With the rapid development of science and technology, technology related to industrial production is also developing rapidly. Once new technologies and equipment appear, they will be quickly and widely used in industrial production. This is the case. Today, air compressors are widely used in industrial production. For example, in the metal packaging industry, it is responsible for providing air sources for all pneumatic components on the production line (including various pneumatic valves); therefore, its operation directly affects the efficient operation of the production line. There are many types of air compressors, but almost all have load and unload controls.

For example, piston air compressors, screw air compressors, and screw air compressors all use this control method. According to our many years of operating experience, although this air supply control method is simple in principle and easy to operate, it has many problems such as high energy consumption, easy damage to the air inlet valve, and unstable air supply pressure. With the development and progress of society, high-efficiency and low-consumption technologies have attracted more and more attention. Whether frequency conversion speed regulation technology can be applied to the field of air compressor air supply to save electric energy and improve air compressor performance and air supply quality has become a topic of concern to us. Based on the actual production, we chose a fixed screw air compressor for research.

Introduction to air compressor loading and unloading air supply control methods

Here, we will briefly introduce the control method of loading and unloading air sources based on the electrical control principle of fixed screw air compressors.

When we run the device, when the start button is pressed, the sb2 and kt1 coils are energized, and their instantaneous closing and delayed opening moving contacts close. When the km4 and km6 coils are energized, the compressor motor begins a Y-start. At this time, the intake control valve yv2 is electrically operated to control the release of gas from the small gas storage tank into the piston cavity of the intake valve, close the intake valve, and start the compressor at light load. When kt2 reaches the set time (usually after 5 seconds), its delayed dynamic breaking contact opens, and the delayed dynamic closing contact closes. After the power is turned off, the km6 coil is released, the km5 coil is energized, and the air compressor motor automatically switches to Y Shaped into a triangle. At this time, YV2 is powered off, the control gas released from the gas storage tank is cut off, the intake valve is fully opened, and the unit operates at full load. (Note: The intake control valve yv2 only works when starting, while the unloading control valve yv1 works after starting).

If the required gas volume is lower than the rated discharge volume, the discharge pressure will rise. When it exceeds the set minimum pressure value pmin (also called loading pressure), the pressure regulator will act to deliver control gas to the intake valve. The piston in the intake valve partially closes the intake valve, reducing the amount of air intake and balancing gas supply and consumption. When the pipeline pressure continues to rise beyond the maximum pressure value pmax (also called unloading pressure) set by the pressure regulating switch (sp2), the pressure regulating switch trips and the solenoid valve yv1 is powered off. In this way, the control gas directly enters the air inlet valve, completely closing the air inlet; at the same time, the exhaust valve opens under the action of the control gas, releasing the compressed air in the separation tank. When the pipeline pressure drops below pmin, the pressure regulating switch sp2 is reset (closed) and yv1 is energized. At this time, the control gas flow to the inlet valve and exhaust valve is cut off. In this way, all inlet valves are opened again, the exhaust valves are closed, and the unit operates at full load.

VFD in the ventilation control system

Problems with loading and unloading gas supply control methods

  • 1. Energy consumption analysis

We know that the control method of loading and unloading causes the pressure of compressed gas to change back and forth between PMIN and PMAX. Pmin is the lowest pressure value, that is, the lowest pressure that can ensure normal operation of the user. Usually, the relationship between pmax and pmin can be expressed by the following formula:

pmax=(1+δ)pmin

δ is a percentage, and its value is roughly between 10% and 25%.

If the variable frequency speed regulation technology can continuously adjust the air supply volume, the pipe network pressure can always be maintained at the working pressure that can meet the air supply volume, that is, around pmin.

Therefore, compared with the air compressor controlled by the frequency conversion system, the energy wasted by the air compressor under the loading and unloading air control mode is mainly reflected in two aspects:
(1) The energy consumed when the compressed air pressure exceeds pmin.

After the pressure reaches pmin, the original control method determines that the pressure continues to rise (until pmax). During this process, more heat will be released to the outside world, causing energy loss. On the other hand, gas pressure above pmin needs to be reduced to close to pmin through a pressure reducing valve before entering the pneumatic components. This process is also an energy-consuming process.

(2) The energy consumed by unreasonable adjustment methods during the unloading process.

Normally, when the pressure reaches pmax, the air compressor can be decompressed and unloaded through the following methods: close the air inlet valve to make the motor idling, and drain the excess compressed air in the separation tank through the exhaust valve. This adjustment method will cause a huge waste of energy. Although closing the air inlet valve and letting the motor idle can eliminate the need for the air compressor to compress the gas, the air compressor still needs to drive the screw to rotate when idling. According to our calculations, the energy consumption when the air compressor is unloaded accounts for about 10% to 15% of the full-load operation of the air compressor (this is still when the unloading time accounts for a small proportion). In other words, the air compressor is idle 10% of the time, doing nothing. Obviously, in the control mode of loading and unloading air supply, the motor of the air compressor has a lot of room for energy saving.

  • 2. Other disadvantages

(1) Use mechanical methods to adjust the air inlet valve so that the air supply cannot be adjusted continuously. When the air consumption continues to change, the air supply pressure will inevitably fluctuate greatly. The accuracy of gas consumption cannot meet the process requirements. Coupled with frequent adjustments to the intake valve, it will accelerate the wear of the intake valve and increase the amount of maintenance and costs.

(2) The exhaust valve is frequently switched on and off, and the durability of the exhaust valve cannot be guaranteed.

Design of constant pressure air supply control scheme

Since there are many problems with the original air supply control method, after comparative analysis of the above content, we believe that variable frequency speed regulation technology should be used for constant pressure air supply control. After the implementation of this plan, we can use the pressure of the pipeline network as the control object. The pressure transmitter converts the pressure of the gas tank into a standard electrical signal, sends it to the regulator, and compares it with the set pressure value. Then based on the difference, calculation is performed according to the established control method, a control signal is generated and sent to the frequency converter. The operating frequency and speed of the motor are controlled by VFD, so that the actual pressure is always close to the set pressure.

At the same time, this solution can also add the function of switching between power frequency and variable frequency, and can also maintain the original control and protection system. In addition, after the solution is implemented, the frequency converter can be used to start the control of the air compressor motor from standstill to rotation, which can not only realize the soft start of the equipment, but also effectively avoid the starting inrush current and the mechanical impact on the air compressor during startup.

With the rapid development of science and technology, technology related to industrial production is also developing rapidly. Once new technologies and equipment appear, they will be quickly and widely used in industrial production. This is the case. Today, air compressors are widely used in industrial production. For example, in the metal packaging industry, it is responsible for providing air sources for all pneumatic components on the production line (including various pneumatic valves); therefore, its operation directly affects the efficient operation of the production line. There are many types of air compressors, but almost all have load and unload controls.

For example, piston air compressors, screw air compressors, and screw air compressors all use this control method. According to our many years of operating experience, although this air supply control method is simple in principle and easy to operate, it has many problems such as high energy consumption, easy damage to the air inlet valve, and unstable air supply pressure. With the development and progress of society, high-efficiency and low-consumption technologies have attracted more and more attention. Whether frequency conversion speed regulation technology can be applied to the field of air compressor air supply to save electric energy and improve air compressor performance and air supply quality has become a topic of concern to us. Based on the actual production, we chose a fixed screw air compressor for research.

Introduction to air compressor loading and unloading air supply control methods

Here, we will briefly introduce the control method of loading and unloading air sources based on the electrical control principle of fixed screw air compressors.

When we run the device, when the start button is pressed, the sb2 and kt1 coils are energized, and their instantaneous closing and delayed opening moving contacts close. When the km4 and km6 coils are energized, the compressor motor begins a Y-start. At this time, the intake control valve yv2 is electrically operated to control the release of gas from the small gas storage tank into the piston cavity of the intake valve, close the intake valve, and start the compressor at light load. When kt2 reaches the set time (usually after 5 seconds), its delayed dynamic breaking contact opens, and the delayed dynamic closing contact closes. After the power is turned off, the km6 coil is released, the km5 coil is energized, and the air compressor motor automatically switches to Y Shaped into a triangle. At this time, YV2 is powered off, the control gas released from the gas storage tank is cut off, the intake valve is fully opened, and the unit operates at full load. (Note: The intake control valve yv2 only works when starting, while the unloading control valve yv1 works after starting).

If the required gas volume is lower than the rated discharge volume, the discharge pressure will rise. When it exceeds the set minimum pressure value pmin (also called loading pressure), the pressure regulator will act to deliver control gas to the intake valve. The piston in the intake valve partially closes the intake valve, reducing the amount of air intake and balancing gas supply and consumption. When the pipeline pressure continues to rise beyond the maximum pressure value pmax (also called unloading pressure) set by the pressure regulating switch (sp2), the pressure regulating switch trips and the solenoid valve yv1 is powered off. In this way, the control gas directly enters the air inlet valve, completely closing the air inlet; at the same time, the exhaust valve opens under the action of the control gas, releasing the compressed air in the separation tank. When the pipeline pressure drops below pmin, the pressure regulating switch sp2 is reset (closed) and yv1 is energized. At this time, the control gas flow to the inlet valve and exhaust valve is cut off. In this way, all inlet valves are opened again, the exhaust valves are closed, and the unit operates at full load.

VFD in the ventilation control system

Problems with loading and unloading gas supply control methods

  • 1. Energy consumption analysis

We know that the control method of loading and unloading causes the pressure of compressed gas to change back and forth between PMIN and PMAX. Pmin is the lowest pressure value, that is, the lowest pressure that can ensure normal operation of the user. Usually, the relationship between pmax and pmin can be expressed by the following formula:

pmax=(1+δ)pmin

δ is a percentage, and its value is roughly between 10% and 25%.

If the variable frequency speed regulation technology can continuously adjust the air supply volume, the pipe network pressure can always be maintained at the working pressure that can meet the air supply volume, that is, around pmin.

Therefore, compared with the air compressor controlled by the frequency conversion system, the energy wasted by the air compressor under the loading and unloading air control mode is mainly reflected in two aspects:
(1) The energy consumed when the compressed air pressure exceeds pmin.

After the pressure reaches pmin, the original control method determines that the pressure continues to rise (until pmax). During this process, more heat will be released to the outside world, causing energy loss. On the other hand, gas pressure above pmin needs to be reduced to close to pmin through a pressure reducing valve before entering the pneumatic components. This process is also an energy-consuming process.

(2) The energy consumed by unreasonable adjustment methods during the unloading process.

Normally, when the pressure reaches pmax, the air compressor can be decompressed and unloaded through the following methods: close the air inlet valve to make the motor idling, and drain the excess compressed air in the separation tank through the exhaust valve. This adjustment method will cause a huge waste of energy. Although closing the air inlet valve and letting the motor idle can eliminate the need for the air compressor to compress the gas, the air compressor still needs to drive the screw to rotate when idling. According to our calculations, the energy consumption when the air compressor is unloaded accounts for about 10% to 15% of the full-load operation of the air compressor (this is still when the unloading time accounts for a small proportion). In other words, the air compressor is idle 10% of the time, doing nothing. Obviously, in the control mode of loading and unloading air supply, the motor of the air compressor has a lot of room for energy saving.

  • 2. Other disadvantages

(1) Use mechanical methods to adjust the air inlet valve so that the air supply cannot be adjusted continuously. When the air consumption continues to change, the air supply pressure will inevitably fluctuate greatly. The accuracy of gas consumption cannot meet the process requirements. Coupled with frequent adjustments to the intake valve, it will accelerate the wear of the intake valve and increase the amount of maintenance and costs.

(2) The exhaust valve is frequently switched on and off, and the durability of the exhaust valve cannot be guaranteed.

Design of constant pressure air supply control scheme

Since there are many problems with the original air supply control method, after comparative analysis of the above content, we believe that variable frequency speed regulation technology should be used for constant pressure air supply control. After the implementation of this plan, we can use the pressure of the pipeline network as the control object. The pressure transmitter converts the pressure of the gas tank into a standard electrical signal, sends it to the regulator, and compares it with the set pressure value. Then based on the difference, calculation is performed according to the established control method, a control signal is generated and sent to the frequency converter. The operating frequency and speed of the motor are controlled by VFD, so that the actual pressure is always close to the set pressure.

At the same time, this solution can also add the function of switching between power frequency and variable frequency, and can also maintain the original control and protection system. In addition, after the solution is implemented, the frequency converter can be used to start the control of the air compressor motor from standstill to rotation, which can not only realize the soft start of the equipment, but also effectively avoid the starting inrush current and the mechanical impact on the air compressor during startup.

continue reading

Related Posts