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The definition of centrifugal (a word, by the way, coined by Newton) is moving, or tending to move, away from a center. Gas moves into the wheel through the inducer, the function of which is to provide a smooth (efficient) transition into the "impeller" section of the wheel. The gas enters the "impeller" section and is moved away from the center, and at the same time velocity is imparted to the gas. The gas exits the impeller and enters the diffusion area, which can be vaned or vane less, and continues into the volute.

In the volute, the gas velocity decreases (due to the increasing area) causing an increase in static pressure. A compressor wheel then has two sections: the inducer and the impeller. There are two factors that will affect impeller size. First is the volume flow, or actual cubic feet per minute (ACFM), or normal cubic meters per hour (Nm3/h). Centrifugal compressors are volumetric devices, and the compressor must be sized for the maximum volume. This value affects the inducer section of the wheel. The inducer must be sized for the correct diameter (D1) or the wheel will choke. (Choke is when you try to stuff too much flow in the given area, and it will not swallow any more.)

The flow is calculated by the formula:

The second factor is the head requirement. The head is primarily determined by the pressure ratio, molecular weight of the gas and inlet temperature. The formula is:

*Head* ≈ *Pr* × *R* × *T*

Therefore the compressor must be designed for the highest inlet temperature and the lowest molecular weight. This will ensure that all operating points will be obtained. The head can then be converted to tip speed by the formula:

The wheel diameter (D2) can then be calculated by: