Rankine Cycle


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Rankine cycle

[′raŋ·kən ‚sī·kəl]
(thermodynamics)
An ideal thermodynamic cycle consisting of heat addition at constant pressure, isentropic expansion, heat rejection at constant pressure, and isentropic compression; used as an ideal standard for the performance of heat-engine and heat-pump installations operating with a condensable vapor as the working fluid, such as a steam power plant. Also known as steam cycle.

Rankine Cycle

 

an idealized thermodynamic cycle in which heat is converted to work or work to heat. The Rankine cycle is adopted as the theoretical basis for an approximate computation of real cycles in steam power plants. The cycle is named after W. J. Rankine, one of the founders of the science of thermodynamics.

In the Rankine cycle, the working fluid (water) is converted to steam in a boiler and then superheated at constant pressure in a superheater. The steam expands adiabatically in a steam turbine, thus performing work. The steam then is condensed at constant pressure in a condenser, and the condensate is first pumped to an economizer for preheating and then returned to the boiler for conversion into steam.

Figure 1. Rankine cycle: (a) curve representing various states of temperature T and entropy S, (b) diagram of the Rankine cycle showing pressure P and volume V; (1–2) conversion of water into steam upon the application of heat, (2–3) superheating of the steam, (3–4) adiabatic expansion of the steam, (4–5) condensation of the steam, and (5–1) preheating of the water

The work performed by 1 kg of steam in a Rankine cycle is represented by the area 1 —2—3—4—5 in the phase diagram (Figure l, a). The thermal efficiency of the cycle is equal to the ratio of this work to the total amount of heat supplied to this kg of steam, which is seen in area 1–2–3–4–6–7–5. The efficiency of a Rankine cycle using saturated steam is 0.29–0.36; with superheated steam it is 0.34–0.46. The Rankine cycle differs from the Carnot cycle in that the heating of the water and the superheating of the steam proceed at constant pressure and increasing temperature. It is inadvisable to execute the Carnot cycle with steam because the condensation would have to be effected to point 5ʹ and the steam-water mixture would then have to be compressed along the adiabatic 5’-l. Additional energy would therefore be required.

I. N. ROZENGAUZ

Rankine cycle

A thermodynamic cycle used as an ideal standard for the comparative performance of heat-engine and heat-pump installations operating with a condensable vapor as the working fluid. Applied typically to a steam power plant, as shown in the illustration, the cycle has four phases: (1) heat addition bcde in a boiler at constant pressure p1 changing water at b to superheated steam at e, (2) isentropic expansion ef in a prime mover from initial pressure P1 to back pressure P2, (3) heat rejection fa in a condenser at constant pressure p2 with wet steam at f converted to saturated liquid at a, and (4) isentropic compression ab of water in a feed pump from pressure p2 to pressure p1.

This cycle more closely approximates the operations in a real steam power plant than does the Carnot cycle. Between given temperature limits it offers a lower ideal thermal efficiency for the conversion of heat into work than does the Camot standard. Losses from irreversibility, in turn, make the conversion efficiency in an actual plant less than the Rankine cycle standard. See Carnot cycle, Refrigeration cycle, Thermodynamic cycle, Vapor cycle

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