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Ragu's Blog

Air Compressor

 

Compressor

A Gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.

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 pressurize and transport liquids.
Types of compressors
 
Air compressors are mainly classified into two types, based on their construction and  operation.
 
  •     Positive displacement type
  •     Dynamic type
  •  

    They are as described below in detail:
     
  •     Centrifugal compressors 
  •     Diagonal or mixed-flow compressors 
  •     Axial-flow compressors 
  •     Reciprocating compressors 
  •     Rotary screw compressors 
  •     Rotary vane compressors 
  •     Scroll compressors 
  •     Diaphragm compressors
  •  
    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 
     

          
    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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