Monika Witt, chairman of Eurammon, the European initiative for natural refrigerants, examines the trend towards using ammonia for lower-capacity refrigeration systems.
AMMONIA has been used in refrigerating technology for nearly 130 years, making it the only refrigerant to be permanently on the market since it was first used. It is typically used in large-scale refrigerating plants on account of its excellent refrigeration effect and its specific evaporation heat value of 1,262 kJ/kg at 0°C - second only to water.

Lower filling quantities with the same performance

Due to the inherent toxicity of ammonia, development has focused on reducing the plant charge. Cascade systems are a possible solution, using CO2 (R744), potassium acetate or potassium formate solution as secondary refrigerants. In these cases the reduction in ammonia charge has no major influence on performance. At the same time, the ammonia is restricted to the refrigerant circuit in the plant room. This solution is also suitable for existing two-stage ammonia cascade systems, as these can usually be easily modified.

Developments in the field of compact tubular and plate heat exchanger systems with high-efficiency exchanger surfaces can reduce quantities of ammonia in the circuit through enhanced heat transfer. For example, tubular heat exchangers can be baffled to take better account of the different boiling behaviour across the heat exchanger so that the ammonia filling quantity can be reduced by around 80% compared to a conventional bare-tube heat exchanger, without any loss in evaporator performance. In addition, various heat exchanger parameters such as dimensions and the number of passes and tubes as well as the price are far more favourable than in comparable machines, thus saving space and costs.

Further potential is offered by exchanger tubes with inner fins, tailor-made for ammonia applications.

New combinations extend the range of applications
Hydrocarbon-based refrigerant blends also extend the applications of ammonia. Experimental tests have shown that compared to pure ammonia, some of the blends tested have a lower discharge outlet temperature, a lower compression pressure ratio, 5-10% better refrigerating capacity and better machine oil solubility.

R723, an azeotropic blend of 60% ammonia and 40% dimethyl ether, has been in practical use for several years. With an almost identical pressure level, it offers a series of advantages over pure ammonia. The discharge temperature can be reduced by about 15-20°C compared to ammonia, permitting the use of air-cooled condensers for example, instead of cylinder head cooling fans or water-cooled cylinder heads.

In addition, the lower temperature on the high pressure side avoids the thermal loads for the materials and refrigerating machine oils. At the same time, there are decisive improvements in the oil solubility for mineral oils which can also be extended into the low temperature range, while preserving miscibility with synthetic oils.

Good problem-free oil recovery is thus possible so that separate oil recovery systems are not needed. Heat transfer during evaporation is also improved. There is no negative impact on the efficiency of the refrigerating process from impurities in the gas phase of the dual fluid blend, such as vapour density, specific heat capacity and evaporation enthalpy. However, the usual precautions taken when working with ammonia must be observed.

Effective exploitation of waste heat

With increasing energy prices, end-users will find considerable potential savings in recovering waste heat to generate hot service water. Depending on how the systems are designed, operators of ammonia plants with medium and large refrigerating capacities can more than half their annual heating costs.

Heat pump applications

Natural refrigerants are also attractive solutions for heat pump applications. New developments in recent years such as hermetic scroll compressors, two-stage and 40bar compressors for ammonia or two-stage centrifugal flow compressors for CO2 all help to improve energy efficiency. In addition, developments in dx systems with soluble oil and plate heat exchangers facilitate reductions of up to 50% in the specific ammonia filling quantity.

Progress has also been achieved for use in private housing applications; initial prototypes of heat pumps with an output of 6kW work reliably with a filling quantity of less than 100g of ammonia.

In the future, ammonia will continue to maintain its established position in large-scale refrigeration, especially at evaporation temperatures above -33°C. But things are also moving in the middle and lower capacity range, with an increasing number of refrigerating companies being involved in this sector.

Eurammon
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