Em - division

View Large There's nothing "critical" about supercritical. "Supercritical" is a thermodynamic expression describing the state of a substance where there is no clear change of state between the liquid & the gaseous phase. Water reaches this state at a pressure above 22.1 Mpa.

Supercritical is not just an elevated pressure as compared to Sub-critical. It crosses critical point & alters the phase change pattern of working fluid in boiler. In Once Through logic with no circulation in water Wall

Sub-Critical Power Plant
  • System with constant evaporation endpoint
  • A typical example of this system is the drum-type steam generator. Natural circulation is produced by heating of the risers. The water/steam mixture leaving the risers is separated into water & steam in the drum. The steam flows into the superheater and the water is returned to the evaporator inlet through down comers.
  • If the system is operated only with natural/assisted circulation, the application range is limited to a max drum pressure of approx. 190 bar.
    If a circulating pump is used (forced circulation), this range can be extended somewhat fixing the endpoint of evaporation in the drum also sets the size of the heating surface in the evaporator & superheater.
Super-Critical Power Plant
  • System with variable evaporation endpoint
  • This type of boiler is Drumless steam generator. Evaporation takes place in a single pass. This principle used in the BENSON/SULZER boiler, the world's most frequently constructed steam generator type. Flow through the evaporator is induced by the feed pump. The system can therefore be operated at any desired pressure i.e. at either subcritical (Close to critical) or supercritical pressure. The evaporation endpoint can shift.
  • The evaporator & superheater areas thus automatically adjust to operational requirements.
  • Basic operation of cycle is same i.e. Rankine Cycle.
  • Constructional Features are also same except supercritical is drumless Boiler.
  • The equipments & philosophy of working are similar in both the cases like:
    • Air preheater
    • Economiser
    • PA, SA & ID fans
    • Electrostatic precipitator
    • Boiler feed pump
    • Feed water heaters
    • Turbine
    • Condenser
    • CEP