The casings on axial compressors are somewhat unusual, because of the disproportionately large inlet and outlet nozzles. This makes the compressors appear to be only nozzles connected by a long tube. Casings can be fabricated or cast, with the fabricated obviously being steel, while the castings can be cast iron or cast steel. In some designs, the casing is an outer shell containing an inner shell, which acts as the stator vane carrier. In other designs, the stators are directly carried on the casing, which are of a one-part construction. With this latter design, the casing is made up of three distinct parts, bolted at two vertical joints. The parts are the inlet section, the center body with the stators, and the discharge section. The three sections are also split horizontally for maintenance. With the three piece-bolted construction, a mixture of fabrications and castings may be used. The mounting feet are attached to the outside casing and so located as to provide a more or less centerline support. As mentioned earlier, some designs use a rectangular inlet section to provide more axial clearance. The reason for using an entire separate casing is that it forms a separate pressure casing that can readily be hydrotested. The disadvantage is that there is more material involved making the cost higher. The compressors that use the integral stator section or single case approach have somewhat of a cost advantage, and in general, may have a slight advantage in being able to keep the stator carriers round, because of the end-bolting to the other casing components. The disadvantage is that there are more joints to seal and maintain. Checking out the entire casing for strength and leakage in a hydro and gas test becomes somewhat more complex.

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