Boilers 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. 

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. 

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. 

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... 

Excessive boiler cycling may result in the exfoliation of high temperature oxide scales from superheater and reheater tubes, and from steam outlet piping. The particles and flakes of exfoliated scale may... 

COMMENTS The advantage of using reheat is to reduce the moisture content at the exit of the low-pressure turbine and increase the net power of the Rankine power plant. The one reheat Rankine basic cycle shown in Fig. 2.13 can be expanded into more than one reheat if desired. In this fashion it is possible to use higher boiler pressure without having to increase the maximum superheater temperature above the limit of the superheater tubes. 

Is the efficiency of a reheat Rankine cycle always higher than the efficiency of a simple Rankine cycle operating between the same boiler pressure and condenser pressure ... 

Take two pumps, a boiler, a turbine, a reheater, another turbine, a splitter, a mixing chamber (open feed-water heater), and a condenser from the inventory shop and connect the devices to form the regenerating Rankine cycle. Switch to analysis mode. 

The Carnot cycle is not a practical model for vapor power cycles because of cavitation and corrosion problems. The modified Carnot model for vapor power cycles is the basic Rankine cycle, which consists of two isobaric and two isentropic processes. The basic elements of the basic Rankine cycle are pump, boiler, turbine, and condenser. The Rankine cycle is the most popular heat engine to produce commercial power. The thermal cycle efficiency of the basic Rankine cycle can be improved by adding a superheater, regenerating, and reheater, among other means. 

Rankine Cycle. The sleam-Rankine cycle employing steam turbines has been the mainstay of utility thermal electric power generation for many years. The cycle, as developed over the years, is sophisticated and efficient. The equipment is dependable and readily available. A typical cycle (Fig. 21) uses superheat, reheat, and regeneration. Heat exchange between flue gas and inlet air adds several percentage points to boiler efficiency in fossil-fueled plants. Modern steam Rankine systems operate at a cycle top temperature of about 800 K with efficiencies of about 40%. All characteristics of this cycle are well suited to use in solar plants. 

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