THE RANKINE CYCLE
A cycle is a series of operations or events which occur repeatedly in the same order. A power cycle or power-plant cycle is such a series of events which regularly repeat themselves for the purpose of converting a portion of the stored energy of a fuel into work. There are two general types of power cycles, the closed cycle and the open cycle.
In the closed cycle a working fluid begins at some initial condition, undergoes certain changes through a series of regular events, and returns to the initial condition. Theoretically, no replenishment of the working fluid is necessary.
The simplest ideal or theoretical power-plant steam cycle is the Rankine cycle. The system contains: 1) a steam-generating unit by which energy is added to the fluid in the form of heat transfer from a burning fuel; 2) a prime mover or steam turbine; 3) a condenser by which energy is rejected to the surroundings by heat transfer, and 4) a boiler feed-water pump.
The following assumptions are made for the Rankine cycle:
1) The working fluid, usually water, is pumped into the boiler, evaporated into steam in the boiler, expanded in the prime mover, condensed in the condenser, and returned to the boiler feed pump to be circulated through the equipment again and again in a closed circuit under steady-flow conditions, that is, at any given point in the system, the conditions of pressure, temperature, flow rate, etc., are constant.
2) All the heat is added in the steam-generating unit, all the heat that is rejected is transferred in the condenser, and there is no heat transfer between the working fluid and the surroundings at any place except in the steam-generating unit and the condenser.
3) There is no pressure drop in the piping system, there is a constant high pressure, p1 from the discharge side of the boiler feed pump to the prime mover, and a constant low pressure, p2, from the exhaust flange of the prime mover to the inlet of the boiler feed pump.
4) Expansion in the prime mover and compression in the pump occur without friction or heat transfer, in other words, they are frictionless adiabatic or isentropic expansion and compression processes in which the entropy of the fluid leaving the device equals the entropy of the fluid entering the device (pump or turbine).
5) The working fluid leaves the condenser as liquid at the highest possible temperature, which is the saturation temperature corresponding to the exhaust pressure p2.
If the steam-generating unit is a boiler only, the steam that it delivers will be wet, and its quality and enthalpy can be determined by throttling calorimeter. If a superheater is included in the steam-generating unit, the steam that is delivered will be superheated and its enthalpy can be determined from its pressure and temperature by use of the superheated steam table or the Mollier chart.
The condensate leaving the condenser and entering the boiler feed pump is always assumed to be saturated water at the condenser pressure, and its enthalpy can be found from the steam tables at the given condenser pressure.
Since this cycle assumes frictionless adiabatic or ideal expansion of the steam in the prime mover, the Rankine-cycle efficiency is the best that is theoretically possible with the equipment. Better theoretical efficiencies are possible by using more equipment in more complex cycles.
It should be noted that only a small part of the energy supplied in the boiler as heat is converted into work and the rest, is lost in the condenser.
The loss resulting from the heat transferred to the condenser cooling water is, to a large extent, inescapable. The temperature of the cooling water varies only with the atmospheric conditions; thus, it remains almost constant. To lower it by artificial means would require the expenditure of additional energy.
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