The Rankine Cycle with Super-Heat

Cycle States

		T		p		ρ		x		h		s
		K		bar		kg/m3		-		kJ/kg		kJ/kgK
1.		373.	124	1.	013 25	958.	365	0.		419.	054	1.	306 92
2.		373.	187	10.		958.	744	-1.		419.	997	1.	306 92
3.		453.	028	10.		5.	144 97	1.		2,777.	11	6.	585 02
4.		638.	473	10.		3.	449 16	-1.		3,190.	74	7.	354 48
5.		373.	125	1.	013 25	0.	597 655	-1.		2,675.	53	7.	353 93

Performance

Parameter	Symb.	Value	Units
Pump Work	W1-2	-0.942550	kJ/kg
Boiler Heat	Q2-3	2357.112907	kJ/kg
Super Heat	Q2-3	413.629961	kJ/kg
Work Out	W3-4	515.208579	kJ/kg
Condenser Heat	Q4-1	-2256.476839	kJ/kg
Efficiency	η	18.56	%

Sometimes referred to as the "steam" cycle, the Rankine cycle drove an industrial revolution and still drives most of our power plants today. In the rankine cycle, a liquid (usually water) is pumped into a boiler, where it is heated to a boil. The super-heater further heats the saturated vapor into a super-heated vapor before routing the vapor to a turbine or piston to do work. In the old steam engines, the work was done by a piston, but in modern power generation systems, it is all done by turbines. Finally, the low pressure vapor/liquid mix is cooled in a condenser. Pure liquid is drawn from the bottom of the condenser, so it will be on the saturated liquid line.

State 1: Reservoir This saturated liquid is just out of the condenser, and it is often at ambient pressure. However, if the reservoir is unvented, then it can be at other pressures.

Process 1-2: Pump The feed-water pump accepts the saturated liquid and pumps it up to the boiler pressure. Liquids require remarkably less energy than gases to compress, so this process is relatively inexpensive in terms of energy.

State 2: Boiler Inlet The liquid is now at pressure and ready to be heated

Process 2-3: Boiler Liquid is heated in the boiler until it comes to a boil.

State 3: Boiler Outlet The outlet of a boiler always draws from the top so that vapor will be removed while the lower-energy liquid is left behind. As a result, the boiler outlet will always a saturated vapor. Some systems combine super-heating with the boiler, but not all.

Process 3-4: Super-Heater The saturated vapor is further heated into the super-heated region. Because the heat transfer characteristics of vapor are so drastically dissimilar to liquid, this is often a separately heat exchanger. A boiler with low water level can also act like a super-heater but with drastically lower mass flow.

State 4: Super-Heated Vapor Safely in the super-heated domain, we are ready to extract work.

Process 4-5: Turbine/Piston Finally, we are ready to extract work. Many power systems suffer when liquid is condensing during the process, so adding a super-heater can really pay dividends.

State 5: Turbine/Piston Exhaust Now back to the reservoir pressure, the working fluid can be a liquid/vapor mix or it can still be a super-heated vapor. Either way, there is still quite a bit of thermal energy to extract to return to state 1.

Process 5-1: Condenser Finally, the condenser cools all the remaining steam back into a saturated liquid. Just like saturated vapor was drawn from the top of the boiler, saturated liquid is drawn from the bottom of the condenser.