Air in steam boiler

[duncan webster 1Participant @duncanwebster1]

Posted by Hopper on 17/06/2023 05:46:11:

…… In a nutshell, I vaguely recall, the latent heat of steam entering the cylinder at say 100PSI is greater than the latent heat of the expanded exhaust steam at say 5PSI………

Edited By Hopper on 17/06/2023 05:48:17

I think you'll find that the latent heat falls as pressure increases until it gets to zero at the critical temperature/pressure. Some power stations operated above this point, with consequent increase in efficiency

 

This point is 374 C, 218 atmospheres, about 3200 psi, so not really of interest to us mere mortals

Edited By duncan webster on 17/06/2023 21:52:42

[John OlsenParticipant @johnolsen79199]

The sensible heat of the incoming steam is higher than that of the exhaust. The latent heat is as close to the same as makes no difference to us, and since we would only want to condense at the cold end anyway, that is the figure that matters to us.

There is still latent heat discarded in the suoercritical plants, since the latent heat that is thrown away is at the temperature and pressure prevailing in the condenser, eg near enough to 0psi and around 25 degrees C, depending on the source of the cooling water. But it will be less than in lower temperature and pressure plants, since less total weight of steam will be required for a given output power.

There is no power to be obtained directly from latent heat, since there is no temperature difference between the gaseous phase and the liquid phase. It follows from Sadie Carnots formula that the efficiency of the plant would be 0%. There is a trick you could do, if you had say a large supply of water at 100 degrees C, and another supply at room temperature, say 25 degrees C. Once you have excluded air from the system, the hot liquid will vaporise and flow to the cold side, and you can use the flow to generate power. But that is actually using the sensible heat, the hot side will get cooler unless you keep pumping in heat.

There have been efforts to find better working fluids than water, but all of the likely candidates have disadvantages much worse than the relatively high latent heat of water.

John

[HopperParticipant @hopper]

Thanks guys. Interesting and informative. Obviously i should have paid more attention to my steam tables at school. The lower latent heat at higher temperatures is quite counterintuitive.

So would we say, then, that steam provides more power to an engine than air because of the greater amount of sensible heat in the steam that can be converted into work? And I suppose if the air was the same temperature as the steam, steam would still create more power because steam has a greater specific heat than air?

Interestingly, quite a few texts and papers seem to state as an assumption that latent heat in steam provides power in engines/turbines but none really go into specifics of how. Perhaps what they are referring to is the use made at the end of the cycle in the condenser where the latent heat is removed by contact with cooling water tubes, thus creating the vacuum "for free" that allows the last of the work to be extracted from the steam at the LP end of the turbine?

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