Flash Gas

Does any one have an idea on how long it takes the liquid to turn into gas once the pressure is dropped.

For instance - through a TXV. Does the gas form instantaneously or is there a delay, a delay of 10ms or 300ms would make a huge differance.

Another example - if a liquid line ruptures does it spray liquid onto the floor which then evaporates or does it come out of the leak as a all gas?

For a pure refrigerant it may be quick but with oil added to the mixture the oil may hold back the flash off for some time?

Any ideas or referances?

Thanks:)

Chef

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Originally Posted by Chef

Another example - if a liquid line ruptures does it spray liquid onto the floor which then evaporates or does it come out of the leak as a all gas?

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This is easy to simulate. Hook manifold hose on gas cylinder and release liquid refrigerant in hose. At exit you will see drops of liquid refrigerant as well as gas. So, it has both phases and it is obvious that liquid is not momentary evaporated.

Then again, when we search for definition of flash gas we get this:

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The gas resulting from the instantaneous evaporation of refrigerant in a pressure-reducing device to cool the refrigerant to the evaporating temperature obtaining at the reduced pressure.

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This calculator could be handy!

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Originally Posted by nike123

This is easy to simulate. Hook manifold hose on gas cylinder and release liquid refrigerant in hose. At exit you will see drops of liquid refrigerant as well as gas. So, it has both phases and it is obvious that liquid is not momentary evaporated.


This calculator could be handy!

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Thanks Nike: I can remember when filling a gas lighter with butane the gas/liquid would spray out if the seal was a bit off. The butane took some some time to evaporate so I get the idea. Not sure if I want to squirt refrigerant onto the floor just yet but the mental image is good.

I tried the calculator you linked to and thanks for that but as it uses EES in the two phase region it sometimes makes mistakes (always actually) and I get 42% liquid and 99.3% gas so it seems to be a little (lot) off.

If the liquid does take some time to evaporate as is now expected, per your thought experiment with the gas bottle, it means that liquid may be squirted out of a TXV valve directly into the evaporator a bit like a fire hose.

That changes a whole lot of assumptions of what is happening. What if the entrance to the evaporator is 90% liquid instead of the average 70% normally assumed? Velocities are off, heat absorption changes.

Somehow I need to get a time constant for liquid to gas.

Chef:confused:

Good day to you Chef

To my way of thinking the rate vaporization would depend almost entirely on the pressure drop (i.e. velocity) at the aperture/rupture and localised energy available. I would envisage that a cone of liquid would be existant at the aperture maintained by the head of liquid behind in conjunction with the cooling effect of the instantaneous evaporation surrounding the cone. I also believe that a proportion of the vapor will re-condense (if localised energy sources allow). The interesting point is the oil, as it is not an emulsion (hopefully) it is only suspended and must make some physical difference to the rate of evaporation.

Thats what I think happens. I might read up about this if I get time.

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Originally Posted by Chef

T

I tried the calculator you linked to and thanks for that but as it uses EES in the two phase region it sometimes makes mistakes (always actually) and I get 42% liquid and 99.3% gas so it seems to be a little (lot) off.

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I dont see where in software you found % Liquid and % Gas.

I see only % volume of flash gas and % mas of flash gas.

In your case that probably mean
42% of mass of flash gas and 58% mass of liquid refrigerant
and in same time
99,3% volume of flash gas and 0,7% volume of liquid refrigerant
and that sound to me perfectly OK!

http://farm4.static.flickr.com/3386/...ba1bb1_o_d.png

Hi Nike

I did not mean to get into a discussion on the program and it should probably have its own thread.

The real interest is in how long it takes for the gas to flash off.

However I greatly appreciate all the effort you put in to post the diagrams and so send in your reply. Its so time consuming to post a picture.

http://www.refrigeration-engineer.co...&pictureid=704


As you can see the % volume is 100% but the mass is only 53%.
Something is wrong here. It does not really compute.

I can explain why this happens in EES if you wish and it may be an intersting topic but its not what I seek at the moment.

Your initial post about the delay of vapourisation is thought provoking but how do we quantify the time it takes. I think it will make a lot of differance to both condensation processes and evaporation processes and might have quite an impact on how we calculate things.

I am trying to see what 100ms and 500ms differance makes on the performance of a system but its proving difficult as I have nothing to go on.

Chef

You are probably right regarding that software maybe need separate thread and I started new thread about that.

To me, it takes about 1-2min. (depending on size of picture and if I add notes to picture).
Snagit software and Flickr pictures server.