Pressure is increased by transferring energy to the gas, accelerating it through the impeller. Note that all work on the gas is done by the impeller; the stationary components only convert the energy added by the impeller. Part of this energy is converted to pressure in the impeller and the remainder is converted to pressure as it decelerates in the diffuser. A typical pressure-velocity profile across a stage is shown in Figure 200-2.
Since the kinetic energy is a function of the square of the velocity, the head (not pressure) produced is proportional to the square of the impeller tip speed:
Note “Head” is a term often used for the work input to a compression process. The units of head are foot-pounds (force) divided by pounds (mass). In general practice, “head” is usually taken as “feet.”
Manufacturers generally define performance of individual impellers in terms of:
• Head coefficient m – a function of actual work input and stage efficiency
• Flow coefficient f – a non-dimensional function of volume flow and rotational speed
Figure 200-3 represents a typical individual impeller curve. The head coefficient typically varies from about 0.4 to 0.6. The surge line in the figure is discussed in Section224. Using the head coefficient, the head can now be shown as: