ME 401 Reverse Rankine (Vapor Compression) Refrigeration Cycle

1. ME 401Reverse Rankine(Vapor Compression) Refrigeration Cycle

2. Rankine Power Cycle Boiler 2’ Qboiler Basic Rankine power cycle 3 2 High P Wturbine Pump Low P 1 Turbine Wpump 4 Condenser Qcondenser

3. Rankine Refrigeration Cycle Reverse the flow of the fluid and the work/heat transfer directions of the power cycle 2’ Condenser Qcond Boiler 3 2 High P Compressor Liq turbine Wcompressor Pump Low P 1 Turbine Wliq turb 4 Qevap Condenser Evaporator

4. Qcond Condenser 2 1 High P Expansion Device Compressor Low P 3 4 Wcomp Evaporator Qevap Rankine Refrigeration Cycle

5. Rankine Refrigeration Cycle • If compressor is reversible and adiabatic, then it follows an isentropic path • Condenser and evaporator should be ~constant pressure devices • 1st Law for the expansion device shows it is “isenthalpic” isentropes isotherms Qcond 1s 2 1 Wcomp 4 3 quality Qevap

6. Liq Thot P Vapor Tcold h Rankine Refrigeration Cycle • Why does a simple pressure drop change a refrigerant’s temperature? • 1st Law for flow through a valve shows hi = he • Pressure drop of liquids and vapors cause small temperature changes • Pressure drop of saturated fluids cause significant temperature changes (forced to change temperature with saturation pressure)

7. Refrigeration Performance • We may consider either the heat absorbed (evaporator) or the heat rejected (condenser) to be the desired effect. Performance depends on which quantity is considered valuable. Refrigeration and air conditioning performance: Heat pump performance: • In some limited applications, both heat flows may be considered valuable.

8. x2=0 Liq Thot=30C 1 2 P Vapor Tcold=0C 3 x4=1 4 h Basic Rankine Refrigeration Cycle Example • Determine the performance of a refrigeration cycle using R12. The low and high temperatures are 0C and 30C. Note that cycle is “pinned” at points 2 and 4 with qualities of 0.0 and 1.0.

9. Basic Rankine Refrigeration Cycle Example Assume: Adiabatic and Reversible compressor Solution: State 2: State 4:

10. 2 3 Basic Rankine Refrigeration Cycle Example Rev State 1: State 3: 1st Law (superheated)

11. Basic Rankine Refrigeration Cycle Example Can also find x3 (expansion valve outlet quality) Therefore, the cycle’s coefficient of performance:

12. Reversible Refrigerator Performance (Carnot COP) • Real cycles are typically less than half of the Rankine cycle due to compressor efficiency, heat transfer, pressure drop in tubes, etc.

13. 1 38C 2 30C 0C 4 3 T s Rankine Refrigerator Performance • Desuperheating of the compressor outlet is a source of entropy generation (heat transfer from 38C to the 30C ambient). • Using an expansion valve (or orifice or “cap” tube) causes entropy generation

14. Rankine Refrigerator Enhancement • Suction line heat exchanger – • A counterflow heat exchanger between the compressor suction gas and the liquid line from the condenser Suction gas to compressor Liquid line from condenser

15. Rankine Refrigerator Enhancement • We will examine how the suction line heat exchanger may (but not always) improve cycle performance and always increases system capacity (refrigeration effect per mass flow rate) • An example using the basic cycle example will be worked to show how to analyze its effects