Compressor rotary piston

The most used mechanical vacuum pumps or compressors are reciprocating, liquid-ring, rotary-vane, rotary blower, rotary piston, and diaphragm. 

Rotary piston or rotary lobe or roots compressors use two counterrotating matching lobe-shaped rotors (Figure 13.11b). Each revolution of the lobes delivers four pulses of gas. Maximum discharge pressures in a single stage are up to 2.5 bar. Suction flowrates of up to 3 m3s 1 are possible. 

Compressors such as reciprocating piston and rotary piston compressors are used to generate the highest pressures. 

LRVPs, similar to all rotary piston displacement compressors, are characterized by small vibrations and low-pulsation delivery. Nevertheless, if the operating performance does not meet the requirements of the customer, the vibration and sound emissions can be reduced by more or less extensive measures. On behalf of the most cost-effective solution possible, it requires well-founded knowledge of the procedures in the vacuum pump. 

The most frequently used are gas ejectors, steam jets with downstream condenser and rotary piston compressor. 

From the point of view of economy, rotary piston compressors are sometimes also used for precompression. These volumetrically working, two-phase, dry-running rotary piston compressors are characterized by insensitivity towards impurities in the medium to be pumped. 

The energy necessary for the compression is provided by a motor drive. Due to the significantly greater degree of efficiency of rotary piston compressors, a smaller total drive power suffices in contrast to gas ejectors in combination with liquid ring compressors (Figure 3.38). 

It is to be taken into account that with the employment of rotary piston vacuum pumps, the temperature of the fluid fed to the liquid ring compressor is higher than with the other combinations. In addition, this heat is to be dissipated by the ring liquid, so that the temperature of the liquid ring will be higher and with it the suction capacity will be lower. 

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). 

Most reciprocating compressors have inlet and outlet valves (on the piston heads) that are actuated by a pressure difference. These are called self acting valves. There are some larger multistage reciprocating piston compressors that do have camshaft-controlled valves with rotary slide valves. 

The main disadvantages to multistage reciprocating piston compressors is that they cannot be practically constructed in machines capable of volumetric flowrates much beyond 1,000 actual cfm. Also, the higher-capacity compressors are rather large and bulky and generally require more maintenance than similar capacity rotary compressors. 

Another important positive displacement compressor is the rotary compressor. This type of compressor is usually of rather simple construction, having no valves and being lightweight. These compressors are constructed to handle volumetric flowrates up to around 2,000 actual cfm and pressure ratios up to around 15 (see Figure 3-69). Rotary compressors are available in a variety of designs. The most widely used rotary compressors are sliding vane, rotary screw, rotary lobe, and liquid-piston. 

For natural gas service, five types of compressors are typically used, that is, diaphragm, piston, rotary, screw, and turbo compressors. The characteristics of each type of compressors are a function of throughput and pressure, as shown in Figure 10.22 [6]. Currently, piston (or reciprocating) and rotary compressors are used to compress hydrogen that are similar to those used for natural gas, with modifications of design and materials. 

In the GM-type PTR (mostly used at present), a compressor produces continuous high and low pressures and uses a rotary valve to generate pressure oscillations in the pulse tube. In the Stirling-type PTR, pressure oscillations are created by the movement of a piston, directly connected to the pulse tube and controlled by a motor. 

Rotary compressors are generally classified as of the straight-lobe type, screw type, sliding-vane type, and liquid-piston type. 

Most screw compressors are of the oil-lubricated type. There are two types—the semihermetic and the open-drive type. In the former, the motor is located in the same housing as the compressor, while in the latter the motor is located outside of the compressor housing and thus requires a shaft seal. The only moving parts in screw compressors are two intermeshing helical rotors. The rotors consist of one male lobe, which functions as a rolling piston, and a female flute, which acts as a cylinder. Since rotary screw compression is a continuous positive-displacement process, no surges are created in the system. 

Devices which are used for transporting fluids (liquids and gases) may be divided into pumps, ejectors, injectors, elevators, conveyors, air and gas pressure devices (such as acid eggs, air lifts, pulsometers etc). Pumps may be divided into piston (reciprocating), centrifugal, propeller, rotary-displacement, density, impact and momentum and turbine pumps. Pumps which are used for compressing gases are called "compressors ... 

EflBlciency.—In practice it is found that compressors with mechanically operated rotary inlet valves show volumetric efficiency varying from 91 per cent at 100 r.p.m. to 88 per cent at 188 r.p.m. Piston inlet machines at 100 r.p.m. give 88 per cent efficiency, and at 188 r.p.m., 79 per cent. 

Rotary blowers are built for air pressures varying from 6 oz. to 10 lb. or even 12 lb. per square inch. The best efficiencies of this type of blower, however are usually secured below 5 lb. pressure, but the simplicity of the machine gives it an advantage over compressors of the piston type and frequently warrants its installation for the higher pressures indicated when designed for this purpose- As the machine operates by displacement, it is usually preferred for cupola practice, and copper and lead blast furnaces, because its positive action will not permit 2 reduction in air supply if the cupola tends to clog. For other uses of air at pressures below 8 oz. the fan is ordinarily more economical. 

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