2019 RETA Breeze Sept-Oct

NUMBER TWO, “Carbon dioxide has a relatively low critical temperature”. Without getting too scientific, critical temperature is the point above which a gas cannot physically condense into liquid. Instead, it becomes a “super- critical fluid”with some of the char- acteristics of both liquid and vapor. It typically has a density very similar to its liquid state, but a viscosity very near its gaseous state. For carbon dioxide, this effect happens whenever it is at a temperature above 87.98°f. compared with ammonia which has a critical temperature of 270.3°f. I would bet that not many of you have seen your condensing tem- perature anywhere close 270.3°f, but most of your systems operate above 87.98°f condensing temperature at some point in the year. This could pres- ent a problem if you were operating a carbon dioxide system. If your refriger- ant cannot turn into a liquid because the condenser is too warm you will certainly have a problem with it being able to do refrigeration with any sort of efficiency. In the old days, condensers were not very efficient, and the properties of supercritical fluids were not so well understood. It was not uncommon that those systems ran trans-critical for many hours each year. Operation at or above trans-critical conditions is very energy intensive, kilowatts or horsepower per ton of refrigeration goes up dramatically. However, an important discovery was made at some point that provides some relief from this penalty. (Disclaimer; this next statement will likely blow your mind if my explanation of critical point above didn’t already do so!) We now know that when a carbon dioxide system is running at or very near trans-critical conditions, if we raise the discharge pressure, two things happen; 1- capac- ity goes up, and 2- energy consumed

per ton goes down. This effect is totally counterintuitive to everything we have been taught about refrigeration. However, charting and comparing the two refrigeration cycles on a carbon dioxide Mollier diagram can prove out this assertion. Modern control technol- ogy is truly amazing, especially when compared with what was available back in the 1880’s. This “smart” tech- nology allows a system to be operated as described above when the controls determine there is an efficiency advan- tage to doing so. A second way that we can avoid trans- critical energy penalties is by using wet bulb based or evaporative type condensing. It is worthwhile to note that at sub-critical conditions, a carbon dioxide system can run more effi- ciently than ammonia due to its lower compression ratio and higher energy density/lower specific volume per unit of refrigeration. Modern evaporative and adiabatic condensers can keep condensing temperatures below the critical point throughout most of the hours in any given year. Even for facilities located in the warmer regions of North America, a system can be designed that will keep the system op- erating in sub critical mode for enough hours that it will make it equal to or more efficient than an ammonia sys- tem (more on that later in this article). There are other innovations coming into the market that promise to help even more with the trans-critical penalty. Two good examples would be ejectors and parallel compression; however, those are topics for future articles. NUMBER THREE, “Carbon dioxide has a relatively high triple point”. Triple point is defined as the pressure & temperature at which a refrigerant can exist in all three possible states; solid, liquid and gas. Carbon dioxide

has a triple point of 60.4psig, much higher than ammonia at 28.15” of vacuum. This could in fact present a problem for someone unfamiliar and untrained on what precautions need to be taken when working on a carbon dioxide system. Issues with a triple point above atmo- spheric pressure can happen under a couple of common circumstances: • When charging liquid refrigerant into a system that has been pulled into a deep vacuum to remove water vapor and non-condensables. • When a vessel or piping that con- tains liquid refrigerant develops a large leak or is opened to atmo- sphere, causing the pressure to rapidly drop below the triple point pressure. In both above scenarios, when the pressure drops below 60.4psig, the liq- uid carbon dioxide in the vessel or pip- ing instantly solidifies, forming dry ice. Needless to say, it is not a good thing when your liquid refrigerant turns into a solid inside your system! The solu- tions to these issues are relatively easy. • When charging an evacuated sys- tem, simply break the vacuum with gaseous carbon dioxide and let the pressure come up above 60.4psig before introducing any liquid. • When you are bleeding off refrig- erant to service a system, use a back-pressure regulator to maintain a pressure of over 60.4psig on the system until all the liquid is gone, then lower the pressure allowing the pressurized vapor to drop to atmo- spheric pressure. • If you have had a large leak, repair the leak and close off the system from atmosphere, allowing the pres- sure to rise on its own in a slow and controlled manner. Do not add pres- sure from any outside source. When the pressure rises above 60.4psig, the dry ice will start to return to a liquid state. Wait until all remaining

22 RETA.com