Greg:
You still haven’t stated what your proposed goal is. For the sake of expediency, I’m forced to assume that you merely want to cool water in what would be described as a small-scale heat exchanger. You want to use liquid CO2 (LCO2) as a cooling source. Again, you fail to state your CO2 temperature as I requested so I have to assume you are using an un-conventional CO2 type of storage since you say the storage temperature is “ambient”. Assuming you are not in West Texas or Phoenix, Arizona, your ambient temperature is below 88 oF (the CO2 critical temperature) and the CO2 exists as saturated liquid at “ambient” temperature (say 85 oF). This is important to note, in case you are not aware of the thermodynamic state of the CO2. The normal method of storing or distributing CO2 has always been to use steel cylinders filled to 90% volume. Under these conditions, the CO2 is considered as High Pressure LCO2 (at 1,084 psig & saturated). The 10% vapor volume is for thermal expansion purposes. There is no practical need to fill a cylinder with a pressure exceeding its critical pressure of 1,084 psig. Why you are storing at 2,700 psig (and how you maintain that in a tank) is something of interest only because it is totally unconventional, unusual, and costly.
A common CO2 fire extinguisher is a perfect example of a conventional, HP CO2 storage cylinder. It would probably suit your purposes better, cheaper, and safer – unless there is a specific reason for your going into pressures above the critical pressure and yet staying below the supercritical phase zone. You don’t achieve the supercritical state with CO2 until you exceed the critical temperature of 88 oF – which I have assumed you stay below. This is the reason I solicited your basic data (temperatures & pressures); it makes all the difference in the world when you need to identify what thermodynamic phase(s) you are dealing with.
Now to describe to you what you will cause and what will happen when you set up your apparatus and open the LCO2 valve on the 1/8” OD copper tubing (I believe it is tubing, not pipe; pipe nominal diameters, while tubing is sized by its OD.). Expanding the 85 oF HP LCO2 down to essentially atmospheric pressure is going to produce a resultant 2-phase product of dry ice “snow” and CO2 gas at -109 oF. This mixture will have a propensity to plug up the rather small, 1/8” OD tubing. It will probably not be a continuous, steady-state flow due to the “sputtering” and plugging going through the tubing. The heat transfer effect by the dry ice snow will be another deterrent in having a good heat transfer effect. It is very difficult to ensure that the snow transfers its latent heat of sublimation. I have found this to be not only frustrating, but very inefficient. Any rough spots, sharp turns, valves, orifices in the tubing will make matters even worse as far as the plugging effect is concerned.
The product mixture you produce is the same thing that a CO2 fire extinguisher produces – except that you will notice that the fire extinguisher has a “horn”, a device that allows for easy dispersion of the solid snow produced by the adiabatic expansion of LCO2 and its conical shape deters any solid plugging downstream of the expansion valve – that’s the reason it’s shaped like it is.
Again, I don’t know your purpose, but you’ve picked a very tough fluid to expand and try to recover its cold effect. In my opinion, it’s the wrong fluid for trying to cool water but you probably have your reasons. Adiabatic expansion of LCO2 always produces a (solid + gas) mixture that is very difficult to control. The crystalline structure of dry ice snow literally has what looks like “barbs” on its surface and this contributes to its ability to form difficult and troublesome clogs that are a deterrent to steady-state flow. And I can personally attest to the difficulty of trying to “melt” (actually, sublime) away the solid blockage using external heating fluids such as water or hot gases. As the solid snow sublimes in the warm or hot surface area it is touching, it forms a gaseous CO2 film that is a natural, excellent insulator. This effect, besides the plugging, is yet another heat transfer problem to expect.
I’m sorry if I’m a bearer of sad news, but what I’ve learned after working with CO2 and Dry Ice for over 25 years is that it isn’t as simple and easy as you first imagine. You have to get deep into the actual Thermodynamic realities of the physical phases involved and the crystalline structures of the compounds.
I hope this hands-on experience of many years is of some help to you in arriving at a successful application. And do not contemplate putting a restriction valve on the 1/8” OD tubing without having everything upstream capable of resisting the full 2,700 psig of your CO2 storage – because it will plug up and effectively block all flow.
Good Luck!
