B. Designs and Operation of Compressors

Read text B. Pay attention to the words in bold type.

Answer the questions to text A.

Decide whether the following statements are true or false according to text A.

1. Compression of a gas decreases its temperature.

2. Compressors are similar tomagnetrons.

3.Liquids are relatively incompressible.

4. The term compressor is used when the pressure of a gas is increased in a motor-driven machine.

5. Compressor is a gas pump.

6.Kinetic devices include centrifugal pumps and compressors.

7.Axial-flow compressors consist of a number of shafts.

8. The function of a liquid ring pump is similar to a rotary vane pump.

9. The liquid ring pump compresses gases by means of an impeller rotating within and eccentric to a cylindrical casing.

10. Typically a multi-stage pump will have up to two compression stages on a common surface.

11. Single stage vacuum pumps typically produce vacuum to 335 torr.

12.Liquid ring compressors are often used in Vapor recovery systems.

1. What kind of device is a gas compressor?

2. Does compression of a gas increase its temperature or pressure?

3. In what wayare compressors similar to pumps?

4. What is the difference between the terms “pump” and “compressor”?

5. What do kinetic devices include?

6. How many stages do axial-flow compressors consist of?

7. How can the liquid-ring pumps be used?

8. What canyou say about the function of a liquid-ring pump?

9. How does the liquid-ring pump compress gases?

10. How many stages can liquid-ring systems have?

11. What is the capacityof a single-stage pump?

12. Where are liquid-ring pumps and compressors used?

· Reciprocating compressors – use pistons driven by a crankshaft. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. But their numbers are declining (they are replaced by less costly rotary screw compressors). The picture of the reciprocating compressor is below.

· Rotary screw compressors – use two meshed rotating positive-displacement helical screws to force the gas into a smaller space. These are usually for continuous operation in commercial and industrial applications and may be either stationary or portable.

· Centrifugal compressors – use a vaned rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section converts the velocity energy to pressure energy. These are for continuous, heavy industrial uses and are usually stationary. Their application can be from 100 hp (75 kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000 lb/in² (69 MPa). Many large snow-making operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers and turbo-charges. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines.

· Diagonal or mixed-flow compressor – are similar to a centrifugal compressor, but have a radial and axial velocity component at the exit from the rotor. The diffuser is often used to turn diagonal flow to the axial direction. The diagonal compressor has a lower diameter diffuser than the equivalent CF compressor.

· Axial-flow compressor – use a series of fan-like rotating rotor to progressively compress the gas flow. Axial-flow compressors are normally used in high flow applications, such as medium to large gas turbine engines. They are almost always multi-staged. See the picture of the Axial-flow compressor below.

· Scroll compressor – similar to a rotary screw device, it includes two interleaved spiral-shaped scrolls to compress the gas. Its output is more pulsed than that of a rotary screw compressor, and this has caused its declining industrial use. It can be used as an automotive supercharger, and in an air conditioner condensing unit.

Since being rediscovered three decades ago, scroll technology has been successfully adapted to vacuum use. Several innovations have allowed scroll-type pumps to produce vacuums lower than 10 m Torr; the pumps are used in many applications requiring an inexpensive, dry, long-lived pump. Look at the picture of the Scroll compressor; pay attention to the special note:

Archimedean Screw is known to be a device consisting of a spiral passage within an inclined cylinder for raising water to a height when rotated.

Thus, a scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved Archimedean spiral shaped scrolls to pump or compress fluids such as liquids and gases. One of the scrolls is fixed, while the other orbits eccentricallywithout rotating, thereby trapping and compressing pockets of fluid between the scrolls.

The device was first patented in 1905 but did not become efficient for fifty years. These devices are known to be used for operating more smoothly, quietly, and reliably than conventional compressors. However, due to their smaller available free volume (to accommodate refrigerant flooding) compared to reciprocating compressors, the reliability of scroll compressors is debatable. Many residential central heat pump and air conditioning systems and a few automotive air conditioning systems employ a scroll compressor instead of the rotary. Traditionally, reciprocating and wobble-plate compressors are used.

· Diaphragm Compressor is used for hydrogen and compressed natural gas (CNG).

Air compressors sold to and used by the general public are often attached on top of a tank for holding the pressurized air. Oil-lubricated and oil-free compressors are also available. Oil-free compressors are desirable because without proper consideration (additional parts) oil can make its way into the air stream. In a given use, for example as a diving air compressor, even minimal oil may be unacceptable.

Дата добавления: 2014-12-26