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typically higher than other types of refrigeration systems. However, according to Mr Chumun, this is just a number. “When you consider that the high side of a typical R410A system reaches a pressure of 25 bar or so, engineers are already dealing with high pressures. You need to understand every refrigeration system, whatever the pressure, and treat it with respect.”
2 In recent years, manufacturers, like Panasonic,
have been improving CO2 technology, equipment and components to a point where R744 is becoming accessible to more and more engineers. There is renewed interest in natural refrigerants for a broader range of large and small applications.
R744 climbs above its critical point of 31°C and it turns into a supercritical fluid – a strange combination of vapour and liquid. In this transcritical state, the refrigerant is no longer able to operate as a liquid. “The first thing I learned on the course was that transcritical has nothing to do with evaporating temperature or what’s going on inside the refrigerated cold room,” explains Mr Chumun. “It’s all to do with the ambient temperature outside. It’s fairly straightforward. “If the ambient temperature stays below
say 23°C, the CO2 system operates in a normal subcritical state, just like any other condensing unit. Above 23°C though, a secondary transcritical valve allows the unit to change from working like a traditional condensing unit into working as a gas cooler. This reduces the temperature (and pressure) of the refrigerant back below its critical point and maintains it as a useable subcritical liquid. “In the UK, this means that in London and the South East where there are more warmer
days each year, a CO2 unit will operate more frequently in a transcritical mode than a unit further north.”
When it comes to working safely with CO2, the recommendation is do things by the book, even for the simplest task. Mr Chumun is clear: “You have to do things in a set way; no shortcuts. If you follow a simple method statement for each procedure, the system will be safe, the site will be safe, everyone will be confident with what it is meant to do.” For example, unlike a standard HFC refrigeration system, liquid R744 should never
be added into a new system - because at
atmospheric pressure, CO2 will immediately turn into solid dry ice. It is essential to add vapour to the system first and get the pressure well above 5.2 bar absolute before any liquid is added. “One of the things I found seriously
interesting is that if you create solid dry ice which blocks a pipe, the last thing you want to do is to try and warm it up and melt it,” says Mr
Chumun. “CO2 has an incredibly high expansion rate, so even a small increase in temperature increases the pressure significantly. What I learnt is that you must never trap liquid in a
CO2 system. If there is ever a situation where dry ice is made, you need to turn off the liquid going in and introduce more vapour, not heat.” Engineers also need to be aware of the pressures associated with CO2, which are
“There is a perception that CO2 is a premium option that’s only ever going to be for supermarkets, never for contractors,” says Mr Chumun. “The improvements in technology
are such that CO2 is a viable option for standard applications, like small cold rooms or convenience stores.”
Mr Chumun concludes: “CO2 is a great refrigerant that’s been around for well over a century. And unlike new low-GWP refrigerants, you’re not trying to get to the next F-Gas
deadline. CO2 has no deadline. As long as you take responsibility and get properly trained, contractors can be confident in offering the
benefits of CO2 to their environmentally- conscious customers.”
Hawco is proud sponsor of the ACR News Trainee of the Year award, which recognises the importance of engineers developing their skills at every stage of their career.
Mervin Chumun inspects a 2HP CO2
unit with Panasonic technical specialist, Indy Tharnvithian. Visit
www.hawco.co.uk/co2 for full product details.
www.acr-news.com
March 2020 21
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