Limitations of Carnot vapour cycle:
This equation dignifies that the wider the temperature range, the more efficient is the cycle.
(a)T3: In practice T3 cannot be reduced below around 300 K (27ºC), corresponding to condenser pressure of 0.035 bar. This is because of two tractors:
(i) Condensation of steam needs bulk supply of cooling water and continuous natural supply below atmospheric temperature of around 15°C is unavailable.
(ii) If condenser is to be of reasonable size and cost, the temperature difference between condensing steam and cooling water should be at least 10°C.
(b) TI: The maximum cycle temperature Tl is limited to around 900 K (627°C) by the strength of the materials available for highly stressed parts of plant, such as boiler tubes and turbine blades. This upper limit is called as metallurgical limit.
(c) Critical Point: In fact the steam Carnot cycle has maximum cycle temperature of well below this metallurgical limit because of the properties of steam; it is limited to critical point temperature of 374°C (647 K). Thus modern materials cannot be used to their best advantage with this cycle when steam is working fluid. Further, because the saturated water and steam curves converge to critical point, a plant operating on carnot cycle with its maximum temperature near critical-point temperature would have a very large s.s.c., that is it would be very large in size and expensive.
(d) Compression Process (4 - 1: Compressing very wet steam mixture would require compressor of size and cost comparable with the turbine. It would absorb work comparable with developed by the turbine. It would have short life owing to blade erosion and cavitations problem. Because of these reasons the Carnot cycle is not practical.