2. Horizontally split Single stage (double-suction)15 (1.03)650，000(18，406)10，000(7，457) Multistage 1000 (69)200，000(5，663)35，000(26，100)
3. Vertically split
Single stage (single-suction) Overhung 30 (2.07)250，000(7，079)10，000(7，457) Pipeline 1200 (82)25，000 (708)20，000(14，914)
Multistage More than 5500 (379)20，000 (566)15，000(11，185)
FPO
Figure 6-45. Horizontally split centrifugal compressor with shrouded rotors. (Cour-tesy of Elliott Company.)
FPO
Figure 6-46.Barrel-type compressor. (Courtesy ElliottCompany，Jeannette， PA.)
The internal parts such asshaft， impellers， bearings， and seals are readily accessible for inspection and repairs by removing the top half. The casing material is cast iron or cast steel.
There are various types of barrel or centrifugal compressors. Low-pressure types with overhung impellers are used for combustion processes，ventilation， and conveying applications. Multistage barrel casings are used for high-pressures in which the horizontally split joint is inadequate. Figure 6-46 shows the barrel compressor in the background and the inner bundlefrom the compressor in front. Once the casing is removed from thebarrel， it is horizontally split.
Compressor Configuration
To properly design a centrifugal compressor， one must know the oper-ating conditions.the type of gas， its pressure， temperature， and molecular weight. One must also know the corrosive properties of the gas so that proper metallurgical selection can be made. Gas fluctuations due to process instabilities must be pinpointed so that the compressor can operate without surging.
Centrifugal compressors for industrial applications have relatively low pressure ratios per stage. This condition is necessary so that the compressors can have a wide operating range while stress levels are kept at a minimum.Because of the low pressure ratios for eachstage， a single machine may have a number of stages in one ..barrel'' to achieve the desired overall pressure ratio. Figure 6-47 shows some of the many configurations. Some factors to be considered when selecting a configuration to meet plant needs are:
1. Intercooling between stages can considerably reduce the power consumed.
2. Back-to-back impellers allow for a balanced rotor thrust and min-imize overloading the thrust bearings.
3. Cold inlet or hot discharge at the middle of the case reduces oil-seal and lubrication problems.
4. Single inlet or single discharge reduces external piping problems.
5. Balance planes that are easily accessible in the field can appreciably reduce field-balancing time.
6. Balance piston with no external leakage will greatly reduce wear on the thrust bearings.
7. Hot and cold sections of the case that are adjacent to each other willreduce thermal gradients， and thus reduce case distortion.
8. Horizontally split casings are easier to open for inspection thanvertically split ones， reducing maintenance time.
9. Overhung rotors present an easier alignment problem because shaft-end alignment is necessary only at the coupling between the compressor and driver.
10. Smaller， high-pressure compressors that do the same job will reduce foundation problems but will have greatly reduced operational range.
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