JPH0159438B2 - - Google Patents

Description

[Detailed description of the invention]

[Object of the Invention] (Industrial Application Field) The present invention relates to a method of operating a screw compressor. (Prior Art) Conventionally, a screw compressor has controlled its capacity from 0 to 100% using a slide valve provided on the compressor without changing the rotational speed of the electric motor directly connected to the screw compressor. As shown in Figure 2, which will be described later, the power efficiency of this technology has a problem in that when the flow rate by capacity control becomes less than 50%, the power increases rapidly and the efficiency deteriorates. Next, when adjusting the amount of conveyance by driving a reciprocating compressor, which is not a screw compressor, by an internal combustion engine with a variable rotation speed, the amount of conveyance is adjusted by the following first to third methods. There is a method of adjusting steplessly (Japanese Patent Laid-Open No. 49-41909), but this conventional technique has the following problems. That is, (a) First method (indicated by the solid line in the drawing) When the conveyance amount Q is between 100% and 90%, the rotation speed n is kept constant and the opening maintaining force p is varied, but when Q is 90%
% to 40%, p is kept constant and n is varied. For this reason, within the operating range where Q is smaller than 90%, the amount of conveyance must be adjusted ``while varying the rotational speed'' of the internal combustion engine. (b) The second method (as indicated by the dashed-dotted line in the drawing) When the conveyance amount Q is between 100% and 70%, the opening maintaining force is set to p=0 and the rotational speed n is varied without changing, and Q is 70%. Between % and 40%, keep n constant and p
Vary. For this reason, within the operating range where Q is 100% to 70%, the conveyance amount must be adjusted "while varying the rotational speed" of the internal combustion engine. (c) Third method (indicated by the broken line in the drawing) The conveyance amount is adjusted while simultaneously varying the rotational speed n and the opening holding force p while the conveyance amount Q is between 100% and 40%. . Therefore, in the entire operating range where Q is from 100% to 40%, the amount of conveyance must be adjusted while "changing the rotational speed" of the internal combustion engine. Therefore, in any case, the method of the prior art has to adjust the conveyance amount of the compressor while changing the rotational speed of the internal combustion engine, which not only makes the structure complicated but also makes the operation difficult. Although it is not a screw compressor, when a rotary type compressor is driven by an electric motor with variable rotation speed to operate the compressor, its compression capacity may be adjusted by the following first to third methods. Method (Unexamined Japanese Patent Publication 1973)
57683), but each of these conventional techniques has the following problems. That is, (a) First method (according to Figures 1 and 2) During normal operation, the motor is operated at high speed and the compressor is operated at constant load, and when the load decreases, the motor is switched to low-speed operation. At the same time, the compressor is operated by bypass. However, with this method, the amount of bypass of refrigerant gas cannot be adjusted in a partial load region where the motor is operated at low speed, and the amount of bypass is fixed. For this reason, when the load fluctuates during the low-speed operation, it is not possible to vary the bypass amount of refrigerant gas to a large or small amount in response to the variation, and therefore it is not possible to respond precisely to variations in the load. (b) Second method (according to Figures 3 and 4) During normal operation, the motor operates at high speed and the compressor operates at a constant load, and when the load decreases, the motor switches to medium-speed operation and compresses at the same time. The machine is put into a slight bypass operation, and when the load further decreases, the electric motor is switched to low-speed operation and at the same time the compressor is put into a large bypass operation. This method not only complicates the structure, but also the amount of refrigerant gas bypass is determined to be constant in the region of moderate partial load when the motor is operated at medium speed, so the load within this region is Even if the load changes, the refrigerant bypass amount cannot be changed to a large or small amount in accordance with the change, and it is not possible to finely respond to load changes within the medium-speed operation region. Even in a small partial load range in which the motor is operated at a lower speed, it is not possible to precisely respond to load fluctuations within the low-speed operation range for the same reason as in the case of medium-speed operation. In this way, the methods (a) and (b) above are
Although the rotation speed of the compressor-driven electric motor is switched from "high to low" or "high to medium to low" according to load fluctuations, the motor rotates at a constant rotation speed selected according to the load. In this case, no adjustment is made for "load fluctuations" within that region. (c) Third method The motor is operated while continuously and steplessly changing both the rotational speed and the amount of bypass. This method inevitably becomes structurally extremely complex. (Problems to be Solved by the Invention) The present invention solves the problems of the prior art,
It is possible to prevent deterioration of power efficiency during capacity control during operation, and to reduce the pressure on the discharge side of the screw compressor when switching the rotation speed of the screw compressor from high to low or vice versa in response to changes in capacity. The purpose of this is to provide an operating method that does not cause fluctuations. [Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the method for operating a screw compressor of the present invention includes the following steps when performing capacity control during operation of a screw compressor driven by an electric motor: While in a predetermined operating region with a large capacity, the electric motor is driven at a constant high rotation speed, and is controlled by a slide valve provided in the compressor according to the capacity, and when in a predetermined operating region with a small capacity. When you reach it,
The number of poles of the motor is changed so that the motor is driven at a constant low rotation speed, and the slide valve is temporarily moved in a direction where the load increases so that the pressure on the discharge side of the screw compressor becomes constant. Then, control is performed by the slide valve according to the capacity, and when the predetermined operation region with a large capacity is reached again, the motor's pole is changed so that the motor is switched to a constant high rotation speed and driven again. At the same time as changing the number of compressors, the slide valve is temporarily set to move in the direction of decreasing the load so that the pressure on the discharge side of the screw compressor becomes constant, and then the slide valve is controlled according to the capacity. It is composed of (Function) The present invention relates to a method for controlling the capacity of a screw compressor by driving an electric motor. Keep it
By opening and closing the slide valve of the compressor according to the capacity, the compressor is adapted to the changed capacity each time. When the capacity of the compressor reaches a small operating range outside of the above-mentioned operating range, the number of poles of the motor is switched and the motor is switched to a constant low rotational speed to drive the compressor. The slide valve is once set to move in the direction where the load increases, and the switching to the lower rotational speed prevents the discharge gas amount from suddenly decreasing and the pressure on the discharge side of the compressor from decreasing. Thereafter, in this operating range, the slide valve of the compressor is opened and closed according to the capacity, and the compressor is adapted to the changing capacity each time. Furthermore, when the compressor reaches the predetermined operating range where the capacity is large again, the number of poles of the motor is switched and the motor is switched to a constant high rotational speed to drive the compressor. The valve is once set to be moved in the direction of decreasing the load, and the switching to the higher rotation speed prevents the discharge gas amount from increasing rapidly and the pressure on the discharge side of the compressor from increasing. Thereafter, the slide valve of the compressor is opened and closed depending on the operating range, and the compressor is adapted to the changing capacity each time. The rotation speed of the motor can be easily changed by changing the number of poles. (Example) The present invention will be explained by referring to an example. In Fig. 1, 1 is a pipeline on the gas suction side, 2 is, for example, a liquid injection screw compressor,
The pressurized gas is sucked into a liquid injection type screw compressor 2 through a pipeline 1. 3 is a speed increaser, 4 is a slide valve, and 5 is a slide valve drive device. This slide valve drive device 5 is mainly driven by hydraulic pressure, and includes an input/output device for driving, an operating device, and the like. Reference numeral 6 denotes a pole number changing motor, which can change the number of poles from 2 poles to 4 poles, or from 4 poles to 6 poles or 8 poles. 7 is a pole number converter, and 8 is a power switch. 9 is a liquid separator that separates the compressed gas and the liquid injected into the liquid injection screw compressor; 1;
0 is a flow meter or pressure gauge and a transmitter based on the amount of change (discharge side detector), 11 is a liquid cooler that cools the circulating liquid, 12 is a liquid pump that circulates the liquid, and 13 is a liquid filter. . Note that a is the inlet side of the gas to be pumped, b is the outlet side of the gas,
Moreover, the dotted line is wiring or piping connecting the transmitter 10 of the detector and the operating side. Conventionally, the capacity of a liquid injection screw compressor is controlled by a slide valve provided on the compressor without changing the rotational speed of an electric motor directly connected to the compressor. In other words, when a liquid injection type screw compressor is normally driven by an electric motor, it is 3550 R/M in the 60 Hz area and 2950 R/M in the 50 Hz area.
is common, and is often carried out by directly connecting a two-pole motor. In some cases, a four-pole motor or the like is used, but in this case, the above-mentioned rotational speed is maintained through a speed increaser. The operating characteristics of the liquid injection screw compressor under normal constant speed rotation are as shown in FIG. This shows the flow rate on the horizontal axis and the shaft horsepower of the motor on the vertical axis, and the flow rate and shaft horsepower at full capacity of the compressor are each 100%.
This shows the change in shaft horsepower when the flow rate is reduced by a slide valve (suction and discharge pressures are constant).
However, as is clear from this figure, as the flow rate decreases, the shaft horsepower deviates more from the 45° slope line, but this means that extra power is consumed for the same amount of discharge. It can be seen that the decrease in power efficiency is particularly significant when the flow rate is approximately 50% or less of the full capacity. Although it is not a screw compressor, there is also a known method in which a reciprocating compressor is driven by an internal combustion engine with a variable rotational speed to adjust the conveyance amount without permission. There is an operating range in which it is necessary to respond to load fluctuations while varying the rotational speed of the electric motor itself.Furthermore, by switching the rotational speed of the drive motor for a rotary compressor, the conveyed amount can be adjusted over a wide range of load fluctuations. There is also a known method for adapting the load to the motor, but this method does not allow for adjustments to be made in response to load fluctuations within the range in which the motor rotates at a constant speed. Moreover, the gas pressure on the discharge side of the compressor changes by changing the rotational speed of the electric motor. In view of such prior art, the present invention uses a screw compressor, combines the characteristics of the screw compressor with the characteristics of a pole conversion motor, uses a relatively simple mechanism, and can handle a wide operating range of gas volume. This is to make it possible to save the power consumption most economically and to prevent the gas pressure on the discharge side of the compressor from fluctuating when changing the rotation speed.The embodiment shown in Fig. 3 will be explained below. do. In this embodiment, the pole number changing motor used was a 2-4 pole motor, but this is just an example, and the motor could be changed from 4 poles to 6 poles as appropriate depending on the load range of the consumer's compressor and the usage time. Poles, 4 poles - 6 poles - 8 poles
Select a polar motor, etc., and use a speed increaser to maintain the compressor rotation speed within a reasonable range. In this example, the liquid jet screw compressor has a suction pressure of 2 Kg/cm ² G and a discharge pressure of 15 Kg/cm ²
The compressor was operated at G, and the liquid injection amount was adjusted so that the compressor discharge outlet gas temperature was 85°C. The flow rate at the discharge port is determined using both an indicator and a pulse transmitter, and the flow rate is increased or decreased at a constant pressure based on the indicator, and the pulse is used to control the fluid pressure valve of the slide valve drive device to operate the slide valve. The pressure balance was maintained. In addition, the slide valve drive device is equipped with a transmitter at the part that controls half the flow rate, and when this is detected, the pole number converter of the electric motor is activated, and the position of the slide valve is set accordingly. Make it. When the liquid injection type screw compressor is operated in this manner, its operating characteristics are as shown in FIG. In the figure, the flow rate is 100% (1010 m ³ /h), the rotation speed is 2950 R/M, the suction pressure is 2 Kg/cm ² G, and the discharge pressure is 15
Kg/cm ² G, operated with the slide valve fully closed, and the shaft horsepower at that time of 229KW was taken as 100%, which is indicated by point A. Next, as the flow rate is gradually reduced, the slide valve begins to open gradually, and the shaft horsepower changes along the line A to B. When the flow rate reaches approximately 50% at point B, the transmitter works,
Switch the pole number converter from 2 poles to 4 poles, reduce the motor rotation speed to 1/2, and temporarily close the slide valve.
Set to move in the direction of increasing load so that the pressure on the discharge side of the screw compressor remains constant. As a result, the shaft horsepower decreases from B to C. If the slide valve continues to perform capacity control to reduce the flow rate, the power consumption situation will now be shown by the action along the line C to D. When the predetermined operating range with large capacity is reached again, the number of poles of the motor is changed so that the motor is driven at a constant high rotational speed again, and the slide valve is temporarily moved in the direction of decreasing the load. Set so that the pressure on the discharge side of the screw compressor remains constant. Then, the capacity is controlled by the slide valve in accordance with the capacity. As shown in Figure 4, the slide valve mechanism is constructed by cutting out a portion of the cylinder case surrounding the screw rotor in the axial direction, inserting a slide valve 4, and moving it in parallel to change the compression stroke to control the capacity. Make it. As is clear from the figure, 100% of A
In the case of load operation, all the gas in the tooth space volume V ₁ is discharged from the discharge port, but in the case of partial load (B),
When the slide valve driving device 5 moves the slide valve 4 in parallel in the direction of the discharge port, a space is created on the suction side end face (between the slide valve 4 and the fixed end 14), so the tooth space volume changes from V ₁ to V ₂ decreases and the amount of displacement decreases. In the case of the conventional method that is not based on the operating method of the present invention, power efficiency deteriorates especially at low loads as shown in A-B-E in Fig. 3, but according to the operating method of the present invention -D, the power efficiency at low loads is significantly improved, resulting in significant savings in power consumption. The magnitude of the savings is expressed by the area of the BCDE. This is a clever combination of the characteristics of the screw compressor that does not significantly reduce its efficiency even at low speed rotation, and the characteristics of the electric motor that adjusts power consumption to the load by changing the number of poles. This was achieved by combining the following. Next, an example of an embodiment in which the present invention is applied to city gas pumping will be described. Example The method of the present invention was carried out under the following conditions for use in compressed city gas transportation. Compressor: Rotor diameter 255mmφ Oil injection screw compressor Motor: 230KW, 2-pole to 4-pole switching motor Maximum displacement: 2100Nm ³ /h Shaft power: 216KW (at maximum air volume) Rotation speed: 2950R/M ~ 1450R/M Compressed gas: City gas Suction pressure: Atmospheric pressure Discharge pressure: 7 Kg/cm ² G The control method was to adjust the amount of gas discharged from the compressor so as to keep the pressure on the discharge side constant at 7 Kg/cm ² G. That is, in FIG. 1, a pressure detection sensor is provided in the transmitter 10, which operates and adjusts the slide valve 4 built into the compressor. Currently, the compressor is operating at 100% load with two poles, i.e., 2950R/M, and as the amount of gas used gradually decreases, the slide valve 4 built into the compressor gradually begins to open and moves in the direction of reducing the load. , the amount of discharged gas is approximately
When the 50% position is reached, the pole converter 7 is activated by the operating signal of the limit switch installed at this position.
and increase the motor rotation speed from 2950R/M.
Switch to 1450R/M. As a result, the amount of discharged gas is halved, so the pressure of the pressure detection sensor of the transmitter 10 decreases, and the slide valve temporarily moves in the direction of increasing the load, and the pressure of the pressure detection sensor decreases to 7 kg/
Adjust so that cm ² G. After that, control is performed by the slide valve according to the capacity at each time. After that, during operation in the above-mentioned operating region with a small capacity, when the amount of city gas used increases again and the slide valve reaches the 100% position, the pole number converter 7 operates again and changes the rotation speed of the motor.
Switch to 2950R/M. As a result, the amount of discharged gas is doubled, so the pressure of the pressure detection sensor of the transmitter 10 increases, and the slide valve is temporarily set to move in the direction of decreasing the load to maintain the pressure, and after that, the pressure is maintained at the respective capacity. Control is performed by the slide valve accordingly. In this way, it was possible to continuously adjust the discharge gas amount of the oil injection screw compressor according to the pressure. Using the same conditions as the compressor for compressed city gas described above (however, the electric motor is not of the pole number switching type but has a constant rotation speed of 2950R/M), it can be slid without changing the rotation speed of the electric motor. The following table shows the results of a comparison between the conventional compressor operating method in which capacity is controlled from 0 to 100% using only a valve and the method of the present invention during peak consumption and other partial loads. It was hot.

[Table] According to this table, it can be seen that there is a difference in power consumption between the method of the present invention and the conventional method when the load factor is 40% and 20%. Its value is (114-97) KW per hour for 40% load factor, the operating time is 9.6 hours per day, and (91-65) KW per hour for 20% load factor. , its operating time is 4.8 hours per day. Therefore, the amount of power saved in one year by this example is {(114-97) x 9.6 + (91-65) x 4.8} x 3
65=105120 (KW), and by implementing the present invention, it is possible to save a lot of power, and the energy saving effect is remarkable. In the above embodiment, the number of poles of the motor was changed when the load reached approximately 50%, but the present invention is not limited to this, and the number of poles may be changed without changing the nature of the present invention. It can also be carried out above or below 50% load. Further, in the above embodiments, the case where the method of the present invention was implemented on a liquid (oil) injection type screw compressor was explained, but the method of the present invention can be similarly implemented on an oil-free screw compressor. [Effects of the Invention] According to the present invention, when performing capacity control in the operation of a screw compressor driven by an electric motor, the slide valve provided in the compressor performs control while the screw compressor is in a large capacity operation region, and the capacity is controlled. When a small predetermined operating range is reached, the number of poles of the motor is changed to reduce the rotational speed of the motor, and the slide valve is temporarily moved and set in the direction of increasing load to reduce the pressure on the discharge side of the screw compressor. Then, the capacity is controlled by the slide valve according to the capacity, and when the predetermined operating range with large capacity is reached again, the rotation of the motor is set to high rotation (constant) again. At the same time, the number of poles of the motor is changed so as to switch back and the slide valve is moved in the direction where the load decreases, so that the pressure on the discharge side of the screw compressor becomes constant, and after that, the pressure on the discharge side of the screw compressor becomes constant. Accordingly, control was performed using a slide valve. Therefore, it is possible to prevent deterioration in power efficiency that occurs when controlling the capacity of a screw compressor over a wide load range, and also to prevent pressure fluctuations on the discharge side of the screw compressor from occurring when changing the number of poles of the motor. can be prevented. In addition, the present invention uses a simple and inexpensive configuration of converting the number of poles of the motor and also uses a slide valve system, so power efficiency is improved especially at low loads, and power consumption is significantly reduced, so it will last for a long time. It is possible to provide an optimal operating method for compressors in departments that require low-load operation over a long period of time, or for cases where load fluctuations are large and are repeated on a daily basis.

[Brief explanation of drawings]

Fig. 1 is a flow sheet diagram when implementing the operating method of the present invention, Fig. 2 is a diagram showing the operating characteristics of a conventional liquid injection screw compressor at constant speed rotation, and Fig. 3 is a flow sheet diagram when implementing the operating method of the present invention. A diagram showing the operating characteristics of the liquid injection screw compressor when the method is implemented, and Figure 4 is an explanatory diagram of the slide valve mechanism, where A shows the case of 100% load operation and B shows the case of partial load. It is something. 2...Liquid injection screw compressor, 4...Slide valve, 5...Slide valve drive device, 6...Pole number conversion motor as an electric motor, 7...Pole number converter.

Claims (1)

[Claims] 1. When performing capacity control during operation of a screw compressor driven by an electric motor, the electric motor is driven at a constant high rotational speed while in a predetermined operation region with a large capacity, and the compressor is equipped with a load according to the capacity. When a predetermined operation region with a small capacity is reached, the number of poles of the motor is changed so that the motor is driven at a constant low rotational speed, and the slide valve is temporarily unloaded. Set the screw compressor to move in the direction of increasing so that the pressure on the discharge side of the screw compressor remains constant.
Subsequently, control is performed by the slide valve according to the capacity, and when a predetermined operation region with a large capacity is reached again, the number of poles of the motor is changed so that the motor is switched to a constant high rotation speed and driven again. At the same time as the switching, the slide valve is temporarily set to move in a direction where the load is reduced so that the pressure on the discharge side of the screw compressor becomes constant, and control is then performed by the slide valve according to the capacity. How to operate a screw compressor.