Short Answer:

Positive displacement machines are mechanical devices that transfer a fixed amount of fluid from the inlet to the outlet with each cycle. They work by trapping a specific volume of fluid and forcing it through the system under pressure. These machines provide a steady flow rate regardless of discharge pressure and are commonly used in hydraulic systems, compressors, and pumps.

They are ideal for applications that require high pressure and precise flow control. Examples include reciprocating pumps, gear pumps, vane pumps, and screw compressors. Positive displacement machines are reliable, efficient, and capable of handling viscous fluids easily.

Detailed Explanation :

Positive Displacement Machines

Positive displacement machines are a type of fluid-handling device used to move liquids or gases by trapping a certain amount of the fluid and then forcing (displacing) it into the discharge pipe or outlet. The fundamental working principle of these machines is based on the repeated mechanical action of a piston, screw, vane, or gear, which traps and moves a fixed volume of fluid during each cycle of operation.

Unlike dynamic machines such as centrifugal pumps, where the flow rate depends on the speed and discharge pressure, the flow rate in positive displacement machines remains almost constant regardless of system pressure. This makes them suitable for applications where precise flow control and high pressure are required.

Positive displacement machines can handle a wide range of fluids, including thick, viscous, and even contaminated liquids, which makes them highly versatile in industrial and hydraulic applications.

Working Principle

The operation of positive displacement machines involves two main steps — intake (suction) and discharge (delivery). During the intake stroke, the machine creates a low-pressure area that allows the fluid to enter the chamber. Then, during the discharge stroke, mechanical motion compresses or pushes the trapped fluid out of the chamber into the outlet pipe.

Because each cycle transfers a fixed amount of fluid, the flow rate is directly proportional to the speed of the machine. For example, doubling the rotational speed of a gear pump doubles the discharge rate. The mechanical parts of the machine, such as pistons, vanes, gears, or screws, ensure that fluid does not leak back into the inlet, maintaining constant efficiency and pressure.

These machines are self-priming and can deliver fluid even when system resistance is high, making them very effective in high-pressure operations.

Types of Positive Displacement Machines

1. Reciprocating Type:
   These machines use a piston or plunger that moves back and forth in a cylinder. During the suction stroke, the fluid enters the cylinder, and during the discharge stroke, the piston pushes it out. Examples include piston pumps and diaphragm pumps. They are commonly used for high-pressure, low-flow applications such as hydraulic presses and oil drilling.
2. Rotary Type:
   In rotary positive displacement machines, rotating elements like gears, vanes, or screws are used to trap and move the fluid. Examples include gear pumps, screw pumps, and vane pumps. Rotary types are used where smooth, continuous flow at moderate pressure is required, such as in lubrication systems and hydraulic circuits.

Applications

Positive displacement machines are widely used in industries and systems where precise and consistent fluid flow is necessary. Common applications include:

• Hydraulic systems for driving actuators and motors.
• Oil and fuel transfer systems.
• Chemical and pharmaceutical industries for controlled dosing.
• Food and beverage industry for viscous product handling.
• Refrigeration compressors and pneumatic systems.

They are also used in construction equipment, marine engines, and process industries where steady output pressure and reliable operation are crucial.

Advantages

• Constant flow rate irrespective of pressure variations.
• High efficiency at high pressures.
• Can handle viscous and non-lubricating fluids.
• Self-priming capability.
• Precise control of fluid volume and direction.

Disadvantages

• Generally more expensive than dynamic types.
• Flow pulsation may occur in reciprocating types.
• Maintenance required for moving parts.
• Limited suitability for large volume flow applications.

Conclusion

Positive displacement machines are essential in mechanical and hydraulic systems for transferring a fixed amount of fluid efficiently and accurately. Their ability to maintain a constant flow under varying pressures makes them highly valuable for industrial operations. Despite their higher maintenance needs, they remain the best choice for high-pressure, precision-demanding, and viscous fluid applications.