Refrigerant NH3 (ammonia) has been used for more than a century in industrial and larger refrigeration plants. It has no ozone depletion potential and no direct global warming potential. The efficiency is at least as good as R-22. In some areas, the efficiency is even more favorable. Its contribution to indirect global warming is therefore small.
There are some negative aspects that restrict the wider use in commercial areas or require costly and sometimes new technical developments. NH3 has a high isentropic exponent (NH3 = 1.31, while R-22 = 1.18) which results in a discharge temperature that is higher than that of R-22. Single-stage compression is therefore already subject to certain restrictions below an evaporating temperature of around -10°C (16°F).
There is also a question of suitable lubricants. The oils used previously were not soluble with complicated technology and seriously limit the use of direct expansion evaporators due to the deterioration in heat transfer. Special demands are made on the thermal stability of the lubricants due to the high discharge gas temperatures. This is especially valid when automatic operation is considered, where the oil should remain for years in the circuit without losing any of its stability.
NH3 has a high enthalpy difference and, as a result, a relatively small circulating mass flow (approximately 13 percent to 15 percent compared to R-22). This feature is favorable for large plants but makes regulation of the refrigerant injection more difficult with small capacities.
A further criterion that must be considered is the corrosive action on copper-containing materials. Pipelines must be made of steel.
Additional characteristics include toxicity and flammability, which require special safety measures in the construction and operation of such plants.
DESIGN CRITERIABased on the present state of technology, industrial NH3 systems need totally different plant technology compared to usual commercial systems.
Due to its insolubility with the lubricating oil and specific characteristics of the refrigerant, high-efficiency oil separators and flooded evaporators with gravity or pump circulation are usually employed. Because of the toxicity-flammability issue, the evaporator often cannot be installed directly at the cold space. The heat transport must then take place with a secondary refrigerant circuit.
Two-stage compressors, or screw compressors with generously sized oil coolers, must be used at medium-pressure ratios. The compressor is usually of an open design, with the drive motor a separate component.
Refrigerant lines, heat exchangers, and fittings must be made of steel. Be aware that such pipelines are subject to examination by certified inspectors.
Corresponding safety measures and special machining rooms are required depending on the size of the plant and the refrigerant charge.
SOME SOLUTIONSEfforts are being made worldwide to develop simpler NH3 systems that can also be used in commercial applications.
One aspect of the research program is dealing with part-soluble lubricants, with the aim of improved oil circulation in the system. Simplified methods for automatic return of nonsoluble oils are also being examined as an alternative.
Also, various equipment manufacturers have developed special evaporators, in which the refrigerant charge can be significantly reduced. In addition, there are also solutions for the sealing of the NH3 plants. This deals with compact liquid chillers (charged below 50 kg), installed in a closed container and partly with an integrated water reservoir to absorb NH3 in case of a leak. This type of compact unit can be installed in areas that were previously reserved for plants with halogen refrigerants.
It is still too early to give a final judgment concerning the extended use of compact NH3 systems in place of plants with HFC refrigerants and conventional technologies. From a purely technical viewpoint and presupposing an acceptable price, it is anticipated that a wider range will be available soon.
Publication date: 04/03/2006