INTRODUCTION
A compressor is a machine that is used to increase the pressure of a gas or vapor. They can be grouped into two major classifications: centrifugal and positive displacement.
The purpose of compressors is to move air and other gases from place to place. Gases, unlike liquids, are compressible and require compression devices, which although similar to pumps, operate on somewhat different principles. Compressors, blowers, and fans are such compression devices.
• Compressors. Move air or gas in higher differential pressure ranges from 35 psi to as high as 65,000 psi in extreme cases.
• Blowers. Move large volumes of air or gas at pressures up to 50 pounds per square inch.
• Fans. Move air or gas at a sufficient pressure to overcome static forces. Discharge pressures range from a few inches of water to about / pound per square inch.
how compressor work
To understand how gases and gas mixtures behave, it is necessary to recognize that gases consist of individual molecules of the various gas components, widely separated compared to their size. These molecules are always traveling at high speed; they strike against the walls of the enclosing vessel
and produce what we know as pressure
Temperature affects average molecule speed. When heat is added to a fixed volume of gas, the molecules travel faster, and hit the containing walls of the vessel more often and with greater force
This then produces a greater pressure. This is consistent with Amonton's Law
If the enclosed vessel is fitted with a piston so that the gas can be squeezed into a smaller space, the molecule travel is now restricted. The molecules now hit the walls with a greater frequency, increasing the pressure, consistent with Boyle's Law
However, moving the piston also delivers energy to the molecules, causing them to move with increasing velocity. As with heating, this results in a temperature increase. Furthermore, all the molecules have been forced into a smaller space, which results in an increased number of
collisions on a unit area of the wall. This, together with the increased velocity, results in increased pressure.
The compression of gases to higher pressures results in higher temperatures, creating problems in compressor design. All basic compressor elements, regardless of type, have certain design-limiting operating conditions. When any limitation is involved, it becomes necessary to perform the work in more than one step of the compression process. This is termed multi staging and uses one basic machine element designed to operate in series with other elements of the machine
This limitation varies with the type of compressor, but the most important limitations include:
1. Discharge pressure—all types.
2. Pressure rise or differential—dynamic units and most displacement types.
3. Compression ratio—dynamic units.
4. Effect of clearance—reciprocating units (this is related to the compression ratio).
5. Desirability of saving power.
METHODS OF COMPRESSION
Four methods are used to compress gas. Two are in the intermittent class, and two are in the continuous flow class
Four methods are used to compress gas. Two are in the intermittentclass, and two are in the continuous flow class.
1. Trap consecutive quantities of gas in some type of enclosure, reduce the volume (thus increasing the pressure), then push the compressed gas out of the enclosure.
2. Trap consecutive quantities of gas in some type of enclosure, carry it without volume change to the discharge opening, compress the gas by back flow from the discharge system, then push the compressed gas out of the enclosure.
3. Compress the gas by the mechanical action of rapidly rotating impellers or bladed rotors that impart velocity and pressure to the flowing gas, (Velocity is further converted into pressure in stationary diffusers or blades.)
4. Entrain the gas in a high velocity jet of the same or another gas (usually, but not necessarily, steam) and convert the high velocity of the mixture into pressure in a diffuser.
Compressors using methods 1 and 2 are in the intermittent class and
are known as positive displacement compressors. Those using method 3 are known as dynamic compressors. Compressors using method 4 are known as ejectors and normally operate with an intake below atmospheric pressure.
Compressors change mechanical energy into gas energy. This is in accordance with the First Law of Thermodynamics, which states that energy cannot be created or destroyed during a process (such as compression of a gas), although the process may change mechanical energy into gas energy. Some of the energy is also converted into nonusable forms such as heat losses
Mechanical energy can be converted into gas energy in one of two ways:
1. By positive displacement of the gas into a smaller volume. Flow is directly proportional to speed of the compressor, but the pressure ratio is determined by pressure in the system into which the compressor is pumping.
2.By dynamic action imparting velocity to the gas. This velocity is then converted into pressure. Flow rate and pressure ratio both vary as a function of speed, but only within a very limited range and then only with properly designed control systems.
In choosing the correct compressor for a given installation, the following factors must be taken into account:
• Maximum, minimum and mean demand. If there is an intermittent requirement for air, but a large compressor set is needed to cater for peak requirements, the
• Ambient conditions-temperature, altitude and humidity. At high altitude, because of the reduced air density, the compressor efficiency and capacity is reduced. High humidity can result in large quantities of water which have to be disposed of.
• Methods of cooling available - availability of cooling water or ambient temperature for air cooling. The possibility of using the waste heat for space heating or for process heating should be considered.
• Environmental factors- noise and vibration. Special foundations may be required, particularly with reciprocating compressors.
• Requirements for skilled maintenance personnel.
Rotary positive displacement compressors are machines in which compression and displacement result from the positive action of rotating elements.
Sliding vane compressors are rotary positive displacement machines in which axial vanes slide radially in a rotor eccentrically mounted in a cylindrical casing. Gas trapped between vanes is compressed and displaced
Liquid piston compressors are rotary positive displacement machines in which water or other liquid is used as the piston to compress and displace the gas handled.
Two-impeller straight-lobe compressors are rotary positive displacement machines in which two straight mating lobed impellers trap gas and carry it from intake to discharge. There is no internal
compression.
Helical or spiral lobe compressors are rotary positive displacement machines in which two inter meshing rotors, each with a helical form, compress and displace the gas.
Dynamic compressors are rotary continuous-flow machines in which the rapidly rotating element accelerates the gas as it passes through the element, converting the velocity head into pressure. This
occurs partially in the rotating element and partially in stationary diffusers
or blades.
Centrifugal compressors are dynamic machines in which one or more rotating impellers, usually shrouded on the sides, accelerate the gas. Main gas flow is radial.
Axial compressors are dynamic machines in which gas acceleration is obtained by the action of the bladed rotor. Main gas flow is axial
Mixed flow compressors are dynamic machines with an impeller form combining some characteristics of both the centrifugal and axial types.
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