Compressors - Types And Mode Of Operation

A mechanical compressor is a mechanical component designed to increase the pressure of a fluid by means or a purely mechanical process. When the fluid is a gas, the compressor is called a "compressor", but when it is a liquid, the appropriate term is "pump". Compressors are used to increase the pressure of a fluid and also enable it to be conveyed through a pipe. Gases are compressible,thus by means of compressors the volume of the gas could be greatly reduced to suit its purpose.

 Gas as a fluid 

 

The behavior of gases can be explain by the kinetic theory of gases where a gas is described as a number of molecules (particles) with very weak connections (intermolecular forces) thus making each molecule practically independent. With this conditions gases are characterized by the

following criteria:

• A gas does not have a shape of its own but always take the shape of its container.

• A gas does not have a volume of its own since always occupies the entire volume available.

The chaotic movements of the molecules that make up the matter makes the matter has no shape and enable it to occupy the entire closed space in which the matter is contained. It also leads to a pressure build up as the molecule hit the walls of their container with great speed in a perfectly elastic manner leading to change in momentum.

The gross behaviour of gases are explain by means of simple laws called GAS LAWS. These laws includes:

1. Boyle’s law that explains the relationship between pressure of a fixed mass of gas and its volume in a situation of constant temperature. It tells us that an increase in the pressure of a gas would result from a commensurate decrease in the volume             

        P ∞ 1/V 

2. Charle’s law that explains the relationship between volume of a fixed mass of gas and its temperature in a situation of constant pressure. It tells us that an increase in the volume of a gas would result from a commensurate increase in the temperature              

    V ∞ T

3. Pressure law that explains the relationship between pressure of a fixed mass of gas and its temperature in a situation of constant volume. It tells us that an increase in the pressure of a gas would result from a commensurate increase in the temperature   

   V ∞ T

The density of matter reaches its minimum value in gaseous state. It decreases under the

effect of a pressure decrease (Gay-Lussac's Law and Charles' Law) or of a temperature

increase (this is gas expansion).

Air as a gas

Air is a mixture of gases. The main gases that make up air are:

• Nitrogen > 78 %

• Oxygen > 21 %

• Inert gases (argon, neon, helium) and hydrogen, carbon dioxide, watervapour; 1 %

The density of air is 1.293 kg/m3Air is present in our environment in unlimited quantities.Its quality is variable because air contains water, oil, dust, etc.Compressed air is easy to convey through pipes.It is relatively insensitive to changes in temperature.Air outlets and leaks generate very little pollution.It is ideal for use in explosive environments.Its compressibility property is close to that of perfect gases: P.v/t = constant. Air is characterised by three factors:

• Pressure.

• Temperature.

• Humidity.   

Mariotte and Gay-Lussac's Law is the most significant in the field of industrial pneumatics since it is the basic law governing compressed air

P . V = K . T

were    P: absolute pressure (Pa)
            V: volume (m3)
            T: absolute temperature (K)
            K: constant
This relationship is used by the compressor: a constant volume of air is drawn into the
compressor, of which the volume decreases. This reduction of volume causes an increase
in the pressure and temperature of the air drawn in. In the case of air moving in a pipe the volume is replaced by flow-rate. where flow rate is product of the velocity of the fluid and the cross sectional area of the pipe.Air flow-rate is the quantity of compressed air that passes through a given cross-sectional area per unit of time. 
                                                                     Q = A . v

Q: flow-rate (m3/s)
A: cross-sectional area (m2)
v: velocity (m/s)

Compressed air is clean, safe, simple and efficient.When air is compressed, there is no risk of any dangerous gases of other noxious products escaping.It is an energy source that is not combustible and does not pollute.Air has an atmospheric pressure of 1 bar. When it is mechanically compressed by a compressor, its pressure is increased. 

                                                    (P1 V1) / T1 = (P2 V2) / T2
P = Pressure
V = Volume
T = Temperature of the gas
1 = Initial state before the change
2 = Final state after the change

By applying this formula to a compressor, the volume of air (or the flow-rate) and the air pressure can be controlled and raised to a level as required for the target application.

Types of Compressor and their mode of operation

Compressed air can be produced using two methods:

• Dynamic compression (transformation of the velocity of the air into pressure):centrifugal compressors and axial compressors.

• Volumetric compression (reduction of volume using a compressing element):reciprocating compressors (usually of the piston type) and rotary compressors(screw compressors, vane compressors, gear compressors and liquid ring compressors).

However, Compressors can be divided into four main categories. Namely:

• Reciprocating (volumetric)

• Rotary (volumetric)

• Centrifugal

• Axial flow 

 A reciprocating compressor or piston compressor is a positive-displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure. The intake gas enters the suction manifold, then flows into the compression cylinder where it gets compressed by a piston driven in a reciprocating motion via a crankshaft, and is then discharged. Applications include oil refineries, gas pipelines, chemical plants, natural gas processing plants and refrigeration plants. One specialty application is the blowing of plastic bottles made of polyethylene terephthalate (PET).

The reciprocating pump could either make use of piston or diaphragm for its compression and suction. 

A rotary screw compressor is a type of gas compressor which uses a rotary type positive displacement mechanism. They are commonly used to replace piston compressors where large volumes of high pressure air are needed, either for large industrial applications or to operate high-power air tools such as jackhammers.

The gas compression process of a rotary screw is a continuous sweeping motion, so there is very little pulsation or surging of flow, as occurs with piston compressors.Rotary screw compressors use two meshing helical screws, known as rotors, to compress the gas. In a dry running rotary screw compressor, timing gears ensure that the male and female rotors maintain precise alignment. In an oil-flooded rotary screw compressor, lubricating oil bridges the space between the rotors, both providing a hydraulic seal and transferring mechanical energy between the driving and driven rotor. Gas enters at the suction side and moves through the threads as the screws rotate. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws.


The effectiveness of this mechanism is dependent on precisely fitting clearances between the helical rotors, and between the rotors and the chamber for sealing of the compression cavities.
Typically, they are used to supply compressed air for general industrial applications. Trailer mounted diesel powered units are often seen at construction sites, and are used to power air operated construction machinery.

Additionally, they are becoming increasingly popular in municipal wastewater treatment facilities, for their increased efficiency and thus, lower power consumption

Centrifugal compressors, sometimes termed radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery. The idealized compressive dynamic turbo-machine achieves a pressure rise by adding kinetic energy/velocity to a continuous flow of fluid through the rotor or impeller. This kinetic energy is then converted to an increase in potential energy/static pressure by slowing the flow through a diffuser. The pressure rise in impeller is in most cases almost equal to the rise in the diffuser section.

 

In a centrifugal compressor the mechanical energy is increased by centrifugal action. The gas enters the suction eye of a high speed rotary element called the impeller which caries radial vanes integrally cast in it. As the impeller rotates, the blades of the impeller force the gas outward from the center the impeller to the outer rim of impeller, the increase in velocity of the gas creates a flow pressure area at the eye of the impeller. The gas at the outer rim of the impeller is forced in to a passage way called a diffuser where the velocity decreases in the pressure of the gas.

      In a multistage compressor different sets of impellers and diffusers the pressure of the gas in stages. Labryrinths are installed between the impellers the prevents gas leakage between the stages, each stage has its own casing drain to insure that no liquid is in the compressor before starting.

  To balance the axial thrust developed from the discharge end to the suction end of machine, a balancing drum connected to the suction discharge end of the machine to stop gas leakage along the shaft, a seal is formed with seal oil.

 

Axial flow compressors produce a continuous decelerating flow of compressed gas, and have the benefits of high efficiency and large mass flow rate, particularly in relation to their cross-section. They do, however, require several rows of airfoils to achieve large pressure rises making them complex and expensive relative to other designs (e.g. centrifugal compressors).

Axial compressors are widely used in gas turbines such as jet engines, high speed ship engines, and small scale power stations. They are also used in industrial applications such as large volume air separation plants, blast furnace air, fluid catalytic cracking air, and propane dehydrogenation. Due to high performance, high reliability and flexible operation during the flight envelope, they are also used in aerospace engines 

References : wikipedia.org , 

                       centrifugalcompressor.blogspot.com

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