Showing posts with label Classification of compressors. Show all posts
Showing posts with label Classification of compressors. Show all posts

Thursday, 14 November 2013

RECIPROCATING COMPRESSORS

    Q.2) Classify the compressors
      (i) On the basis of operations employed
      (ii) On the basis of pressure achieved
      (iii) On the basis of pressure ratio
      (iv) On the basis of capacity of compressors.
    A.2) Depending upon different parameters, compressors can be classified on the basis of operations employed, the delivery pressure achieved, pressure ratio and capacity of compressors as follows.
    On the basis of operations employed, compressors are classified into two groups:
      i) :Reciprocating compressors : It uses piston cylinder arrangement and due to positive displacement of air in the cylinder, the air is compressed and delivered to a vessel called Receiver. These are capable to produce high delivery pressure with low volume flow rate.
      ii) Rotary Compressors : These compressors operate at high speeds, therefore, can handle large volume flow rates compared to reciprocating compressors.

      In rotary compressors, the dynamic head is imparted to the gas with the help of very high speed impeller rotating at a confined space so that the air is compressed due to centrifugal action.
    On the basis of delivery pressure, compressors are classified into three categories
      i) Low Pressure Compressors : Delivery pressure upto 1.1 bar
      ii) Medium Pressure Compressors : Delivery pressure upto 7 bar
      iii) High Pressure Compressors : Delivery pressure between 7 to 10 bar.
    On the basis of pressure ratio, we can classify the devices as follows,
      Fans : Pressure ratio upto 1.1
      Blower : Pressure ratio upto 1.1 to 4.0
      Compressors : Pressure ratio above 4.0
    On the basis of capacity, compressors can be classified as follows,
      Low capacity compressors : Volume flow rate upto 10 m3/min, or less
      Medium capacity compressors : Volume flow rate 10 m3/min to 300 m3/min
      High capacity compressors : Volume flow rate above 300 m3/min

    Q.3) Find an expression for required work done to drive a compressor, when compression is,
      adiabatic in nature
      isothermal compression
      polytropic compression

    A.3) During the analysis of the operations of a reciprocating air compressor, we consider some assumptions to simplify the analysis. They are as follows
      i) There is no Clearence Volume
      ii) Working substance air is an ideal gas
      iii) There is no frictional loss.
      iv) There is no wire drawing in the valve or pipe lines.

WORK REQUIRED TO DRIVE A COMPRESSOR

Suppose, we are running a single stage air compressor, which draws air at pressure P1 and temperature T1 during the suction or induction process. The air thus drawn inside the cylinder then compressed to achieve a delivery pressure, P2 by adiabatic process. In the adjacent figure, process a - b is the suction process. Process b - c is the adiabatic compression of the air from pressure P1 to pressure P2. Process c - d is the delivery stroke, delivering the compressed air at a pressure P2.

    Wad = P2V2 + {( P2V2 - P1V1)/(γ - 1)} - P1V1
    => (P2V2 - P1V1){1 + 1/(γ - 1)}
    => {γ /(γ - 1)}P1V1 {(P2V2)/(P1V1) - 1}
    => {γ /(γ - 1)}mRT1{(P2/P1){(γ - 1)/γ} - 1}

POLYTROPIC WORKDONE IN RECIPROCATING COMPRESSORS

    Wpoly = P2V2 + {( P2V2 - P1V1)/(n - 1)} - P1V1
    => (P2V2 - P1V1){1 + 1/(n - 1)}
    => {n/(n - 1)}P1V1 {(P2V2)/(P1V1) - 1}
    => {n/(n - 1)}mRT1{(P2/P1){(n - 1)/n} - 1}
ISOTHERMAL WORKDONE REQUIRED TO DRIVE RECIPROCATING COMPRESSOR
    Wiso = P2V2 + {P1V1ln(V1/V2)} - P1V1
but as the process is isothermal, P1V1 = P2V2
    Wiso = P1V1ln(V1/V2)
    Wiso = mRT1ln rp
    V1/V2 = P2/P1 = rp (pressure ratio)

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