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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% 7

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posted: | 3/28/2010 |

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PRESSURE ENTHALPY

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