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

A. Kinetic Devices

A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume. Compression of a gas naturally increases its temperature. Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas. Liquids are relatively incompressible, so the main action of a pump is to move the liquid elsewhere.

The term pump is generally used for liquid-handling or hand-operated devices while the term compressor is used when the pressure of a gas is increased in a motor-driven machine. A gas pump is generally called a compressor.

Kinetic devices include centrifugal pumps and compressors and axial-flow devices. In centrifugal pumps and compressors, the vanes of the impeller, or rotor take fluid in near the axis. Centrifugal pumps and compressors can handle at moderate flow rates and pressure increases.

Axial-flow compressors consist of a number of stages, each with a set of stationary and rotating blade passages.

Liquid ring pumps are known to be rotating positive displacement pumps. They are typically used as vacuum pumps but can also be used as gas compressors. The function of a liquid ring pump is similar to a rotary vane pump. They are an inherently low friction design, with the rotor being the only moving part.

The liquid ring pump compresses gases by means of an impeller rotating within and eccentric to a cylindrical casing. Liquid (usually water) is fed into the pump and by centrifugal action, forms a moving cylindrical ring against the inside of the casing. This liquid ring creates a series of seals in the space between the impeller vanes to form compression chambers. The eccentricity between the impeller rotating axis and the casing geometric axis results in a cyclic variation of the volume enclosed by the vanes and the ring.

Gas (or air) drawn into the pump via an inlet port in the end of the casing, is trapped in the space formed by the impeller vanes and the liquid ring. Liquid ring systems can be single or multi-stage. Typically a multi-stage pump will have up to two compression stages on a common shaft. In vacuum service, the attainable pressure reduction is limited by the vapor pressure of the ring-liquid. Single stage vacuum pumps typically produce vacuum to 35 torr (mm Hg), assuming air is being pumped and the ring-liquid is water at 15ºC (60F) or less.

These simple, but highly reliable pumps have a variety of industrial applications. For example, soil remediation, where contaminated ground water is drawn from wells by vacuum. In petroleum refining, vacuum distillation also makes use of liquid ring vacuum pumps to provide the process vacuum. Liquid ring compressors are often used in Vapor recovery systems.

Comprehension Check – 1

Дата добавления: 2014-12-26