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The so-called unloader is a device that uses mechanical means to forcefully open the intake automatic valve of the piston compressor, keeping the intake port of the corresponding cylinder fully open at all times. There is no compression process of the gas inside the cylinder; the gas drawn in by the cylinder returns to the intake chamber during the exhaust process, resulting in a zero exhaust volume for that working volume. This method of regulating air volume is commonly used in double-acting piston compressors, where there is a working volume of the same size on both sides of the piston. By using a unloader, it is possible to render the working volume on either side inactive, thereby turning the double-acting cylinder into a single-acting one and reducing the exhaust volume by half. Conversely, when the unloader is not in operation, the exhaust volume of that cylinder can return to 100 percent. Of course, unloaders can also be used on both sides of the piston to reduce the displacement of the corresponding cylinder to zero. The unloaders commonly used today usually consist of an actuator fixed to the cylinder housing of the piston compressor, in combination with a mesh-type intake automatic valve equipped with unload forks inside the cylinder. The ejector rod in the actuator is used to push the unloading fork, forcing the intake valve to open and thus achieving unloading. As for the source of force that drives the push rod, common options include pneumatic and electromagnetic types, that is, power is provided by a pneumatic diaphragm, a pneumatic plunger, or an electromagnetic method. The type that is used more frequently nowadays is the pneumatic diaphragm-type unloader, and its working principle is shown in the figure below. Working principle: When high-pressure gas enters the diaphragm box through the interface at its top, it increases the pressure of the gas inside the box. This causes the elastic diaphragm within the box to deform in the direction of the pressure difference, pushing the push rod outward. Once this push rod comes into contact with the unloading fork on the intake valve, it causes the unloading fork to move axially, until the prongs on the unloading fork fully open the valve leaf of the intake valve. As a result, the intake valve cannot close on its own, and the intake passage remains completely open. The gas that enters the cylinder can flow back freely along this passage; since the gas is not compressed, the exhaust valve cannot be opened, which leads to a zero exhaust volume for this working volume. When high-pressure gas is discharged from the diaphragm box through the same interface at its top to empty it, the elastic membrane inside the diaphragm box causes the rod of the unloader to return to its original position. The unloading fork on the intake valve, no longer under the push of the rod, returns to its original position thanks to its own spring, causing the forks to disengage from the valve plates. The valve then returns to normal operation, no longer under the control of the unloader, and at this point the working volume returns to its designed state, allowing normal intake and exhaust of air. Regulation feature: A unloader is used to adjust the compressor’s displacement. In double-acting cylinders, regulation at 0%, 50%, and 100% levels can be achieved by either not pressing open the intake valve, pressing open one of the intake valves, or pressing open both intake valves simultaneously. As gas continuously flows in and out through the intake valve, the resistance loss is greater than when the valve is operating normally; as a result, the power consumption per unit volume of gas increases, but the overall power consumption is significantly reduced. Remote control can be achieved when the unloader adjustment is controlled by electromagnetic valves for the intake and release of instrument air. In multi-stage compressors, it is necessary for the unloaders in each stage to operate in unison in order to achieve the desired control effect. Since the unloader is a moving part, the overall adjustment reliability is reduced. For lubrication-free compressors, special consideration must be given to the fact that the moving parts within the unloader must be oil-free lubricated, especially in the case of special compressors used with flammable and explosive gas media. Generally speaking, using a unloader to regulate the exhaust volume results in a indicated work amount that corresponds to the energy required to overcome the resistance of the fully open intake valve, thus offering higher efficiency. The diagram showing how the intake valve is adjusted by using a unloader is as follows: Over the years, piston compressors have developed various methods for adjusting the volume of exhaust gas, such as variable speed control, adjustment by opening the intake valve (unloader control falls under this category), regulation of the connection between intake and exhaust (backflow control), and adjustment by changing the supplementary volume (adjustment of the clearance volume). These methods differ significantly in terms of cost-effectiveness, reliability, and ease of operation. Among them, variable speed regulation offers the best economic efficiency, but it is less used due to the high initial cost associated with the use of variable frequency motors. Most users still employ the compressor return pipeline for regulating the exhaust volume (as a form of backflow control), by connecting the inlet pipe at the front of the compressor to the outlet pipe at the back, and using automatic valves (pneumatic or solenoid valves) to control the amount of air that flows back. When the supply of air decreases, this backflow is used to compensate for the reduced intake volume, thereby keeping the compressor operating under its rated conditions. This control method requires less investment and still enables remote control; it offers high reliability in operation and allows for stepless adjustment. However, the compressor’s power consumption does not decrease as the exhaust volume decreases, resulting in poor economic efficiency. Theoretically, it is possible to achieve load relief regulation by partially opening the intake valve, allowing for stepless adjustment of the exhaust volume from 0 to 100%, which thus makes the adjustment more flexible. However, there are also obvious disadvantages: 1. As the valve leaf in the valve moves continuously due to changes in the pressure difference before and behind the valve, it repeatedly impacts the load relief fork, leading to fatigue and damage of the valve leaf. 2. It is difficult to match the degree of unloading adjustment for each stage of a multi-stage compressor. 3. An increase in the complexity of control components reduces reliability. Therefore, this type of control method is hardly used in the field of piston compressors. Currently, the manufacturers of unloading devices are all valve producers. Since very few users employ unloading devices to regulate exhaust volume, most of these devices have little to no actual usage. However, considering economic factors, there should be a certain market for this type of regulation method. There are various ways to adjust the displacement of piston compressors. The user should consider factors such as adjustment characteristics, cost-effectiveness, and reliability in order to select the most suitable method of adjustment.