Reheat cycle

The Reheat Cycle The regenerative cycle improves the efficiency of a gas turbine but does not provide any added work per pound of air flow. To achieve this latter goal, the concept of the reheat cycle must be utilized. The reheat cycle utihzed in the 1990s has pressure ratios of as high as 30 1 with turbine inlet temperatures of out 2100° F (1150° C). The reheat is done between the power turbine and the compressor trains. The reheat cycle, as shown in Fig. 29-35, con-... 

The Intercooled Regenerative Reheat Cycle The Carnot cycle is the optimum cycle between two temperatures, and all cycles try to approach this optimum. Maximum thermal efficiency is achieved by approaching the isothermal compression and expansion of the Carnot cycle or by intercoohng in compression and reheating in the expansion process. The intercooled regenerative reheat cycle approaches this optimum cycle in a practical fashion. This cycle achieves the maximum efficiency and work output of any of the cycles described to this point. With the insertion of an intercooler in the compressor, the pressure ratio for maximum efficiency moves to a much higher ratio, as indicated in Fig. 29-36. 

FIG. 29-35 Performance map showing the effect of pressure ratio and turbine inlet temperature on a split-shaft reheating cycle. 

The reheat cycle increases the turbine work, and consequently the net work of the cycle, can be increased without changing the compressor work or the turbine inlet temperature by dividing the turbine expansion into two... 

This cycle produces an increase of 30% in work output, but the overall efficiency is slightly decreased as seen in Figure 2-15. An intercooling regenerative cycle can increase the power output and the thermal efficiency. This combination provides an increase in efficiency of about 12% and an increase in power output of about 30%, as indicated in Figure 2-16. Maximum efficiency, however, occurs at lower pressure ratios, as compared with the simple or reheat cycles. 

Fig. 11. DSC of miciocryslalline, paraffinic, and Fischer-Tropsch waxes on the reheat cycle. Samples were first heated to 175°C, then cooled to —50°C, followed by reheating to 175°C, all at 20°C/min. Heat of fusion for each is shown in the legend.

To further understand the thermodynamic philosophy of the improvements on the EGT cycle we recall the cycle calculations of Chapter 3 for ordinary dry gas turbine cycles—including the simple cycle, the recuperated cycle and the intercooled and reheated cycles. 

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. 

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. 

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. 

Figure 2 Single-reheat cycle with co-generation of hydrogen in a nuclear power plant (Mokry, 2009)...

Figure 3-8. DSC of 5m and 5s polymorphs. Metastable phase 5m shows phase transition to the sublimation polymorph 5s and transformation to the thermodynamic form upon heating up to 200 C. Polymorph 5s does not show phase changes under the same conditions except the sublimation endotherm. The reheating cycle endotherm is shifted to lower T than the first heating cycle due to better contact...

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