Reheat cycle

Reheat cycle

If  very wet steam flows through a turbine, the hydrodynamic conditions for the turbine blades and nozzles deteriorate sharply, causing a reduction of the internal relative efficiency of the turbine, ; this in turn leads to a reduction of the effec­tive (thermal) efficiency of the power plant as a whole. For modern turbines the admissible dryness fraction of exhaust steam (at the turbine exit) should be not less than x = 0.86 to 0.88.

As has already been mentioned, one of the ways to reduce the wetness of exhaust steam at the turbine exit is to superheat the steam in the boiler. Superheating leads to an increase in the thermal efficiency of the cycle reali­zed, and at the same time, on the T-s diagram it shifts the point corresponding to the conditions of exhaust steam to the right, into the region of greater dryness fractions, as illustrated in Fig. .

We have also found that with the same superheat temperature the use of high pressures increases the cycle areas ratio and, consequently, the thermal efficiency of the cycle, but simultaneously a higher pressure diminishes the dryness fraction of the exhaust steam and the internal relative efficiency of the turbine.

Fig. 

One solution could be to further increase the superheat temperature (the dotted line in Fig. . However, as was already mentioned, further temperature increases are restricted by the properties of construction mate­rials. The economic advantage of this undertaking should also take into consideration increased investments involved in building such a plant.

One way to reduce the final wetness of exhaust steam is to reheat the steam. After the flow of steam, performing work in the turbine, expands to some pressure p* > p₂, it is extracted from the turbine and directed to flow into an additional superheater, or reheater, installed, for instance, in the boiler flue. In this reheater, steam temperature rises to T*, and then the steam flows back into the turbine, in which it expands to the pressure p₂. As can be seen from the T-s diagram, shown in Fig. , the final wetness of steam diminishes.

The diagram of a power plant with steam reheating is shown in Fig. , in which the reheat superheater, or reheater, is designated by RS. When reheating the steam, the turbine is a two-cylinder unit, comprising a high-pressure turbine and a low-pressure turbine arranged on a common shaft along with a generator.

Figure  shows on a T-s diagram an internally reversible reheat cycle of the steam power plant, practising superheating. It is clear that this cycle can be visualized as consisting of two individual cycles, the conventional Rankine cycle (main) 5-4-6-1-2-3-5 and an additional cycle 2-7-8-9-2 (the line 7-8 is an isobar p* = const). It can be assumed that the work done along the section 7-2 of the expansionadiabat in the main cycle is spent to ensure adiabatic compression of the working medium on the section 2-7 of the additional cycle.

Fig. 

Fig. 

The expression for the thermal efficiency of the reheat cycle can be pre­sented in the following form:

                                                                                     

If the thermal efficiency of the additional cycle,

                                                                              

is greater than the thermal efficiency of the main cycle,

                                                                                        

then the thermal efficiency of the reheat cycle, , will be greater than the thermal efficiency of a Rankine cycle without reheating (i.e. greater than that of the main cycle):

In fact, if , it means that the area ratio of the additional cycle is greater than that of the main cycle and, consequently, the area ratio of the total reheat cycle is larger than the area ratio of the main cycle.

Steam reheating, practised at one time mainly to do away with the high wetness of steam in the last stages of turbines, is now used to increase the thermal efficiency cycles.

 Analyzing the T-s diagram, we see that if steam is returned for reheating at a temperature not very low and it is being reheated to a temperature close to T₁, the thermal efficiency of the additional cycle will be higher than the thermal efficiency of the main cycle; in this case the area ratio of the additional cycle will be far greater than that of the main cycle .

Fig. 

A cycle with steam reheating to a temperature T* = T₁ is shown on the i-s diagram in Fig. .

Fig. 

Modern steam power plants are usually operated not only with single but with double steam reheating.

Steam reheating used in steam power plants as a means for raising the thermal efficiency of the plant, is similar to the two-stage heat addition in gas-turbine plants, considered