Chapter 11 Power and Refrigeration Systems

Transcription

1 Capter 11 Power and Refrigeration Systems Cycle : working fluid undergoes a series of processes and finally ly returns to te initial state (ex. Steam power plant,cf)internal-combustion combustion engine) OPEN & CLOSED CYCLES OVERVIEW POWER CYCLE Te Rankine Cycle termal efficiency effects of pressure and temperature Reeat cycle Regenerative cycle Losses and Cogeneration Air-Standard Power Cycles (open cycle) Te Brayton cycle Simple gas-turbine cycle wit regenerator Gas turbine power cycle configurations Jet propulsion Reciprocating Engine Power Cycles Otto cycle Diesel cycle Stirling cycle REFRIGERATION SYSTEMS Vapor-compression refrigeration cycle Actual vapor-compression refrigeration cycle Ammonia Ammonia absorption refrigeration cycle Air-standard refrigeration cycle OTHER SYSTEMS : combined-cycle cycle power and refrigeration systems

2 A power system (4 steady processes) Saft work w vdp Boundary movement work w Pdv Const.P isentropic isentropic Const.P w vdp+ 0 vdp+ 0 vdp+ vdp net net w Pdv + Pdv + Pdv + Pdv RANKINE CYCLE 4 steady processes 1-22 reversible adiabatic pumping processes in te pump 2-33 constant-pressure transfer of eat in te boiler 3-44 reversible adiabatic expansion in te turbine 4-11 constant-pressure transfer of eat in te condenser η t wnet area 1 2 2' q area a 2 2' 3 b a H η < η t, rankine t, carnot Wy not select te Carnot cycle? A. pumping process : very difficult to andle te mixture and deliver saturated liquid at 2 2 B. supereating te vapor : Carnot cycle (all eat transfers ; isotermal) but in process pressures are dropping (expansion work is done) te eat must be transferred to te vapor : P drops but T increases impossible to acieve it in practice

3 Ex will be solved 11.3 EFFECT OF PRESSURE AND TEMPERATURE ON THE RANKINE CYCLE Effect of exaust pressure on Rankine-cycle efficiency Pressure drop temperature decreases Network increases Heat transferred to te steam troug te boiler : increased by a -2-2-a-a a But erosion problem : lower quality tan 4 state (moisture content) Effect of supereating on Rankine-cycle efficiency Supereating effect in boiler Network increases efficiency increases Steam quality increases preventing erosion Effect of boiler pressure on Rankine-cycle efficiency Supereating effect in boiler Network increases efficiency increases Steam quality increases preventing erosion Ex and 3 will be solved

4 Summary : For enancing te termal efficiency of Rankine cycle - lowering te exaust temperature - increasing te pressure during eat addition - supereating te steam For controlling te quality of te steam leaving te turbine - quality increases by supereating te steam - quality decreases by lowering te exaust pressure and increasing ng te pressure during eat addition 11.4 REHEAT CYCLE Discussion - tis cycle focuses on avoiding excessive moisture in te low-pressure stages of te turbine - relatively little gain in efficiency - Example 11.3

5 11.5 REGENERATIVE CYCLE : using feedwater eaters Liquid pase vaporization Super eated vapor T-S S diagram sowing te relationsips between Carnot-cycle efficiency and Rankine-cycle efficiency RANKINE cycle : (compression, isentropic) (eating at constant P in liquid pase) (eating at constant P, vaporization) (expansion, isentropic) (condensation, isotermal, constant P) CARNOT cycle : (compression, isentropic) (eating at const. P, vaporization) (expansion, isentropic) (constant P condensation) Average temperature concept : for enancing te Rankine-cycle efficiency, te increased average temperature at wic eat t is supplied or te decreased average temperature at wic eat is rejectedr A. During te process between states 2 and 2, 2, te working fluid is eated wile in te liquid pase : average temperature of te working fluid is muc lower tan during te process 2-32 (vaporization process) B. It is evident tat te Rankine cycle efficiency is lower tan te Carnot cycle efficiency Regenerative Cycle concept is tat te working fluid enters te boiler at some state between en 2 and 2,, : average temperature at wic eat is supplied is iger Heat transfer between ig temperature gas in te turbine and liquid from te pump. Turbine Bolier At eac point, te vapor temperature is only infinitesimally iger tan te liquid temperature. : in tis case, eat transfer instantly occurs same patterns IDEAL Reversible is assumed process 4-5 is parallel to but impossible To te liquid From te vapor Two areas are te same : means te eat transfer from te vapor and to te liquid

6 T Lecture note for general termodynamics, 2003 Idealized regenerative cycle efficiency is equal to te Carnot cycle efficiency Area 3-4-d-b-3 : eat transfer in 3-4 process Heat transfer to te liquid Heat transfer from te vapor a b c d Area 1-5-c-a-1 : eat transfer from te working fluid during 5-1 IMPOSSIBLE : reversible and moisture problem. s 1 5 carnot b Rejected eat d PRACTICAL REGERERATIVE CYCLE USES FEEDWATERS Note tat x does not mean quality but extraction fraction!

7 PRACTICAL REGERERATIVE CYCLE USES FEEDWATERS (OPEN) Two pumping + one feedwater eater m6 Extraction fraction x m 5 pumping Feed water eater Average temperature at wic eat is supplied as been increased by feed water eater.. pumping m + m m m (1 x) m m m m + m m Sould be taken as te limit of saturation (1 xm ) + xm m liquid because, in te two-pase region, some damage occur in te pump 2. x Non-mixed feedwater eater (closed feedwater) ANALYSIS ; no drip pump and x a T3 T4 T6a m m m m m xm m m ; 6 5 6a 6c m + xm m + xm a Permitting liquid but not vapor to flow to a region of lower pressure OPEN : less expensive and better eat transfer caracteristics

8 Arrangement of eaters in an actual power plant utilizing regenerative rative feedwater eaters Dual goals : eating and removing air from te feedwater because of erosion in te boiler 11.5 DEVIATION OF ACTUAL CYCLES FROM IDEAL CYCLES Turbine and pump losses (eat transfer and friction) Piping loss due to pressure drop resulting from eat transfer and friction Condenser loss relatively small (minor loss)