Out

Figure 5-4. Diagram of a single-stage overhung type centrifugal compressor.

A less common form of the single stage is shown in Figure 5-5. In this form, the impeller is located between two bearings, as is the multistage. This type of compressor is sometimes referred to as a beam type single stage. The flow enters and leaves in a tangential direction with the nozzles located in the horizontal plane. The between-bearing single stage is found most commonly in pipe line booster service where the inherent rigidity of the two outboard bearings is desirable.

Figure 5-6 is a flow diagram and schematic layout of the integrally geared compressor, and Figure 5-7 shows exploded view. It consists of three impellers, the first located on one pinion, which would have a lower speed than the other pinion that has mounted the remaining two impellers. This arrangement is common to the plant air compressor. Configurations such as this are used in process air and gas services, with the number of stages set to match the application.

Figure 5-8 shows the multistage arrangement. The flow path is straight through the compressor, moving through each impeller in turn. This type of centrifugal compressor is probably the most common of any found in process service, with applications ranging from air to gas. The latter includes various process gases and basic refrigeration service.

Figure 5-5. Diagram of a beam type single-stage compressor.

Figure 5-6. Flow diagram and schematic of an integrally geared compressor.
Figure 5-7. An exploded view of an integrally geared compressor. (Courtesy of

Cooper Turbocompressot)

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Figure 5-8. Diagram of a multistage centrifugal compressor with a straight-through flow path.

Figures 5-9 and 5-10 depict the two most common forms of in-out arrangements. This arrangement is also referred to as a compound compressor. In these applications, the flow out of the compressor is taken through an intercooler and back to the compressor. The arrangement is not limited to cooling because some services use this arrangement to remove and scrub the gas stream at a particular pressure level. Provision for liquid removal must be made if one of the gas components reaches its saturation

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Figure 5-9. Diagram of an in-out arrangement with intercooling

Figure 5-10. Diagram of a double-cooled centrifugal compressor

temperature in the process of cooling. Figure 5-10 shows a double-cooled or double compound compressor. This arrangement is used mostly when the gas being compressed has a temperature limit. The limit may be imposed by the materials of construction or where the gas becomes more reactive with an increase in temperature and thus sets the limit in a given application. Polymer formation is generally related to temperature and may form the basis for an upper temperature limit. However, with the external cooling, the amount of compression needed can be accomplished in a single case. The physical space needed to locate the multiple nozzles normally limits the number of in-out points to the two shown.

The arrangement shown in Figure 5-11 is referred to as a double-flow compressor (see also Figure 5-12). As indicated in the figure, the flow enters the case at two points, is compressed by one or more stages at each end, and then enters the double-flow impeller. The flow passes through each individual section of the double-flow impeller and joins at

Figure 5-12. A double-flow compressor with Inlets on each end and a common center discharge. {Courtesy of Elliott Company

IN OUT IN

Figure 5-11, Diagram of a double-flow compressor with two inlets.

Figure 5-12. A double-flow compressor with Inlets on each end and a common center discharge. {Courtesy of Elliott Company the ditfuser. There are various physical arrangements to accomplish the double-flow compression. One variation is to use two back-to-back stages for the final compression and join the flow either internally, prior to leaving the case, or join two separate outlet nozzles outside the case.

From a process point of view, the flow should be joined prior to exiting the discharge nozzle.

Another variation of this arrangement is to use it in the single-stage configuration, where only a single inlet and outlet nozzle is used. The flow enters the case and is divided to each side of the double-How impeller and then joins at the impeller exit prior to entering the diffuser. Figure 5-13 shows a schematic diagram of the flow in this machine. The advantage of the double-flow arrangement is, of course, that in the same casing size, it doubles the flow. However, the realization of the advantage is more complex. The losses in the flow paths through the double-flow impeller must, in theory, be identical. In practice, of course, this is not possible. The sensitivity is a function of the total head level. The lower the levels, the more nearly the paths must be the same.

The single-stage configuration, the lower head compressor, will exhibit the highest degree of sensitivity to the flow imbalance and have its performance most adversely affected. The multistage configuration, while not as sensitive to the flow anomalies because of the higher head generated, will benefit from careful flow path design to keep the flow balanced to each section of the double flow inlets. If a number of options are open for a given application, the double-flow option should not be the first choice; although, it should be evaluated because successful applications in service indicate that with careful design the compressor will perform satisfactorily.

The arrangement in Figure 5-14, generally called "back to back," is normally considered useful in solving difficult thrust balance problems where the conventional thrust bearing and balance drum size are inade

Figure 5-13. Diagram of a double-flow compressor with flow split internally.

Figure 5-13. Diagram of a double-flow compressor with flow split internally.

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Figure 5-14. Diagram of an arrangement used to overcome a thrust balance problem.

quate or become excessively large. The balance drum will be described in detail in a following section. The flow is removed part way through the compressor and reintroduced at the opposite end, then allowed to exit at the center. Because centrifugal impellers inherently exhibit a unidirectional thrust, this arrangement can be used to reduce the net rotor thrust. The obvious use is for applications generating high thrusts, higher than can be readily controlled by a normal size thrust bearing and balance drum. An evaluation of the cross leakage between the two discharge nozzles must be made and compared to the balance drum leakage to determine the desirability of the "back to back." It can be combined with the sidestream modes, discussed in the next paragraph, to possibly help sway a close evaluation. In some rare cases, this design has been used for two different services. Unfortunately, it is difficult to totally isolate the two streams because of the potential cross leakage. In cases where the two services may have a common source, or the mixing of the streams does not cause a problem, it is possible to generate savings by using only one compressor case.

A very common compressor design used in the chemical industry, particularly in large refrigeration systems, is the sidestream compressor (see Figure 5-15). Gas enters the first impeller and passes through two impellers. As the main stream approaches the third impeller, it is joined by a second stream of gas, mixed, and then sent through the third impeller. The properties of the gas stream are modified at the mixing point, as the sidestream is rarely at the same temperature as the stream from the second impeller. In refrigeration service, this stream is taken from an exchanger where it is flashed to a vapor, resulting in a stream temperature near saturation. As such, the sidestream would act to cool

Figure 5-15. Diagram of flow path through a sidestream compressor.

the total stream. The weight flow to the third impeller is the combined weight flow of the two streams.

The second sidestream follows the same logic. To show the flexibility of the arrangement, the last sidestream is indicated as an extraction. This stream could be used where heated gas at less than discharge pressure is required. Using the extraction saves the energy needed to compress this quantity of gas to the full discharge pressure and then throttling for the heating service. One potential application of an extraction stream is for use in a reboiler. The arrangement shown was arbitrarily chosen to illustrate the available options. The total number of sidestream nozzles is limited only by the physical space required to locate them on the case. Three nozzles are not uncommon.

When applications are more complex than can be accommodated by a single-case compressor, multiple cases can be used. The most frequently used is the tandem-driven series flow arrangement using a common driver (see Figure 5-16). A gear unit may be included in the compressor train, either between cases or between the driver and the compressors. The individual compressor cases may take the form of any of the types described before. The maximum number of compressors is generally limited to three. Longer, tandem-driven series-connected compressor trains tend to encounter specific speed problems. In the longer trains, the double-flow arrangement can be useful in permitting more compressors to run at the same speed. At the inlet, where flow is the highest, the gas stream is divided into parallel streams and the volume is reduced by compression to a value within the specific speed capability of a single-flow compressor. The

Figure 5-16. A tandem driven mufti-body centrifugal compressor train with a steam turbine driver. (Courtesy of Demag Delaval Turbomachinery Corp)

alternative to the double-flow arrangement is the use of a speed increasing gear between compressor bodies to permit the flow matching of downstream stages. This is one case where the double-flow compressor should be considered first. When longer trains are needed, the cases are grouped with several individual drivers, maintaining the series flow concept. One installation that can be recalled used nine individual cases, separately driven and series connected, for a very high pressure air application.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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