2019 RETA Breeze Sept-Oct

enabled the industry to take a second look at the natural refrigerants, and we are seeing them beginning to be used in areas where they have either never been used, or haven’t been used for ages. Carbon dioxide is the latest natu- ral compound to be scrutinized, and shows a lot of promise for the future, even when compared to ammonia. Let’s look at the four most commonly stated “disadvan- tages” of carbon dioxide from the old days. NUMBER ONE, “Carbon dioxide has high operating pressures”. As any of us refrigeration savvy people know every refrigerant has a pressure/ temperature relationship curve, and

carbon dioxide is no exception. When compared with its natural cousin am- monia, carbon dioxide has a lot higher pressure at any given temperature. For instance, at -15°f., ammonia is saturat- ed at 6.2psig, while carbon dioxide will be at 220.5psig. At +85°f., ammonia will be at 151.4psig and carbon diox- ide at a whopping 1,018psig. The num- bers for carbon dioxide might alarm anyone who has been around am- monia systems for any length of time, considering that the typical ammonia system is equipped with 250psig relief valves, only slightly higher than the suction pressure of a carbon dioxide system running a 0°f. freezer! Similarly, on a warm day, the carbon dioxide system can have discharge pressures of well over 1,000psig, while we get really nervous if our ammonia system condensing hits 200psig.

In a relative sense though, 1,000psig is not that high. Consider that the forklifts running around most facilities have hydraulic oil flowing through flexible plastic hoses placed directly in front of the operators face at pressures of over 3,200psig. Seems scary, but no one worries about pulling back on the lift lever on a fully loaded forklift to raise a pallet up for stacking. The important takeaway is that any piping system simply needs to be designed to operate at the pressures it will experience by the fluid it contains. This is achieved on a carbon dioxide system by using stainless-steel or a special copper/iron alloy tubing, both of which are designed with adequate pressure safety factors. If the system is designed and constructed correctly, it poses no additional risk due to higher operating pressures.

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- capacity goes up, and 2- energy consumed per ton goes down. This effect is totally counter- intuitive to everything we have been taught about refrigeration.