- 2067 Mangsir · 6 marks
Power vs refrigeration cycle; Rankine components and diagrams
Short answer
Power cycles produce net work output, operate clockwise; refrigeration cycles absorb heat from a cold space, operate anticlockwise. Rankine cycle components: pump, boiler, turbine, condenser. T-s diagram shows isentropic compression (pump), constant pressure heat addition (boiler), isentropic expansion (turbine), constant pressure condensation.
Differentiate between power cycle and refrigeration cycle. Sketch components, P-v and T-s diagrams for Rankine cycle.
Power cycle versus refrigeration cycle
| Point | Power cycle | Refrigeration cycle |
|---|---|---|
| Purpose | Convert heat into net work | Remove heat from a cold space |
| Direction on P-v / T-s | Clockwise | Anticlockwise |
| Net work | Output (W_net > 0) | Input (W_net < 0) |
| Heat flow | Q_H absorbed at high T, Q_L rejected at low T | Q_L absorbed at low T, Q_H rejected at high T |
| Performance | Thermal efficiency η = W/Q_H | COP = Q_L/W |
| Value | η < 1 | COP can be > 1 |
| Examples | Rankine, Otto, Diesel, Brayton | Vapour-compression, reversed Brayton |
| Device | Heat engine | Refrigerator or heat pump |
Rankine cycle components
Boiler (Q_in)
2 +----------+ 3
| |
Pump Turbine --> W_out
| |
1 +----------+ 4
Condenser (Q_out)
- Boiler (with superheater): heats the water at constant pressure to steam (2-3).
- Turbine: expands the steam isentropically and produces work (3-4).
- Condenser: condenses the exhaust at constant pressure to saturated liquid (4-1).
- Pump: raises the liquid pressure isentropically (1-2).
Diagrams
T-s diagram P-v diagram
T P
| 3 | 2 ____________________ 3
| .-'''-. .'| | | \
| .' '.' | | | '.
| 2' /____________3' | | | '-.
| / \ | | 1 _________________________ 4
| / \| +----------------------------------> v
| 2 / |\
| 1/___________________4 \
| / \
+-----------------------------> s
(2 lies just above 1; 2'-3' boiling, 3'-3 superheating)
Efficiency: η = [(h₃ − h₄) − (h₂ − h₁)]/(h₃ − h₂).
✓ Answer checked by Claude Opus 5.5 (high effort)
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