Reciprocating positive compressors

In much the same manner as pumps, compressors are classified as one of two general classes positive displacement or dynamic (see Figure 3-68) [23]. These two general classes of compressors are the same as that for pumps. The positive displacement class of compressors is an intermittent flow device, which is usually a reciprocating piston compressor or a rotary compressor (e.g., sliding vane, screws, etc.). The dynamic class of compressor is a continuous flow device, which is usually an axial-flow or centrifugal compressor (or mix of the two). 

For reciprocating positive pumps and compressors the fluid displacement per stroke depends on the geometry, the fluid compressibility, the polytropic coefficient, and the internal leakages and volumetric losses, but basically not on the speed of machine. 

Three main classes of gas compressors are centrifugal and axial, rotary continuous positive displacement, and reciprocating positive... 

Reciprocating positive displacement machines exist in versions such as piston pumps, membrane pumps, piston compressors, and membrane compressors. These machines achieve the highest pressures and the pumps are additionally able to reach a high conveying precision. The main reason for these features is the leakproof discharge chambers (operating areas of the positive displacement machines). 

Compressors used in a typical refrigerator/freezer are of the positive displacement type, utilizing either reciprocating or rotary motion (Eig. 5) and... 

Many users consider rotaiy compressors, such as the Rootes -type blower, as turbomachines because their behavior in terms of the rotor dynamics is very close to centrifugal and axial flow machineiy. Unhke the reciprocating machines, the rotary machines do not have a veiy high vibration problem but, like the reciprocating machines, they are positive displacement machines. 

Positive-Displacement Compressors Reciprocating compressors are built in different sizes (up to about one megawatt refrigeration capacity per unit). Modern compressors are high-speed, mostly direct-coupled, single-ac ting, from one to mostly eight, and occasion-aUy up to sixteen c inders. 

Compressors using the intermittent compression mode are referred to as positive displacement compressors, of which there are two distinct types reciprocating and rotary. Continuous-mode compressors are also characterized by two fundamental types dynamic and ejector. 

The reciprocating compressor is a positive displacement, intermittent flow machine and operates at a fixed volume in its basic configuration. 

The reciprocating compressor has several strikes against it when it comes to reliability, if the main consideration of reliability is long run times as would normally be expected from the dynamic type machines. It has a lot of parts and the parts are subject to wear. Before one jumps to the conclusion and totally excludes this compressor from consideration because of these factors, the positive aspects of this machine should be... 

Type Comparison. Reciprocating Compressors. Positive Displacement Rotary Compressors. Centrifugal Compressors. Axial Compressors. 

For positive displacement compressors, discharge PR valves are nearly always required. Reliance on stalling of a reciprocating compressor is generally not economically attractive, since driver stalling pressures are usually quite high in comparison to operating pressure. 

A reciprocating compressor is a positive-displacement machine in which the compressing and displacing element is a piston moving linearly within a cylinder. Figure 10-1 shows the action of a reciprocating compressor. 

Here are two more examples of penny-pinching. The piston of a reciprocating engine was secured to the piston rod by a nut, which w as locked in position by a tab washer. When the compressor was overhauled, the tightness of this nut was checked. To do this, the tab on the washer had to be knocked down and then knocked up again. This weakened the washer so that the tab snapped off in service, the nut worked loose, and the piston hit the end of the cylinder, fracturing the piston rod. 

Positive-displacement compressors can be divided into two major classifications rotary and reciprocating. 

Reciprocating compressors are positive-displacement types having one or more cylinders. Each cylinder is fitted with a piston driven by a crankshaft through a connecting rod. As the name implies, compressors within... 

Crankshafts on positive-displacement reciprocating compressors have offsets from the shaft centerline that provide... 

Positive displacement Two major types of positive-displacement compressors are used in industrial applications reciprocating and screw. 

In turbomachineiy the centrifugal flow and the axial-flow compressors are the ones used for compressing gases. Positive-displacement compressors such as reciprocating, gear-type, or lobe-type are widely used in the industry for many other applications such as slurry pumping. 

After a lively development in the past two decades during which positive-displacement screw-type and radial-turbo-compressors have replaced piston-type compressors in certain areas. The trend is now towards reciprocating compressors, again owing to their lower power consumption for many applications. 

The reciprocating compressor is a positive-displacement compressor. It is cheaper to purchase and install than a centrifugal compressor. Also—in theory—far more efficient (90 percent) than a centrifugal compressor (70 percent). Certainly, reciprocating compressors are more simple to understand and engineer than centrifugal machines. Best of all, they are not subject to surge. 

Figure 3.24. Control of positive displacement compressors, rotary and reciprocating, (a) Flow control with variable speed drives, (b) Pressure control with bypass to the suction of the compressor, (c) Reciprocating compressor. SC is a servomechanism that opens some of suction valves during discharge, thus permitting stepwise internal bypass. The clearance unloader is controllable similarly. These built-in devices may be supplemented with external bypass to smooth out pressure fluctuations.

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