What is the most efficient way to create steam with the desired properties?

[physicsnoobian]

Hello!
new here. recently I've developed an interest in the way electricity is produced, I've seen some mechanisms, most of them are steam turbine based.

i have a few questions about the topic, mainly about thermodynamics.

if i want to create enough energy to move a steam turbine that requires the following steam properties:
pressure of 10 bar, temperature of 220 celsius, in the rate of 0.05kg steam per second.
to create the steam i use a 2cm diameter aluminium tube, 20 cm in length. inside there is molten salt in temperature of 280 celsius. in the first half of the tube there is constant heat that is done by gas, and in the other 10 cm, i pour 10ml of purified water every 1 second to create steam. the steam creating part of the tube is inside a container in the volume of 20X20X20 cm, and there's a nozzle at the side wall of the container for steam to come out.
let's say that the whole thing takes place on sea level in 24 celsius.

1. how much time will it take for the water to turn into steam in by the conditions i described?
2. how much heat will be lost by the molten salt throughout the steam production process?
3. will reducing the volume of the container accelerate the time it takes to the steam create the desired pressure of 10 bar?
4. can this situation cause the Leidenfrost effect to the water? what shape does the surface of the aluminium tube should be to prevent it?
5. let's say the inside of the container is dry. will having high pressured steam in the container prevent the ability to constantly pour water over the hot tube? I'm assuming the evaporation process will be faster and will include less heat loss of the molten salt if pouring water is done instead of putting the tube inside a water tank as there is time gap between the time that more cold water if poured on the tube, the export of pressured steam from the container and the general temperature inside the container as the steam is hotter than the water in a water tank - please correct me if I'm wrong.
6. is there more efficient way to create steam with these properties?
7. how one can calculations these problems?

Thank you very much for the help! fascinating subject, will be great if explained in simple language.

 

[phyzguy]

I don't know the answer to some of these, but I have a question. You are adding 10 ml of water per second. This is 10 g of water per second. How do you expect this to yield 50g of steam per second?

 

[physicsnoobian]

sorry! missed one 0. meaning 100ml or 0.1l/0.1kg.

thank you for the interest and quick reply!

 

[russ_watters]

in the first half of the tube there is constant heat that is done by gas...

1. how much time will it take for the water to turn into steam in by the conditions i described?

Your steam output is a straightforward function of your heat input rate, which you haven't specified.

Also, this is a continuous process, so the way you worded the question is as an irrelevancy. It doesn't matter how long it takes to boil a certain amount of water once it is injected. What matter is how much you are boiling per unit of time. That means that ultimately your flow rate of water in is controlled to be equal to your steam output and amount of water in the boiler (and how long it stays there) is essentially an arbitrary choice.

2. how much heat will be lost by the molten salt throughout the steam production process?

All of it.

...

I feel like you are thinking there's a lack of efficiency in steam production...but do you know what the efficiency of steam production currently is? It feels like you are thinking that a heat transfer method can be produced which could significantly increase steam production efficiency. But it doesn't seem like you are considering conservation of energy and just how straightforward the issue is:

For the overall device:
Heat in * efficiency = Heat out

And for a heat exchanger itself:
Heat in = Heat out

That's it. No matter how complicated your device is, that's all it boils down to (pun intended).

 

[anorlunda]

What happens to the steam after going through the turbine?

There are open cycle plants where the stream its discarded.

Closed cycle plants are more efficient. They condense the steam into water after the turbine, then pump it back to the start to be used again.

 

[russ_watters]

What happens to the steam after going through the turbine?

There are open cycle plants where the stream its discarded.

Closed cycle plants are more efficient. They condense the steam into water after the turbine, then pump it back to the start to be used again.

Yes, though these are the efficiency of the cycle thermodynamic cycle once you have steam, which is different from the efficiency of the steam generation. However, to your point, the efficiency of the steam cycle is of far more importance than the efficiency of the steam generation...though also it is obviously easier to generate steam from hot water than from cold water!

 

[jrmichler]

Here's a good place to start to learn about boilers and steam generating efficiency: http://cleaverbrooks.com/reference-center/insights/Boiler Efficiency Guide.pdf. It covers a range of factors from heat transfer area to combustion efficiency to excess air, although it does not cover feedwater temperature. Another reference, this one for coal fired power plant boilers: http://article.sciencepublishinggroup.com/html/10.11648.j.ajmie.20170201.15.html.

The key point is that total heat in equals total heat out. This is true whether you are heating a kettle on a kitchen stove, evaporating maple syrup, or running a large power plant boiler. A more basic place to start is to search heat transfer. Learning heat transfer by internet does not work well, but the alternative is to go to college and study mechanical or chemical engineering. There you will get thermodynamics and heat transfer in your junior year.

 

[russ_watters]

I guess I can try to help a little more by just putting a finer point on this:

I feel like you are thinking there's a lack of efficiency in steam production...but do you know what the efficiency of steam production currently is?

First, per @anorlunda's point, the biggest factor in cycle efficiency is generally what you do with the condensate or any remaining steam: returning it to the boiler is a lot better than dumping it. So let's assume a boiler that does that. So:

Heating can be done by burning a fuel or by using an electric heating element. Electric heating is 100% efficient. Period. Standard combustion boilers operating at relatively low steam pressure are limited to roughly 80% efficiency due to the fact that the exhaust from the combustion contains steam. If you use the combustion exhaust to preheat the incoming combustion air or use a lower temperature working fluid like water or air instead of steam, you can condense the water vapor out and boost the efficiency to about 95%. The upper limit can vary substantially by system, but there are zillions of standard boilers out there that run at 80% with only slight variations.

If you use a heat pump cycle, depending on your cycle's needs/constraints, the sky's; the limit: 3:1, 4:1, 5:1 COP (500% "efficiency") is possible.