Cap Tube Design Program - Results

[Chef]

The basics of the program have been resolved and tested in a limited way and the results are promising.

As the only system I have that has schreader valves is an R134a units running at 0.001Kg/s and Dx=8Bar Sx=1bar and a subcool of 5C I can only test the code against one known cap tube length. It was very close so it is a good confirmation of the base priciples but I need other examples to correlate against. Only R134a at the moment though please.

Then I can find out which part of the code is sensitive and try to smooth it out.

It must be a running system where you know the following:
Cap tube ID, Cap tube length, Discharge pressure (absolute or gauge) and temperature and evap P or T plus the mass flow rate.
Compressor volume and RPM would suffice if you dont have the mass flow.
Really - no guesses as it is the physical reality that proves the maths!

If any one has them please let me have them. Thanks.
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The other items that would be very useful are:-

• Viscosity data versus temp for fluid and gas states in saturated region.
• Hf and Hfg for the saturated part versus temp
• Any R134a tables

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To date both EES and Refprop dont provide this data and so I have had to generate an equation of state parametric table for R134a and there seems to be a few areas where it seems to be not quite right and some correlation to another source would be useful.

What would be really cool is a table with P, T, H, Hf, Hfg, Hg, viscosity f, viscisty g, spec vol g, spec vol f, and of course quality! (A Christmas wish list?)

If any one knows what are the standard P and T values for h=0 for R134a it would also help immensely as that means I can develop the equations of state from principles.

Ice - The term for static head is actually quite small and as most systems have the tube running horizontally or close to it. Its not often one see's a cap tube fridge unit where the compressor and condensor is downstairs and the evaporator is upstairs? So for this cut it is not included but it can be added if want me to.

Your comment about entry and exit losses from the tube are relevant but the results shows them to be very small for the inlet and for the outlet it is equivalent to adding just 4mm of tube length. This seems too small to consider at the moment. (But I can add it in if you want me too)

Peter - The actual negative tube length does occur in this model as well as Stoecker's but it is when it actually reaches sonic velocities of 170m/s. This seems reasonable as the speed of sound I calculated is close to this. I think the Stoecker model omits acceleration Pressure drop at its peril but the momentum part was useful. Thanks for that.

Us Iceman - So far its up to 320 equations so the solver could not make it with just 50 but maybe later we can look at it.

The input module and output module I have worked up for the solver defeats its 'save' and 'load' and 'output' functions so does mean it is useable by anyone. If anyone needs it please send a PM.

Bottom line - it works.

Chef

[US Iceman]

I suspect you have spent a fair amount of time on this little research project. Congratulations on some promising first runs.

I don't know if you have spent a lot of time reading about the capabilities of EES or not, but in the Professional version (which I have) it is possible to incorporate the file into a standalone executable file.

That might make it useful and put your hard work into a format more could use.

Note: One thing that might produce some useful comparison results is the review of existing cap tube tables from the manufacturers. Or, the comparison of your work to that of the manufacturers for distributor tubing, which I think is similar.

[Chef]

I have been reading up on EES and have been using it a little but I cant get Hf and Hfg and Hg from its lookup functions nor does it have viscosity for the fluid and gas states otherwise it would be fairly easy to implement. I see it has parametric look up tables and so that may be a way to get the data into EES or just write it equations of state. The only issue here would be if others want to use it for another gas then all that part needs to be changed each time?

The paper was interesting and may help solve some wobbles in the code at low temperature - i will see how it works later.

Do you have anything on R134a viscosities?

Chef

[Chef]

The results are very interesting and not what one would expect.

I have a system charged with R134a and it has a selectable speed compressor. This means I have a setup where I can change the mass flow rate for the same size and length cap tube and measure the conditions at the condenser outlet and do some comparisons. Keeping the evap at the same load and temp. Handy.

The pressure drop in a cap tube is made up of 2 main sections.

• First is the subcool region, this is where the liquid from the condenser has a pressure above the saturation point. This pressure is then lost as it passes down the tube until saturation point is reached. It then turns to 2 phase flow. (The idea of this being called subcooling is odd and I prefer to think of it as over-pressure, makes more sense)

• Second the 2 phase flow starts and as more gas evaporates the velocity increases very quickly and so does the pressure drop. This is the main portion of pressure drop.

Here are the conditions: Evap is -8C and Condenser outlet is 32C
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So for a m=0.0009 (2500 revs) the tube length just for the 2 phase flow is 1.4m but my tube is 2.4 meters long.
This means that in the first 1m of the tube it must be be liquid, so it has to be subcooled (ie over pressured) and it must lose this excess pressure over this length of 1m of tube to become saturated.
The program shows this to be 6C of subcooling. The same as measured, nice to know.
So nearly half the tube is filled with liquid and moving at just 2m/s but as it gets to the sat point things start to change and the pressure drop really begins to start, but nearly all of the pressure is lost in just the last 0.5 metres of the tube – interesting.
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Now switch the compressor up to 3500 revs and a mass flow of 0.00126. The tube length calculated for the 2 phase flow part is now only 0.7 metre long so that means 1.7 metres is needed to be flled with subcooled liquid. The program shows that to be 10C and I measured 9C so close enough.

This means that the first 1.7 metres of the tube is filled with liquid and only in the last 0.7 is it 2 phase and most of the pressure is lost in the last 0.3 metre. Quite amazing and not expected.
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So what does this mean, well the tube dimensions are fixed but the onset point of saturated liquid is not and by moving this up and down the tube the system establishes equilibrium at the expense of excess head pressure (subcooling). Conclusions - it seems one can determine how the cap tube is functioning directly from the subcool number.

So much more work to do.

Chef