Working fluid (heat of vaporization) in the Rankine cycle

[skittlesmonkey]

I don't understand why you would ever run the Rankine cycle with a working fluid that has a high heat of vaporization. The fluid is heated, evaporates, runs across a turbine, condenses on the cold sink, and is pumped back into the boiler. why would one use something like water or a refrigerant to run this cycle when it is thermally much cheaper to generate a unit of gas of lets say heptane, which has the same boiling point as water but a lower heat of vaporization. Just as i fill a steam engines boiler with heptane ( pretty much the same boiling point as water) it runs faster and does more work per unit energy compared to water. So why would one use working fluids like water or refrigerants ( when there are alternatives with lower heats of vaporization and the same boiling point)? I keep being told a high heat of vaporization (per mole that is) is desirable because it makes for a low flow rate, but don't u want a high flow rate and put more flow across your turbine which is the point of the whole thing. When mole of liquid is evaporated it makes 22.4 liters of gas with very little variation from this law no mater if high or low heat of vaporization so why use such a working fluid?

[lawcat]

Simple I guess. Enthalpy of the fluid, or heat of vaporization is the Energy required to vaporize the liquid. That differential energy being transfered into gas is then converted into turbine's output at the condensation side. For fluids with low heat of vaporization, the energy is lower, and such fluids therefore transfer less energy onto turbine. Think of it this way, low heat of vaporization equals low energy in the system, which equals lower turbine power. Imagine heptane hitting the turbine and condensing immediately even before it can transfer any work on the turbine.

[skittlesmonkey]

i don't see the heat as having anything to do with it

if a volume of gas rushes past a turbine ( the turbine is hot anyway so it doesn't condense on it) it causes it to spin. Whether the gas is hot, cold, brown, or purple only the volume matters, and u can generate more volume per unit energy with a fluid that has a lower heat of vaporization.

[nattyb52]

i don't see the heat as having anything to do with it

if a volume of gas rushes past a turbine ( the turbine is hot anyway so it doesn't condense on it) it causes it to spin. Whether the gas is hot, cold, brown, or purple only the volume matters, and u can generate more volume per unit energy with a fluid that has a lower heat of vaporization.

 

The heat differential across the turbine is what produces the power. The lower the heat difference, the less the power produced. Steam won't move except across a temperature differential.

 

The Barber Nichols turbine design site has lots of answers on turbine design.

[skittlesmonkey]

ok i get it as the vapor expands ( does work) it cools causing some condensation, but as it condensates the heat given off by the heat of vaporization is enough to keep most of it as a gas. the water will remain a gas for longer. if i have a piston with 1 mole of heptane gas and next to it a piston with one mole of water gas and i let them both expand from 1 bar to 2 bars while doing work, the water one will expand further doing more work. the heptane condensates much faster as it cools. thank you so much for explaining