expander test rig PDF

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I am still looking at ring leakage, as I ran across the above link, I thought a few might find of interest. I am not too sure about all that mercury splatering about one's test set up, but the effort is interesting.
Garry

Hi Garry

That is a very interesting aritical. It inspiried me to do a little analysis. In the charts Fig 4 where he compared saturated steam to supper heated steam he showed ideal gas expansion lines for comparisons. There was a great deal of differance with the saturated steam. That got me to wondering how an isentropic expansion steam line would compare to the ideal gas formula isentropic expansion line. The results are very interesting.

The supper heated steam line comes very close to the ideal gas steam line. But what is really interesting is the deviation of 0 supperheat looks very much like that shown in Halls chart.

In Halls the saturated and ideal gas chart lines are close at first. I didn't origionally get that result. But then I figured throttling may have slightly heated the inlet steam.

I think that what this analysis shows is that there is not as much a loss due to condensation as comparing to an ideal gas would lead you to believe. There is still loss. But using ideal gas formula to predict steam performance can be way off and missleading.

Just looking at the expansion line differances here. A more complete analysis can be done to compare steam rates. A complete analysis would be a better gage. I believe Hall's experment shows the advantage of super heat. And theory is in agreement. The question is how much much of a gain theory predicts to actual when using actual steam properties.

Andy

Hi Andy,

Andy what I see is that the main difference between an ideal gas and steam, or most vapors,is that the constant enthalpy lines curve upward with steam as one gets closer to the saturation point (looking at a T-S chart) and that enthalpy for a ideal gas parallels the temperature.

As the pressures get higher around the saturation curve the constant enthalpy lines get steeper. As we add temperature at any constant pressure (superheat) the lines get less steep. So wouldn't steam more approach an ideal gas at either lower pressures or higher temperatures?

If I understood the parameters the experiment was conducted in the neighborhood of 40 psia, an area where constant enthalpy closely (comparativly) parallels constant temperature. Wouldn't steam and an ideal gas further part company in an area like say 1400 psia close to saturation?

My understanding from what I've read so far is that using any one gas constant for steam in an ideal gas formula is only good over a short range of expansion.

How far in to saturation would an ideal gas formula be good for?

Anyway, hope the world finds you well---------------Bill G.

I use steam properties in my calculations. In that Hall analysis he used ideal gas lines for referance on is plots of expermantal data. It peked my curiosity. I wondered how his expermental data compared to ideal steam lines.

I was looking at isentropic expansions there. Supper heated steam is much closer to the ideal gas curves. I would say that in general it is the gas's temperature that effects it's relation to the ideal gas. The more supperheat the closer it is to an idal gas.

Right in an ideal as enthalpy and temperature have a 1:1 relation.

With a saturated mixture the ideal gas formula do not work vary well.

Hi Bill

My contacts were geting a bit dry last night. Sorry. My sight wasn't to good.

You are right about the ideal gas in relation to steam except I would say it is the superheat of the steam that deturmins how close steam is to the ideal gas. So at lower pressure the temperature can be lower. With out much supper heat at any pressure steam doesn't match the ideal gas curves very well. The fact that enthalpy are not 1:1 in steam is what really seperates them on the isentropic lines. Steam does have a coieficient of isentropic expansion. But it varies. I should have gave the ending pressures of thoes isentropic expansions in the PDF.

The expansion is not all that much of an expansion. To get close to the charts in Hall's experments I used an expansion ratio of 2.87 and used an initial pressure of 40 PSIA. Maybe should have used gage pressure.

So all expansion were start ed from 40 PSIA with a volumetric expansion ratio of 2.87. The ending pressures,initial supper heat were:

09.637,----- ideal gas n = 1.35
10.158,----- ideal gas n = 1.30

10.681,112.2 60 PSIA 93.1 F supperheat
10.888,093.1 40 PSIA 93.1 F supper heat
11.851,015.2 saturated steam at 60 PSIA
12.067,000.0 saturated steam at 40 PSIA

As you can see the more supper heat the closer to the ideal gas it gets.

I change my work sheet to do the anaysis on gage pre4ssure and attached it here. Using gage pressure I had to reduce the expansion ratio. I was targeting the 1.30 ideal gas expansion to end at 10 PSIG As Hall's did. The expansion ratio for expasnion from 40 PSIG to 10 PSIG is now: 1.8435 Quite a reduction from 2.87. Anyway the gage ending pressures are now

09.26,----- ideal gas n = 1.35
10.0,----- ideal gas n = 1.30

09.8,107.8 60 PSIA 93.1 F supperheat
09.9,093.1 40 PSIA 93.1 F supper heat
12.2,011.1 saturated steam at 60 PSIA
12.5,000.0 saturated steam at 40 PSIA

What I see is the trend of saturated steam water mixture to have less and less pressure drop for the same expansion ratio. If I set the initial steam quality to 33% I wound up with an ending pressure of 15 PSIG. It's!! quality is higher after expansion 0.344. Now that interesting.

Andy

Hi Andy,

Acording to a look at the ole T-S chart in the saturated area of it ( it does look like a bell curve doesn't it? I wonder if it has anything to do with probabilitys) I see two things:

One is that as the constant volume lines head to the right toward high moisture, they get closer together horizontally, so that greater volumetric expansion is necessary to acheive a pressure and coresponding temperature difference.

Two is that the constant enthalpy lines get more and more vertical as they approch higher moisture, so it becomes harder and harder to get energy out of an expansion.

Three is that the constant moisture (quality) lines change slope from down and to the right at high moisture to down and to the left at low moisture, the dividing line is roughly in the middle at around 50-55% moisture.

In the spot you picked at 33% the moisture would go down with expansion at around 66% the moisture would go up.

All in all it appears from the chart that the extraction of usable work from the saturation zone can be a demanding design and execution problem.

Also in the superheat area notice that the constant enthalpy lines start to climb much more vertical as we go over one to four hundred pounds, making engine design and execution much more exacting above this area also.

And "Hey, how about that expander test rig?" Ha Ha

See you Andy---------------Bill G.

PS. When I proof read this there was an extra comma I was going to put somewhere, but I forgot where it went.

All files from this thread

File Name	File Size		Posted by	Date
ideal gas Vs steam ex.pdf	86 KB	open | download	Andy	02/05/2005	Read message
ideal gas Vs steam ex.pdf	97.7 KB	open | download	Andy	02/05/2005	Read message