Reheat and intercooling

Fig. 1.10 shows the processes of heat exchange (or recuperation), reheat and intercooling as additions to a JB cycle. Heat exchange alone, from the turbine exhaust to the compressed air before external heating, increases and lowers so that the overall... 

In the ultimate version of the reheated and intercooled reversible cycle [CICICIC- HTHTHT- XJr, both the compression and expansion are divided into a large number of small processes, and a heat exchanger is also used (Fig. 3.6). Then the efficiency approaches that of a Carnot cycle since all the heat is supplied at the maximum temperature Tr = T ax and all the heat is rejected at the minimum temperature = r,nin. 

While the lowest and highest optimum pressure ratios are for these two plants, the addition of reheating and intercooling increases the optimum pressure ratios above that of... 

Finally, carpet plots of efficiency against specific work are shown in Fig. 3.16, for all these plants. The increase in efficiency due to the introduction of heat exchange, coupled with reheating and intercooling, is clear. Further the substantial increases in specific work associated with reheating and intercooling are also evident. 

Figure 4.13 Reheat and intercool Brayton cycle T-s diagram.

Figure 4.14 Ideal reheat and intercool Brayton cycle.

COMMENTS Comparing with Example 4.1, we see that (1) the efficiency of the reheat and intercooler cycle does not increase, and (2) the net power of the reheat and intercooler cycle does increase. 

Review Problems 4.4 Reheat and Intercool Brayton Cycle... 

The pressurized hybrid cycle provides the basis for the high electric efficiency power system. Applying conventional gas turbine technology, power system efficiencies in the 55 to 60 percent range can be achieved. When the pressurized hybrid system is based on a more complex turbine cycle— such as one that is intercooled, reheated, and recuperated—electric efficiencies of 70 percent or higher are projected. 

An ideal Brayton cycle is modified to incorporate multistage compression with intercooling, multistage expansion with reheating, and regeneration. As a result of these modifications, does the efficiency increase ... 

The thermal cycle efficiency of a Brayton cycle can be increased by adding more intercoolers, compressors, reheaters, turbines, and regeneration. However, there is an economic limit to the number of stages of intercoolers, compressors, reheaters, and turbines. 

If an infinite number of intercoolers, compressors, reheaters, and turbines are added to a basic ideal Brayton cycle, the intercooling and multicompression processes approach an isothermal process. Similarly, the reheat and multiexpansion processes approach another isothermal process. This limiting Brayton cycle becomes an Ericsson cycle. 

A four-stage reheat and four-stage intercool Brayton air cycle as shown in Fig. 4.44a has been designed by a junior engineer with the following design input information ... 

Figure 4.44a Four-stage reheat and four-stage intercool Brayton air cycle.

The main cause of the high efficiency is the use of a gas turbine with intercoolers, recuperation, reheater, and a 100 C higher turbine inlet teijperature than currently applied. 

The reference AHTR design" employs a recuperated helium Brayton cycle (Fig. 1) with three stages of reheat and three stages of intercooling. The helium pressure is reduced through three tuibines in series, with reheating of the helium to its maximum temperature with hot molten salt before each turbine. The respective efficiencies at salt exit temperatures of 750, 850, and 1000 C salt are 48, 56, and 59%. 

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. 

The way to enhance the power output of a gas turbine can be achieved by intercooling and reheat. 

Intercooling and Reheat Effects. The net work of a gas turbine cycle is given by... 

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. 1.10. Temperature-entropy diagram. showing reheat, intercooling and recuperation.

Fuller analyses of a/s cycles embracing intercooling and reheating were given in a comprehensive paper by Frost et al. [3], but the analysis is complex and is not reproduced here. 

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. 

Fig. 3.16 showed carpet plots of efficiency and specific work for several dry cycles, including the recuperative [CBTX] cycle, the intercooled [CICBTX] cycle, the reheated [CBTBTX] cycle and the intercooled reheated [CICBTBTX] cycle. These are replotted in Fig. 6.17. The ratio of maximum to minimum temperature is 5 1 (i.e. T nx 1500 K) the polytropic efficiencies are 0.90 (compressor), 0.88 (turbine) the recuperator effectiveness is 0.75. The fuel assumed was methane and real gas effects were included, but no allowance was made for turbine cooling.

The CHAT cycle may be seen as a low loss evaporative development of the dry intercooled, reheated regenerative cycle [CICBTBTX]. It offers some thermodynamic advantage—increase in turbine work (and heat supplied ) with little or no change in the compressor work, leading to an increased thermal efficiency and specific work output. 

Several of the gas turbine cycle options discussed m this section (intercooling, recuperation, and reheat) are illustrated in Figure 4. These cycle options can be applied singly or in various combinations with other cycles to improve thermal efficiency. Other possible cycle concepts that are discussed include thermochemical recuperation, partial oxidation, use of a humid air turbine, and use of fuel cells. 

An open, split-shaft, air bleed Brayton cycle with two compressors (CMPl and CMP2), three turbines (TURl, TUR2, and TUR3), one intercooler (CLRl), one combustion chamber (HTRl), one reheater... 

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