Текст № 5. Power plant cycles

A cycle is a series of operations, which regularly repeat themselves for the purpose of converting a portion of the stored energy of a fuel into a work. There are two general types of power cycles: the closed cycle and the open cycle.

In the closed cycle the working fluid begins at some initial condition, under- goes certain changes and returns to the initial condition.

The Rankin cycle

The simplest ideal of theoretical power plant steam cycle is called the Ran- kin 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 sur- roundings by the heat transfer, and 4) a boiler feed water pump.

The following assumptions are made for the Rankin cycle:

1) The wording fluid, usually water, is pumped into the bailer evaporated info steam in the boiler, expanded in the prime mover, condensed in the condenser and returned to the boiler feed pump to be recirculated 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-generated unit, all the heat that is re- jected 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.

5) The working fluid leaves the condenser as liquid at the highest possi- ble 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 super heater 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 tem- perature by use of the superheated steam table.

The condensate leaving the condenser and entering the boiler feed pump is always saturated water at the condenser pressure, and its enthalpy can be found from the steam tables at the given condenser pressure.

The Rankin-cycle efficiency is the best that is theoretically possible with the equipment. Better theoretical efficiencies are possible by using more equipment in the more complex cycles.

If 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 additional energy.