Machinery's Handbook, 31st Edition
COMPRESSOR TYPES 2819 Compressor Types: Compressors can be centrifugal, multi-stage centrifugal, multi- stage axial, or positive displacement. Centrifugal compressors are similar to centrifugal blowers. Positive displacement compressors are essentially positive displacement pumps, used to deliver high pressures at moderate flow rates. These compressors can employ a va - riety of mechanisms, like pistons, lobes, screws, and vanes. Small positive displacement compressors are often sold to homes and businesses. Positive displacement compressors compress a fixed amount of gas within a closed chamber by decreasing its volume. Volumetric efficiency is affected by leakage and me - chanical losses. Positive displacement compressors tend to be selected for low to mod erate flow applications with high compression requirements. Compressor types in this category include: reciprocating piston, rotary vane, rolling piston, rotary twin screw, and orbiting scroll compressors. Screw compressors are commonly used in the range between 100 and 150 psig. Reciprocating compressors can be either single acting or double acting, and are commonly used for pressures in excess of 200 psig. These types of compressors deliver a pulsating flow of gas, unlike other types of compressors. Multi-stage positive displacement compressors with intercooling are common at pressures exceeding 80 psig. Many state laws and general good practice requires the use of an appropriately sized relief valve ahead of the first stop valve in positive displacement compressors. This relief valve should be set to release at 125 percent of the maximum discharge pressure of the compressor, or at the maximum working pressure of the compressor, whichever is lower. Dynamic compressors impart momentum to the gas by means of a spinning rotor. This momentum energy is converted to pressure energy when the gas is slowed again. This is a constant flow process. Dynamic compressors are subject to the fan laws, and the specific speed is used to select the geometry. Dynamic compressors tend to be selected for moder ate to high flow applications with low to moderate compression requirements. Multi-stage dynamic compressors can provide high flow and high compression. Compressor Sizing: When sizing a compressor for a given application, one must analyze the piping system to the point of use to account for any pressure drops between it and the compressor. Any pressure loss must be added to the compressor range to make sure the de- sired pressure is delivered at the point of use. If the pressure drop is significant (exceeding 10 percent) the change in specific weight of the gas should be taken into account. The maximum capacity of a compressor should exceed the maximum mean air consumption of the system. Because the maximum capacity of an air compressor will exceed the minimum air con sumption in the system, the compressor must modulate its capacity during normal work. This will cause variation of system pressure, or pressure pulsation. As a result, a receiver should be installed after the compressor to stabilize system pressure. Like with fans and pumps, compressors are usually sized by graphing the characteristic curve. Discharge pressure, head, or pressure ratio is graphed against inlet flow rate. For dynamic compressors, the graph includes values at various speeds and efficiencies. The system curve(s) is then plotted over the characteristic curve(s) and the intersection is the operating point. Multiple operating points may be encountered. Each stage of compres- sion will have its own characteristic curve, and in an ideal case each stage will operate at a high efficiency. For dynamic compressors, inlet pressure variation has a direct effect on horsepower. To correctly size the compressor drive, the full range of inlet pressures must be consid- ered as well as discharge pressures. Dynamic compressors are subject to a minimum flow rate called the surge limit. Surge is seen to the left of the characteristic curve(s). The demand limits of the application must be considered and operation below the surge limit prevented.
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