COLD STORES
Natural refrigerants: five questions to ask before designing a system
Natural refrigerants are here to stay says Miriam Solana Ciprés of Carel, and with them, new challenges have arisen in the HVAC/R industry.
T
he 'price to pay' for avoiding refrigerants with high global warming potential is not so low: high pressure, fl ammability and/or toxicity. The search for more and more effi cient systems presents an additional challenge. In this context, the choice of the right technologies to optimise the use of natural refrigerants becomes essential. The following fi ve questions can help as a starting point when designing a system:
1. Which natural refrigerant is the most suitable for my application?
This question is generally easy to answer, considering the diff erent characteristics of each type of natural refrigerant. Hydrocarbons have been proven to be suitable refrigerants for small units, such as beverage coolers or plug-in units, where the refrigerant charge is low enough as to make fl ammability issues less relevant. CO2
has started to be widely used in
applications ranging from centralised systems in supermarkets to air conditioning systems for vehicles.
Ammonia has been mainly used in industrial refrigeration for many years, however its use is now being extended to other applications, such as commercial refrigeration as a secondary fl uid. In any case, developments are allowing the use of refrigerants in applications where they had previously not been used, and thus a detailed
34 July 2021 •
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study of the requirements of each system is recommended before choosing the refrigerant.
2. Where will the application be located? On the one hand, when using CO2
, it is important
to know the climate conditions of the place where the system will be located. In general, for systems with a condenser/gas cooler located outdoors, the outside temperature can determine the most suitable type of cycle and technologies. In practice, if the outside temperature reaches the critical temperature of CO2
(31.2°C), the
system will work in transcritical mode (except for cascade or pumped systems) and thus effi ciency will be lower. There are diff erent technologies available that can help increase effi ciency at high temperatures: parallel compressors, ejectors, FTE etc.
On the other hand, installing the equipment outdoors and/or out of the reach of people when using a fl ammable or toxic refrigerant (hydrocarbons: fl ammable; ammonia: toxic and mildly fl ammable) can make it easier to comply with the relevant standards and make the installation safer.
3. Which regulations and standards need to be respected?
This is one of the toughest issues to face. As regards regulations, these are currently focused
on reducing the global warming impact of refrigerants, thus the use of natural refrigerants is highly recommended. Attention therefore needs to be paid to the safety standards. The safety standards to be complied with when using natural refrigerants depend on the application, the type of refrigerant and the country. There are general standards, such EN 378 in Europe, and product standards, such EN 60335-2-89 for refrigerating appliances with up to a certain refrigerant charge. These mainly include special requirements for systems using fl ammable refrigerants, but there are also some requirements for systems with CO2
.
4. What type of cycle should I design? The evolution of natural refrigerants has led to diff erent types of systems for each refrigerant.
Starting with the traditional layout for CO2 – essentially comprising a compressor, an expansion valve and two heat exchangers – its low effi ciency has led to the development of diff erent alternatives.
On the one hand, it is possible to keep CO2 in a subcritical state by designing a cascade or
pumped system. CO2 is used in a secondary low temperature loop: either vapour compression
(cascade system), pumped loop of liquid CO2 (pumped system) or both (hybrid system). On the other hand, transcritical CO2
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