Superheated steam and water together

[pranj5]

How the compression limit can be determined by the T-S diagram?

 

[Chestermiller]

How the compression limit can be determined by the T-S diagram?

You move up a constant entropy line until you hit the 100% dry envelope. You determine the temperature at that point. Then you determine the corresponding saturation vapor pressure, and take the ratio. For a temperature higher than that, the compressed gas would be superheated (i.e., beyond your desired limit).

 

[pranj5]

If the amount of water increases with pressure, then how can we hit the 100% dry envelope?

 

[Chestermiller]

If the amount of water increases with pressure, then how can we hit the 100% dry envelope?

The amount of liquid water increases with pressure if there isn't enough liquid water around to start with. Mech_Engineer showed this (i.e., if it's pretty wet to start with). Otherwise, the mixture will become drier as you compress. So the amount of water increasing or decreasing depends on the initial wetness. Check out the T-S diagram.

 

[pranj5]

The amount of liquid water increases with pressure if there isn't enough liquid water around to start with.

That's contrary to what you have said in post 42.

 

[Chestermiller]

That's contrary to what you have said in post 42.

I'm going to try to say this as clearly as I can.

1. If the initial mixture is low enough quality (high wetness), the amount of liquid increases as you compress the mixture no matter how much you compress.
2. If the initial mixture is high quality (low wetness), the amount of liquid decreases as you compress the mixture, and the mixture can reach 100% dryness (if you compress it enough). But, if you don't compress it the necessary amount, you will still have some liquid left over in the end.
3. There is a cross over in these two kinds of behavior at an initial quality in the 40 - 60 % range (depending on the starting temperature/pressure).

I strongly urge you to spend time examining the temperature entropy diagram for water so that you can see for yourself how this all works out.

 

[pranj5]

Thanks! Though the actual scenario isn't much clear. Whatsoever, at least it can be said that with sufficient water, the steam will remain saturated during the compression process and part of it will be added to the water to increase the enthalpy of water to reach to saturation level of higher temperature. In such case, power needed to compress that steam will be less as it will remain saturated and power needed to compress steam will be least if the steam can be preserved at saturated state during the process.

 

[pranj5]

Well, the amount of water can be calculated if we consider that the amount of steam will be mixed with water sufficient amount of water so that all the latent heat of steam will add to the final volume and raise its temperature and pressure. As for example, if we take 1 kg steam at 20C and have to mix it with water by pressurisation so that the latent heat of the steam will be contained in the final volume of water, then we will need 32 kg of water for that.

 

[pranj5]

Let's discuss this matter from another point of view. What will happen if instead of compressing the steam, the water will be heated (consider the whole system to be totally insulated). First small amount of steam will rise from the water and this will add to the amount of steam inside and thus increasing the pressure and temperature inside and at the same time the boiling point of the water will increase too. In short, the boiling point of the water increases along with the pressure and temperature inside and at the same time both water and steam will remain in saturated state.
In fact, what I have said above is the principle behind heat pipe, which are well tested and now is even used in space too.

 

[Nidum]

Boiler ? As in what supplies steam to a steam engine ?

 

[pranj5]

Can't understand what you want to mean be "Boiler". Though in sense you can say that if the Boiler is supposed to supply saturated steam instead of superheated steam.
Though this process is basically different from Boilers in one point. In Boilers, the heating is isobaric i.e. the pressure remains constant while the temperature increases. But, in this process, the volume remains constant and both the temperature and pressure increase.