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Why do we have to condense the gas after a turbine or other heat engine, to feed it back to the heater?

Wouldn't it be more efficient to feed it to the heater as warm as it goes out of the thermal engine?

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  • $\begingroup$ Could it be because the air needs to be reasonably dry and rid of moisture? $\endgroup$ – am304 Mar 7 '16 at 16:42
  • $\begingroup$ @am304, I have edited ergon's question to say "gas" instead of "air". Now it is clearer that he is referring to a closed loop system that does not have any non-volatiles like air; only steam or other vapor. $\endgroup$ – ericnutsch Mar 8 '16 at 2:15
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Somewhat counter intuitively, it is not more efficient to use warmer air (or any working fluid) in the inlet to a heat engine. Two similar questions were asked (and answered) regarding steam in a Rankine cycle (here and here). Hopefully these explain why this is.

Aside from reasons of efficiency, it would be difficult to recycle the exhaust from something like a jet engine or gas turbine because the working fluid is also the source of oxygen for the combustion reaction. The oxygen content of the fluid at the inlet would decrease to the point where little or no heat could be generated.

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  • $\begingroup$ So, a steam engine would be more efficient if it operates in the North Pole, right? $\endgroup$ – ergon Mar 8 '16 at 2:32
  • $\begingroup$ Not necessarily. There are a lot of factors that efficiency depends on, and it would be hard to predict how each changes with ambient temperature. However, if you read up on the Carnot cycle, you'll see that efficiency is a function of the difference between the heat addition and heat rejection temperatures. So, operating a Carnot engine at the north pole would probably be better because you could get a bigger temperature difference. $\endgroup$ – Carlton Mar 8 '16 at 16:41
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I'll try another approach that's maybe more intuitiv, and less 'thermodynamic'. That said, you should definitly learn some thermodynamics because it's a cool subject to know about!

For a steam engine, you need to understand the concept of vapor pressure: Take a body of water and giv it enough time, there will be water vapor at a certain presure above the water surface. The higher the temperature, the higher the pressure.

Now, let's look at the steam engine: enter image description here

The pressure difference between points 3 and 4 drive the turbine. For there to be a high pressure difference, you need a high temperature difference. So you need to cool after point 4.
The steam also cools when expanding in the turbine, but that only goes so far.

This is a simple explanation that hopefully helps, but you won't understand important things like steam engine efficiency with it, so again, look into thermodynamics by following the links in the other answers.

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A condenser is an important part of a closed-loop phase change thermodynamic cycle. Thermodynamic cycles generate mechanical energy from the transfer of heat energy. Transfer is the key word here. The larger the differences in temperature in a closed loop system the more efficent it is. If there is no difference in temperature, there is no power to be extracted. I recommend reading the thermodynamics wiki article for more information. Also have a look at Carnot's theorem.

I will explain a steam engine in layman's terms which may help. Have a look at the following image from the steam engine wiki article. enter image description here

At step 2 we can see heat is added to the fluid in the boiler making it a gas. Then in step 3 it expands through the turbine giving us power output. Then on step 4 we need the condenser to remove the heat and condense this expanded gas back into a fluid. That way the fluid can be easily pumped with a small amount of power input into the high pressure boiler. The pump and the turbine have roughly the same pressure drop across them, but since a much larger volume moves through the turbine, it produces more power than is consumed by the pump.

Conversely if we tried to not condense the gas and pump it back into the boiler, it would take more energy to pump it than we would get out of the turbine.

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