Exploring where CO₂’s unusual properties deliver clear advantages in refrigeration design.

Having dealt with A and B, we can now turn to the fun part of this trilogy: the applications where the benefits of using CO₂ as the refrigerant are so overwhelming that we can honestly say, “you would be crazy not to…”

This is not a coincidence. Carbon dioxide has some very unusual properties in comparison with all other refrigerants and this can be used to great advantage in the right circumstances. Presentation of these oddities is not sufficient to explain their power so some examples will be given. The saturated pressure at a given temperature is extremely high. This is often cited as a disadvantage, but since it also delivers many of the good things about CO₂, it is worth a second look.

The high pressure means that the density of gas, even at low temperatures, is much higher than for other refrigerants. This, in turn, means that relatively large pressure drops can be tolerated in pipes, valves and heat exchangers without affecting system performance too much. As a result, it is virtually impossible to overload a carbon dioxide evaporator, which can happen with ammonia or fluorocarbons. This produces a much higher cooling load at the start of a freezing cycle, for example in a blast freezer where a rapid pulldown for the first 30 minutes of the cycle can save several hours at the end of the process.

When freezing boxed beef, we shortened the cycle from 20 hours to 16 hours in this way. In plate freezers, the unusual pressure-temperature characteristic means that the pressure drop incurred in the required flexible hoses connecting the freezer plates to the suction header doesn’t create such a temperature gradient as with other refrigerants. The result is that the freezer plates, which are in direct contact with the product to be frozen, are much colder. When we compared a standard ammonia design for freezing 3 in. (75 mm) slabs of pet food, we found that using carbon dioxide enabled the freezing cycle to be reduced from three hours to just under one hour. The performance was so good that the automated crane in the factory couldn’t keep up. To balance the logistics we raised the suction pressure setting and reduced electricity demand by about 20%.

At the other end of the temperature scale for refrigeration, we also had great success using CO₂ as a volatile secondary coolant for IT equipment, including blade servers and dealer desks. This was counterintuitive because at such high temperatures the pressure is extremely high: when evaporating at 50°F (10°C) the pressure is 645 psi(g) or 44 bar(g) (4.5 MPa), but again it was the very high gas density that opened up the possibilities. In blade server cooling, we had evaporators mounted on the server rack with vertical tubes in a multi-pass configuration. This worked extremely well, even in a thermosyphon arrangement with no pump or compressor to circulate the refrigerant. In the dealer desk installations, we had up to fourteen evaporators connected in series with a negligible temperature difference between the first and the last in line. When we applied a performance assessment factor developed by Professor Hans Quack and his team at the University of Dresden, we found that CO₂ outperformed R-134a by a factor of about 7:1 in this application.

These benefits are confined within strict physical limits. At the freezing end, it is difficult to go below the triple point of -70°F (-56.6°C), although not impossible. For IT cooling, the performance factor drops sharply as the evaporating temperature approaches the critical point of 87.8°F (31°C). Between these limits, CO₂ offers incredible advantages provided the system is designed to take advantage of them.