RankineCycles_web - ENU 4134 Rankine Cycles D. Schubring...

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Unformatted text preview: ENU 4134 Rankine Cycles D. Schubring December 1, 2009 Rankine Cycles I Ideal, simple Rankine cycle (SG/boiler, turbine, condenser, pump) I Non-ideal simple Rankine cycle (real turbine, real pump) I Regeneration and moisture separation I Reheat cycles Ideal, Simple Rankine Cycle It is usual to number the thermodynamic states in a system in the direction of flow. Flow splits complicate the numbering. State 1 can be assigned to be any state convenient to starting. Well go with the same numbering as Problem 6-2, even though T&K are not consistent in the book. Exit conditions of the steam generator usually saturated steam at a given pressure ( P 1 given, x 1 = 1). h 1 = h g ( P 1 ), s 1 = s g ( P 1 ), T 1 = T sat ( P 1 ). Outside of the nuclear industry, state 1 is generally superheated, so all properties are f ( P 1 , T 1 ). (Recommendation: T&Ks superheated steam tables are essentially non-existent. Use a thermo book or TK Solver or EES or whatever better tables you have available.) Ideal, Simple Rankine Cycle (2) State 2 is the exit of the turbine. Ideal cycle implies an isentropic turbine, so s 2 = s 1 . P 2 = P 3 (the condenser pressure), generally given. P 2 and s 2 are sufficient to define a thermodynamic state, generally in the vapor dome. The outflow from the turbine is a mist flow (high velocity vapor flow with entrained droplets). This is a very bad regime for turbine blades, so any x 2 less than 0.9 and certainly any less than 0.85 is problematic. Outside of the nuclear industry, state 2 is sometimes still (slightly) superheated. Ideal, Simple Rankine Cycle (3) State 3 is the exit of the condenser. P 3 = P 2 (given in the problem, usually; otherwise T 3 given). x 3 = 0 (saturated liquid). The flow from the condenser heads directly to a pump, so a quality of larger than zero is problematic. Efficiency is enhanced by P 3 (and T 3 ) being low, but there are two limits: I Very low pressures are challenging to maintain I T 3 must be higher than ultimate heat sink to ensure heat transfer in the correct direction. Ideal, Simple Rankine Cycle (4) State 4 is the exit of the pump and entrance to the steam generator. s 4 = s 3 , P 4 = P 1 . State 4 will be a sub-cooled liquid . Such tables are often sparse, even in good thermo books. A good approximation for h 4 is: h 4 h 3 + vol 3 ( P 4- P 3 ) (1) Ideal, Simple Rankine Cycle Example SG and turbine inlet pressure: 10 MPa SG exit conditions: saturated vapor Condenser pressure: 30 kPa States: 1. SG Exit 2. Turbine Exit 3. Condenser Exit 4. Pump Exit Ideal, Simple Rankine Cycle Example (2)...
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RankineCycles_web - ENU 4134 Rankine Cycles D. Schubring...

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