Steam Cycles
The simplest cycle for the conversion of heat, which may be derived from full or partial combustion or other sources such as nuclear energy, is the open steam cycle depicted as a flow scheme and T-s diagram in Figures 7-11 and 7-12, respectively. The corresponding points in the cycle are labelled A-F in both figures. The top of the curve in the T-s diagram is the critical point. Points on the curve to the right of the critical point represent saturated steam, and points to the left water.
|
Figure 7-11. Simple Steam Cycle |
|
Figure 7-12. T-s Diagram: Simple Steam Cycle |
In the open cycle, corresponding to an atmospheric back-pressure turbine, the pump raises the pressure of the water from atmospheric pressure at the point A to the working pressure of the boiler at point B. The water is then heated to boiling temperature (C) and evaporated at constant temperature (D). Superheating is represented in the T-s diagram by the line D-E. Expansion to exhaust at atmospheric pressure (the vertical line E-F) takes place in a steam engine such as a turbine. The work extracted from the process is represented by the upper shaded area of the T-s diagram. The heat input is represented by the shaded area plus that of the rectangular area below the line A-F.
In the closed cycle with a condenser, known as the Rankine cycle, the expansion continues to the point F’ at a subatmospheric pressure, and the steam is condensed, returning it to the state A’. As can be seen from the T-s diagram, the work extracted is increased by the area of the lower shaded area.
In the last century many improvements have been made to the Rankine cycle. As regards equipment the most important improvement has been the use of steam turbines instead of piston expansion machines. Another feature that has increased the efficiency is the use of low level heat from the turbine for preheating the boiler feed water, thus using less high level heat in the boiler. High-level heat should preferably be used only where topping-heat is required. Doing this is making good use of the exergy present in the fuel. For a good understanding of exergy, the reader is again referred to treatises on thermodynamics (Shvets etal. 1975). Knowledge about exergy is of utmost importance for energy efficient designs (Dolinskovo and Brodianski,
1991) . For the purpose of understanding the cycles discussed in this book it is sufficient to know that high-level heat should preferably not be used for low-level heat requirements, and furthermore, that the pressure energy in media should not be throttled away unnecessarily. This last point is especially important for gases.
Apart from adding a condenser and boiler feed water preheat, there are three major other improvements to the steam cycle. These are the application of higher steam pressures, higher superheat temperatures, and reheat cycles. For details the reader is referred to the literature (e. g., Shvets etal. 1975).