6 replies to this topic

[Pronab]

At 35 .0 Barg pressure and 240 deg. C temperature saturated steam is let down at 4..8 barg pressure . What will be the temperature. Is it an isentropic sexpansion.
Can anyone help me to find out the final temperature at the said final pressure ( 4.8 Barg).

Thanks in advance.

Pronab Mistry.

[Art Montemayor]

pronab:

Nice to hear from you again.

The free expansion of a vapor such as steam through a throttling device such as a valve constitutes an irreversible, ISENTHALPIC process - not an isentropic process (where useful work is done). This, of course, means that the fluid's enthalpy remains constant during the free expansion.

When saturated steam is expanded to a lower pressure, as in your case, it drops down in temperature as well as pressure and falls into the superheated category -i.e., it is no longer saturated. I think you already know all this, but I want to make sure our thinking concurs all the way through what I'm going to explain as a convenient technique to resolve this type of thermodynamic problem:

1) go to: http://webbook.nist....hemistry/fluid/
2) Select the following options
Fluid = Water
Pressure = bar (note that this is absolute – or bara)
Temp = Celsius
Energy = kJ/kg
3) Choose: “Saturation properties -- pressure increments”; press to continue
4) Enter P(low) = 35 bar & P(high) = 37 bar; make P increments of 0.1.
5) Press for Data” and the answers will appear in a nice HTML tabular table that you can copy or download into a spreadsheet within a workbook.
6) Go back to the NIST homepage and this time choose “Isobaric properties” instead of “Saturation properties -- pressure increments” and press to continue.
7) Enter the end pressure of (4.8 +1.01) = 5.81 barA; also enter your initial temperature as 170 oC and your end temperature as 185 oC, all in increments of 1.0 oC;
8) Press for data and you’ll get the usual nice table which you copy and paste to another spreadsheet in the same workbook.

Now look at the initial steam conditions where you’ll see that the initial enthalpy is 50.485 kJ/kg for the vapor phase. Since the enthalpy stays constant, go to the second spreadsheet (which is really a listing of superheated steam conditions) and look for the enthalpy of 50.485. You can see that this enthalpy, at 5.81 barA, corresponds to a temperature of approximately 178 oC. That’s your answer. This is the same as working out the problem on a Mollier Diagram for steam, except that I believe this is more accurate. Plus, since I do all my engineering calculations on spreadsheets, I'm used to being fast on them and I don't have to go hunting for my old steam Mollier diagram or steam tables. All the data is at my finger tips within the NIST database, waiting for me - and you!

The National Institute of Standards and Technology (NIST), an official branch of the US government, operates their Website for the public's benefit. It is our tax money so if don't use it, it's wasted.

This website gives FREE, accurate thermodynamic properties on 34 fluids. Who is going to argue with the accuracy or the source of this official data?

I think you will find this data source and methodology of resolving thermodynamic problems to be very convenient and accurate.

I hope this has helped you in obtaining the answer you seek.

[Pronab]

Art
Thankyou for your reply. The mistake I had , I assume as an isentropic process rather than isenthalpic process.

pronab.

[Pronab]

Art
I found enthalpy of saturated steam at 35 barg and 240deg. C. is ~2802 KJ/Kg. Can you tell me how you choose 170 to 185 deg. C.
I find it from steam table the result is same as from the web you reffer( 178 deg.C). I got it from 4.8 barg pressure at enthalpy 2802 KJ / Kg .
It give me some (~ 1% ) error.

Regards.
Pronab.

[siretb]

I agree that T will be 178°C. I think the the 170-185 are selected so as to bracket the answer. Any other range with answer lying in between the limits would do.

Just to mention that according both to steam tables and a process simulator, the temperature of saturated steam at 35 barg would be 244.2°C. so 240°C lookks a bit low.

[Art Montemayor]

siretb & pronab:

Thanks siretb, that's exactly the reason I chose the temperature range.
You can choose a wider temperature range and probably not waste any noticeable time in getting a larger table; but you'll spend more time looking through more data. It's a "feel" and a skill you develop in making a rough judgement on what the answer should be. With more experience, you start to knowing beforehand what the answer will/should be.

The NIST database agrees, I believe, with siretb's note about the saturated temperature of 35.0 barG steam. But I think that pronab knows that and simply used a close value to the actual number. To me, pronab's description using the word "saturated steam" does the actual definition of his fluid vis-a-vis the temperature. That's why I didn't worry about whether his temperature value was valid/exact or not. "Saturated" says it all. Good discussion guys on a very basic, but important industrial application of saturated steam.

Regards
Art Montemayor
Spring, TX

[Pronab]

I always calculate water sturated temperature using thumb rule ( T = 100*P^0.24 , T is in deg.C and P is in Barg) which is 1 or 2 deg. C error and never mind.
Thanks to Art and Siretb.

Regards.
Pronab.