Resulting design and construction criteria

Based on the present "state of technology", industrial NH3 systems demand a completely different plant technology, compared to usual commercial systems.

Due to the insolubility with the lubricating oil and the specific characteristics of the refrigerant, high efficiency oil separators and flooded evaporators with gravity or pump circulation are usually employed. Because of the danger to the public and to the product to be cooled, the evaporator often cannot be installed directly at the cold space and the heat must be transported by a secondary refrigerant circuit.

Due to the thermal behaviour, two stage compressors or screw compressors with generously sized oil coolers must be used even at medium pressure ratios.

Refrigerant lines, heat exchangers and fittings must be made of steel; welded joints in pipelines of larger dimensions are subject to inspection by a certified inspector. In some cases, aluminium can also be used as a material.

Depending upon the size of the plant and the refrigerant charge, corresponding safety measures and special machine rooms are required.

The refrigeration compressor is usually of "open" design, the drive motor is a separate component.

These measures significantly increase the expenditure for NH3 plants, especially for medium and smaller capacities.

Efforts are therefore being made world-wide to develop simpler systems which can also be used in the commercial area.

Some research programs are dealing with partly soluble lubricants, with the aim of improving oil circulation in the system. Simplified methods for automatic return of non-soluble oils are also being examined as an alternative.

BITZER experiences with NH3:

BITZER is strongly involved in these projects and is operating a larger number of compressors. The experiences up to now have revealed that systems with partly soluble oils are difficult to manage. The moisture content in the system has an important influence on the chemical stability of the circuit and the wear of the compressor. Besides, high refrigerant solution in the oil (wet operation, insufficient oil temperature) leads to strong wear on the bearings and other moving parts. This is due to the enormous volume change when NH3 evaporates in the lubricated areas. These research developments are being continued, with focus also on alternative solutions for non-soluble lubricants.

Various equipment manufacturers have developed special evaporators, allowing significantly reduced refrigerant charge. There is a strong trend towards "low charge" systems, i.a. with regard to safety requirements, which are also largely determined by the refrigerant charge.

In addition to this, there are developments for the "sealing" of NH3 plants: compact liquid chillers (charge 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 which were previously reserved for plants with refrigerants of safety group A1 due to safety requirements. An assessment of NH3 compact systems – instead of systems using HFC refrigerants and conventional technology – is only possible on an individual basis, taking into account the particular application. From a merely technical viewpoint and presupposing an acceptable price level, a wider range of products will supposedly become available in the foreseeable future.