Thread: Thermodynamics - Heating process in Boiler

Hello everyone.. I saw some examples of Rankine cycle where the boiler heats the water at constant pressure so that it's stay in one pressure line as in the temperature-specific volume diagram or temperature-entrophy diagram. Is it a must to heating at constant pressure? Is pressure of fluid normally constant while we're heating it? or does it require some method to keep it constant?

Also, what's the different between normal vapor and superheated vapor?

thanks for ur attention

Originally Posted by thejoker

Hello everyone.. I saw some examples of Rankine cycle where the boiler heats the water at constant pressure so that it's stay in one pressure line as in the temperature-specific volume diagram or temperature-entrophy diagram. Is it a must to heating at constant pressure? Is pressure of fluid normally constant while we're heating it? or does it require some method to keep it constant?

Also, what's the different between normal vapor and superheated vapor?

thanks for ur attention

As far as I know, the Rankine cycle is indeed the practical operating cycle of commercial steam power. A conventional steam engine or steam turbine does operate with a more or less constant pressure of steam from the boiler. Boilers have, I believe, a pressure sensor on them to adjust the heat input so as to maintain this working pressure. (Though this would not have been the case with steam locomotives, I grant, as the heat from the firebox would not have been that accurately controllable.)

As to the difference between normal and superheated vapour, if you just take off the vapour at the temperature it boils at (N.B. This will be above 100C, due to the operating pressure of course), you have saturated steam. If you heat this vapour further (making it hotter but now unsaturated), you create extra pressure which can do extra work when the steam is allowed to expand in the engine or turbine. This is superheating. Its great advantage is that, being unsaturated, its expansion in the working cycle does not cause condensation to start, which would lead to an abrupt drop in pressure and can cause damage due to impingement of droplets on turbine blades etc There is more about superheating of steam here: Superheated steam - Wikipedia, the free encyclopedia

If you heat this vapour further (making it hotter but now unsaturated), you create extra pressure which can do extra work when the steam is allowed to expand in the engine or turbine. This is superheating. Its great advantage is that, being unsaturated, its expansion in the working cycle does not cause condensation to start, which would lead to an abrupt drop in pressure and can cause damage due to impingement of droplets on turbine blades etc

I don't believe you raise the pressure at all, I do know that the excess heat is carried into the turbine and the heat is dispersed through the assembly makes the surfaces hotter then the condensing temperature of the steam, as you said preventing the possibility of condensate droplets damaging the blades. The removal of noncondensables (air) is critical in the turbines for this reasons as well.