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DaBit
17-07-2003, 01:03 PM
A common question that pops up amongst us overclockers is 'What size captube do I need?', or even worse: 'I have captube XXX; how long does it need to be?'
To answer those questions, there are tables with information, such as the Cubigel captube sizing table (http://www.cubigel.com/english/tecinf3.htm).

I tried to pour that table with some interpolation into a nice easy-to-use piece of software. I normalised the values in those tables to 1mm I.D. captube, and plotted the data to see what function could be used for interpolation. Then I noticed that there are large jumps in the plot, so it it no good.

Further research pointed me to a model developed by Wolff et al. (1995). They used the Buckingham pi theorem to predict mass flow. According to them, less than 1% of the experimental results was more than 5% off from the predicted values. Now, that's close enough. And estimating refrigeration capacity from mass flow is no big deal once you know parameters like subcooling and evaporation temperature. Thus, it seems like a good model to pour into a piece of software.

Wolff's model only uses a parameter which I cannot find in refrigerant datasheets: the surface tension.

Who can provide this parameter for various refrigerants?

Prof Sporlan
17-07-2003, 05:04 PM
Surface tension values for many refrigerants can be obtained from the NIST RefProp program. One can also write code to query the RefProp DLL to obtain these values. Also, as the Prof recalls, the ASHRAE thermophysical properities of refrigerants guide has correlations for surface tension.

DaBit
18-07-2003, 08:42 AM
And where could a DIY guy obtain the RefProp program or the correlations mentioned in the ASHRAE manual?

Prof Sporlan
19-07-2003, 01:22 AM
Ask, and it will be given you; seek, and you will find; knock, and it will be opened to you. (Matthew 7:7)

The Prof finds these values for saturated liquid:

Temp (F) s (lbf/ft)
-40 0.001236
-20 0.001113
0 0.000992
20 0.000973
40 0.000758
60 0.000646
80 0.000538
100 0.000433

Prof Sporlan
21-07-2003, 01:53 AM
Quoted from a book that, when all things added up, means nothing... Prof :)
A fine source of adages, regardless of one's beliefs. :)

DaBit
21-07-2003, 10:11 AM
Originally posted by Prof Sporlan
The Prof finds these values for saturated liquid:

Can I use these values for all the refrigerants? More specific: is the difference in surface tension between various refrigerants low enough to be insignificant?

Prof Sporlan
21-07-2003, 05:56 PM
Can I use these values for all the refrigerants?

It would be better for your cap tube calculator to use actual values. The Prof has tables for a number of refrigerants.

DaBit
22-07-2003, 08:33 AM
Originally posted by Prof Sporlan
It would be better for your cap tube calculator to use actual values. The Prof has tables for a number of refrigerants.

Is it possible to send me a set of tables? Personally I think that a cap tube calculation tool would be handy for many people, not just overclockers.

Prof Sporlan
22-07-2003, 11:00 PM
For one sizing cap tubes, the complete book will be handy:

Thermophysical Properties of Refrigerants (http://resourcecenter.ashrae.org/store/ashrae/newstore.cgi?itemid=8250&view=item&categoryid=174&page=1&loginid=300917)