Turbines, steam reheat cycle

Reheat involves steam-to-steam heat exchange using steam at boiler discharge conditions. In the reheat cycle, after partially expanding through the turbine, steam returns to the reheater section of the boiler, where more heat is added. After leaving the reheater, the steam completes its expansion in the turbine. The number of reheats that are practical from a cycle efficiency and cost colisidcratioli is two. 

If a steam bottoming cycle is appropriate, many design decisions need to be made, including the selection of the turbine cycle (reheat or non-reheat) and the operating conditions. Usually, steam turbines below 100 MW are non-reheat, while turbines above 150 MW are reheat turbines. This generalization is subject to a few exceptions. In fact, a small (83 MW) modem reheat steam turbine went into operation (June 1990) as a part of a gas turbine combined cycle repowering (43). 

Superheaters and Reheaters A superheater raises the temperature of the steam generated above the saturation level. An important function is to minimize moisture in the last stages of a turbine to avoid blade erosion. With continued increase of evaporation temperatures and pressures, however, a point is reached at which the available superheat temperature is insufficient to prevent excessive moisture from forming in the low-pressure turbine stages. This condition is resolved by removing the vapor for reheat at constant pressure in the boiler and returning it to the turbine for continued ejq)ansion to condenser pressure. The thermodynamic cycle using this modification of the RanTine cycle is called the reheat cycle. 

A steam reheat Rankine cycle operates between the pressure limits of 5 and 1600 psia. Steam is superheated to 600° F before it is expanded to the reheat pressure of 500 psia. Steam is reheated to 600°F. The steam flow rate is 8001bm/hr. Determine the quality of steam at the exit of the turbine, the cycle efficiency, and the power produced by the cycle. 

COMMENTS The sole purpose of the reheat cycle is to reduce the moisture content of the steam at the final stages of the turbine expansion process. The more reheating processes, the higher the quality of the steam at the exit of the last-stage turbine. The reheat temperature is often very close or equal to the turbine inlet temperature. The optimum reheat pressure is about one-fourth of the maximum cycle pressure. 

Determine the efficiency and power output of a regenerative Rankine (without superheater or reheater) cycle, using steam as the working fluid, in which the condenser temperature is 50° C. The boiler temperature is 350°C. The steam leaves the boiler as saturated vapor. The mass rate of steam flow is 1 kg/sec. After expansion in the high-pressure turbine to 100°C, some of the steam is extracted from the turbine exit for the purpose of heating the feed-water in an open feed-water heater the rest of the steam is then expanded in the low-pressure turbine to the condenser. The water leaves the open feed-water heater at 100°C as saturated liquid. 

A four-stage turbine with reheat and three-stage regenerative steam Rankine cycle as shown in Fig. 2.36a was designed by a junior engineer. The following design information is provided ... 

Air cooled condensing plants, steam turbines with multiple reheating cycle are described and analyzed. Diagrams and plates illustrating design and plants are appended. 4 refs, cited. 

The efficiency of the Rankine cycle can be improved by returning the steam to the boiler after partial steam expansion in a process referred to as reheat. Reheat cycles add significant complexity to the turbine, the boiler, and the controls but, at large scale, the increased complexity and cost can be justified by the increase in efficiency of a few percent. Other significant factors affecting overall... 

The steam cycle is designed with three LP preheaters, condensing steam that is extracted from the LP turbines, and with four HP preheaters, condensing steam from the HP and IP turbines. The reheat pressure is 4.25 MPa, achieving a reheat temperature of 442°C. The design pressure of the deaerator is 0.55 MPa. Four parallel feed-water pumps are foreseen, of which three are needed to provide the mass flow of 1179 kg/s at fiiU power and the fourth one is kept on hot standby. 

The Canadian SCWR concept is a pressure-tube type of concept. It adopts the direct cycle, which includes a 2540-MWth core that receives feed water at 315°C and 1176 kg/s and generates supercritical steam at 625°C and 25 MPa. The cycle includes steam reheat using a moisture separator reheater (MSR) between the IP turbine and LP turbine. The MSR separates the moisture from the steam and reheats the steam to ensure an acceptable moisture level at the outlet of the LP turbine. Four LP condensate heaters are included in the cycle as well as a deaerator and four HP feed-water heaters. The gross electrical output is calculated as 1255 MWg, giving a gross thermal efficiency of 49.4%. A schematic diagram of the direct cycle is shown in Fig. 8.4 (Zhou, 2009). 

Figure A1.18 Simplified iayout of typicai BWR NPP (courtesy of US NRC) geneiai basic features (1) thermal neutron spectrum (2) UO2 fuel (3) fuel enrichment about 3% (4) direct cycie with steam separator (steam generator and pressurizer are eiiminated), ie, singie-flow circuit (singie loop) (5) RPV with verticai fuel rods (elements) assembled in bundle strings cooled with upward flow of fight water (water and water—steam mixture) (6) reactor coolant, moderator, and power cycle working fluid are the same fluid (7) reactor cooiant outlet parameters pressure about 7 MPa and samration temperature at this pressure is about 286°C and (8) power cycle subcritical-pressure regenerative Rankine steam turbine cycie with steam reheat.

SFR (based on Russian BN-600 reactor) NPP (Generation IV) fast neutron spectrum no moderator reactor coolant P = 0.1 MPa and = 380°C and = 550°C indirect cycle (triple loop, ie, sodium—sodium—water/ superheated steam) Rankine power cycle with single steam reheat—primary steam (turbine inlet) Pin = 14.2 MPa (Psat = 338°C) and Pjn = 505°C secondary steam Pi = 2.45 MPa (Pgat = 223°C) and Pin = 505°C (P = 374°C) Up to 40... 

PWR NPP (Generation III+, to be implemented within next 1—10 years) thermal neutron spectrum moderator and reactor coolant—H2O P = 15.5 MPa (P at = 345°C) and Pout = 327° C indirect cycle (double loop, ie, water—water/saturated steam) Rankine power cycle with single steam reheat—primary steam (turbine inlet) Pin = 7.8 MPa Pin/sat = 293°C Up to 38... 

ABWR NPP (Generation ni+) thermal neutron spectrum direct cycle (single loop) moderator/reactor coolant/ working fluid in Rankine power cycle with single steam reheat—H2O primary steam (turbine inlet) Pin = 6.97 MPa Pin/sat = 286°C secondary steam Pin = 1.7 MPa (Paat = 204°C) Pin = 259°C Up to 34... 

SCWR NPP (one of Canadian concepts, ie, pressure channel reactor) thermal neutron spectrum moderator—D2O reactor coolant—H2O P = 25 MPa (Pq. = 22.064 MPa) and Pin = 350-C and Pout = 625°C (P = 374°C) direct cycle, ie, single loop supercritical pressure Rankine power cycle with single steam reheat—primary steam (turbine inlet) Pj = 25 MPa (P = 22.064 MPa) and P = 625°C (Per = 374°C) secondary steam Pi = 5.7 MPa (Psat = 252°C) and Pi = 625°C possible backup—indirect supercritical pressure Rankine steam cycle with single steam reheat (superheat) 45-50... 

LFR NPP (based on Russian design Brest-300 reactor) fast neutron spectrum no moderator reactor coolant—liquid Pb P = 0.1 MPa and Pj = 420°C and = 540°C primary power cycle—indirect supercritical pressure Rankine steam cycle Pi = 24.5 MPa (P = 22.064 MPa) and Pj = 340° C and Pout = 520° C (Per = 374° C) or subcritical pressure Rankine steam cycle with superheated steam single steam reheat possible backup in some other countries—indirect supercritical pressure CO2 Brayton gas turbine cycle 43... 

This is a direct cycle layout. Steam from a reactor flows directly to a turbine. The turbine does not require an intermediate steam reheat. 

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