Capacity Control

The reciprocating compressor is a fixed displacement compressor in ils basic configuration; however, several methods are used to overcome this limitation to permit running at multiple operating points. In the discussion on cylinders, mention was made of clearance pockets. By use of the clearance pockets, the cylinder capacity can be lowered (see Equation 3.5). If the pocket is connected directly to the clearance area, the clearance term e can be increased. Increasing the clearance reduces the capacity by lowering the volumetric efficiency. Control of the pocket addition is by either a manual valve or by a remotely operated valve. If multiple pockets are used, a step unloading system can be designed (see Figure 3-22). The variable volume clearance pocket can provide an alternate unloading method. This device is normally attached to the outboard head. It consists of a piston-cylinder arrangement where the piston rod is threaded and

Full Load % Load 1/2 Load

1/4 Load No Load

Figure 3-22. A 5-step clearance pocket unloading scheme. (Courtesy of Dresser-Rand)

Figure 3-22. A 5-step clearance pocket unloading scheme. (Courtesy of Dresser-Rand)

attached to a handwheel. Turning the handwheel changes the clearance volume in an infinite number of steps up to the total pocket volume.

On cylinders lacking the physical space for pockets, the same effect can be achieved by using external bottles and some piping. Care must be taken to keep the piping close-coupled and physically strong enough to prevent accidental breakage. Remotely operated valves permit the capacity reduction to be integrated into an automatic control system.

An additional capacity control method is the unloader. This method can be used in conjunction with clearance pockets to extend the range of control to zero capacity. On double-acting cylinders, unloading the individual sides one at a time will provide a two-step unloading of the cylinder. On multicylinder arrangements, the cylinders can be unloaded one at a time providing as many steps as cylinders operating in parallel. The unloaders can also be used to totally unload the compressor, as is necessary for electric motor driver startup.

Three types of unloaders will be described, the plug type, the port type, and the plunger type. The plug type, shown in Figure 3-23, is normally used on all inlet valves for the unloaded end. The center of the valve is used for the unloader plug and port. The port type, shown in Figure 3-24, is used to replace one of the inlet valves on multiple inlet valve cylinders. It is normally used with low molecular weight applications. This unloader consists of a plug and port using the entire space of the valve it replaces. The plunger type, shown in Figure 3-25, is used on heavier gas applications where the maximum unloaded flow area is needed. The unloader operates by using the plunger fingers to hold the valve

Figure 3-23. Plug type unloader. (Courtesy of Dresser-Rand)
Figure 3-24. Port type unloader. (Courtesy of Dresser-Rand)

plates open. Control of all the described unloaders is the same, in that a piston operator is used. Additional control may be obtained by using a cooled bypass line from the discharge to the compressor suction. The bypass is normally used with discrete unloading steps.

Figure 3-25. Plunger type unloader. Note the plunger finger used to hold the valve open when energized. (Courtesy of Dresser-Rand)

A few words of caution when using the valve unloading method: A problem arises with the possible loss of rod load reversals. Rod reversals are needed to provide lubrication to some of the bearings, as discussed earlier in this chapter. While the reversal problem is generally associated with unloading a double-acting cylinder from one side, it should be checked for all unloaded cases, including pocket unloading. If operation without rod reversals is absolutely mandatory, auxiliary lubrication must be brought to the bearings affected. The second caution is the anticipated duration of a totally unloaded condition. While the capacity has been reduced to zero, the gas in the outer end of the cylinder is being moved about in a reciprocating manner following piston movement. The movement of uncompressed gas will generate heat, and prolonged unloaded operation without proper cooling may cause severe overheating. In any case, investigation of potential problems should be undertaken with the equipment manufacturer.

From the foregoing discussion, it should be clear that cylinder capacity can be controlled. While the automatic control is normally limited to certain finite steps, the steps can be selected in size or number to minimize any adverse effect especially in conjunction with prudent use of the variable volume pocket.

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