Natural refrigerants
There are many natural refrigerants. The best known are composed of the elements oxygen, carbon, nitrogen, and hydrogen, have a low environmental impact, and possess good thermodynamic properties. Compared to synthetic refrigerants, such as the well-known R134a, they also have special characteristics that must be taken into account during product development. An overview.
Hydrocarbons
For cooling applications down to approx. –20 °C, there are generally two refrigerants to choose from: the hydrocarbons isobutane (R600a) and propane (R290). Such substances are ubiquitous in society; we carry butane around in our backpacks as camping gas and fuel our barbecues with propane. The choice of refrigerant must be made carefully: isobutane requires approximately three times the compressor displacement to achieve the same cooling capacity as a propane circuit.
In principle, propane can be used to build more compact systems. However, these must withstand higher pressures and be equipped with more safety devices, which entails greater development and approval costs.
For ULT applications down to -100°C, the two refrigerants ethylene (R1150) and ethane (R170) have become established.
Carbon dioxide CO 2 (R744)
The substance carbon dioxide CO 2 (R744) is now also familiar to end consumers, for example through household appliances that add carbon dioxide to tap water. As a refrigerant, the substance has two special features: In common cooling applications, relatively high system pressures arise (see comparison table), which all components must be able to withstand. In addition, heat dissipation is usually supercritical, which in turn requires special equipment. The use of CO₂ is often discussed in the design phase, but usually fails due to the availability of components for the low-capacity range or because the device would simply become too large and too heavy.
Ammonia (R717)
Although ammonia (R717) has excellent thermodynamic properties, its toxicity and aggressiveness towards non-ferrous metals mean that it is neither panic-safe nor suitable for cost-efficient equipment development. This refrigerant is currently not relevant in our field of activity with outputs <5 kW.
| Example operating point | Isobutan R600a | Propan R290 | Carbon dioxide R744 Limited selection |
| Pressure at 45°C | 604 kPa | 1534 kPa | 9000 kPa |
| Pressure at -25°C | 58 kPa | 203 kPa | 1683 kPa |
| Pressure ratio π | 10.4 | 7.54 | 5.35 |
| Volumetric efficiency based on π (model: 0.94 – 0.035∙π) | 58% | 68% | 75% |
| Cooling capacity at 10 cm3 compressor displacement (∆Tsh = ∆Tsc = 0 K) | 103 W | 350 W | 765 W (gas cooler outlet = 45°C) |
| Flammable | yes | yes | no |
| Manufacturing/development costs | low | middle | high |
| Efficiency | middle | good | good |
| Power-to-weight ratio | low | middle | middle |
How to: We can handle pressure
At first glance, the flammability of hydrocarbons is off-putting. But industry can handle it – and so can we. This is because the cooling unit is a hermetically sealed system that can withstand at least three times the maximum pressure.
In connection with flammable refrigerants, one often hears of a filling quantity limit of 150 g. Up to this amount, the components do not need to have ATEX protection. The limit is to be raised to 400-500 g per system in the near future. Will this simplify things? Not necessarily, because our devices are often subject to the requirement that they can be transported by air freight – which limits the filling quantity to a maximum of 100 g per refrigeration circuit. The concentration in the room in the event of a potential leak in the system must also be taken into account. In this case, 8 g/m³ must not be exceeded.
Selection
From applications as high-temperature heat pumps to ultra-low temperature refrigeration systems, there are various suitable natural refrigerants to choose from. We use customized simulation tools to calculate whether a customer’s requirements can be met with one of these refrigerants, whereby the fill quantity forecast is particularly important in the case of flammable refrigerants.
Applicable standards
IEC61010-2-011 or IEC61010-2-12 always applies to laboratory and medical applications in this area. This series of standards applies to refrigeration systems in laboratories with flammable refrigerants and fill quantities < 150 g. For larger systems with higher fill quantities, EN378 must be consulted. The differences between these standards are quite significant, which is why the requirements for the refrigeration unit must be defined from the outset.
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