Task 1: Refrigerator with ammonia A compression refrigeration unit with ammonia as refrigerant operates as follows: The low-pressure compressor LC gets dry saturated vapor from the evaporator of pressure p1 = 0.72 bar. The LC compresses the refrigerant to intermediate pressure p2 = 4 bar. This vapor is fed to an isobaric mixing chamber MC. The high-pressure compressor HC takes in dry- saturated steam from state 3 and compresses it to the pressure p4 = 14.3 bar. Then, at constant pressure p4, ammonia is condensed, whereby the specific heat q4-5 is released. At the outlet of the condenser, the ammonia is just completely condensed and has state 5. A part of the refrigerant is first subcooled to state 6 by flowing through a tube in the mixing chamber. This part of the refrigerant is then expanded in a throttle T1 to the pressure p7 = p and a vapor content x7 = 0.14. Then the refrigerant enters the evaporator, where it receives the specific heat q7-1 and evaporates to state 1. The other part of the refrigerant is supplied to the mixing chamber via the throttle T2. The amount is just sufficient to keep the liquid level constant and to operate the process stationary. The specific work of the low-pressure compressor is W1-2 = 236 kJ/kg, the specific heat transferred in the condenser is q4-5 = -1305 kJ/kg. Condenser 94-5 50 x=0 P = 14.3 bar Temp. Press. Steam table for ammonia in the saturated state Specific enthalpy T h' h" C bar kJ/kg kJ/kg HC T2 3 40 0.72 317.6 1706.7 x = 1 -2 4 490.8 1758.2 8 P = 4 bar M 37 14.3 673.2 1785.7 2 6 MC LC T1 x=0.14 97-1 P = 0.72 bar 7 Evaporator Assumptions: All components are adiabatic to the ambient. The heat exchangers and the mixing chamber are isobaric. The compressors are isentropic. The throttles are isenthalp. 1. Sketch the process in a T-s diagram. 2. Determine the specific enthalpy h7 and the specific heat q7-1 transferred in the evaporator. 3. What is the specific work W3-4? 4. Determine the ratio of the mass flows m/m2. 5. What is the coefficient of performance of the compression refrigeration unit?