CENTRIFUGAL / AXIAL COMPRESSORS

engineers March 11, 2021

Centrifugal Compressors are used for process applications from about 500 to 200,000 acme-inlet, and axial-flow compressors from about 75,000 to 600,000 acme (inlet). Smaller axial are available (down to 20,000 acme), but centrifugal are preferred at these capacities.

Axial-flow compressors are more efficient than centrifugal, and can be provided in higher capacities. However, centrifugal are far more common in the process industry. Centrifugal compressors have wider operating ranges.

Axial are generally used only for air, or clean, non-corrosive gases, since these machines are more susceptible to deposits, corrosion and erosion than centrifuges. The outlet temperature limits the maximum compression ratio per stage. The usual limit is 400° to 450° F for centrifuges and about 600° F for axial. The temperature limit is lower for centrifuges because they have diaphragms between impellers, which tend to expand radially due to the temperature rise of the compressed gas.

For some applications, the compression ratio is set by the maximum number of impellers the manufacturer can provide within a single casing (up to about 6 to 8 for centrifuges and about 12 to 24 for axials). For centrifugal machines, a single impeller will usually develop up to 10,000 to 12,000 ft.-Ibs. /lb. of polytrophic head. The corresponding value for an axial is about half (4,500 to 6,000 ft.-lbs./lb. per impeller). To compensate for this, an axial unit will usually have about twice as many impeller stages (sets of rotating and stationary blades) as a corresponding centrifugal.

F1 shows approximate ranges of application for centrifugal and axial flow machines. Maximum discharge pressures range from about 10,000 psia for small centrifuges down to considerably lower pressures for large machines. Axials above 75,000 acfm are usually limited to about 130 psia at the outlet. As noted earlier. Fig. 1 should be regarded "only as a rough guide to current process industry practice