber (43) is in connection with the oil separator
(10).
16. Screw-type compressor as claimed in claim 14 and 15, wherein
at one extremity, the valve body (48) has a piston-forming part
(48A) which is movable in a chamber (53) which is in
connection with a part of the compressor where the control
pressure (PI) for opening the inlet valve (7) is prevailing.
17. Screw-type compressor as claimed in claim 16, wherein the
other extremity of the valve body (48) cooperates with two
springs (56, 57), whereby the one (57) thereof is stronger than
the other and only can be compressed by the valve body (48)
after the other (56) has been partially compressed.

This invention relates to a method for controlling the oil recirculation in
an oil-injected screw-type compressor which, between the oil separator
(10) and the compressor element (1), comprises an oil recirculation
conduit (17) in which an oil cooler (18) is installed which is bridged-over
by a passage or bypass (30), said control taking place by means of a
thermostatic valve (24) having a valve element (26) which can be moved
by means of a temperature-sensitive element (34). During the transition
of the screw compressor from the unloaded to the loaded condition, the
effect of the temperature-sensitive element (34) temporarily is switched
off at least partially, such that the valve element (26) takes a position in
which, regardless of the temperature of the oil, at least the bypass (30) is
open and thus the recirculation of oil from the oil separator (10) to the
compressor element (1) takes place through this bypass (30).

Documents:

974-kolnp-2004-granted-abstract.pdf

974-kolnp-2004-granted-claims.pdf

974-kolnp-2004-granted-correspondence.pdf

974-kolnp-2004-granted-description (complete).pdf

974-kolnp-2004-granted-drawings.pdf

974-kolnp-2004-granted-examination report.pdf

974-kolnp-2004-granted-form 1.pdf

974-kolnp-2004-granted-form 18.pdf

974-kolnp-2004-granted-form 2.pdf

974-kolnp-2004-granted-form 5.pdf

974-kolnp-2004-granted-pa.pdf

974-kolnp-2004-granted-reply to examination report.pdf

974-kolnp-2004-granted-specification.pdf

974-kolnp-2004-granted-translated copy of priority document.pdf

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Patent Number

228104

Indian Patent Application Number

974/KOLNP/2004

PG Journal Number

05/2009

Publication Date

30-Jan-2009

Grant Date

28-Jan-2009

Date of Filing

12-Jul-2004

Name of Patentee

ATLAS COPCO AIRPOWER, NAAMLOZE VEN-NOOTSCHAP

Applicant Address

BOOMSESTEENWEG 957, B-2610 WILRIJK

Inventors:

#	Inventor's Name	Inventor's Address
1	TRUYENS, FRANCOIS, LOUIS, JOSEPHINE	VLINDERSTRAAT 11, B-2880 BORNEM
2	NOUKENS, IVAN, ALICE, DANIEL	LEGERSTRAAT 219, B-2610 WILRIJK

PCT International Classification Number

F04C 29/02

PCT International Application Number

PCT/BE03/00013

PCT International Filing date

2003-01-30

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	20020077	2002-02-08	Belgium