Vane-Type Rotary Compressors

A relatively large quantity of oil is injected into the flow stream to lubricate the vanes. This is normally captured by a discharge cooler and after-scmbber and recycled to the inlet. 

Vane compressors tend to be limited to low pressure service, generally less than 100 to 200 psi discharge. They are used extensively as vapor recovery compressors and vacuum pumps. Single-stage vane compressors can develop 27 in. Hg vacuums, two-stage compressors can develop 29.0 in. Hg, and three-stage compressors can develop even higher vacuums. 

Screw compressors can handle moderate amounts of liquid. They can also handle dirty gases because there is no metallic contact within the casing. 

It tends to be limited to 250 psig discharge pressures and a maximum of 400 hp in hydrocarbon service, although machines up to 6,000 hp are available in other service. Screw compressors are not as good as vane com tres-sors in developing a vacuum, although they are used in vacuum sen icc. 

Non-lubricated screw compressors have very close clearances and thus they are designed for limited ranges of discharge temperature, temperature rise, compression ratio, etc., all of which can cause changes in thesc clearances. Lubricated compressors have a somewhat broader tolerance to changes in operating conditions, but they are still more limited than reciprocating compressors. 

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Slow speed requires high degree of vibration and pulsation suppression. Vane-Type Rotary Compressors... 

Figure 10-7. Vane-type rotary compressor. ICovrtesy of Dresser-Rand Company.)...

This type of compressor is classified as a constant volume, variable-pressure machine that is quite similar to the vane-type rotary in general characteristics. Both have a built-in compression ratio. 

Depending on the number of stages, the gas that flashes in the lowci pressure separators can be compres.sed and then recombined with the gas from the high-pressure separator. Both reciprocating and centrifugal compressors are commonly used. In low-horsepower installations, especially lor compressing gas from stock tanks (vapor recovery), rotary aiuf vane type compressors are common. 

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. 

Reciprocating compressors These cause very high noise levels at low frequency (typically, below 420 Hz). The low-frequency noises are very difficult to attenuate, and the most popular solution is to use rotary (vane type) compressors. These are inherently quieter and have the further advantage that the noise they generate is at high frequency (typically, above 1 kHz) and is therefore easy to attenuate. 

For higher pressures, rotary compressors of the sliding vane type will give delivery pressures up to 1 MN/m2. In a compressor of this type, as illustrated in Figure 8.35, the compression ratio is achieved by eccentric mounting of the rotor which is slotted to take sliding vanes which sub-divide the crescent-shaped space between the rotor and... 

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

Positive-displacement compressors, such as rotary lobe, screw type, or sliding-vane types, pump a relatively constant volume of vapor against whatever back pressure (resistance to heat transfer) is developed by the eveporator. The mass flow rate depends upon the inlet vapor density. Rotary lobe compressors are used for compression ratios up to about 2.0, the compression ratio being limited by mechanical and thermal distortion of small clearances inside the machine. The efficiency of positive-displacement compressors is between 60 and 75%. 

Table 8.2 lists the common failure modes of rotary-type, positive-displacement compressors. This type of compressor can be grouped into two types sliding vane and rotary screw. 

There are four different types of rotary compressors avdlable in the market straight lobe (Figure 14a), sliding vane (Figure 14b), liquid-ring (Figure 4c), and screw (Figure 14d, 14e). 

Volume 1 explains that pumps ean be classified as either positive-displacement or kinetie. The same is true for compressors. In a positive displacement compressor the gas is transported from low pressure to high pressure in a device that reduces its volume and thus inereases its pressure. The most common type of positive displacement eompressors are reeiprocating and rotary (serew or vane) just as was the ease for pumps. Kinetic compressors impart a veloeity head to the gas, which is then converted to a pressure head in accordance with Bernoulli s Law as the gas is slowed down to the velocity in the discharge line. Just as was the case with pumps, centrifugal compressors are the only form of kinetic compressor commonly used. 

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