Variable-frequency helium compressor capable of adjusting circulating flow of lubricating oil

Abstract

The invention discloses a variable-frequency helium compressor capable of adjusting the circulating flow of lubricating oil. The variable-frequency helium compressor comprises an oil way system and a flow control part, the oil way system comprises a pressing bag, an oil supplementing opening, a heat exchanger, a first filter, a second variable flow control device and a first fixed flow control device. The flow control part is connected between the frequency signal output end of the pressing bag and the control end of the second variable flow control device, and the flow control part is used for recognizing the rotating frequency of a pressing bag motor and outputting a control signal to the control end of the second variable flow control device according to the rotating frequency of the pressing bag motor. Therefore, the opening degree of the second variable flow control device is adjusted, and the circulating flow of lubricating oil in the oil way system is controlled. The fixed flow control device in the oil way is reserved on the basis of the variable flow control device, the adjusting precision of the variable flow control device can be improved, and the efficiency of the helium compressor is improved.

Description

Technical Field

[0001] The invention belongs to the field of GM refrigerator research and development, and specifically relates to a variable frequency helium compressor capable of adjusting the circulation flow of lubricating oil. Background Art

[0002] Scroll helium compressor is the core power source of GM refrigerator, with advantages such as high efficiency and stability, and is widely used in nuclear magnetic resonance inspection equipment and semiconductor field of ultra-high vacuum pump. As a 24 / 7 non-stop equipment, GM refrigerator has attracted the attention of the industry for its high energy consumption. At present, variable frequency helium compressor has become the mainstream of new product development as the main means of energy saving for GM refrigerator.

[0003] Since helium has the characteristics of small molecular weight and large adiabatic index, a large amount of lubricating oil needs to be added to the scroll when compressing helium using a scroll helium compressor, which has the effect of increasing the oil seal and cooling the scroll. In order to achieve the energy saving and variable operating conditions of the GM refrigerator, the variable frequency helium compressor needs to change the speed of the scroll pack according to the load or terminal control, thereby changing the exhaust volume of the helium compressor.

[0004] The change in the speed of the scroll compressor will also change the demand for the circulation volume of lubricating oil. When the helium compressor is at high frequency and high speed, the amount of helium compressed is large, and the required lubricating oil also needs to be increased. When the helium compressor is at low frequency and low speed, the amount of helium in the compressor is small, and the required lubricating oil also needs to be reduced.

[0005] At present, conventional lubricating oil flow regulation technology uses a single fixed flow control device to throttle and reduce the pressure of the lubricating oil. However, the change in the operating frequency of the variable frequency compressor will adjust the helium flow by changing the motor speed, which ultimately affects the output power of the refrigerator. The fixed throttling device alone cannot meet the requirements for lubricating oil flow under different helium flow rates. When the operating frequency increases, the pressure and flow will increase, resulting in large throttling losses; when the operating frequency decreases, the lubricating oil exceeding the demand will enter the pressure pack, pressurizing it and doing work, resulting in a decrease in the working efficiency of the pressure pack. Therefore, a control method for an adjustable lubricating oil flow has become a difficult problem that current equipment needs to solve. Summary of the invention

[0006] Purpose of the invention: The purpose of the present invention is to provide a variable frequency helium compressor capable of adjusting the circulation flow of lubricating oil, in view of the fact that conventional single fixed flow control devices throttle and reduce the pressure of lubricating oil cannot cope with the problem of helium flow changes in variable frequency compression packages.

[0007] Technical solution: A variable frequency helium compressor capable of adjusting the circulation flow of lubricating oil of the present invention comprises an oil circuit system and a flow control unit for controlling the flow of lubricating oil of the oil circuit system;

[0008] The oil circuit system includes a pressure pack, the oil pool at the bottom of the pressure pack is connected to the oil replenishment port and one end of the heat exchanger; the other end of the heat exchanger is connected to one end of the first filter, and the other end of the first filter is connected to one end of the second variable flow control device and one end of the first fixed flow control device respectively; the other end of the second variable flow control device and the other end of the fixed flow control device are connected to the oil return port at the top of the pressure pack after merging;

[0009] The flow control unit is connected between the frequency signal output end of the pressing package or the output end for controlling the operating frequency of the pressing package and the control end of the second variable flow control device. The flow control unit is used to identify the rotation frequency of the pressing package motor and output a control signal for controlling the opening of the second variable flow control device to the control end of the second variable flow control device according to the rotation frequency of the pressing package motor, thereby adjusting the opening of the second variable flow control device and further controlling the circulation flow of the lubricating oil in the oil circuit system.

[0010] Furthermore, the flow control unit includes a frequency identification unit and a valve control unit, the input end of the frequency identification unit is connected to the frequency signal output end of the press pack, and the output end is connected to the input end of the valve control unit, and the output end of the valve control unit is connected to the control end of the second variable flow control device; the frequency identification unit identifies the rotation frequency of the press pack motor and transmits it to the valve control unit, the valve control unit converts the identified rotation frequency of the press pack motor into an opening signal of the second variable flow control device, and adjusts the opening of the second variable flow control device based on the opening signal, thereby adjusting the flow rate of the lubricating oil entering the press pack to meet the cooling requirements of the lubricating oil under different helium flow rates due to the high and low press pack frequencies.

[0011] Furthermore, when the frequency recognition unit identifies that the rotation frequency of the pressing motor is at a low frequency, the valve control unit reduces the opening of the second variable flow control device; when the frequency recognition unit identifies that the rotation frequency of the pressing motor is at a high frequency, the valve control unit increases the opening of the second variable flow control device; reducing the opening at a low frequency reduces the circulation flow of the lubricating oil and improves the working efficiency of the pressing; increasing the opening at a high frequency increases the circulation flow of the lubricating oil and reduces throttling losses.

[0012] Furthermore, the opening degree of the first fixed flow control device is set to the opening degree corresponding to the lowest frequency, thereby reducing the power loss caused by the low-frequency variable flow control device being in the closed state.

[0013] Furthermore, it also includes a gas circuit system, which includes an exhaust pipe, the exhaust pipe is connected to one end of the exhaust buffer tank, the other end of the exhaust buffer tank is connected to one end of the pressure bag, the other end of the pressure bag is connected to the first thermistor and one end of the heat exchanger in sequence; the other end of the heat exchanger is connected to the third thermistor, the oil separator, the safety valve, the adsorber, and one end of the intake pipe in sequence, and the other end of the intake pipe is used to transport high-pressure helium to the refrigerator.

[0014] Furthermore, an air supply pipe and a second pressure sensor are provided on the air pipeline between the exhaust pipe and the exhaust buffer tank.

[0015] Furthermore, the output end of the oil separator is connected to the first pressure sensor and one end of the first variable flow control device in sequence through an air pipeline; the other end of the first variable flow control device is connected to the exhaust buffer tank to form a high-pressure internal bypass air path of the helium compressor.

[0016] Furthermore, the output end of the oil separator is connected to one end of the second filter through an oil pipeline, and the other end of the second filter is connected to one end of a second fixed flow control device; the other end of the second fixed flow control device is connected to the air pipeline between the exhaust buffer tank and the pressure bag, which can transport the oil mist separated by the oil separator to the pressure bag, ensuring that the circulating oil volume remains basically stable, and no additional lubricating oil needs to be injected during the normal service life.

[0017] Furthermore, it also includes a cooling water system, which includes a water inlet pipeline connected to one end of the heat exchanger, and the other end of the heat exchanger is connected to the water outlet pipeline.

[0018] Furthermore, a second thermistor is connected to the water outlet pipeline to detect the outlet temperature of the cooling water. When the outlet temperature is too high, it is fed back that the cooling water flow rate provided by the outside is insufficient.

[0019] Beneficial effects: Compared with the prior art, the significant technical effects of the present invention are as follows:

[0020] Aiming at the problem that the conventional single fixed flow control device for lubricating oil throttling and pressure reduction cannot cope with the change of helium flow in the variable frequency compression package, the present invention adopts a variable flow control device in parallel with a fixed flow control device to control the flow of lubricating oil, adopts a flow control part to receive the frequency signal from the vortex compression package through the frequency recognition part, and changes the opening of the variable flow control device through the output signal of the valve control part to control the circulation flow of the lubricating oil, reduces the throttling loss caused by excessive frequency, and improves the energy conversion efficiency of the compression package under low frequency.

[0021] On the basis of the opening adjustment of the variable flow control device, the adjustment method of the fixed flow control device is retained, and the opening of the fixed flow control device is reduced to the opening corresponding to the lowest frequency. The combination of variable flow and fixed flow can not only protect the pressure bag and prevent the lubricating oil circulation from being blocked due to circuit failure and causing helium to exceed the safe working range, but also reduce the adjustment range of the flow corresponding to the variable flow control device, refine the valve opening at each frequency, and improve the control accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0023] Figure 2 is a curve diagram showing the change of the opening degree of the flow control part with the frequency in the present invention;

[0024] Figure 3 This is a flow chart of the flow control unit controlling the lubricating oil flow rate in the present invention. DETAILED DESCRIPTION

[0025] The technical solution of the present invention is described in detail below in conjunction with specific implementation methods and the accompanying drawings.

[0026] like Figure 1 As shown, a variable frequency helium compressor capable of adjusting the circulation flow of lubricating oil of the present invention involves the following components: a water outlet pipeline 1, a water inlet pipeline 2, a first filter 3, a first fixed flow control device 4, a pressure pack 5, a heat exchanger 6, an air inlet pipe 7, an exhaust buffer tank 8, an oil separator 9, and an adsorber 10; an exhaust pipe 11, an air supply pipe 12, a second filter 13, a second fixed flow control device 14, a first pressure sensor 15, a first variable flow control device 16, an oil supply port 17, a first thermistor 18, a second thermistor 19, a third thermistor 20, a second pressure sensor 21, a safety valve 22, a flow control unit 23, a second variable flow control device 24, a frequency identification unit 25, and a valve control unit 26. Among them, the pressure pack 5 adopts a variable frequency vortex pressure pack, and the pressure pack 5 is used for variable frequency supercharging of helium. The heat exchanger 6 is used to cool the high-temperature helium and lubricating oil at the outlet of the pressure pack 5. The oil separator 9 is used to separate oil mist from the helium. The adsorber 10 is used to absorb the oil carried by the helium at the outlet of the oil separator 9; the exhaust buffer tank 8 plays a buffering role; the first filter 3 and the second filter 13 filter the impurities in the return oil. The first variable flow device 16 is used to change the bypass gas volume of the variable frequency helium compressor and adjust the pressure of the system operation. The safety valve 22 is used to release the pressure to the atmosphere when the gas pressure is too high.

[0027] The variable frequency helium compressor with adjustable lubricating oil circulation flow rate includes an air circuit system, an oil circuit system, a cooling water system, and a flow control unit 23. The flow control unit 23 is used to control the lubricating oil flow rate of the oil circuit system, and the cooling water system conducts heat and cools the helium and lubricating oil discharged from the pressure package 5 through the heat exchanger 6. The air circuit and the main oil return circuit operate independently.

[0028] In the gas circuit system, the exhaust pipe 11 is connected to one end of the exhaust buffer tank 8, the other end of the exhaust buffer tank 8 is connected to one end of the pressure bag 5, and the other end of the pressure bag 5 is connected to the first thermistor 18 and one end of the heat exchanger 6 in sequence. The other end of the heat exchanger 6 is connected to the third thermistor 20, the oil separator 9, the safety valve 22, the adsorber 10, and one end of the intake pipe 7 in sequence, and the other end of the intake pipe 7 is used to transport high-pressure helium to the refrigerator. The gas pipeline between the exhaust pipe 11 and the exhaust buffer tank 8 is provided with a gas supply pipe 12 and a second pressure sensor 21. The output end of the oil separator 9 is connected to the first pressure sensor 15 and one end of the first variable flow control device 16 in sequence through the gas pipeline; the other end of the first variable flow control device 16 is connected to the exhaust buffer tank 8 to form a high-pressure internal bypass gas path of the helium compressor.

[0029] In the main oil circuit system, the oil pool at the bottom of the pressure pack 5 is connected to the oil replenishment port 17 and one end of the heat exchanger 6; the other end of the heat exchanger 6 is connected to one end of the first filter 3, and the other end of the first filter 3 is respectively connected to one end of the second variable flow control device 24 and one end of the first fixed flow control device 4. The other end of the variable flow control device 24 and the other end of the fixed flow control device 4 are connected to the oil return port at the top of the pressure pack 5 after merging.

[0030] In the oil return oil circuit system, the output end of the oil separator 9 is connected to one end of the second filter 13 through the oil circuit pipeline, and the other end of the second filter 13 is connected to one end of the second fixed flow control device 14. The other end of the second fixed flow control device 14 is connected to the gas circuit pipeline between the exhaust buffer tank 8 and the pressure bag 5, and the other end of the second fixed flow control device 14 is connected to the gas circuit pipeline at the other end of the exhaust buffer tank 8 and merges into the pressure bag 5. The oil mist separated by the oil separator 9 is connected to one end of the filter 13 from one end of the oil separator 9.

[0031] In the cooling water system, the water inlet pipe 2 is connected to one end of the heat exchanger 6, and the other end of the heat exchanger 6 is connected to the cooling water outlet pipe 1. The outlet pipe 1 is connected to a second thermistor 19.

[0032] The flow control unit 23 is connected between the frequency signal output end of the press package 5 and the control end of the second variable flow control device 24. The flow control unit 23 is used to identify the rotation frequency of the motor of the press package 5, and output a control signal for controlling the opening of the second variable flow control device 24 to the control end of the second variable flow control device 24 according to the rotation frequency of the motor of the press package 5, thereby adjusting the opening of the second variable flow control device 24, and then controlling the circulation flow of the lubricating oil in the oil circuit system.

[0033] In this embodiment, the flow control unit 23 includes a frequency identification unit 25 and a valve control unit 26. The input end of the frequency identification unit 25 is connected to the frequency signal output end of the pressure pack 5, and the output end is connected to the input end of the valve control unit 26. The output end of the valve control unit 26 is connected to the control end of the second variable flow control device 24. The frequency identification unit 25 identifies the rotation frequency of the motor of the pressure pack 5 and transmits it to the valve control unit 26. The valve control unit 26 outputs a control signal for controlling the opening of the second variable flow control device 24 to the second variable flow control device 24 according to the identified rotation frequency of the motor of the pressure pack 5, and adjusts the opening of the second variable flow control device 24 based on the opening control signal, thereby controlling the circulation flow of the lubricating oil, and then adjusting the temperature of the helium discharged from the vortex pressure pack 5.

[0034] In this embodiment, when the frequency recognition unit 25 recognizes that the rotation frequency of the motor of the press bag 5 is at a low frequency, the valve control unit 26 reduces the opening of the second variable flow control device 24. When the frequency recognition unit 25 recognizes that the rotation frequency of the motor of the press bag 5 is at a high frequency, the valve control unit 26 increases the opening of the second variable flow control device 24.

[0035] In this embodiment, the opening degree of the first fixed flow control device 4 is set to the opening degree corresponding to the lowest frequency, so as to reduce the power loss caused by the low-frequency second variable flow control device 24 being in the closed state.

[0036] The second variable flow control device 24 may be an electronic expansion valve, and the first fixed flow control device 4 may be a throttle orifice.

[0037] The variable frequency helium compressor capable of adjusting the lubricating oil circulation flow rate of the present invention has the following working process:

[0038] When the pressure pack 5 is started to work at the lowest rated speed of the rotating motor, helium enters the variable frequency helium compressor through the exhaust pipe 11 and the air supply pipe 12, enters the pressure pack 5 through the exhaust buffer tank 8, and after being heated and pressurized in the pressure pack 5, it is cooled by the heat exchanger 6, and filtered by the oil separator 9 and the adsorber 10, and the high-pressure helium is transported to the refrigerator through the intake pipe 7. When the second pressure sensor 21 and the first pressure sensor 15 recognize that the inlet and outlet pressure difference is too large, the bypass first variable flow control device 16 is opened to adjust the circulating helium pressure. When the pressure is too high, the excess helium will be discharged through the safety valve 22. The lubricating oil is cooled by the heat exchanger 6 from the oil pool at the bottom of the pressure pack 5, filtered by the impurities of the first filter 3, and then throttled and reduced in pressure by the first fixed flow control device 4 to enter the pressure pack 5. Part of the oil is separated by the filter element of the oil separator 9 in the form of oil mist, and enters the pressure pack together with the outlet end of the exhaust buffer tank 8. When the lubricating oil flow is insufficient, it is necessary to stop the machine and replenish oil through the oil replenishment port 17. The cooling water passes through the water inlet pipe 2 and the heat exchanger 6 to transfer heat and cool the helium and lubricating oil discharged from the pressure bag 5, and the outflowing hot cooling water returns to the chiller through the water outlet pipe 1.

[0039] When the motor rotation frequency of the pressure package 5 increases, the operating frequency increases, resulting in an increase in the helium circulation flow rate. The frequency identification unit 21 in the flow control unit 23 identifies the change in the current operating frequency of the pressure package 5, converts the signal into a control signal and outputs it to the valve control unit 26. The valve control unit 26 adjusts the opening of the second variable flow control device 24 connected in parallel to the first fixed flow control device 4 to increase the valve hole opening, thereby increasing the circulation flow of the lubricating oil.

[0040] When the motor rotation frequency of the pressure package 5 decreases, the operating frequency decreases, resulting in a decrease in the helium circulation flow rate. The frequency recognition unit 25 in the flow control unit 23 recognizes the change in the current operating frequency of the pressure package 5, and converts the signal into a control signal and outputs it to the valve control unit 26. The valve control unit 26 adjusts the opening of the second variable flow control device 24 to reduce the valve hole opening, thereby reducing the circulation flow of the lubricating oil.

[0041] The second variable flow control device 24 adjusts the flow area by adjusting the opening to adjust the flow of lubricating oil, so as to meet the cooling requirements of lubricating oil under different helium flow rates at different frequencies of the pressure pack 5. At low frequencies, the opening is reduced to reduce the circulation flow of lubricating oil and improve the working efficiency of the pressure pack 5; at high frequencies, the opening is increased to increase the circulation flow of lubricating oil and reduce throttling losses.

[0042] The flow control unit 23 receives the frequency signal from the vortex pressure pack 5 and adjusts the circulation flow of the lubricating oil and the temperature of the helium discharged from the pressure pack 5 by changing the opening of the second variable flow control device 24. The opening of the second variable flow control device 24 and the operating frequency of the pressure pack 5 satisfy a specific positive correlation curve.

[0043] The lubricating oil circulation flow control method of the present invention retains the adjustment method of the fixed flow control device 4 of the fixed frequency compressor on the basis of the opening adjustment of the second variable flow control device 24, and controls the circulation flow of the lubricating oil by combining the variable flow and the fixed flow, so as to avoid the second variable flow control device 24 being completely closed due to a circuit failure, causing the helium temperature to be too high and damage the equipment.

[0044] like Figure 2 As shown, the corresponding relationship between the rotation frequency of the motor of the press pack 5 and the circulation flow rate of the lubricating oil is given. The scale value of the horizontal axis H1 represents the lowest operating frequency when the press pack 5 is started. When the operating frequency is within the range of H2, the flow control unit 23 receives the frequency signal from the press pack 5 by the frequency identification unit, and outputs a signal through the valve control unit 26 to change the opening of the variable flow control device 24 to control the circulation flow rate of the lubricating oil. L1 is the fixed flow rate of the fixed flow control device 4, and L2 is the variable flow rate of the variable flow control device 24. The sum of the two constitutes the circulating oil flow rate in the lubricating oil circulation system.

[0045] like Figure 3 As shown, it is a flow chart of the flow control unit 23 controlling the flow rate of lubricating oil. As can be seen from the figure, the frequency recognition unit 25 of the flow control unit 15 will automatically recognize the frequency signal transmitted by the pressure bag at every time step T0, and after processing, it will be output to the second variable flow control device 24 by the valve control unit 26 to adjust the opening of the second variable flow control device 24, thereby changing the flow rate of the lubricating oil.

Claims

1. A variable frequency helium compressor capable of adjusting the circulation flow of lubricating oil, characterized in that: It includes an oil circuit system and a flow control unit (23) for controlling the flow of lubricating oil in the oil circuit system; The oil circuit system comprises a pressure pack (5), wherein the oil pool at the bottom of the pressure pack (5) is connected to an oil replenishment port (17) and one end of a heat exchanger (6); the other end of the heat exchanger (6) is connected to one end of a first filter (3), and the other end of the first filter (3) is respectively connected to one end of a second variable flow control device (24) and one end of a first fixed flow control device (4); the other end of the second variable flow control device (24) and the other end of the fixed flow control device (4) are connected to the oil return port at the top of the pressure pack (5) after merging; The flow control unit (23) is connected between the frequency signal output end of the pressure pack (5) and the control end of the second variable flow control device (24). The flow control unit (23) is used to identify the rotation frequency of the motor of the pressure pack (5) and output a control signal for controlling the opening of the second variable flow control device (24) to the control end of the second variable flow control device (24) according to the rotation frequency of the motor of the pressure pack (5), thereby adjusting the opening of the second variable flow control device (24) and further controlling the circulation flow of the lubricating oil in the oil circuit system.

2. The variable frequency helium compressor with adjustable lubricating oil circulation flow rate according to claim 1, characterized in that: The flow control unit (23) comprises a frequency identification unit (25) and a valve control unit (26), wherein the input end of the frequency identification unit (25) is connected to the frequency signal output end of the pressure bag (5), and the output end thereof is connected to the input end of the valve control unit (26), and the output end of the valve control unit (26) is connected to the control end of the second variable flow control device (24); The frequency identification unit (25) identifies the rotation frequency of the motor of the pressure bag (5) and transmits it to the valve control unit (26). The valve control unit (26) converts the identified rotation frequency of the motor of the pressure bag (5) into an opening signal of the second variable flow control device (24), and adjusts the opening of the second variable flow control device (24) based on the opening signal.

3. The variable frequency helium compressor with adjustable lubricating oil circulation flow rate according to claim 2, characterized in that: When the frequency recognition unit (25) recognizes that the rotation frequency of the motor of the pressure bag (5) is low, the valve control unit (26) reduces the opening of the second variable flow control device (24); When the frequency recognition unit (25) recognizes that the rotation frequency of the motor of the pressing bag (5) is high frequency, the valve control unit (26) increases the opening of the second variable flow control device (24).

4. The variable frequency helium compressor with adjustable lubricating oil circulation flow rate according to claim 1, characterized in that: The opening degree of the first fixed flow control device (4) is set to the opening degree corresponding to the lowest frequency.

5. The variable frequency helium compressor with adjustable lubricating oil circulation flow rate according to claim 1, characterized in that: It also includes an air circuit system, the air circuit system including an exhaust pipe (11), the exhaust pipe (11) is connected to one end of an exhaust buffer tank (8), the other end of the exhaust buffer tank (8) is connected to one end of a pressure pack (5), and the other end of the pressure pack (5) is connected to a first thermistor (18) and one end of a heat exchanger (6) in sequence; The other end of the heat exchanger (6) is connected in sequence to a third thermistor (20), an oil separator (9), a safety valve (22), an adsorber (10), and one end of an air inlet pipe (7); the other end of the air inlet pipe (7) is used to transport high-pressure helium to a refrigerator.

6. The variable frequency helium compressor with adjustable lubricating oil circulation flow rate according to claim 5, characterized in that: An air supply pipe (12) and a second pressure sensor (21) are provided on the air pipeline between the exhaust pipe (11) and the exhaust buffer tank (8).

7. The variable frequency helium compressor with adjustable lubricating oil circulation flow rate according to claim 5, characterized in that: The output end of the oil separator (9) is connected in sequence to a first pressure sensor (15) and one end of a first variable flow control device (16) through an air pipeline; the other end of the first variable flow control device (16) is connected to an exhaust buffer tank (8) to form a high-pressure internal bypass air path of the helium compressor.

8. The variable frequency helium compressor with adjustable lubricating oil circulation flow rate according to claim 5, characterized in that: The output end of the oil separator (9) is connected to one end of a second filter (13) through an oil pipeline, and the other end of the second filter (13) is connected to one end of a second fixed flow control device (14); The other end of the second fixed flow control device (14) is connected to the gas pipeline between the exhaust buffer tank (8) and the pressure bag (5).

9. The variable frequency helium compressor with adjustable lubricating oil circulation flow rate according to claim 1, characterized in that: It also includes a cooling water system, which includes a water inlet pipeline (2), the water inlet pipeline (2) is connected to one end of the heat exchanger (6), and the other end of the heat exchanger (6) is connected to the water outlet pipeline (1).

10. The variable frequency helium compressor with adjustable lubricating oil circulation flow rate according to claim 9, characterized in that: The water outlet pipeline (1) is connected to a second thermistor (19).

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