RACA Journal June 2016 - Page 67

Essentials Process 2-3 Here two processes take place that of removing sensible heat from point 2 to the saturation line and de-superheating the refrigerant. From the saturation line the actual condensation process starts, which is a change of phase from a high pressure vapour to high pressure liquid (transfer of latent heat). The process from 2-3 takes place at constant pressure and follows the constant pressure line on the p-H diagram. There is de-superheat of 10°C needed and then condensation starts at 40°C. Process 3-4 This takes place in the expansion valve at constant enthalpy. Work is not done to the refrigerant and some of the heat in the refrigerant is used to evaporate the liquid refrigerant. Some of the heat may cause flash gas. The pressure is reduced from condensing pressure to evaporating pressure. Scientific laws and processes were used during the invention of refrigeration systems which made quantifying possible Process 4-1 This takes place at constant evaporating pressure and heat is absorbed by the refrigerant from the surroundings. This increase in enthalpy is called refrigerating effect. So far we read only heat of compression from the p-H diagram but the following can also be read: 1 Refrigerating effect = h1- h4 = 415kJ/kg – 245kJ/kg = 170kJ/kg 2 Heat rejection = h2- h3= 445kJ/kg – 245kJ/kg = 200kJ/kg Please note that heat rejected is equal to refrigerating effect plus heat of compression 3 Coefficient of performance = Heat absorbed in the evaporator / Heat equivalent of power input into compressor (heat of compression) = refrigerating effect / Heat of compression = 170 / 30 = 5.67 (no units since is a ratio, the higher the figure the higher the efficiency of the system) 4 We have so far considered only one kilogram of refrigerant flow per second (1kg/s) The following may be calculated if total refrigerant flow per point is known Plant Capacity (kW) = mass flow (kg/s) x refrigerating effect (kJ/kg) = 1 x 170 = 170kW Power Input (kW) = mass flow (kg/s) x heat of compression (kJ/kg) = 1 x 30 = 30kW Total Heat Rejection (kW) = mass flow (kg/s) x Total Heat Rejection (kJ/kg) = 1 x 200 = 200kW It can be seen that Total Heat rejection = Plant capacity + Power Input. RACA References: ASHRAE www.hvacronline.co.za RACA Journal I June 2016 65