Rotary type

1 1 Centrifugal type Axial flow (radial flow) type

Single-stage Multi-stage (single- or double-acting)

Rotary screw Sliding vane Rotary lobe

Single-stage Multi-stage

Fig. 30-3 Common Compressor Types.

Fig. 30-3 Common Compressor Types.

action. They offer multi-stage and high-pressure capability and can range in capacity from fractional hp (or kW) to several thousand hp (or kW), although most applications are below 250 hp (185 kW). Pressures range from low vacuum at suction to more than 10,000 psi (700 bar). Reciprocating compressors are categorized as either single-acting or double-acting.

In single-acting compressors, air is compressed only on the upstroke of the piston. They may feature single or multiple cylinders, with single-stage or multi-stage compression. Single-stage units are typically rated at discharge pressures of 25 to 100 psig (2.7 to 7.9 bar). Two-stage units are typically rated at discharge pressures of 100 to 250 psig (7.9 to 18.3 bar), though designs for higher pressure are not uncommon.

Air-cooled units reject the heat of compression from cylinders, heads, and intercoolers to cooling air driven from the compressor fan. Liquid-cooled compressors have jacketed heads, cylinders, and intercoolers through which the heat of compression is rejected to the circulating coolant.

Figure 30-4 is an illustration of a single-acting reciprocating compressor, which reveals the characteristic automotive-type pistons, driven through connecting rods from the crankshaft, with compression taking place on the top of the pistons on each revolution of the crankshaft. Pistons typically use heat-resistant, nonmetallic guides and piston rings. Figure 30-5 is an illustration of a two-stage compressor with a liquid-cooled intercooler.

In double-acting compressors, air is compressed on both the upstroke and downstroke of the piston. The double-acting piston is driven by a piston rod extending through a packing gland to a crosshead, which is driven through a connecting rod from the main crankshaft.

Fig. 30-2 Performance Characteristics of Centrifugal vs. Reciprocating Compressors. Source: Compressed Air and Gas Institute
Fig. 30-4 Single-Acting, Reciprocating Compressor with AutomotiveType Skirted Pistons. Source: Compressed Air and Gas Institute
Fig. 30-5 Two-Stage Reciprocating Compressor with Liquid Cooled Intercooler. Source: Compressed Air and Gas Institute

Double-acting compressors may employ single or multiple cylinders. Discharge pressure may range to several thousand psig. For 100 psig (8 bar) service, two-stage double-acting units begin at about 75 hp (56 kW).

Typically, double-acting compressors are used for heavy-duty continuous service and employ cooling water-jacketed cylinders and heads. Figure 30-6 illustrates a double-acting compressor cylinder separated from the compressor frame by a structural member called a distance piece. Figure 30-7 shows a 125 hp (93 kW), two-stage double-acting reciprocating compressor installed in a paper mill.

Reciprocating compressors are essentially constant

Fig. 30-6 Double-Acting Reciprocating Compressor Cylinder Separated from the Frame by a Distance Piece. Source: Compressed Air and Gas Institute
Fig. 30-7 125 hp (93 kW) Two-Stage Double-Acting Reciprocating Air Compressor. Source: Compressed Air and Gas Institute

capacity, variable pressure machines. Capacity regulation can be achieved with one or more of the following methods:

• Automatic stop-start control by means of a pressure-actuated switch. This method is typically used when compressed air demand is light and intermittent.

• With constant-speed control, unloading can be accomplished in several different ways. Two common methods are inlet valve unloaders and clearance unloaders. Inlet valve unloaders mechanically hold the cylinder inlet valve open, thereby preventing compression. Clearance unloaders open pockets or small reservoirs, which increases the clearance volume of the cylinder, thereby reducing the volume of air being compressed. For higher capacity motor-driven reciprocating compressors, this process can be accomplished in discrete steps varying from full load to no load. This process is referred to as step control.

• With variable-speed control, efficient unloading can be accomplished through much of the operating regime. By varying speed in response to changes in air demand, the compressor will operate at near 100% of full-load volumetric efficiency. The ability to reduce speed will be limited by the driver's capability. Once the lower speed limit of the driver is reached, one of the previously mentioned compressor control methods will be required to further reduce output. All of these control systems utilize an air discharge pressure set point that actuates a pressure-sensor or pilot. A falling pressure indicates that air is being used faster than it is being compressed and that more air is required. A rising pressure indicates that more air is being compressed than is being used and that less air is required.

With prime mover drives, speed control is a common method of varying reciprocating compressor capacity. In these cases, the regulator actuates the fuel- or steam-admission governor valve on the driver to control the speed. Electric motor-driven compressors generally operate at constant speed, although it is possible to apply variable frequency drives (VFDs) to achieve variable speed operation. On reciprocating compressors of small and intermediate capacities, both constant-speed control and automatic start-stop control are typically used.

Reciprocating compressors typically require constant oil feed to the cylinders. Because this oil can contaminate the air stream, heavy-duty filtration is required to keep the oil out of the compressed air system. Oil-free reciprocating units, which are more costly, are used for applications requiring high-purity, non-oil contaminated compressed air.

Advantages of reciprocating compressors include their high efficiency and excellent part-load performance, as well as the large range of available sizes and pressures. Disadvantages include relatively high maintenance requirements and downtime because of their many moving parts and the vibration and stress caused by their reciprocating action. Reciprocating units are also physically larger and more expensive than alternative compressor types in larger capacities. A substantial foundation is required for their support and to dampen the dynamic loading. Pulsation is inherent in reciprocating compressors because suction and discharge valves are open during only part of the stroke.

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