This is a common type of positive displacement pump. While most people are likely more familiar with centrifugal pumps in their daily lives, they may know little about positive displacement pumps. The concept is actually quite simple: just as a centrifugal pump uses centrifugal force to move liquid, a positive displacement pump moves liquid by changing the internal volume of the pump chamber. Common examples include diaphragm pumps, plunger pumps, and rotor pumps. Of course, there is much more to discuss regarding positive displacement pumps, but I will not go into further detail today; instead, the main focus here is the
principle of the pneumatic diaphragm pump,
leaving a detailed explanation of positive displacement pumps for another time.
A pneumatic diaphragm pump consists primarily of two main sections: the drive mechanism and the diaphragm pump head.
The drive mechanism powers the reciprocating movement of the diaphragm; common transmission methods include mechanical, hydraulic, and pneumatic systems, with hydraulic transmission being the most widely used. The working section of the pump comprises a crank-connecting rod mechanism, a plunger, a hydraulic cylinder, a diaphragm, the pump body, and suction and discharge valves. Notably, the drive assembly—consisting of the crankshaft, connecting rod, plunger, and hydraulic cylinder—is very similar to that of a reciprocating plunger pump. During operation, the crank-connecting rod mechanism, driven by an electric motor, causes the plunger to move back and forth. This motion is transmitted to the diaphragm via the working fluid (usually oil) inside the hydraulic cylinder, causing the diaphragm to flex back and forth.
The pump head section utilizes a diaphragm to separate the fluid being pumped from the working fluid. When the diaphragm moves toward the drive mechanism, negative pressure is created within the pump chamber, drawing in the fluid; conversely, when the diaphragm moves in the opposite direction, the fluid is discharged. Because the pumped fluid is isolated from the working fluid by the diaphragm, it comes into contact only with the pump chamber, the valves, and one side of the diaphragm itself—never the plunger or sealing components. This ensures that critical parts like the plunger operate entirely within an oil medium, maintaining optimal working conditions. Diaphragms require good flexibility and corrosion resistance; they are typically made from materials such as PTFE or rubber. Bowl-shaped components featuring perforations are positioned on either side of the diaphragm to prevent excessive localized deformation; these are generally known as diaphragm limiters. Pneumatic diaphragm pumps offer excellent sealing performance, easily achieving leak-free operation, and are suitable for conveying corrosive liquids—such as acids, alkalis, and salts—as well as high-viscosity fluids. Each of the pump's two symmetrical working chambers contains a diaphragm, and the two are linked together by a central connecting rod. Compressed air enters the air distribution valve from the pump's inlet and is directed by the distribution mechanism into one of the chambers, driving the diaphragm to move while exhausting gas from the other chamber. Upon reaching the end of the stroke, the distribution mechanism automatically redirects the compressed air to the opposite working chamber, driving the diaphragm in the reverse direction; this results in continuous, synchronized reciprocating motion of both diaphragms. As compressed air enters the distribution valve and drives the diaphragm to the right, suction draws the medium in through the inlet, lifting the ball valve to allow entry before the valve reseats; simultaneously, the medium within the chamber is compressed, forcing the outlet ball valve open for discharge while the inlet ball valve remains closed to prevent backflow. This cycle repeats, continuously drawing the medium in through the inlet and discharging it through the outlet.
The working principle of pneumatic diaphragm pumps
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