Steam Quality vs Pressure 1

[warickwrx]

Hi All

After conducted a throttling experiment with water and noting the results. I would like to verify if it they make sense. What I noticed as that when the pressure inside the sealed vessel increased from say 1 to 7 bar in 1 bar increments the quality factor of the steam remained fairly constant. It was around 0.96 or 96% and when it decreased from 7 bar to 1 bar the quality factor also remained fairly constant but was slightly higher around 0.97. Any Ideads

 

[EmmanuelTop]

You need to describe the experiment a bit better.

Were you producing steam at various pressures? What did you measure and where/at which point?

Dejan IVANOVIC
Process Engineer, MSChE

 

[warickwrx]

I used the armfield TH3 apparatus to conduct the experiment, you measure pressure and tempreture inside the vessel, and then throttle and measure outlet tempreture I would assume at atmospheric pressure. I attached a picture of the setup.

 

[Latexman]

I think the difference between 96% and 97% steam quality on that apparatus would be meaningless, i.e. the same number. It's just not that accurate. Ask your lab instructor/teacher/professor.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[warickwrx]

I checked the steam quality cals for increasing it is 93% and decresing is 97%, I am try to verify if this is acttually valid.

I am not sure if I am calculation the enthalapies correctly, for example if the pressure inside (p1) the vessel is 8 bar, I use the hl (enthalapy of saturated liquid to be 721.02 kJ/Kg at P1), and the enthalapy of vaporization to be hfg ( 2047.28 kJ/KG at P1), lastly the enthalpy of the gas at the exit is hg (2636.29 kJ/KG at T2) these are taken from the tables and used to caculate the quality of the steam.

 

[IRstuff]

If you had insufficient total water mass to start with, and/or your boiler was too hot, then changing the pressure might not affect the vapor fraction

TTFN
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[EmmanuelTop]

I agree with Latexman. I am not sure how did you manage to accurately measure 96% vs 97% steam quality, since you mention that only temperature was measured at the outlet? The difference between the two states is essentially negligible.

http://www.engineeringtoolbox.com/wet-steam-quality-d_426.html

Dejan IVANOVIC
Process Engineer, MSChE

 

[IRstuff]

I think the OP's question was about why the vapor fraction didn't change over a 7x variation in pressure. Per

https://upload.wikimedia.org/wikipedia/commons/3/33/Phase_diagram_of_water_simplified.svg

if the temperature was just at 100C, the vapor fraction would should have dropped drastically going from 1 bar to 7 bar. Any temperature above about 200C would have kept everything in vapor phase.

TTFN
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[EmmanuelTop]

Fully agree. That is why we asked the OP to clarify the experiment - I assume he was producing saturated steam from 1 barg to 7 barg and reverse, and expanding it to atmospheric pressure. Boiling temperatures are different in those two cases, of course.

If the experiments were performed at non-equilibrium conditions, then any value for steam quality is possible, I guess.

Dejan IVANOVIC
Process Engineer, MSChE

 

[25362]

Although examples and graphs in the following link are in imperial units, it may be, nevertheless, instructive:

http://www.plantservices.com/articles/2003/378/?start=1

 

[zdas04]

The term "steam quality" only has meaning in reasonably steady conditions. The rate of condensation is very much a statistical function with a fairly large random component.

If I trap a quantity of saturated steam at a given pressure and temperature, then I can get a decent measure of quality every couple of minutes. A plot of those quality measurements would make sense. If I take the readings every second, the plot gets far jaggy-er and the trend may very well appear to be towards increasing quality even though at steady state that would be impossible.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[25362]

The process by which water droplets are entrained in boilers is explained in:

http://www.nationalboard.org/index.aspx?pageID=164&ID=235

 

[Latexman]

warickwrx,

Did you mean "the quality factor of the steam" or the vapor fraction of the steam? They are two entirely different physical characteristics.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[25362]

Dejan IVANOVIC,
IMHO, the table you brought to show the irrelevance of % vol of liquid droplets in wet steam, should be accompanied by the pressure at which the reported three-decimals % vol/vol were estimated. Dou you agree?

 

[EmmanuelTop]

Absolutely correct, 25362.

Dejan IVANOVIC
Process Engineer, MSChE

 

[warickwrx]

Hi regarding the following two pints 1. "the quality factor of the steam" and secondly 2. " the vapor fraction of the steam " They are two entirely different physical characteristics. Could someone perhaps explain the difference ?

 

[Latexman]

The literal definitions of quality and vapor fraction are the same, but traditionally they are used differently most of the time. These different uses are nuances that I have observed in my chemical industry experience, so bear with me.

Vapor fraction is usually used in the situation where a known quantity (batch) or flow rate (steady-state) of liquid is partially vaporized or condensed, but the majority of the time it's used when vaporizing. For example, a vapor fraction of 0.4 could mean that an initial mass of 100 lbs. of water was heated to yield 40 lbs of steam and 60 lbs of water, or 100 lbs/hr of water was heated to yield 40 lbs/hr of steam and 60 lbs/hr of water.

Steam quality is used only where a flow rate (steady-state) of vapor and liquid is partially condensed or vaporized, but the majority of the time it's used when condensing. For example, coming out of a boiler the steam is usually saturated and very high "quality", say 98+ % vapor. The < 2% water in the stream is usually due to imperfect mechanical separation of water droplets that is entrained in the mostly vapor stream. As this steam flows downstream in the piping there is heat loss to the surroundings and some of the vapor condenses to liquid. As such, the quality decreases to, say, 96%, 94%, or further.

There may be industries where the usage of the two terms are different than I described. That would not be surprising to me.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[zdas04]

My experience is "quality = vapor fraction * 100%". The explanation above seems to be focused on different uses prefer working with different terms. I don't believe that that convention is universal.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist