The most common types of fluid machinery are turbines and pumps, compressors or fans, positive displacement machines and rotodynamic machines.
Each type of fluid machine can be classified into one of two categories: energy conversion direction or principle of operation. Turbines, pumps and fans are all fluid machines established on direction of energy conversion. Positive displacement machines and rotodynamic machines are fluid machines classified on principles of operation.
According to RoyMech.co.uk, "A machine for converting mechanical energy into fluid flow is called a pump." Turbines convert energy from fluids into rotating shaft energy. Compressors convert mechanical energy for the purpose of increasing a compressible fluid's potential energy by increasing its pressure. A fan machine is used to increase a compressible fluid's kinetic energy. Positive displacement machines draw fluid at low pressures into a chamber. When the chamber inlet is closed, the outlet opens, and the volume of the fluid is reduced and forced out of its chamber. Rotodynamic machines pass fluids through rotating components; in turbines, the rotating component is called a rotor.
Some fluid machines are called either "radial flow" or "axial flow" machines based on the direction of the flow of fluid through a rotor. In radial flow turbines, the fluid flows toward the rotor's center. In pumps and compressors, the fluid flows away from the rotor's center. Radial flow machines are referred to as "inward flow machines" and radial flow machines are referred to as "outward flow machines."
Positive displacement machines feature nearly zero fluid slippage during the energy transfer process. They can generate high pressures in their compact design. Rotodynamic fluid machines provide smooth and continuous action, but they can easily be damaged if discharge valves are abruptly closed. These pumps are often used to handle high-flow, low-discharge applications.