Axial Compressors
The axial designation is used when the gas flow is parallel to the compressor shaft. Energy transfer is caused by the action of a number of rows of blades or a rotor, each row followed by a fixed row fastened to the casing.
The axial-flow dynamic compressor is shown in Figure 14-19. It is essentially a large capacity, high-speed machine with characteristics quite different from those of the centrifugal compressor. Each stage consists of two rows of blades, one row rotating and the next row stationary. The rotor blades impart velocity and pressure to the gas as the rotor turns, the velocity being converted to pressure in the stationary blades. Fre quently about half the pressure rise is generated in the rotor blades and half in the sta tor. Gas flow is predominantly in an axial direction and there is no appreciable vortex action.
Dynamics
The dynamic forces generated by an axial-flow compressor are also similar to those of other rotating machinery. The major difference is that the dominant orientation of pro cess forces is parallel to the shaft. As a result, the axial forces and vibration profile will be dominant.
Special attention must be given to the axial movement and vibration of axial-flow compressors. A variation in the operating dynamics and stability of this design will be dominant in the axial direction. Radial readings will help in understanding the operat ing conditions, but will be less definitive than in other designs.
Mixed-Flow Compressors
The mixed-flow designation is used when the gas flow is between radial and axial. It combines the design features of each with characteristics also lying between the two. This type is not applied as frequently as the others. Because of the long length required for each stage, this type is generally not found in multistage designs.
COMPRESSOR SELECTION
Air power compressors generally operate at pressures of 500 psig or lower, with the majority in the range of 125 psig or less. All major types of compressors (i.e., recipro cating, vane, helical lobe, and dynamic) are used for this type of service. Choice is limited somewhat by capacity at 100 psig of about 10,500 cubic feet per minute but can be built to approximately 28,000 cfm. The vane-type rotary has an upper listed size of 3,700 cfm as a twin unit and the helical lobe rotary can be used to nearly 20,000 cfm. The centrifugal can be built to very large sizes. It is currently offered in the proven, moderate speed designs starting at a minimum of about 5,000 cfm.
Selection Criteria
The following guidelines should be used for the selection process. Although the crite ria listed are not all-inclusive, they will provide definition of the major considerations that should be used to select the best compressor for a specific application.
Application
The mode of operation of a specific application should be the first consideration. The inherent design of each type of compressor defines the acceptable operating envelope or mode of operation that it can perform with reasonable reliability and life cycle costs. For example, a bullgear-type centrifugal compressor is not suitable for load-fol lowing applications but will prove exceptional service in constant-load and -volume applications.
Load factor is the ratio of actual compressed air output, while the compressor is oper ating, to the rated full-load output during the same period. It should never be 100 per-
cent, a good rule being to select an installation for from 50 to 80 percent load factor, depending on the size, type, and number of compressors involved. Proper use of load factor results in: more uniform pressure, a cooling-off period, less maintenance, and ability to increase use of air without additional compressors.
Load factor is particularly important with air-cooled machines where sustained full load operation results in an early buildup of deposits on valves and other parts. This buildup increases the frequency of maintenance required to maintain compressor reli ability. Intermittent operation is always recommended for these units. The frequency and duration of unloaded operation depend on the type, size, and operating pressure of the compressor. Air-cooled compressors for application at pressures higher than 200 psig are usually rated by a rule that states that the compressing time shall not exceed 30 minutes or less than I0 minutes. Shutdown or unloaded time should be at least equal to compression time or 50 percent.
Rotary screw compressors are exceptions to this 50 percent rule. Each time a rotary screw compressor unloads, both the male and female rotors instantaneously shift axially. These units are equipped with a balance piston or heavy-duty thrust bearing that is designed to absorb the tremendous axial forces that result from this instantaneous movement, but they are not able to fully protect the compressor or its components. The compressor’s design accepted the impact loading that results from this unload shifting and incorporated enough axial strength to absorb a normal unloading cycle. If this type of compressor is subjected to constant or frequent unloading, as in a load-following application, the cycle frequency is substantially increased and the useful life of the compressor is proportionally reduced. There have been documented cases where either the male or female rotor actually broke through the compressor’s casing as a direct result of this failure mode.
The only compressor that is ideally suited for load-following applications is the reciprocating type. These units have an absolute ability to absorb the variations in pressure and demand without any impact on either reliability or life-cycle cost. The major negative of the reciprocating compressor is the pulsing or constant variation in pressure that is produced by the reciprocating compression cycle. Properly sized accumulators and receiver tanks will resolve most of the pulsing.
Life-Cycle Costs
All capital equipment decisions should be based on the true or life-cycle cost of the system. Life-cycle cost includes all costs that will be incurred, beginning with specifi cation development before procurement to final decommissioning cost at the end of the compressor’s useful life. In many cases, consideration is given only to the actual procurement and installation cost of the compressor. Although these costs are important, they represent less than 20 percent of the life-cycle cost of the compressor.
The cost evaluation must include the recurring costs, such as power consumption and maintenance, that are an integral part of day-to-day operation. Other costs that should be considered include training of operators and maintenance personnel who must maintain the compressor.