Every compressed air system begins with a compressor, the source of airflow for all the equipment and processes. Air compressors are utilized to raise the pressure of a volume of air. Air compressors are versatile mechanical tools that use one or numerous pistons to pump compressed air into a defined space. An air compressor is defined as a component that takes in air at atmospheric pressure and delivers it at a higher pressure.
Compressed Air
Compressed air is widely used throughout industry and is considered one of the most useful and clean industrial utilities. It is simple to use, but complicated and costly to create. Compressed air is air that is condensed and
contained at a pressure that is greater than the atmosphere. The process takes a given mass of air, which occupies a given volume of space, and reduces it into a smaller space. In that space, greater air mass produces greater
pressure. Compressed air is used in many different manufacturing operations.
Applications of Compressed Air
Compressed air is used in almost every industry like automotive, construction, universities, hospitals, mining, agriculture, food and beverage, consumer goods, pharmaceutical, electronics, and more
1.For maintenance work, plants can use air-operated drills, screwdrivers,
and wrenches.
2.Painting can be done using paint-spraying systems.
3.Pneumatic tools are used on a production line.
4.Used in the foundry for cleaning large castings, and to remove weld scale, rust, and paint in other industries.
5.Grinding, wire brushing, polishing, sanding, shot blasting, and buffing are performed efficiently with compressed air in the automotive, aircraft, rail car, locomotive, vessel shops, shipbuilding, other heavy machinery,
and other industries.
TYPES OF AIR COMPRESSORS
Air compressors are of two types:
Positive displacement air compressors
Non positive/dynamic air compressors
In the positive displacement type, a given quantity of air or gas is trapped in a compression chamber and the volume it occupies is mechanically reduced, causing a corresponding rise in pressure prior to
discharge. At constant speed, the air flow remains essentially constant with
variations in discharge pressure.
Dynamic compressors impart velocity energy to continuously flowing air or gas by means of impellers rotating at very high speeds. The velocity energy is changed into pressure energy both by the impellers and the
discharge volutes or diffusers.The compressors are further segmented into several compressor types as shown in Figure 4.19.
The function of all of them is to draw in air from the atmosphere and produce air at substantially higher pressures.
POSITIVE DISPLACEMENT COMPRESSORS
Positive displacement compressors decrease the volume and increase the pressure of a quantity of air by mechanical means. These are classified
as:
1.Rotary compressors
2.Reciprocating compressors
ROTARY COMPRESSORS
Rotary air compressors are positive displacement compressors. Rotary
compressors have rotors that give a continuous pulsation free discharge.They operate at high speeds. Their capital costs are low. They are compact in size, have low weight, and are easy to maintain. These units are basically oil cooled (with air cooled or water cooled oil coolers) where the oil seals the internal clearances. Because the cooling takes place right inside the compressor, the working parts never experience extreme operating temperatures. Rotary compressors are classified into three general groups:
1.Screw compressor
2.Lobe compressor
3.Vane compressor
Screw Compressor
Rotary screw compressors are positive displacement compressors.Compression is achieved via the meshing of two helically cut rotor profiles. One rotor is cut as a male profile, and the other as a female profile. These two rotors spin in opposite directions. The rotary screw compressor can be single screw or twin screw.
A single-screw compressor uses a single main screw rotor meshing with two gate rotors with matching teeth. The main screw is driven by the prime mover, typically an electric motor. (Refer to Figure 4.20.)
Air is drawn through the inlet port into the voids between the rotors. As the rotors move, the volume of trapped air is successively reduced and compressed by the rotors coming into mesh.
These compressors are available as dry or wet (oil-flooded) screw types.
In the dry-screw type, the rotors run inside of a stator without a lubricant (or coolant). The heat of compression is removed outside of the compressor,limiting it to a single-stage operation. In the oil-flooded screw type compressor, the lubricant is injected into the air, which is trapped inside of the stator. In this case, the lubricant is used for cooling, sealing, and lubrication. The air is removed from the compressed gas-lubricant mixture in a separator.
Because of simple design and few wearing parts, rotary screw air compressors are easy to maintain, operate, and provide great installation flexibility. Advantages of the rotary screw compressor include smooth, pulse-free air output in a compact size with high output volume over a long life
Lobe Compressor
A schematic diagram for a rotary lobe compressor is provided in Figure
4.22. In this type of compressor, the rotors do not touch and a certain amount of slip exists. The slip increases as the output pressure increases. The principle of operation is analogous to the rotary screw compressor, except that with the lobe compressor, the mating lobes are not typically lubricated
for air service. As the lobe impellers rotate, gas is trapped between the lobe impellers and the compressor case, where the gas is pressurized through the rotation of lobes and then discharged. The bearings and timing gears are lubricated using a pressurized lubricating system or sump.
A rotary vane compressor is schematically illustrated in Figure 4.23. Rotary vane compressors consist of a rotor which is mounted offset in a larger housing which can be of circular or a more complex shape. The rotor has slots along its length, each slot contains a vane. The vanes are thrown
outwards by centrifugal force when the compressor is running and the vanes move in and out of the slot because the rotor is eccentric to the casing. The vanes sweep the cylinder, sucking air in on one side and ejecting it on the other.
RECIPROCATING COMPRESSORS
Reciprocating compressors were the first of the modern air compressor designs. Reciprocating air compressors are positive displacement machines in which pistons are driven by a crankshaft. This means they are taking in successive volumes of air that is confined within a closed space and
elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by using a piston within a cylinder as the compressing and displacing element. As the piston enters the down stroke, air is drawn into
the cylinder from the atmosphere through an air inlet valve. During the up stroke, the piston compresses the air and forces it through a discharge control valve and out of the compressor. These are available in single-acting and double-acting configurations. The reciprocating air compressor is considered
single acting when the compressing is accomplished using only one side of
the piston. A compressor using both sides of the piston is considered double acting. Reciprocating compressors are of two types:
1.Piston compressors
2.Diaphragm compressors
Advantages
Good for small applications.
Cheap and simple to operate.
Operates over a wide range of pressures.
Disadvantages
Noisy.
Maintenance problems.
Not good for small applications.
Oil free air units are expensive.
PISTON COMPRESSORS
Piston compressors are of two types:
Single-Stage Piston Compressor
The single-stage reciprocating compressor has a piston that moves downward during the suction stroke, expanding the air in the cylinder asshown in Figure 4.24. The expanding air causes pressure in the cylinder to drop. When the pressure falls below the pressure on the other side of the inlet
valve, the valve opens and allows air in until the pressure equalizes across the inlet valve. The piston bottoms out and then begins a compression stroke. The upward movement of the piston compresses the air in the cylinder,
causing the pressure across the inlet valve to equalize and the inlet valve to reset. The piston continues to compress air during the remainder of the upward stroke until the cylinder pressure is great enough to open the
discharge valve against the valve spring pressure. Once the discharge valve is open, the air compressed in the cylinder is discharged until the piston completes the stroke.
Two-Stage Piston Compressor
To avoid an excessive rise in temperature, multistage compressors are provided with intercoolers. These heat exchangers remove the heat of compression from the gas and reduce its temperature to approximately the
temperature existing at the compressor intake. Such cooling reduces the volume of gas going to the high pressure cylinders, reduces the power required for compression, and keeps the temperature within safe operating limits. These compressors can generate high pressures than single-stage compressors. The most common type is two-stage or double-acting compressors. This style of reciprocating air compressor utilizes a doubleacting piston (compression takes place on both sides of the piston), piston
rod, crosshead, connecting rod and crankshaft. (Refer to Figure 4.25.)
Double-acting compressors are available in single-and multi-cylinder and single-and multistage configurations. Most double-acting air compressors are available either water-cooled or air-cooled. Lubrication of double-acting compressors is accomplished with a positive pressure oil pump. The oil
required to both lubricate and protect the compressor is circulated via the oil
pump to the cylinders and the crankshaft bearings. Discharge air temperatures in these machines may often exceed 300°C in some of the applications. On the whole, the reciprocating compressors run hotter than
other types of compressors
cylinder at greatly reduced temperature after passing through the intercooler,
thus improving efficiency as compared to that of a single-stage unit.
Advantages
Highest efficiency.
Longest service life.
Field serviceability
DIAPHRAGM COMPRESSOR
A diaphragm compressor (also known as a membrane compressor) is a variant of the conventional reciprocating compressor. The compression of gas occurs by the movement of a flexible membrane, instead of an intake
element. The back and forth movement of the membrane is driven by a rod and a crankshaft mechanism. (Refer to Figure 4.26.) Only the membrane and the compressor box come in touch with the gas being compressed.
Diaphragm compressors are used for hydrogen and compressed natural gas
(CNG) as well as in a number of other applications.
The oil pressure required to bend the diaphragm is generated by a crank
drive with a reciprocating piston. During the compression stroke, the piston pushes oil from the cylinder into the diaphragm head where it flows through the perforated plate to the back side of the diaphragm. The diaphragm is thus forced to bend into the concave diaphragm head cover surface. As the piston moves back, it pulls the diaphragm against the surface of the perforated plate, which is also concave. So the oscillation frequency of the diaphragm corresponds to the compressor speed.
DYNAMIC COMPRESSORS
These compressors raise the pressure of air or gas by imparting velocity, energy, and converting it to pressure energy. First, rapidly rotating impellers (similar to fans) accelerate the air. Then, the fast flowing air passes through a diffuser section that converts its velocity head into pressure by directing it
into a volute. Dynamic compressors include centrifugal and axial types. These types of compressors are widely used in the chemical and petroleum refinery industries for specific services. They are also used in other industries
such as the iron and steel industry, etc. Compared to positive displacement
type compressors, dynamic compressors are much smaller in size and produce much less vibration.
Centrifugal/Axial Compressors
The results accomplished by centrifugal compressors are the same as by
previously explained compressor types, but the centrifugal compressors go about it in an entirely different way. Whereas reciprocating and screw compressors compress air by squeezing the air from a large volume into a
smaller one, centrifugal compressors raise pressure by increasing the air’s velocity. For this reason, centrifugal compressors are referred to as dynamic
compressors.
Centrifugal compressors raise the pressure of air by imparting velocity, using a rotating impeller, and converting it to pressure. Each stage of compression in a centrifugal compressor consists of an impeller which
rotates and a stationary inlet and discharge section. The air enters the eye of the impeller, designated D as shown in Figure 4.27. As the impeller rotates, air is thrown against the casing of the compressor. The air becomes
compressed as more and more air is thrown out to the casing by the impeller blades. The air is pushed along the path designated A, B, and C. The pressure of the air is increased as it is pushed along this path.
Centrifugal compressors produce high-pressure discharge by converting
angular momentum imparted by the rotating impeller (dynamic displacement). In order to do this efficiently, centrifugal compressors rotate at higher speeds than the other types of compressors. These types of compressors are also designed for higher capacity because flow through the compressor is continuous. Compare to other type of compressors, axial flow compressors are mainly used for applications where the head required is low.
Advantages
High-quality air.
Moderate efficiency.
Longer service life.
Disadvantages
Higher initial cost.
Must be water-cooled.
Airflow is sensitive to changes in ambient conditions.
Axial Flow Compressor
The axial flow type air compressor is essentially a large capacity, high
speed machine with characteristics quite different from the centrifugal air
compressor. Axial flow compressors are used mainly as compressors for gas
turbines. The component of an axial flow compressor consists of the rotating
element that constructs from a single drum to which are attached severalrows of decreasing height blades having airfoil cross sections. Between each rotating blade row is a stationary blade row. All blade angles and areas are designed precisely for a given performance and high efficiency. The
efficiency in an axial flow compressor is higher than the centrifugal compressor. The operation of the axial flow compressor is a function of the rotational speed of the blades and the turning of the flow in the rotor. The
stationary blades (stator) are used to diffuse the flow and convert the velocity increased in the rotor to a pressure increase. One rotor and one stator make up a stage in a compressor.
SPECIFICATIONS OF COMPRESSORS
Following parameters are taken into account while selecting a compressor:
1.Pressure range (psi)
2.Flow rate
3.Receiver size (gallons)
4.Power supply (may be electric or mechanical)
5.Size of installation
6.Number of stages (in case of reciprocating)