The trunk type compressor is generally arranged with the cylinder vertical in the basic single-stage arrangement, in the vertical, "in line," multistage configuration, the number of cylinders is normally limited to two. Most multi-cylinder arrangements are in pairs in the form of a V, usually at 45° from the vertical. These compressors usually have up to eight cylinders and are normally used in compressing organic refrigerants.

The few single-acting crosshead compressors are normally single stage machines with vertical cylinders. The more common double-acting type, when used as a single-stage, commonly has a horizontal cylinder. The double-acting cylinder compressor is built in both the horizontal and the vertical arrangement. There is generally a design trade-off to be made in this group of compressors regarding cylinder orientation. From a ring wear consideration, the more logical orientation is vertical; however, taking into account size and the ensuing physical location as well as maintenance problems, most installations normally favor the horizon tal arrangement.

There is wide variation in multistage configuration. The most common is the horizontally opposed. Probably the next most common is the vertical arrangement. Other variations include V, Y, angle or L type. These later arrangements are not too common and are mentioned only to complete possible configurations. Another modification is the tandem-cylinder arrangement, which is almost always horizontal. In this configuration, the cylinders are oriented in line with one another with the innermost cylinder having a piston rod protruding from both ends. This outboard rod in turn drives the next cylinder. While somewhat compact and more competitive in price than the side-by-side arrangements, it is not too popular with maintenance people.

Drive Methods

Another feature of reciprocating compressors that is somewhat unique when compared to the rest of the compressor family is the number of available drive arrangements, which is almost as complex as the cylinder arrangements. In single and multistage arrangement small compressors, particularly the trunk type, are usually V-belt driven by electric motors. The single-acting crosshead type and the small, double-acting, singlestage compressor are also driven in a similar manner. Larger, multistage, trunk type compressors can be sized to operate at common motor speeds and therefore are direct coupled. The larger, crosshead, double-acting, multistage compressors present the most variations in drive arrangements. If it has an integral electric motor sharing a common shaft with the compressor, it is called an engine type. These compressors can also be directly coupled to a separate electric motor in a more conventional manner. Gear units may be involved in the drive train where speed matching is required. Multiple frames are sometimes used with a common crankshaft in a compound arrangement to use a common driver.

Variable frequency motor drives are becoming more popular because of the ability to provide capacity control.

Reciprocating compressors are available with a large variety of other drivers, which include the piston engine, steam turbine, or, in rare cases, a gas turbine. Next in popularity to the electric motor is the piston engine. The arrangement lends itself to skid mounting, particularly with the semi-portable units found in the oilfield. The unit is also popular as a "lease" unit, which may be lifted onto a flat bed trailer and moved from one location to another as needed. The engine is either direct-coupled or. as with smaller compressors, it may be belt-connected.

A variation of the smaller, skid-mounted, engine-driven compressor is a larger, engine-driven version in the form of the integral engine compressor tsee Figure 3-4). The compressor and the engine share a common frame and crankshaft. When the engine cylinders are vertical or in a V configuration and the compressor cylinders are horizontal, the machine is called an angle engine compressor.

Figure 3-4. Cutaway of a two-stage piston engine driven compressor. (Courtesy of Dresser-Rand)

A more rare form of driver is the steam cylinder. Most arrangements combine the steam driver and compressor on the same frame with the steam cylinder opposite the compressor cylinder. Each cylinder's connecting rod is connected to a common throw on the crankshaft. A flywheel is used to provide inertia. For air service, the units are built as single- and two-stage units, with other combinations available for process service.

Figure 3-5. Steps in the cycle of reciprocating compressor.

complete crankshaft revolution and encompasses a complete compression cycle. To begin the cycle, refer to the figure at (a) the location where the piston is at the lower end of the stroke (bottom dead center) and is at path point 1 on the indicator diagram. At this point, the cylinder has filled with gas at intake pressure Pj. Note that the valves are both closed. At (b), the piston has started to move to the left. This is the compression portion of the cycle and is illustrated by Path 1-2. When the piston reaches point 2 on the indicator diagram, the exhaust valve starts to open. The discharge portion of the cycle is shown at (c). This is shown on the indicator diagram Path 2-3. Note that the discharge valve is open during this period while the intake valve is closed. The gas is discharged at the discharge line pressure P2. When the piston reaches point 3, it has traveled to the upper end of its stroke (top dead center). Physically, at this point in the stroke, there is a space between the piston face and the head. This space results in a trapped volume and is called the clearance volume. Next in the cycle, the piston reverses direction and starts the expansion portion of the cycle, as illustrated at (d) in the figure. Path 3-4 shows this portion of the cycle. Here the gas trapped in the clearance volume is re-expanded to the intake pressure. Note that the discharge valve has closed, and the intake valve is still closed. At point 4, the expansion is complete and the intake valve opens. The intake portion of the cycle is shown at (e). This is indicated by Path 4-1 on the indicator diagram. The cylinder fills with gas at intake line pressure P[. When the piston reaches point 1, the cycle is complete and starts to repeat.

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