Short Answer:

Non-positive displacement machines are devices that transfer fluid by increasing its velocity and converting this velocity into pressure. They do not trap a fixed amount of fluid but instead move it continuously through the machine. The flow rate depends on the speed of the impeller and the resistance offered by the system.

Examples of non-positive displacement machines include centrifugal pumps, axial flow pumps, and mixed flow pumps. These machines are commonly used for large volume, low-pressure applications such as water supply, irrigation, and cooling systems where continuous and smooth flow is needed.

Detailed Explanation :

Non-Positive Displacement Machines

Non-positive displacement machines are fluid handling devices that operate on the principle of imparting velocity to the fluid and then converting this velocity energy into pressure energy. Unlike positive displacement machines, they do not deliver a fixed quantity of fluid during each cycle. Instead, the flow is continuous and depends on the machine’s speed and the system pressure.

These machines are mainly used in situations where a large quantity of fluid must be moved at relatively low or moderate pressures. The most common examples include centrifugal pumps and axial flow pumps. Non-positive displacement machines are widely used in domestic, agricultural, and industrial applications for pumping water, cooling fluids, and other similar tasks.

They are also known as dynamic machines, as they rely on the dynamic action of impellers or rotors to transfer energy to the fluid. The performance of these machines is strongly influenced by the discharge pressure, fluid viscosity, and impeller speed.

Working Principle

The basic principle behind non-positive displacement machines is conversion of kinetic energy into pressure energy. The impeller or rotor rotates at high speed, imparting velocity to the fluid. As the fluid passes through the impeller blades, it gains kinetic energy. This high-velocity fluid then moves into a diffuser or volute casing, where the velocity decreases, and the kinetic energy is converted into pressure energy according to Bernoulli’s principle.

The discharge rate or flow in these machines depends on:

The speed of rotation of the impeller.
The resistance of the discharge system (pressure head).
The design and geometry of the impeller and casing.

If the resistance at the outlet increases, the flow rate automatically reduces, and if resistance decreases, the flow rate increases. Therefore, non-positive displacement machines are not suitable for applications requiring constant flow under varying loads.

Types of Non-Positive Displacement Machines

Centrifugal Pump:
This is the most common type of non-positive displacement machine. It consists of an impeller mounted on a shaft and enclosed within a casing. As the impeller rotates, it draws fluid into the center (eye) and throws it radially outward due to centrifugal force. The fluid velocity increases, and this velocity is converted into pressure in the diffuser section. Centrifugal pumps are widely used for water supply, irrigation, and industrial cooling.
Axial Flow Pump:
In this type, the fluid moves parallel to the pump shaft (axially) instead of radially. The impeller consists of propeller-like blades that push the fluid in the direction of the shaft. Axial flow pumps are suitable for applications that require a very high flow rate at a relatively low head, such as flood control and circulation systems.
Mixed Flow Pump:
This type combines features of both centrifugal and axial flow pumps. The fluid experiences both radial and axial motion. Mixed flow pumps are used where moderate head and high discharge are needed, such as in irrigation or cooling tower circulation.

Characteristics of Non-Positive Displacement Machines

The discharge rate is not constant; it varies with the system head.
They are best suited for handling large quantities of low-viscosity fluids.
They are lighter, simpler, and less expensive than positive displacement machines.
The efficiency depends on speed and load conditions.
They cannot handle high-viscosity or compressible fluids effectively.
They usually require priming before operation to remove trapped air and ensure proper suction.

Applications

Non-positive displacement machines are widely used in:

Domestic water supply systems.
Irrigation and drainage systems.
Cooling systems in engines and power plants.
Chemical and process industries for fluid circulation.
Marine and wastewater treatment plants.

Their ability to provide continuous and smooth fluid flow makes them ideal for general-purpose pumping and circulation tasks.

Advantages

Simple design and easy maintenance.
Continuous and smooth flow without pulsation.
Suitable for large flow rate applications.
Compact and lightweight construction.
Economical for low-pressure systems.

Disadvantages

Flow rate depends on system pressure.
Cannot handle viscous fluids effectively.
Require priming before use.
Less efficient at high heads or variable loads.
Not suitable for precise flow control.

Conclusion

Non-positive displacement machines play an important role in moving fluids where a constant flow rate is not essential. They operate efficiently for low-pressure, high-volume fluid transfer. Although they cannot provide high-pressure output like positive displacement types, their simplicity, reliability, and ability to handle large fluid volumes make them very useful in practical engineering applications such as irrigation, cooling, and water transport systems.