Wave Speed in Liquid Ammonia

[dbevil]

I'm again in the process of doing surge studies. This time on an anhydrous liquid ammonia line (-28°F., ~50 psig). I can't seem to find a source for the wave speed (acoustic velocity) of liquid ammonia nor can I find the bulk modulus for ammonia. Can anyone point me to a reference for either of these values?

Thanx in Advance...

 

[25362]

It shouldn't be too difficult to estimate the sub-cooled liquid's isothermal compressibility (the inverse of the bulk modulus) from a thermodynamic chart.

Using Perry VI's fig. 3-17 (log P vs enthalpy) I tried with my old eyes to do just that, selecting a density of 650 kg/m³ going up isothermically from one pressure to another and found a modulus of about 12,300 kg/cm².
I obtained the sound velocity from:

[(12,300*10⁴)(9.8/650)]^0.5 = 1,360 m/s

I'd assume an error of +/- 5% due to my inaccuracy in reading the charted values. If you find tabulated values, kindly let me know, to see how far-off was my estimate.
Thanks.

 

[Montemayor]

dbevil:

The answer is: 4995 ft/sec as the acoustic velocity in liquid, saturated ammonia at 65 psia.

The source for this valuable information is the NIST website that gives you all the thermodynamic data on the major refrigerants for FREE! This source is not only free, but it is THE authoritative and recognized standard for our nation's technical information.

I don't have the URL handy right now, but can furnish it readily if you are interested. You (and others) should not do any serious engineering without knowledge of this information availability to you (through our tax dollars!)



Art Montemayor
Spring, TX

 

[25362]

Montemayor, is that saturated ammonia ? The query refers to subcooled ammonia. Would this mean a change of some kind ? Thanks.

 

[Montemayor]

25362:

You're absolutely right. In my haste to try to get out the door to take my granddaughters to the Circus today, I didn't spot the fact that at -28°F, the 50 psig NH3 is supercooled. The NIST website will give you all the data you want on the Thermo values, saturated or otherwise.

The acoustic velocity will be different from what I gave. I downloaded all the NIST data I could justify into Excel tables and have created a lot of tables this way. Since I'm in a hurry I'll check back tonight to see if someone's gone there and gotten the actual value.

Art Montemayor
Spring, TX

 

[25362]

I must apologize to Montemayor and to dbevil because:

first, the web site given is correct and gives speeds of sound at any pre-selected T,P conditions;

secondly, because the right figure for the speed of sound as tabulated is 5800 fps, a bit higher than the previous figure submitted by AM, and 30% higher than the one I estimated from reading the charts.

BTW, the web even gives graphs on the change of speeds as a function of pressure.

 

[Montemayor]

dbevil:

The NIST website that will give you the Thermodynamic properties for a host of compounds, including ammonia, is:

http://webbook.nist.gov/chemistry/fluid/

As 25362 relates, it has all kinds of information and in a variety of forms and formats. You can download in tabular form (for Excel), in ASCII, HTML, or as curves. It is the authoritative site that I use for all NH3 refrigeration calculations I make. Who is going to dare to challenge the data and the source?

Art Montemayor
Spring, TX

 

[25362]

to dbevil, using NIST data one can estimate the bulk modulus at the given conditions. My estimate: 21,770 kg/cm², similar to that of water at 15^oC. Ammonia density given by NIST: 682.14 kg/m³ at the requested T,P conditions.

 

[dbevil]

Thanks to all who have contributed here. I now have the info I need to complete my calculations. I had found a value at another location listing 4900 ft/sec, but didn't think it was accurate, since that's higher than water! The wave speed is a measure of the compressibility of a fluid and I was having trouble believing that liquid ammonia was less compressible than water. But I can't dispute the facts, it does seem to be less compressible than water (at least, at these conditions). Oddly enough, it becomes more compressible as the pressure rises, dropping to 2200 ft/sec at 500 psig!

 

[25362]

to dbevil something is wrong with your conclusions:

primo From MIST data, at -28^oF and 500 psig the sound speed is higher than at 50 psig: 5840 f/s vs 5804 f/s.
secondo The speed of sound in liquid ammonia at the conditions you stated is indeed about 20% higher than in water at ambient conditions.
tertio The density of liquid ammonia at the higher pressure is, of course, higher, 683.6 kg/m³ vs 682.1 kg/m³.
quarto The modulus of elasticity is proportional to the square of the sound velocity times the density, therefore, since both go up with pressure, so does the bulk modulus, meaning that the isothermal compressibility (the inverse of the modulus) goes down as the pressure increases.
quinto The bulk modulus for water at 15^oC is 22,000 kg/cm² that for ammonia 21,770 kg/cm² meaning there is no large difference between the compressibilities of liquid ammonia and water.

Please discuss my findings, and tell me whether you agree; and if you don't please tell me why.

 

[dbevil]

25362:

The higher pressure system I spoke of was also at higher temperatures as well, since ammonia is liquid at those conditions.

Your right for -28°F but the wave speed does go down for higher temperatures. This chart, however, does give much higher wave speeds even at higher temperatures than I encountered before. There does not seem to be a great deal of agreement between the different sources. I have a bit of a dilemma in deciding what data is the most accurate. But for now, I'll treat it as a worst case and see if I can resolve the surge issues.