Page 432 - Compression Machinery for Oil and Gas

Downstream Chapter 10 411

impellers are typically used for low flow coefficients (smaller inlet flow areas)
this effect is not severe. In a high flow high inlet eye diameter 3D impeller, the
blade angles along the leading edge may be varied from disk to shroud to match
the incoming flow field and obtain optimum incidence angles. This typically
occurs at the impeller’s design flow rate, and any increase or decrease in flow
rate will result in nonoptimal incidence and lower performance. The efficiency
of an impeller is a function of the inlet flow coefficient. As the flow coefficient
increases, there is an increase in impeller efficiency. While it would seem that
only 3D impellers should be used due to their higher efficiency, it generally
results in a larger stage spacing (axial length), which could increase rotordy-
namic complexity and cost. Consequently 3D impellers are typically used only
for the first or first few impellers unless an incoming sidestream calls for a high
flow coefficient stage further down the machine. High flow and high-efficiency
design staging may require reduced speeds due to higher stress levels encoun-
tered. The compressor supplier’s and designer’s experience in selecting various
flow coefficient impellers should be carefully reviewed.

Machine Mach Number and Inlet Relative Mach Number

As flow increases, the impeller eye diameter also increases and because of the
high molecular weight of some refrigerants such as propane (MW¼44) and low
inlet temperatures, high Mach conditions often exist for these compressors.
Mach number is generally defined as the ratio between the gas speed and the
speed of sound. In centrifugal compressor applications two Mach numbers com-
monly referred to are the peripheral Mach number (M u ) and the inlet relative
Mach number (M 1rs ). The peripheral Mach number (also known as machine
Mach number) is commonly used because it easy to calculate and it gives a
rough indication about the criticality of the stage and provides a benchmark
for the impeller exit velocity into the diffuser. The inlet relative Mach number
is much more important and gives an indication of the gas behavior at the impel-
ler eye. It is based on the inlet relative velocity at the shroud as shown in
Fig. 10.8. It is very important to carefully evaluate different manufacturer’s cal-
culations of inlet relative Mach numbers taking into consideration the location
of measurement and other details. Some manufactures use the mean or root
mean square (RMS) diameter while others use the tip as the reference location.
Sorokes and Kopko [5] have provided a valuable paper covering this important
area and examined the issues involved when making comparisons between
compressor designs.
The change in operating envelope as the Mach numbers change are shown in
Fig. 10.9. The sonic condition (Mach number¼1) for an impeller designed for
subsonic flow constitutes a “stonewall” barrier to any further flow increase,
which shortens the operating range.