Non-flow process with steam (Constant Pressure)

1. Feb 18, 2013

freshbox

Question 1
In a closed system, 0.6kg of wet steam at a pressure of 5 bar and a dryness fraction of 0.2 is heated at constant pressure until the volume occupied is 0.18m3.
(A)Calculate the change in internal energy, the work energy and the heat energy transferred during the process.

Question 2
In a closed system, 0.25m3 superheated steam at a pressure of 10 bar and a temperature of 300°C cools at a constant pressure until the volume becomes 0.15m3. Determine:
(A)The mass of the steam in the system
(B)The dryness fraction of the steam after cooling.

For question 2, the question says that find the dryness fraction of the steam after cooling, this shows that the steam is wet steam.

But for question 1, the question did not give me a hint to find dryness fraction of the steam. How do I know that I need to find the dryness fraction in order to solve the question?

And what confuses me is that how did the wet steam from question 1 remains as wet steam after heating? I thought it will be either dry saturated steam or superheated steam.

Thanks..

2. Feb 18, 2013

voko

What happens to the wet steam depends on how much heat goes into it. It may not have received enough to become dry. That's what you need to find out.

3. Feb 18, 2013

freshbox

Do you mean that after heating the wet steam may still become a wet steam because it has not received enough to become dry?

4. Feb 18, 2013

voko

Exactly.

5. Feb 18, 2013

freshbox

Woah that solve part of the mystery hehe... Ok if you look at question 2, i know it is a wet steam through "determine The dryness fraction of the steam after cooling."

For question 1, how do i determine what kind of steam it is because it did not give me any hint at all.

Thanks.

6. Feb 18, 2013

voko

If a wet steam is heated under constant pressure, what happens to its temperature and volume (before it gets completely dry)?

7. Feb 18, 2013

freshbox

temperature rises and volume getting lesser?

8. Feb 18, 2013

voko

The wet steam is in water-vapor equilibrium. At constant pressure, can the equilibrium temperature change?

9. Feb 18, 2013

freshbox

I'm sorry I don't understand. I only know in a constant pressure process, p1=p2=p.

Last edited: Feb 18, 2013
10. Feb 18, 2013

voko

When you boil water in your kitchen, you add heat to the water-vapor system held at a constant (atmospheric) pressure. Does its temperature change while it is boiling?

11. Feb 18, 2013

freshbox

No the temperature will not change because it reaches it's boiling point already. So what are you trying to tell me?

Am I right to say:
Compressed Water/Saturated Water/Wet Steam: Have Volume
Dry Saturated Steam/Superheated Steam: No Volume

12. Feb 18, 2013

voko

Wet vapor, when held at a constant pressure, won't change in temperature when heated: the heat will be use to "boil" water in it.

I am not sure what you are trying to say about "having volume". Everything has some volume.

13. Feb 18, 2013

freshbox

So do you mean to find out whether stage 1 steam is the same as stage 2 steam, i have to work out the temperature first?

So for wet steam when heated at a constant pressure remain as it is. Does this applies to wet steam only or how about the other steam like example: dry saturated.

Thanks

Last edited: Feb 18, 2013
14. Feb 18, 2013

voko

Wet steam, when heated at constant pressure, does not remain as it is. It becomes drier, which means there is more steam in the water-steam mixture. And if you have more steam, with pressure and temperature constant, what happens to its volume?

15. Feb 18, 2013

freshbox

with more steam, i think the volume will increase.

Since you say wet steam when heated at constant pressure, it becomes drier and does not remain as it is but how come for question 1 it remains as wet steam? And you say that because it has not received enough to become dry.

So how do i determine whether it has changed into a different steam or remain the same steam?

Sorry I'm very confused

16. Feb 18, 2013

voko

Start from the beginning. 0.6kg of wet steam at a pressure of 5 bar. What is its temperature? Remember, the water in this mixture must be at the boiling point for the pressure given.

17. Feb 18, 2013

freshbox

Ok if according to the steam table @ 5bar, the temperature should be 151.8°C

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18. Feb 18, 2013

voko

Very well. Now that you know the temperature and pressure, what is the volume? Compute separately the volume of the saturated steam component, and the volume of the water component.

19. Feb 18, 2013

freshbox

Stage 1 working:

Given from question: mass =0.6kg, P=5 Bar, x=0.2

vx=xvg
vg@5bar=0.3748 from chart.
0.2=vx/vg
0.2=vx/0.3748
vx=0.07496m3/kg

v=V/m
0.07496=V/0.6
V=0.04496m3 (Ans)

20. Feb 18, 2013

voko

That is not exactly what I asked, but that is a start. Now, assume that all the water in the wet steam becomes steam, so you have saturated steam. What volume would that be? Compare with the volume given. What is your conclusion?