Limitations This equation shows that the wider the temperature range, the more efficient is the cycle.
(a) T3: In practice T3 cannot be reduced below about 300 K (27ºC), corresponding to a condenser pressure of 0.035 bar. This is due to two tractors:
(i) Condensation of steam requires a bulk supply of cooling water and such a continuous natural supply below atmospheric temperature of about 15°C is unavailable.
(ii) If condenser is to be of a reasonable size and cost, the temperature difference between the condensing steam and the cooling water must be at least 10°C.
(b) TI : The maximum cycle temperature Tl is also limited to about 900 K (627°C) by the strength of the materials available for the highly stressed parts of the plant, such as boiler tubes and turbine blades. This upper limit is called the metallurgical limit.
(c) Critical Point : In fact the steam Carnot cycle has a maximum cycle temperature of well below this metallurgical limit owing to the properties of steam; it is limited to the critical-point temperature of 374°C (647 K). Hence modern materials cannot be used to their best advantage with this cycle when steam is the working fluid. Furthermore, because the saturated water and steam curves converge to the critical point, a plant operating on the carnot cycle with its maximum temperature near the critical-point temperature would have a very large s.s.c., i.e. it would be very large in size and very expensive.
(d) Compression Process (4 - 1) : Compressing a very wet steam mixture would require a compressor of size and cost comparable with the turbine. It Would absorb work comparable with the developed by the turbine. It would have a short life because of blade erosion and cavitations problem. these reasons the Carnot cycle is not practical.
Uses of Carnot Cycle
1. It is useful in helping us to appreciate what factors are desirable in the design of a practical cycle; namely a maximum possible temperature range.
- maximum possible heat addition into the cycle at the maximum cycle temperature
- a minimum possible work input into the cycle.
2. The Carnot cycle also helps to understand the thermodynamic constraints on the design of cycles. For example, even if such a plant were practicable and even if the maximum cycle temperature could be 900K the cycle thermal efficiency would be well below 100%. This is called Cartrot lintitation.
A hypothetical plant operating on such a cycle would have a plant efficiency lower than this owing to the inefficiencies of the individual plant items.
ηplant = ηth × ηitem 1 × ηitem 2 × ηitem3 × ...
