PRESSURE ENTHALPY

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					PRESSURE ENTHALPY
Pressure Enthalpy
The Pressure Enthalpy Diagram is also known as Mollier Diagram and Pressure Heat
Chart.

The Pressure Enthalpy diagram provides a means of determining a great deal of
information about the characteristics (properties) of ONE KILOGRAM of refrigerant
moving through the vapour compression system and changing state as it goes.




                           R22 Pressure Enthalpy Diagram

As can be seen from the above diagram, the following variables form the basis of the
diagram:
       Absolute Pressure kPa absolute (i.e. gauge pressure PLUS 101.3kPa)
       Specific Enthalpy kJ/kg (kilojoules per kilogram)
       Temperature OC
       Specific Entropy kJ/kgK (kilojoules per kilogram Kelvin)
       Specific Volume L/kg (litres per kilogram)

The diagram can be divided into three (3) distinct areas:
       Sub-cooled liquid
       Saturated liquid / vapour
       Superheated vapour




                                 The Three (3) Areas



                                           1
  One kilogram (1kg) of R22 liquid refrigerant at 0OC would absorb 85kJ of heat to
                      change state from a liquid to a vapour.

Plotting a Cycle on the Pressure Enthalpy Diagram
A basic vapour compression system pressure enthalpy plot should look similar to the
diagram below:




   1. Draw a CONSTANT PRESSURE LINE at a pressure corresponding to the
      CONDENSING TEMPERATURE.

   2. Draw a CONSTANT PRESSURE LINE at a pressure corresponding to the
      EVAPORATING (VAPOURISING) TEMPERATURE.

   3. Plot the condition of the refrigerant ENTERING the TX VALVE.

   4. Draw a VERTICAL LINE through this point to indicate ENTHALPY
      VALUE.

   5. Position the condition of the refrigerant ENTERING the EVAPORATOR. The
      enthalpy of the liquid vapour mixture leaving the TX Valve is the same as the
      enthalpy of the liquid ENTERING the TX Valve at the intersection of the
      constant pressure and enthalpy line.



                                         2
   6. Plot the condition of the vapour entering the compressor:
              Superheated
              At absolute pressure
              At temperature from the reading

   7. Position the condition of the compressor discharge:
             Superheated vapour
             At absolute pressure
      The ENTROPY of the vapour entering the compressor is the same as the
      ENTROPY of the vapour leaving the compressor. Draw a line parallel to the
      entropy lines for the compressor entry to the constant condensing pressure
      line.

Once the cycle of a system is plotted on a Pressure Enthalpy Diagram, considerable
information can be obtained such as: pressures; temperatures; enthalpies; and specific
volumes.

These values can be used to calculate:
   1. Compression Ratios                            5.   Refrigeration Effect
   2. Mass Flow Rate                                6.   Displacement Volumes
   3. Power Needed to Compress Refrigerant          7.   System Capacity
   4. Evaporator Capacity                           8.   Condenser Capacity

For most practical applications, the diagram may be drawn neglecting pressure drops.




                                          3
4
Calculations
An R22 system operates between the temperatures of 40OC and -10OC with liquid
sub-cooling of 10K and the vapour superheated to 30OC.

Refer to the 2 diagrams below to understand the structure of the calculations. The
R22 P.E. Diagram is the plot based on the above figures whereas the 2nd diagram
indicates the points at which the calculations are based.




                                          5
Note: Figures may be ‘rounded’ for simplification of calculations.

   1. Calculate the compression ratio.

              A
       CR =
              B

       where A = Absolute Discharge Pressure kPa abs.
             B = Absolute Suction Pressure kPa abs.

              1550
       CR =
               350

       CR = 4.4268:1

   2. Calculate the refrigeration effect.
      RE = C – B

       Where C = Enthalpy of refrigerant entering compressor kJ/kg
             B = Enthalpy of refrigerant entering evaporator kJ/kg

       RE = 430 – 235

       RE = 195 kJ/kg

   3. Calculate the mass flow rate if the compressor has a displacement of 5 L/s
      operating at the above conditions.

            Dg
       m=
              v

       Where m = mass flow rate
             Dg = displacement
                  v = specific volume

            5
       m=
            75

       m = 0.0667 kg/s

   4. If capacity is m x RE calculate the capacity.

       Q = m × RE

       Q = 0.0667 × 195

       Q = 13.0065 kJ/s or 13.0065 kW


                                            6
5. Calculate the Latent Heat of Vaporisation at -10OC.

   LH = C1 – B1

   LH = 405 – 185

   LH = 220 kJ/kg

6. Calculate the amount of flash gas at specific conditions.

   FG = B – B1

   FG = 235 – 185

   FG = 50 kJ/kg

7. Calculate the % of Flash Gas

                 FG 100
    % of FG =       ×             FG = Flash Gas    LH = Latent Heat
                 LH   1

                 50 100
    % of FG =       ×
                 220 1

    % of FG = 22.727%




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