The trend towards lower refrigerant charges in ammonia systems and the emergence of new blends to further improve the performance and application range of ammonia solutions is changing the R717 industry. The blend R723, a mixture of ammonia and dimethylether, promises further potential in terms of heat transfer properties and miscibility with oil. However, after several years of manufacturing and distribution, the industry is still skeptical about the advantages and drawbacks of R723. This was confirmed by Dr. Dieter Krauss from Schick, a German producer of “schick R723” presenting its product at last year’s industry trade fair Chillventa. In the latest edition of “Kälte Klima Aktuell” (KKA), Krauss answers to some remaining questions concerning the special characteristics, safety issues and performance of R723.
Safety specifications according to DIN EN378
One of the major concerns regarding R723 are safety requirements resulting from a mixture of flammable hydrocarbons and toxic ammonia. It must be pointed out, Krauss states, that ammonia blend has some differences in safety technology and standards in comparison to R717. The parameters lower explosive limit and specific combustion heat are important to classify the refrigerants in safety groups according to DIN EN378. With a lower explosive limit of 6,0 Vol-%, the refrigerant blend is assigned to group L2 (for DIN EN378) - the same group as R717. However, the specific combustion heat amounts to 22,800 kJ/kg, which is higher than the limit value for group L2 (19,000kJ/kg). Therefore, the blend has to be assigned to group L3 (like e.g. propane) and requires higher safety technology regarding the system construction.
One of the general concerns mentioned regularly is the toxicity of ammonia. In fact, it can be toxic for humans at high concentrations. However, the human sense of smell can detect even the slightest traces of ammonia and in concentrations not dangerous at all for humans. Moreover, with today's state of gas warning systems and safety technology, systems using ammonia as a refrigerant can be operated without any risk, says Krauss.
Azetropic blend with performance advantages
Krauss contradicts the argument that leakages change the composition of the blend. “schick R723” consists of 60 mass-% ammonia and 40 mass-% Dimethylether and is a azeotropic mixure. This means that its composition cannot be changed by simple distillation and this occurs when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture. Therefore, R723 can be handled as a mono component refrigerant.
Another advantage of the blend is the performance efficiency. Compared to pure ammonia, R723 has a larger volumetric refrigeration output which can lead to a 7% improvement in COP under real operating conditions.
Moreover, it opens up additional fields of application for natural refrigerants, including in plants with refrigeration outputs below 100kW, and in portable refrigeration units. Thanks to the low compressor discharge temperature systems can frequently do without cylinder cooling. In many cases, air-cooled condensers may be used instead of water-cooled condensers. The lower temperature on the high-pressure side reduces the thermal strain on materials and refrigeration lubricating oils.
Refrigeration systems using halogenated refrigerants cannot be retrofitted to use R723. However, any engineer experienced with ammonia as a refrigerant can do the retrofitting of ammonia systems. The same materials can be used and only the sealing compounds have to be verified. Particularly suitable for the blend are PTFE, Buna N, aramid fibre AF400 and HN-BR. Due to the slightly higher vapour pressure, the thermostatic valves open more and have to be adapted. Oils that can be used are in general mineral oils, poly-alpha-olefins and PAG-oils. Before bringing into service the new refrigerant, attention should be paid to the cleaning of the system. As old oil residues and decomposition products can be unblocked and evoke mechanical disturbances.
Availability of components and compressors
Most ammonia system manufacturers finished their tests with the product and provide already all different types of components needed to install a refrigeration system with R723 as natural refrigerant. To ensure workers health and safety standards are met, several additional components can be received at Schick. The same compressors as with R717 can be used, however, the manufacturer is advised to consult the refrigerant manufacturer before installation and use.