A Gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.
Types of compressors
Air compressors are mainly classified into two types, based on their construction and operation.
They are as described below in detail:
They are as described below in detail:
Positive displacement type air compressors
Positive displacement compressors mechanically displace a fixed volume of air into a reduced volume. They deliver a nearly constant volume, when operated at a fixed speed; while the discharge pressure is determined by the system load conditions. The different types of positive displacement compressors are as follows:
Reciprocating air compressors:
Compressed air is generated by the to & fro movement of the piston in the compression chamber (Just like an IC Engine). Each movement compresses a fixed quantity of free air at a specific pressure. According to the type of construction, reciprocating compressors can be further classified as single stage / double stage and single acting / double acting compressors. The vertical type air compressors are suitable for applications ranging between 50 - 150 CFM, and the horizontal balance type is most suited for applications ranging from 200 - 5000 CFM.
Rotary type air Compressors:
Air is compressed by two rotating, intermeshing rotors (in some cases one rotor is kept stationery and the other rotates). The action of the rotary screw / lobe can be compared to a reciprocating compressor.
Rotary screw compressors use two meshed rotating positive-displacement helical screws to force the gas into a smaller space These are usually used for continuous operation in commercial and industrial applications and may be either stationary or portable. Their application can be from 3 horsepower (2.2 kW) to over 1,200 horsepower (890 kW) and from low pressure to very high pressure (>1200 psi or 8.3 MPa).
Rotary vane compressors consist of a rotor with a number of blades inserted in radial slots in the rotor. The rotor is mounted offset in a larger housing which can be circular or a more complex shape. As the rotor turns, blades slide in and out of the slots keeping contact with the outer wall of the housing. Thus, a series of decreasing volumes is created by the rotating blades. Rotary Vane compressors are, with piston compressors one of the oldest of compressor technologies. With suitable port connections, the devices may be either a compressor or a vacuum pump. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Dry vane machines are used at relatively low pressures (e.g., 2 bar) for bulk material movement whilst oil-injected machines have the necessary volumetric efficiency to achieve pressures up to about 13 bar in a single stage. A rotary vane compressor is well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor.
Dynamic type air compressors
Dynamic compressors mechanically impart a velocity to the air, through the use of impellers rotating at high speed, in an enclosed housing. The air is forced into a progressively reduced volume. The volumetric flow will vary inversely with the differential pressure across the compressor. The dynamic type of air compressors are classified into:
Centrifugal air compressors:
The centrifugal air compressor consists of an impeller, mounted on a shaft and positioned within a housing, consisting of an inlet duct, a volute and a diffuser. The impeller rotates at high speed and imparts a velocity to the air. The diffuser surrounds the impeller and acts to convert the kinetic energy of the air into potential energy at a higher-pressure level.
Axial flow air compressors
The axial flow type air compressor is essentially a large capacity, high speed machine, with characteristics quite different from the centrifugal. Each stage consists of two row of blades, one row rotating and the next row stationery. The rotor blades impart velocity and pressure to the gas as the motor turns, the velocity being converted to pressure in the stationery blades.
Roots compressors
This is really a blower and is generally limited to a pressure of 1 bar in ingle stage and a pressure of 2.2 bar in two stage combination.
scroll compressor
A scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved spiral-like vanes to pump or compress fluids such as liquids and gases. The vane geometry may be involute, archimedean spiral, or hybrid curves. They operate more smoothly, quietly, and reliably than other types of compressors in the lower volume range
Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid or gas between the scrolls.
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. Only the membrane and the compressor box come in touch with the gas being compressed.
Compressor Cooling System
Cooling of air compressors merits tremendous improtants since it affects the energy efficiency. Cooling systems could be either air cooled or water cooled :
a) Air cooled compressors: These compressors use fan for forced cooling of the compressors. Due to the low cooling efficiency, this type of cooling is mostly used for low capacity compressors having intermittent usage.
b) Water cooled compressors: For heavy duty or continuous applications water cooling system is adopted, as the efficiency of cooling is high.
Location of compressors
Ideally compressors should be located where it can induct clean, dry and cool air. One of the major issues related to the compressor location is the ambient temperature. The compressed air system exposed to extremely high temperatures can result in higher specific power consumption, unscheduled shutdowns, increased maintenance and decrease in lubricant life. Ventilation is equally important for all compressors regardless of type of cooling.
It would be interesting to note that for every 4oC reduction in intake air temperature there is 1% reduction in compressor power consumption.
At design stage, it is important to plan for proper ventilation and access to compressor location. The locations exceeding temperature of 45oC should be avoided. A free space of about 1 m around the compressor needs to be provided for maintenance and 1 m for motor starter access panel.
Addressing the issue of ventilation at the design stage can help in increasing the life of the compressor, Lubricating oil and the coolant.
The following points should be considered while locating the compressors.
- Low humidity to reduce water entrainment
- Adequate ventilation especially for air cooled unit.
- Minimum suction piping
- Minimum of bends
The quantity of free air delivered by the compressor can be calculated using the following formula.
Free air delivered Qf = k x π x d2/4 x T1/P1 x 2 (P3-P4) (P2xRa) ½
T3
Where
K - Coefficient of discharge of nozzle as per IS standards
d - Diameter of the nozzle
Ra - Gas constant 287.1 J/kg k
P1 - Absolute inlet pressure kg/cm2
T1 - Inlet temperature K
P2 - Absolute pressure in receiver kg/cm2
T3 - Absolute temperature before nozzle kg/cm2
P3 - Absolute pressure before nozzle kg/cm2
S - Speed of the compressor in rpm
P3 - P4 - Differential pressure across the nozzle kg/cm2
T3
Where
K - Coefficient of discharge of nozzle as per IS standards
d - Diameter of the nozzle
Ra - Gas constant 287.1 J/kg k
P1 - Absolute inlet pressure kg/cm2
T1 - Inlet temperature K
P2 - Absolute pressure in receiver kg/cm2
T3 - Absolute temperature before nozzle kg/cm2
P3 - Absolute pressure before nozzle kg/cm2
S - Speed of the compressor in rpm
P3 - P4 - Differential pressure across the nozzle kg/cm2
Volumetric efficiency = Free air delivered (m3/min) x 100
Compressor swept volume (m3/min)
Compressor swept volume = π x D2 x L x S x n x N
4
D - Diameter of the cylinder in m
L - Stroke length in m
S - Compressor speed in rpm
n - Number of cylinders in compressor
N = 1 for single acting compressor and 2 for double acting
Specific power consumption = Actual power consumption
Measured free air delivered
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