CN111794969A - Unloading control method of screw compressor - Google Patents

Abstract

The invention belongs to the technical field of compressors, and aims to solve the problems that the existing screw compressors often have problems such as shutdown and alarm, which affect their normal operation. To this end, the present invention provides an unloading control method for a screw compressor, the unloading control method includes: when the screw compressor starts to unload from a fully loaded state, controlling the oil supply valve to stop the oil supply from the oil supply pipe and controlling the third Drain valve to make the third oil drain pipe drain oil and maintain the set time, so that the piston moves from the full load position to the position of the second oil drain port; when the piston moves to the position of the second oil drain port, control the oil supply valve In order to restore oil supply to the oil supply pipe and control the third oil drain valve to keep the third oil drain pipe draining oil, so as to keep the piston at the position of the second oil drain port. The invention can improve the unloading speed of the screw compressor, shorten the unloading time, and avoid problems such as shutdown or alarm of the screw compressor due to the excessively long unloading time.

Description

Unloading control method of screw compressor

technical field

The invention belongs to the technical field of compressors, and specifically provides an unloading control method of a screw compressor.

Background technique

Compressor is a kind of fluid mechanical equipment that can lift low-pressure gas into high-pressure gas. According to the category, it can be divided into reciprocating compressor, screw compressor, rotary compressor, scroll compressor and centrifugal compressor, etc. Screw compressors are typically used in large commercial and industrial systems.

Taking the screw compressor of a commercial air conditioner as an example, a commercial air conditioner is a device that can adjust the indoor ambient temperature. The working cycle of the screw compressor can be divided into three processes: suction, compression and exhaust, which can be used for the refrigeration of commercial air conditioners. However, in the process of practical application, the screw compressor of commercial air conditioner often has problems such as shutdown and alarm, and restarting and disarming the alarm will waste a lot of time and affect the normal operation of commercial air conditioner. However, these problems However, no specific cause has been found, which has also become a difficult problem that commercial air conditioners have been unable to overcome.

Therefore, there is a need in the art for an unloading control method for a screw compressor to solve the above problems.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems in the prior art, that is, in order to solve the problem that the existing screw compressor often has problems such as shutdown and alarm, which affects its normal operation, the present invention provides an unloading control method for a screw compressor. The compressor includes a piston cavity, a piston, a piston rod, a capacity-adjusting slider, an oil supply pipe, a first oil drain pipe, a second oil drain pipe, a third oil drain pipe and an oil groove. The oil pipe is provided with a first oil drain valve, the second oil drain pipe is provided with a second oil drain valve, the third oil drain pipe is provided with a third oil drain valve, and the two ends of the piston rod are respectively connected with the piston and the capacity adjustment slider. , the piston is set to move the capacity-adjusting slider relative to the piston chamber according to the change of the oil amount in the piston chamber, and the capacity-adjusting slider can cooperate with the rotor of the screw compressor to adjust the load of the screw compressor. The inlet is communicated with the oil tank, the outlet of the oil supply pipe is communicated with the piston cavity, the inlet of the first oil drain pipe is communicated with the oil supply pipe, the inlet of the second oil drain pipe is communicated with the first oil drain port of the piston cavity, and the inlet of the third oil drain pipe is communicated with It communicates with the second oil drain port of the piston cavity, the outlet of the first oil drain pipe, the outlet of the second oil drain pipe and the outlet of the third oil drain pipe are all connected with the oil groove, and the first oil drain port and the second oil drain port are opposite along the piston. The unloading control method includes: when the screw compressor starts unloading from a full load state, controlling the oil supply valve to stop the oil supply from the oil supply pipe and controlling the third oil drain valve to make the third drain valve The oil pipe is drained and maintained for a set time, so that the piston moves from the full load position to the position of the second oil drain port; when the piston moves to the position of the second oil drain port, the oil supply valve is controlled to restore the oil supply to the oil supply pipe And control the third oil drain valve to keep the third oil drain pipe draining oil, so as to keep the piston at the position of the second oil drain port.

In a preferred technical solution of the above-mentioned unloading control method, while the steps of "controlling the oil supply valve to stop the oil supply from the oil supply pipe and controlling the third oil drain valve to drain oil from the third oil drain pipe and maintaining the set time" , the unloading control method further includes: controlling the second oil drain valve to drain oil from the second oil drain pipe for a set time.

In a preferred technical solution of the above-mentioned unloading control method, after the step of "controlling the oil supply valve to restore oil supply to the oil supply pipe and controlling the third oil drain valve to keep the third oil drain pipe draining oil", the unloading control method It also includes: acquiring the input control instruction; if the input control instruction is an unloading instruction, causing the screw compressor to continue unloading.

In a preferred technical solution of the above-mentioned unloading control method, a part of the oil supply pipe is set as a capillary.

In a preferred technical solution of the above-mentioned unloading control method, the oil supply valve is arranged on the upstream side of the capillary along the flow direction of the oil in the oil supply pipe.

In a preferred technical solution of the above-mentioned unloading control method, along the flow direction of the oil in the oil supply pipe, the oil supply valve is arranged on the downstream side of the capillary tube.

In a preferred technical solution of the above-mentioned unloading control method, a filter is provided at the suction port of the screw compressor.

In a preferred technical solution of the above-mentioned unloading control method, the oil groove is formed at the inner bottom of the casing of the screw compressor.

In a preferred technical solution of the above-mentioned unloading control method, the suction port of the screw compressor is arranged at the end of the casing.

In a preferred technical solution of the above-mentioned unloading control method, the discharge port of the screw compressor is arranged on the top of the casing.

Those skilled in the art can understand that, in the preferred technical solution of the present invention, by arranging an oil supply valve on the oil supply pipe, when the screw compressor starts to unload from a full load state, the oil supply valve is controlled to stop the oil supply pipe Supply oil and control the third oil drain valve to make the third oil drain pipe drain oil and maintain the set time, so that the piston moves from the full load position to the position of the second oil drain port; when the piston moves to the position of the second oil drain port , control the oil supply valve to restore oil supply to the oil supply pipe and control the third oil drain valve to keep the third oil drain pipe draining oil, so as to keep the piston at the position of the second oil drain port. When the screw compressor starts to unload from the full load state, while the oil is drained through the third oil drain pipe, the oil supply pipe is also stopped, so that the piston can quickly move to the position of the second oil drain port to reach the corresponding load state, which is the same as the current one. Compared with the prior art, which can only be drained through one oil drain pipe, it not only greatly improves the unloading speed, but also shortens the time for unloading to the corresponding load state, and avoids the unloading time of the screw compressor being too slow under low temperature or high pressure conditions. If it is too long, the air-conditioning unit will have problems such as shutdown or alarm, and there is no need to set up other structures, which will not increase the production cost; The pipe restores oil supply and controls the third oil drain valve to keep the third oil drain pipe draining oil, so that the piston can remain at the position of the second oil drain port, so that the screw compressor can maintain the current load state and avoid the screw compressor not receiving oil. When the control command is reached, the current load state is changed to improve the user experience.

Further, by controlling the second oil drain valve at the same time as the steps of "controlling the oil supply valve to make the oil supply pipe stop oil supply and controlling the third oil drain valve to drain oil from the third oil drain pipe and maintain the set time" In order to drain oil from the second oil drain pipe and maintain the set time, through this setting, the screw compressor drains oil through the third oil drain pipe and stops the oil supply from the oil supply pipe, and also drains oil through the second oil drain pipe. Compared with the prior art, it not only increases the pipeline for oil leakage from the piston cavity, but also stops the oil feeding into the piston cavity, further increasing the unloading speed, shortening the time for unloading to the corresponding load state, and avoiding the screw compressor at low temperature or high pressure. Under the working conditions, the unloading speed is too slow, which leads to the unloading time being too long, so that the air-conditioning unit has problems such as shutdown or alarm, which further improves the user experience.

Description of drawings

Preferred embodiments of the present invention are described below with reference to the accompanying drawings, in which:

Fig. 1 is the flow chart of the unloading control method of the screw compressor of commercial air conditioner of the present invention;

FIG. 2 is a partial structural schematic diagram of the screw compressor of the commercial air conditioner of the present invention.

Reference numerals: 1. Piston cavity; 2. Piston; 3. Piston rod; 4. Capacity adjustment slider; 5. Oil supply pipe; 6. First oil drain pipe; 7. Second oil drain pipe; 8. Third drain Oil pipe; 9, oil tank; 10, oil supply valve; 11, first oil drain valve; 12, second oil drain valve; 13, third oil drain valve; 14, rotor; 15, capillary; 16, suction port; 17. Filter; 18. Housing; 19. Exhaust port.

Detailed ways

First of all, those skilled in the art should understand that these embodiments are only used to explain the technical principle of the present invention, and are not intended to limit the protection scope of the present invention. For example, although the present invention is described in conjunction with a screw compressor of a commercial air conditioner, the unloading control method of a screw compressor of the present invention is obviously also applicable to screw compressors in food production, pharmaceutical production, textile and other equipment. The adjustment and change of the application objects do not constitute a limitation of the present invention, and should be limited within the protection scope of the present invention.

It should be noted that, in the description of the present invention, the terms “upper”, “lower”, “left”, “right”, “inner”, “outer” and other terms indicated in the direction or the positional relationship are based on the drawings. The direction or positional relationship shown is only for the convenience of description, rather than indicating or implying that the device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. Furthermore, the terms "first", "second", and "third" are used for descriptive purposes only and should not be construed to indicate or imply relative importance.

In addition, it should be noted that, in the description of the present invention, unless otherwise expressly specified and limited, the terms "arranged", "installed", "connected" and "connected" should be understood in a broad sense, for example, it may be fixed The connection can also be a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, and it can be internal communication between two components. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

Based on the problems pointed out by the background art that the screw compressor of the existing commercial air conditioner often has problems such as shutdown, alarm and other problems that affect its normal operation, the present invention provides an unloading control method of the screw compressor, aiming at avoiding the screw compressor in Under low temperature or high pressure conditions, the unloading speed is too slow, resulting in a long unloading time, which causes the air conditioning unit to stop or alarm.

Specifically, as shown in FIG. 2 , the screw compressor of the present invention includes a casing, a piston cavity 1, a piston 2, a piston rod 3, a capacity adjusting slider 4, an oil supply pipe 5, a first oil drain pipe 6, and a second oil drain pipe 7. The third oil drain pipe 8 and oil groove 9, piston cavity 1, piston 2, piston rod 3, capacity adjustment slider 4, oil supply pipe 5, first oil drain pipe 6, second oil drain pipe 7, third oil drain pipe 8 and oil tank 9 are arranged in the casing, the casing is provided with a suction port (as a refrigerant inlet) and an exhaust port (as a refrigerant outlet), the oil supply pipe 5 is provided with an oil supply valve 10, and the first oil drain pipe 6 is provided with There is a first oil drain valve 11, a second oil drain valve 12 is provided on the second oil drain pipe 7, a third oil drain valve 13 is provided on the third oil drain pipe 8, and the two ends of the piston rod 3 are respectively connected with the piston 2 and the container. The adjusting slider 4 is connected, and the piston 2 is arranged to move relative to the piston chamber 1 according to the change of the oil amount in the piston chamber 1 to move the capacity adjusting slider 4, and the capacity adjusting slider 4 can cooperate with the rotor 14 of the screw compressor. To adjust the load of the screw compressor, the inlet of the oil supply pipe 5 is connected with the oil groove 9, the outlet of the oil supply pipe 5 is connected with the piston chamber 1, the inlet of the first oil drain pipe 6 is connected with the oil supply pipe 5, and the outlet of the second oil drain pipe 7 is connected with the oil supply pipe 5. The inlet is connected to the first oil drain port of the piston chamber 1, the inlet of the third oil drain pipe 8 is connected to the second oil drain port of the piston chamber 1, the outlet of the first oil drain pipe 6, the outlet of the second oil drain pipe 7 and the third oil drain port The outlets of the oil drain pipes 8 are all communicated with the oil grooves 9 . The working principle of the screw compressor is that the oil in the oil tank 9 flows into the piston cavity 1 through the oil supply pipe 5, the oil in the piston cavity 1 flows into the oil tank 9 through the oil drain pipe, and the oil pressure is adjusted by adjusting the oil amount in the piston cavity 1, Under the action of oil pressure, the piston 2 in the piston chamber 1 is pushed to move, and the movement of the piston 2 drives the movement of the capacity adjustment slider 4 through the piston rod 3, and the volume is realized by adjusting the position of the capacity adjustment slider 4 and the cooperation with the rotor 14. To adjust the ratio, for example, the capacity adjustment slider 4 can be adjusted to 25% load, 50% load, 75% load or 100% load (correspondingly, the first oil relief valve is a 25% oil relief valve, the second oil relief valve is 50% oil drain valve, the third oil drain valve is 75% oil drain valve), the process of increasing the load of the screw compressor is called the loading process, for example, from 75% load to 100% load, the oil tank 9 The oil flows into the piston chamber 1 through the oil supply pipe 5, the oil volume in the piston chamber 1 increases, the oil pressure increases, the piston 2 moves out relative to the piston chamber 1, and drives the capacity adjustment slider 4 to correspond to the 75% load. The process of reducing the load is called the unloading process. For example, from 100% load to 75% load, the amount of oil flowing into the piston chamber 1 through the oil supply pipe 5 is less than that through the oil supply pipe 5. The amount of oil flowing out of the oil drain pipe from the piston chamber 1, the amount of oil in the piston chamber 1 decreases, the oil pressure decreases, the piston 2 moves in relative to the piston chamber 1, and drives the capacity adjustment slider 4 to move from the position corresponding to the 100% load to the position corresponding to 75% load (that is, the position of the second drain port). In the prior art, when the screw compressor is unloaded from 100% load to 75% load, the third oil drain valve 13 is controlled to make the third oil drain pipe 8 begin to drain oil until the piston 2 moves to the position of the second oil drain port, At this time, the capacity adjustment slider 4 is at the position corresponding to 75% load. When the screw compressor is unloaded from 75% load to 50% load, the second oil drain valve 12 is controlled to make the second oil drain pipe 7 begin to drain oil until the piston 2 Move to the position of the first oil drain port. At this time, the capacity adjusting slider 4 is located at the position corresponding to 50% load. When the screw compressor is unloaded from 50% load to 25% load, the first oil drain valve 11 is controlled to make the first oil drain valve 11. The oil drain pipe 6 starts to drain oil until the piston 2 moves so that the capacity adjusting slider 4 is at the position corresponding to the 25% load. During this process, the oil supply pipe 5 always supplies oil.

For the existing commercial air conditioners pointed out in the background art that sometimes shutdowns, alarms, etc. occur, and no specific cause has been found for these situations, the inventor has found that the problem has been repeatedly and repeatedly tested, analyzed and compared. The main reason is that the unloading speed of the screw compressor is too slow under low temperature or high pressure conditions, resulting in too long unloading time.

In view of this, the present invention proposes an unloading control method for a screw compressor. As shown in Figures 1 and 2, the unloading control method includes: when the screw compressor starts to unload from a fully loaded state, controlling the oil supply valve 10 to make The oil supply pipe 5 stops oil supply and controls the third oil drain valve 13 to drain oil from the third oil drain pipe 8 and maintain the set time, so that the piston 2 moves from the full load position to the position of the second oil drain port; when the piston 2 When moving to the position of the second oil drain port, control the oil supply valve 10 to restore oil supply to the oil supply pipe 5 and control the third oil drain valve 13 to keep the third oil drain pipe 8 draining oil, so that the piston 2 is kept at The position of the second oil drain port, that is, when the oil is drained through only one oil drain pipe in the prior art, the oil is drained through one oil drain pipe, and the oil supply of the oil supply pipe is also stopped, the oil drain speed is increased, and the screw compressor Quickly unload to the corresponding load, shorten the unloading time, and avoid problems such as the shutdown or alarm of the air-conditioning unit due to the slow unloading speed of the screw compressor causing the unloading time to be too long. The set time here refers to the time required for the piston 2 to move from the fully loaded position to the position of the second oil drain port. For this set time, those skilled in the art can obtain it through repeated tests. 8. After the oil is drained and the oil supply pipe 5 stops supplying oil and maintains the set time, the piston 2 has moved to the position of the second oil drain port, that is, it has been unloaded from the full load position to the load position corresponding to the second oil drain port. , at this time, the oil supply pipe 5 is restored to supply oil and the third oil drain pipe 8 is kept to drain oil, so that the piston 2 is kept at the position of the second oil drain port under the action of pressure. It should be noted that the piston 2 is relative to the piston cavity. The moving-in movement of 1 can be achieved by means of a spring. In a possible situation, similar to the foregoing, the load position corresponding to the second oil drain port is the 75% load position, and the load position corresponding to the first oil drain port is the 50% load position. The load position corresponding to the oil port and the second oil drain port is not limited to this, and those skilled in the art can set it flexibly. The load position corresponding to the oil drain port is the 75% load position. This specific adjustment and change of the load position corresponding to the first oil drain port and the second oil drain port does not deviate from the principle and scope of the present invention. should be limited within the protection scope of the present invention.

It should be noted that the state of the oil supply valve 10 can be set arbitrarily, for example, the oil supply valve 10 can be set to be closed in the power-on state to stop the oil supply pipe 5, and open in the power-off state to start the oil supply pipe 5 For oil supply, of course, the oil supply valve 10 can also be set to open in the power-on state to make the oil supply pipe 5 start supplying oil, and close in the power-off state to make the oil supply pipe 5 stop oil supply; similarly, the first oil drain valve The state of 11 can be set arbitrarily. For example, the first oil drain valve 11 can be set to be closed in the power-on state to stop the first oil drain pipe 6 from draining oil, and open in the power-off state to make the first oil drain pipe 6 start to drain oil, of course. , the first oil drain valve 11 can also be set to open in the power-on state to make the first oil drain pipe 6 start to drain oil, and close in the power-off state to stop the oil drain from the first oil drain pipe 6; similarly, the second oil drain valve The state of the valve 12 can also be set arbitrarily. For example, the second oil drain valve 12 can be set to be closed in the power-on state to stop the second oil drain pipe 7 from draining oil, and open in the power-off state to make the second oil drain pipe 7 start to drain oil. Of course, the second oil drain valve 12 can also be set to open in the power-on state to make the second oil drain pipe 7 start to drain oil, and close in the power-off state to stop the oil drain from the second oil drain pipe 7; similarly, the third oil drain valve The state of the valve 13 can also be set arbitrarily. For example, the third oil drain valve 13 can be set to be closed in the energized state to stop the third oil drain pipe 8 from draining oil, and open in the power-off state to start the third oil drain pipe 8 to drain oil. Of course, the third oil drain valve 13 can also be set to open in the power-on state to start the third oil drain pipe 8 to drain oil, and close in the power-off state to stop the oil drain from the third oil drain pipe 8 .

Preferably, as shown in FIG. 1 , in the step of “controlling the oil supply valve 10 to stop oil supply from the oil supply pipe 5 and controlling the third oil drain valve 13 to drain oil from the third oil drain pipe 8 and maintain the set time” At the same time, the unloading control method of the present invention also includes: controlling the second oil drain valve 12 to make the second oil drain pipe 7 drain oil and maintain the set time, which not only stops the oil supply of the piston chamber 1, but also increases the piston chamber 1 The oil drain pipeline further improves the unloading speed and shortens the time for unloading to the corresponding load state. After the second oil drain pipe 7 is drained and maintained for a set time, the piston 2 has moved to the position of the second oil drain port, that is, the load has been unloaded from 100% to 75% of the load, and the second oil drain valve 12 is controlled to make the second oil drain valve 12 The oil drain pipe 7 stops oil draining, so that the piston 2 is kept at the position of the second oil drain port under the action of pressure, so that the capacity adjusting slider 4 is kept at the position corresponding to the 75% load.

Preferably, after the step of "controlling the oil supply valve 10 to restore oil supply to the oil supply pipe 5 and controlling the third oil drain valve 13 to keep the third oil drain pipe 8 draining oil", the unloading control method of the present invention further comprises: : Get the input control command; if the input control command is an unload command, make the screw compressor continue to unload; if the input control command is a loading command, make the screw compressor load; if no control command is obtained, make the screw compressor continue to unload; The screw compressor maintains the existing load state.

Preferably, as shown in FIG. 2 , a part of the oil supply pipe 5 is set as a capillary 15 to slow down the oil supply speed of the oil supply pipe 5 and prevent the oil supply speed of the oil supply pipe 5 from affecting the loading and unloading of the screw compressor. . The capillary 15 can be arranged on the side of the oil supply pipe 5 close to the outlet, or can be arranged in other positions of the oil supply pipe 5 .

In a possible situation, as shown in FIG. 2 , the oil supply valve 10 is provided on the upstream side of the capillary tube 15 along the flow direction of the oil in the oil supply pipe 5 .

In another possible situation, the oil supply valve 10 is provided on the downstream side of the capillary tube 15 in the flow direction of the oil in the oil supply tube 5 .

Preferably, as shown in FIG. 2 , a filter 17 is provided at the suction port 16 of the screw compressor to filter the gas entering the interior of the screw compressor. Of course, the suction port 16 of the screw compressor may not be A filter 17 is provided, and the gas directly enters the inside of the screw compressor.

Preferably, as shown in FIG. 2, the oil groove 9 is formed at the inner bottom of the casing 18 of the screw compressor, the inlet of the oil supply pipe 5 is communicated with the oil groove 9, the outlet of the oil drain pipe is communicated with the oil groove 9, and the oil in the oil groove 9 passes through the supply The oil pipe 5 enters the piston chamber 1, and the oil in the piston chamber 1 enters the oil groove 9 through the oil drain pipe to form an oil circuit circulation. Of course, the oil groove 9 can also be formed in other positions of the screw compressor.

Those skilled in the art can flexibly set the position of the suction port 16 of the screw compressor. In a possible situation, as shown in FIG. 2, the suction port 16 of the screw compressor is arranged at the end of the casing 18. Of course, The suction port 16 of the screw compressor can also be arranged on the top of the casing 18 or at other positions. The specific adjustment and change of the setting position of the suction port 16 of the screw compressor does not deviate from the principle of the present invention. and scope should be limited within the protection scope of the present invention.

Those skilled in the art can flexibly set the position of the exhaust port 19 of the screw compressor. In a possible situation, as shown in FIG. 2 , the exhaust port 19 of the screw compressor is arranged on the top of the casing 18. Of course, the screw compressor The exhaust port 19 of the compressor can also be arranged at the end of the casing 18 or at other positions. The specific adjustment and change of the setting position of the exhaust port 19 of the screw compressor does not deviate from the principle of the present invention. and scope should be limited within the protection scope of the present invention.

So far, the technical solutions of the present invention have been described with reference to the preferred embodiments shown in the accompanying drawings, however, those skilled in the art can easily understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present invention.

Claims (10)

1. An unloading control method for a screw compressor, wherein the screw compressor comprises a piston cavity, a piston, a piston rod, a capacity-adjusting slider, an oil supply pipe, a first oil drain pipe, a second oil drain pipe, a Three oil drain pipes and oil grooves, the oil supply pipe is provided with an oil supply valve, the first oil drain pipe is provided with a first oil drain valve, the second oil drain pipe is provided with a second oil drain valve, the A third oil drain valve is provided on the third oil drain pipe, two ends of the piston rod are respectively connected with the piston and the capacity adjustment slider, and the piston is arranged to be able to change according to the oil amount in the piston cavity moving relative to the piston cavity to move the capacity adjustment slider, the capacity adjustment slider can cooperate with the rotor of the screw compressor to adjust the load of the screw compressor, The inlet of the oil supply pipe is communicated with the oil tank, the outlet of the oil supply pipe is communicated with the piston cavity, the inlet of the first oil drain pipe is communicated with the oil supply pipe, and the second oil drain pipe is in communication with the oil supply pipe. The inlet is communicated with the first oil drain port of the piston cavity, the inlet of the third oil drain pipe is communicated with the second oil drain port of the piston cavity, the outlet of the first oil drain pipe, the second oil drain pipe The outlet of the oil drain pipe and the outlet of the third oil drain pipe are both communicated with the oil groove, and the first oil drain port and the second oil drain port are arranged in sequence along the moving direction of the piston relative to the piston cavity, The unloading control method includes: When the screw compressor starts to unload from a full load state, control the oil supply valve to stop oil supply from the oil supply pipe and control the third oil drain valve to drain oil from the third oil drain pipe and maintain it. setting the time to move the piston from the fully loaded position to the position of the second drain port; When the piston moves to the position of the second oil drain port, the oil supply valve is controlled to restore oil supply to the oil supply pipe and the third oil drain valve is controlled to make the third oil drain pipe Drain oil is maintained so that the piston remains in the position of the second oil drain port. 2. The unloading control method according to claim 1, characterized in that "controlling the oil supply valve to stop oil supply to the oil supply pipe and controlling the third oil drain valve to make the third oil At the same time as the step of draining oil from the oil drain pipe and maintaining the set time, the unloading control method further includes: The second drain valve is controlled to drain the second drain line for the set time. 3. The unloading control method according to claim 1, characterized in that "controlling the oil supply valve to restore oil supply to the oil supply pipe and controlling the third oil drain valve to make the third oil After the oil drain pipe keeps draining oil", the unloading control method further includes: Get the input control command; If the input control command is an unloading command, the screw compressor will continue to be unloaded. 4 . The unloading control method according to claim 1 , wherein a part of the oil supply pipe is configured as a capillary. 5 . 5 . The unloading control method according to claim 4 , wherein the oil supply valve is provided on the upstream side of the capillary along the flow direction of the oil in the oil supply pipe. 6 . 6 . The unloading control method according to claim 4 , wherein the oil supply valve is provided on the downstream side of the capillary along the flow direction of oil in the oil supply pipe. 7 . 7 . The unloading control method according to claim 1 , wherein a filter is provided at the suction port of the screw compressor. 8 . 8 . The unloading control method according to claim 1 , wherein the oil groove is formed at the inner bottom of the casing of the screw compressor. 9 . 9 . The unloading control method according to claim 8 , wherein the suction port of the screw compressor is provided at the end of the casing. 10 . 10 . The unloading control method according to claim 8 , wherein the discharge port of the screw compressor is arranged on the top of the casing. 11 .

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