Tiny Density Gradients, a Song by the Transcriticals

Brent Cullimore

Tiny Bubbles was a hit song by Don Ho (before even my time). Tiny Density Gradients just won’t make the charts.

But that’s what happens when you take a pure fluid substance to high enough pressure or temperature: the distinction between liquid and vapor disappears. Instead, you instead get a dense, spongy state called a “supercritical fluid.” It happens easily with carbon dioxide (aka CO2 in the chemical world or r744 in the refrigerant world) because of its relatively low critical temperature (31°C, 87.8°F).

This fun and instructive video shows what is happening:

One of the key points here is that the vapor density isn’t much less than the liquid density just below the critical point.

CO2 and I go way back. I’m not old enough for Don Ho, but I am old enough to have played with dry ice as a kid. (I also played with liquid mercury, but that’s another story.) Dry ice taught me the wonders of sublimation: going right from a solid to a vapor, with no liquid state in between. This was before I moved to high and dry Colorado and experienced sublimation all winter long.

CO2 has been on my mind a lot lately, and not just because it is smothering us in a greenhouse blanket along with its friend CH4.

It started last fall with my brother-in-law asking why CO2 was used in preference to N2 for gas cartridges. Except perhaps for whipped cream canisters or for smoothing the head and reducing the acidity of beer, pressurized CO2 is the “default choice.” Why?

I didn’t know at first. But it turns out that the little BB gun cartridges don’t store gas, they start full of liquid. At least it is technically liquid if you are in subcritical Colorado, while it is a dense spongy fluid if you are in supercritical Florida during the summer. A CO2 cartridge is preferred because it is a source of lots of potential gas, not actual gas (until it is almost empty anyway). It also has a heck of a Joule-Thomson coefficient, so next time you’re hit by a BB, see if it feels cold too.

Then I ran into CO2 it again while working with my colleague, Tim Panczak, on modeling various aspects of a home kegerator. The most common choice is to use a CO2 bottle to pressurize and carbonate the beer. The bottle starts supercritical but drops into the saturation dome as it is depleted. The gas also slowly dissolves into the beer, and potentially foams up in the spigot during a pour event … but you’ll have to wait for another blog for that story and sample problem.

For that same kegerator, I decided to size a transcritical CO2 vapor compression refrigeration system instead of what you can buy today: an r134a or r600a cycle. You can see the model and the r134a comparison here

What is a transcritical cycle? It is basically like a subcritical cycle, except the pressure at the compressor outlet is supercritical instead of subcritical. This means there is no condenser since the tiny bubbles have been replaced by a tiny density gradient: the heat rejection component is instead a “gas cooler.” Tiny bubbles appear again downstream of the throttle in the low-pressure side of the loop, at the inlet of the evaporator. This page has a nice, short description with diagrams.

If you can’t buy such a system, you might ask why we are going to use it in our otherwise realistic kegerator series of sample models. Transcritical CO2 systems have been under intense R&D for at least 20 years as a more environmentally-wise cycle, which means any clever comment I could make about CO2 being both the villain and the hero was probably first made decades ago.

Still, I was astonished to learn that the #1 ranked target for a group dedicated to reducing atmospheric warming was refrigerants. Not just less use of refrigeration, and not just more efficient refrigeration, but also replacement of the refrigerant itself.

You can argue about these relative rankings (I certainly do), but there something to this. That fact was brought home by a recent article showing the importance of the ozone hole to climate change. The ozone hole, created in part by chlorofluorocarbon refrigerants and to a lesser extent to their replacement hydrochlorofluorocarbons (like r134a), has contributed to about half of artic warming and about a third of the warming in the world overall. Yikes.

To top it off, this great detective story appeared in the news. The article is about a group of scientists tracking down a treaty cheat who was still making r11.

Even if the words “great detective story” aren’t enough to make you read the article, you just have to look at the place where a lot of the story happens: the Jungfraujoch research station in Switzerland:

Having watched so many James Bond movies, all my training says that this is the bad guy’s secret hideout, not the good guys' headquarters.

If I want to go visit that place, and now of course I absolutely do, I felt like I had to offer an r134a alternative or they wouldn’t let me in. To me, that's more than enough justification to use a transcritical CO2 cycle.

One final thought: don’t invest in supercritical champagne. It doesn’t sparkle, and no one at your party will be entertained with stories of flow visualization using schlieren density waves. Trust me on that one.

Vapor Compression Cycles

Tuesday March 10th, 2pm MST

This webinar explains how the toolbox approach of Thermal Desktop and FloCAD can be used to design and simulate vapor compression cycles at various levels of detail. Applications include heat pumps, automotive climate control, and refrigeration systems.

Click here to register

Working Fluid Mixtures

Thursday March 12th, 2pm MST

Working fluid mixtures can be as simple as air and water. Or as complex as ... well, air and water.

"Air" might be a simple perfect gas or a collection of real gases ... itself a mixture. "Water" might be a simple nonvolatile approximation of liquid water, or it might be a volatile liquid.

This webinar discusses mixture types, and repercussions such as pressure and temperature range limits. It illustrates both how to set initial conditions and how to determine what is going on in results.

Click here to register