advertisement

Reversible Steady-flow Work Some questions can be answered • Why do they dump all that energy out by condensing the steam in a steam power generating system before boosting the pressure back up? also: • Why don’t they like the steam to condense in the turbine? Equations for work are similar, but quite different! What does that difference mean? The larger the specific volume, the larger the work! Example 6-12 It takes more than 500 times the work to compress the vapor than it takes to pump the liquid. Look over proof in the text. Similarly, a reversible compressor uses less work. • So, can make a compressor more efficient by: – removing as many irreversibilities as possible. • not too economic or practical – Keeping v as small as possible • means keeping the gas as cold as possible. Comparison of work done by different reversible compressors between two pressures. adiabatic Maximum cooling The area to the left of the curves represents the work done. Multistage compression with intercooling. Ideally, the intercooling is at constant pressure and gets you back to T1 each time. Where do you break it to minimize work? • Intermediate pressure is Px • differentiate equation for work wrt/Px • Px/P1 = P2/Px • Gives the result that compression work is equal at each stage. Example 6-13 – Work for various compression processes What should pressure be here? Isentropic = 263 kJ/kg Isothermal = 189 kJ/kg Polytropic = 246 kJ/kg Two stage polytropic = 215 kJ/kg The more stages, the closer it gets to isothermal.