Thermal efficiency turbine cooling

The condensing steam turbine has a relatively low thermal efficiency because about two-thirds of the steam enthalpy is lost to cooling water in the condenser. Expensive boiler feedwater treatment is required to remove chlorides, salts, and silicates, which can be deposited on the blades causing premature failure. The blades are already under erosion conditions because of water drops present in the condensing steam. Even with these disadvantages, the condensing turbine is still selected, especially in a process that requires very large compressor drivers and relatively low amounts of process steam. 

The [CBT]ig efficiency is replotted in Fig. 3.14, against (Tt,ITx) with pressure ratio as a parameter. There is an indication in Fig. 3.14 that there may be a limiting maximum temperature for the highest thermal efficiency, and this was observed earlier by Horlock et al. [8] and Guha [9]. It is argued by the latter and by Wilcock et al. [10] that this is a real gas effect not apparent in the a/s calculations such as those shown in Fig. 3.9. This point will be dealt with later in Chapter 4 while discussing the turbine cooling effects. 

In this chapter, cycle calculations are made with assumed but realistic estimates of the probable turbine cooling air requirements which include some changes from the uncooled thermal efficiencies. Indeed it is suggested that for modern gas turbines there may be a limit on the combustion temperature for maximum thermal efficiency [2,3]. 

Thus the cooled reversible cycle [CHT]rci with a first rotor inlet temperature, Tj, will have an internal thermal efficiency exactly the same as that of the uncooled cycle [CHTJru with a higher turbine entry temperature 3 = Tr, and the same pressure ratio. There is no penalty on efficiency in cooling the turbine gases at entry but note that the specific work output, w = (wj — wc)/CpT = [(0 /x) — 11(j — 1), is reduced, since 0 < 0. 

However, it is important to note that this conclusion becomes invalid if the air for cooling the LP turbine is taken from compressor delivery (as in Fig. 4.3b) and then throttled at constant temperature (T2 = Ty) to the lower pressure before being mixed with the gas leaving the HP turbine. The thermal efficiency drops as another internal irreversibility is introduced it can be shown [5] that... 

For the various reversible cycles described in Section 4.2.1, the thermal efficiency was the same, independent of the number of cooling. steps. This is not the case for the irreversible cycles described in this section. Both the thermal efficiency and the turbine exit temperature depend on the number and nature of cooling steps (whether the cooling air is throttled or not). 

But it app>ears that thermal efficiency does tend towards a maximum level with increasing combustion temperature. More realistic calculations of highly cooled turbines are given in the next chapter, after a brief description of the heat transfer analysis involved in the determination of cooling flow quantities. 

Here, we first discuss whether such semi-closure (which is introduced so that CO2 separation can be undertaken more easily) is likely to lead to higher or lower thermal efficiency, and in this discussion it is helpful to consider recirculation in relation to an air standard cycle (see Fig. 8.4). Fig. 8.4a shows a closed air standard cycle with unit air flow Fig. 8.4b shows an open cycle similarly with unit air flow and an air heater rather than a combustion chamber. The cycles are identical in every respect except that in the former the turbine exhaust air from the turbine is cooled before it re-enters the compressor. In the latter, the turbine exhaust air is discharged to atmosphere and a fresh charge of air is taken in by the compressor. The quantities of heat supplied and the work output are the same for each of the two cycles, so that the thermal efficiencies are identical. 

A power plant operating on heat recovered from the exhaust gases of internal-combustion < uses isobutane as the working medium in a modified Rankine cycle in which the upper pressure I is above the critical pressure of isobutane. Thus the isobutane does not undergo a change of p" as it absorbs heat prior to its entry into the turbine. Isobutane vapor is heated at 4,800 kPa to 2 and enters the turbine as a supercritical fluid at these conditions. Isentropic expansion in the turh produces superheated vapor at 450 kPa, which is cooled and condensed at constant pressure, resulting saturated liquid enters the pump for return to the heater. If the power output of the modi Rankine cycle is 1,000 kW, what is the isobutane flow rate, the heat-transfer rates in the heater condenser, and the thermal efficiency of the cycle ... 

The idealization of the gas-turbine engine (based on air, and called the Brayton cycle) is shown on a PF diagram in Fig. 8.12. Step AB is the reversible adiabatic compression of air from Pa (atmospheric pressure) to Pb- In step BC heat Qbcz replacing combustion, is added at constant pressure, raising the air temperature prior to the work-producing isentropic expansion of the air from pressure Pc to pressure Po (atmospheric pressure). Step DA is a constant-pressure cooling process that merely completes the cycle. The thermal efficiency of the cycle is ... 

The major difficulty to limit the higher thermal efficiency of the indirect cycle is the lower core inlet temperature. For the direct cycle, the cold gas leaving the precooler can be extracted to cool the reactor pressure vessel (RPV) and the other steel structures. Therefore the core inlet temperature can be as higher as 500- 600°C. For the indirect cycles proposed before, such as MGR-GTI proposed by Yan and Lidskyl, the core inlet temperature is kept as lower as 310°C in order to cool RPV The plant busbar efficiency of MGR-GTI is about 42.1%. In order to achieve higher power plant efficiency, it seems special designs of RPV cooling should be provided for the indirect gas turbine cycle. 

The concept of HTGR (High Temperature Gas-Cooled Reactor) plant with Closed Cycle Gas turbine is taking an increasing interest because of its unique characteristics such as highly excellent safety and high thermal efficiency which reaches about 50 percents. The necessity of the demonstration test of the full scale power conversion module to be used in this plant has been pointed out. This paper presents a new proposal of demonstration test facility concept. 

Steam turbines typically have a thermal efficiency of about 35%, meaning that 35% of the heat of combustion is transformed into electricity. The remaining 65% of the heat either goes up the stack (typically 10%) or is discharged with the condenser cooling water (typically 55%). 

K. The net power output of the turbine is 35 kW. The discharged steam is at lOkPa. Cooling water is used in the condenser at a rate of 750 kg/s. Determine (a) The thermal efficiency (b) The work loss at each unit. 

The Steam turbine is a tandem compound unit, generally consisting of a double-flow, high-pressure turbine and three double-flow, low-pressure turbines, which exhaust to a high vacuum condenser for maximum thermal efficiency. The condenser may be cooled by sea, lake, or river water, or by use of atmospheric cooling towers. 

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