Aluminium as Construction Material in Ammonia Refrigeration Cycles

Transcription

1 Aluminium as Construction Material in Ammonia Refrigeration Cycles Experiences With Aluminium Compared to other metals, aluminium has only a brief history as an engineering material. While, about 150 years ago, it was possible to produce aluminium as metal for the first time, the first heat exchanger with aluminium pipes was used in an ammonia cycle about 60 years ago. Other construction elements made of aluminium, which have proved reliable for years, are sealings and structural components for compressors (pistons, connecting rods, cylinder heads, crankcases, etc.). These components are made of plastic alloys and cast alloys. Plastic alloys are semifinished and finished products with a sufficient elongation at break (mostly greater than 8%) shaped by rolling, drawing, pressing and forging. Cast alloys are suitable for sand casting and chill casting and apart from other alloy components most contain some percentage of silicon. As ammonia is widely used above all in industrial refrigeration systems, steel has predominantly been used for the essential construction elements, such as pipes, heat exchangers and containers in ammonia cycles. In order to gain ground for ammonia applications in the sector of commercial refrigeration, technologies are in demand, which refrigeration technicians are familiar with from HFC and CFC refrigeration systems (soldering and separable fasteners). In this respect aluminium offers the best prerequisites as a construction material. Properties of aluminium (Al) In comparison to those of copper (Cu) predominantly used in HFC cycles Al Cu Density 10³ kg/m³ thermal conductivity W/m K electrical conductivity ca. m/ mm² yield strength ca. N/mm²

2 thermal expansion /K melting point C Advantageous properties of aluminium are: cost efficient cold plasticity good machinability toughness at sub-zero and, therefore, suitable for the evaporation temperature range of below 10 C solderability and weldability hygienic safeness reusability Aluminium alloys Aluminium ist not just aluminium. In fusible condition it can be alloyed to copper, magnesium, manganese, silicium, iron, titan, berrylium, lithium, chromium, zinc, zirconium and molybdenum in order to foster certain characteristics or prevent other unrequested characteristics. According to the standard EN 573-3/4 it can be distinguished between the following forgeable alloys of aluminium: Group 1xxx with min. 99 % of alumium with a solidity from 70 to 190 N/mm2 Group 2xxx with copper proportion from 0,7 to 6,8 % and with a solidity from 190 to 570 N/mm2 Group 3xxx with manganese proportion with a solidity from 100 to 350 N/mm2 (is often used as material for core tubes of heat exchangers) Group 4xxx with silicium proportion from 0,6 to 21,5 % and with a solidity from 170 to 380 N/mm2 Group 5xxx with magnesium proportion from 0,2 to 6,2 % and with a solidity from 100 to 450 N/mm2 Group 6xxx with magnesium and silicium proportion of about 1 % and with a solidity from 100 to 450 N/mm2 Group 7xxx with zinc proportion from 0,8 to 12,0 % and with a solidity from 220 to 700 N/mm2 Group 8xxx with proportion of other elements (alloys of aluminium and lithium)

3 Regulations for the use of aluminium According to the draft of EN 378, aluminium and aluminium alloys can be used in any component of the refrigeration cycle if their solidity is sufficient high and they are compatible with ammonia and the employed lubricants. Contact substances for Al engineering components In the refrigerant cycle of an ammonia system, Al construction elements get into contact with ammonia, refrigeration oil, traces of water and other metals. Refrigeration oils used in ammonia systems are on the one hand mineral oils and synthetic oils insoluble in ammonia and on the other hand the synthetic oils on the basis of polyalkylene glycol (PAG) soluble in ammonia, which have been available since ca Due to the different hygroscopicity of these oils, different prerequisites for corrosion exist. For secondary refrigerant cycles, the compatibility of water and aqueous solutions of antifreeze agents with the used aluminium alloys needs to be proved in each individual case. Outside of the cycle, surfaces of Al construction elements are exposed to corrosion if the temperature drops below the dew-point and condensate forms, which in the case of NH 3 leakages by absorbing ammonia turns into an NH 3 -poor solution. Corrodibility A thin but tight oxide layer makes the originally base medal aluminium relatively corrosionresistant against attacks by air and humidity. For Ammonia cycles, aluminium alloys with the following corrosion resistances are available: Construction elements in the refrigerant cycle No measurable corrosion in pure, dry NH3 Negligible corrosion from a technical viewpoint (few µm/a) in contact with NH3 and refrigeration oil with different corrosion rates depending on the type of oil, how old the oil is, and the water content If insoluble mineral and synthetic oils are used, due to their limited absorptiveness of water (100 to 200 ppm), no problems with corrosion may be expected. If soluble PAG oils are used, the absorptiveness of water increases. Via changes in the oil, this can lead to reactions with aluminium components in the compressor resulting in a considerably reduced operating lifetime.

4 Construction elements in the secondary refrigerant cycle The corrosion behaviour of heat exchanger pipes, when ethylene glycol water mixtures are used as intermediate substances, can be disregarded. For other secondary refrigerants, extensive studies and tables now exist, in which the authors propose possibly necessary inhibitors for the different fluids. If other metals are used (e.g. copper) in the secondary refrigerant cycle, considerable corrosion must be expected. Especially, if cycles are not saturated with nitrogen, a complete breakdown is possible, in extreme cases after only 0.5 to 2 years. Therefore, the combinations of materials have to be analysed. Construction elements in contact to the environment Aqueous solutions of ammonia cause corrosion not acceptable from a technical viewpoint. This puts especially aluminium components at risk, which due to temperatures below the dew-point and leakages get into contact with aqueous solutions. For such components, however, manufacturers offer various coatings as corrosion prevention. Stress corrosion cracking behaviour The aluminium alloys intended for the use in ammonia cycles are not affected by stress corrosion cracking. Connecting technologies The inseparable connections of pipes by hard-soldering of the couples aluminium/aluminium and aluminium/austenitic Cr Ni steels are mechanically solid and corrosion-resistant. In the case of soldered joints of aluminium with ordinary steels or low-alloy steels, corrosion has to be expected at the junction. By rolling aluminium pipes into steel panels, shell-and-tube heat exchangers can be manufactured permanently tight. For separable connections, there are a number of safe solutions, which both after several disconnections and also under stress of temperature changes are tight and corrosionresistant. Aluminium in electric motors The hermetic compressors with copper motors used in HFC systems cannot be employed in ammonia cycles. Since 1993 semi-hermetic compressors, whose motors have aluminium windings and a special insulation system, have successfully been used in ammonia cycles.

5 Thus, these compressors can do without the wear-sensitive slide ring sealing required with open compressors. The ammonia cycle is only closed by static sealings. Correspondingly, due to the lower conductivity of aluminium in comparison to copper (approximately 65%), the dimensioning for these motors needs to be different. Conclusion Aluminium alloys have proved reliable as construction materials in ammonia cycles for years. For the application of technologies, which refrigeration technicians are familiar with from HFC systems, construction elements of aluminium alloys offer safe solutions with regard to corrosion resistance and mechanical resistance. Therefore, the advantages of the refrigerant ammonia with regard to considerably smaller dimensioning of pipes, fittings and valves can be combined with the advantages of the construction material aluminium. In case of doubt, the German-language original should be consulted as the authoritative text. Published by eurammon P. O. Box D Frankfurt Phone Fax