Temperature and Pressure to Dryness Fraction 1

[Dmanley]

Hello,
Could someone help me refresh my memory from college days?

How would one find out the dryness fraction/Steam quality when you know the temperature and pressure?

I know it has something to do with a load of formulas and charts but for the life of me I can't remember what to do with them!?!


Thank you for any assistance,

Derry Manley

 

[sailoday28]

Pressure and temp are not enough. You must have a property such as enthalpy, h or specific volume v.

h=hf + x (hg-hf)
where hf =enth of sat liq
hg= enth of sat vapor
hg and hf are functions of P or T and available from steam tables.
Substiture in above and solve for x, the dryness fraction.
Using Mollier Chart can also provide an approximate value for x.

 

[mbeychok]

Dmanley:

Usually, you will only know the temperature and pressure of your wet steam. Almost all steam tables include a Mollier diagram. Use it as follows:

(1) Find the intersection of your steam temperature (T) and 14.696 psia. Read the enthalpy at that point.

(2) Move along the constant enthalpy line to your left until you reach your steam pressure (P) and, at that point, read the moisture percentage.

As an example, let's assume steam at 280 °F and 200 psia pressure:

(1) Steam at 280 °F intersects the 14.696 psia pressure line at an enthalpy of 1183 Btu/lb.

(2) Move to the left along that enthalpy line of 1183 Btu/lb until you reach the pressure line of 200 psia. At that point, the moisture percentage can be read as 1.7% and therefore the dryness percentage is 98.3% which is a dryness fraction of 0.983

If the intersection of your steam temperature and the 14.696 psia pressure line is at a constant enthalpy line which does not cross the steam saturation line, then your steam is not wet (i.e., the moisture percentage is zero).

My steam tables (published by Combustion Engineering) actually has the above example shown alongside the upper left section of the Mollier Diagram sheet.

Hope this helps,

Milton Beychok
(Contact me at www.air-dispersion.com)
.

 

[sailoday28]

mbeychok (Chemical)---I believe you have a typo for the example "steam at 280 °F and 200 psia pressure". This condition would be subcooled water. Could you correct it and continue with your explination?

 

[rmw]

Well this one sent me scrambling for my old CE steam tables which I rarely use anymore. (I keep them in the drawer with my slide rules.) They are not as comprehensive as Keenan and Keyes which I prefer when I have to use paper tables.

However, I note from the example given in the upper left that the two process conditions used by Milton for the example in his post are based on a combination of calorimeter readings (the 280 °F) and actual process conditions (200 psia).

The process steam at 200 psia process steam at unknown enthalpy and moisture has been adiabatically expanded to atmospheric pressure in the calorimeter, and the resulting vapor is superheated at 280 °F. That determines the enthalpy, and thus following an isenthalpic expansion line back to 200 psia gives the quality of the initial process steam which I would then agree with sailoday28, cannot be at 280 °F and 200 psia, but had to be at 200 psia/387.8 °F but 1.944% wet as noted.

(For these last values I used my Vacworks by Graham because it is quick, but I could have used WASP.)

So, in order to completely agree with Milton's post, I would recommend the addition of a statement regarding the use of a calorimeter to the steps that he outlines.

Meanwhile, if DerryM is still following this, he may be getting the answer to his question. If not, go find a good thermodynamics text book and read the chapter on calorimeters. They are what are used for determining steam quality.

rmw

 

[mbeychok]

Sailoday:

Thanks for picking up the fact that I neglected to say that the 280 °F was the throttling calorimeter temperature after a sample of the steam was throttled at constant enthalpy down to atmospheric pressure.

Thanks to rmw for also picking up on this and explaining it quite thoroughly.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[Dmanley]

Ok...
I'm getting a little put off here because I’m in Metric and when i try to do what you’re saying it gets a bit messy.

I have:
Temperature 126.1 (deg C) 258.98 (F)
Pressure 2.47 (Bar A) 35.8 (PSI)

Ok from this I think my value goes inside the envelope.
I don't have a copy of the steam tables so I'm usiong whats at this link.

http://www.chemicalogic.com/steamtab/duo/downloads.htm

Could someone just look up a value of 126.1 C and see what they get.

Or even better could someone look it up a tell me cause if I had a hard copy of the charts, they would be out the window by now!!!

Thank you,
Derry

P.S. I am really a patient person but...
This is something I should be able to do in my sleep. This is what happens when you spend too long in the pharma industry annoying people about paper work!

 

[onuigbo]

Why do we have condensate in a superheated steam line? Apart from steam traps, how can we eliminate the formation of condensate in a superheated steam line.

We deliver steam as if your life depends on it.

 

[rmw]

onuigbo,

You need to ask this question in a new thread, and give more specific details.

dmanley,

For the temperature and pressure you show, there is very slight moisture, too little for my Vacworks II mollier program to even indicate a value.

This is when I plug in the English units you gave.

If I plug in the pressure in psia and the temperature in metric, I get that the saturation temperature would be 127 °C.

Unfortunately Vacworks II does not have a pressure field for Bar A.

I suspect that you might have some decimal error in your conversions, or if you are reading gages and thermometers, normal gage error margin.

I would consider it to be saturated.

Go to

http://www.graham-mfg.com

and I think you can get them to send you a Vacworks II via the internet.

rmw