Let’s briefly define what an axial pump is. Axial pumps are a type of pump characterized by low discharge pressure and high flow rate. They are widely used in many applications, particularly where high flow at low pressure is required. An axial flow pump is a pump in which the fluid flows parallel to the shaft axis. For example, consider the propeller of a ship or a boat—this is an axial pump. It operates on the same principle as a ceiling fan. An axial pump is also a type of centrifugal pump that uses the momentum and velocity of the fluid to generate the pressure needed for higher flow rates. As with all centrifugal pumps, energy transfer in axial flow pumps occurs entirely through the fluid flow.
Working Principle and Characteristics of Axial Flow Pumps
An axial pump is a type of dynamic pump. It pushes the working fluid in a direction parallel to the shaft or impeller axis. This is different from centrifugal pumps, where the fluid exits perpendicular to the impeller axis. Some axial pump designs include blades at the pump outlet that form a diffuser. These blades remove the rotational effect of the fluid and convert its kinetic energy into working pressure. Axial pumps are used in systems requiring high fluid discharge at low head, such as flood control and drainage systems.
Characteristic Curves: Centrifugal vs. Axial Pumps
There are several differences between centrifugal and axial pumps, which can be illustrated through characteristic curves. Some key points include:
- Both pump types can achieve similar maximum efficiency levels.
- In a centrifugal pump, as fluid flow decreases, the input power also decreases. In contrast, in an axial pump, the input power reaches its maximum when the flow stops. This means that variations in fluid flow do not affect the axial pump’s operation—it continues functioning efficiently at all times.
- Centrifugal pumps generally produce higher heads compared to axial pumps.
Factors to Consider When Selecting an Axial Pump
Pump selection depends on the application and intended use. For long service life and optimal performance, the following performance characteristics should be considered:
Flow Rate: Defines how fast the pump moves fluid through the system, typically expressed in gallons per minute (gpm). The pump’s rated capacity should match the required system flow rate.
Pressure: As with all centrifugal pumps, the pressure generated by an axial flow pump depends on the specific weight of the fluid being pumped.
Net Positive Suction Head (NPSH): The difference between the pump’s inlet static pressure head and the vapor pressure head of the fluid. The required NPSH is critical to preventing cavitation.
Applications of Axial Pumps
Axial pumps are widely used across many industries. Some common applications include:
- Agricultural machinery
- Irrigation systems
- Sewage treatment plants
- Cooling water pumps
- Drainage systems