Configuration refers to the relationship between the inlet, discharge, and side streams to the mechanical arrangement of the compressor. This will be clarified by the following examples.
Figure 200-60 shows a typical cross-section of a multistage centrifugal compressor. This is called a “straight-through” compressor because flow goes in one end and out the other.
Another common configuration is the “compound,” or “Out-and-In” type (Figure 200-61). This arrangement allows removal of the total gas stream for intercooling, power savings, or processing, and re-entry for additional compression. Note the additional spacing required for flow extraction and re-entry. Although some designs can minimize the effect, this reduces the maximum number of impellers available for compression.
The “sidestream compressor” shown in Figure 200-62 allows the introduction or extraction of partial flows at intermediate levels to satisfy various process requirements. The number of sidestreams in a single casing is limited only by available spacing. This arrangement adds the complexity of requiring mixed temperature calculations to determine impeller performance downstream of sidestream inlets.
The “double-flow” configuration effectively doubles the capacity of a given frame size (Figure 200-63). The compressor is divided into two sections, the inlet flow entering at either end, and discharging through a common discharge nozzle at the center of the casing. The impellers in each section face in opposite directions, achieving thrust balance at all operating conditions. While flow is doubled, the number of stages available for increasing head is cut in half. The use of the doubleflow option should be carefully evaluated against other alternatives.
The compressor in Figure200-64 utilizes what is commonly called the “back-toback” impeller arrangement. This type has advantages in high pressure-rise applications where thrust balancing becomes difficult using a conventional thrust bearing and balancing drum. Since the back-to-back impellers produce opposing thrust forces, the net thrust is significantly reduced, eliminating the need for a balance piston to provide thrust compensation. This arrangement must, however, be carefully reviewed with respect to division wall-flow disturbances, bearing span, and seal design on rotor stability.
One other configuration to note is a combination series/parallel unit, Figure 200-65. Eastern Region has one of these in booster-compression service, and reports good performance, and flexibility switching back and forth in order to obtain higher flows, or discharge pressure, as needed for system operation.