Development of Semi-Hermetic Screw Compressors

The once “simple“ refrigeration world became much more complex due to a large number of substitute refrigerants, which not only had different properties, but also required the redesign of compressors. In 1997 an important step towards reducing global warming was taken with the adoption of the Kyoto Protocol: the emission of greenhouse gases has been continuously reduced since then. The consequence for the air conditioning and refrigeration industry is the attempt to contain direct emissions both by reducing the amount of refrigerant in the system and by improving the leak protection of the systems. Increased plant efficiency reduces energy consumption and thus CO2 emissions into the atmosphere. By classifying the refrigerants by to GWP equivalent to the release of CO2 and partly by taxation, the switch to natural refrigerants is motivated. Replacement refrigerants with ever lower GWP values have been and will be brought onto the market in rapid succession, which has caused uncertainty in the industry.

In order to establish natural refrigerants (CO2 = R744, ammonia = R717, propane = R290) as a long-term alternative, standards had to be introduced or adapted and components had to be developed especially for the high pressure levels of CO2. These components were initially responsible for a cost increase, which led to the realization of saving potentials elsewhere to maintain margins and thus to cost pressure.

In the meantime, the natural refrigerants are available everywhere at low cost at refrigeration quality and the component prices have fallen due to the quantity effect. Nevertheless, these effects had an impact on distribution channels and the value chain:

In recent years, every market participant has tried to increase the value added. Today many component manufacturers produce complete systems, wholesalers produce their own systems and plant manufacturers integrate services. Large end-users have brought the expertise in-house and specify/plan their systems in detail; plant builders often became pure installers, who had to specialize within one industry niche.

In addition to and as a result of increased price pressure, the main consequence is the tendency towards standardised, modular systems that come with factory guarantees and leave no room for on-site adjustments or repairs.

The trend has accelerated with the discussions about climate change; new installations are now equipped with natural refrigerants in the vast majority of cases, with CO2 dominating the supermarket sector in particular.

The traditional dividing line between the commercial and industrial refrigeration industry is becoming increasingly blurred, CO2 systems are penetrating into larger capacity ranges, compact ammonia systems are being used in water chillers from 200 to 800 kW, and due to the changes in standards, propane systems with a small refrigerant charge are being increasingly used.

The rapid progression of standards is forcing “ex-freon“ system builders, installers and users to constantly learn and upgrade their skills. Suppliers of systems and components must provide better technical operating instructions and support. Complete packages become more comprehensive and complex with integrated “commercial“ features that go beyond traditional industrial standards.

The quest for higher energy efficiency levels makes the previous methods of condensing questionable by emitting heat to atmosphere. The holistic approach of heating and cooling as the two sides of the energy equation will lead to new grid-based systems where the waste heat will be fed in either directly or boosted up by heatpumps.

Ammonia as a non-ozone-depleting substance without direct influence on greenhouse effects with an energy efficiency at least equal to R22 makes this refrigerant to appear as a long-term alternative. The low price and availability makes it commercially very interesting. However, the high enthalpy difference, which is an advantage for large industrial applications, can make control somewhat more difficult for smaller capacities. Flammability and toxicity do require special safety measures, in particular dedicated plant rooms with automatic forced ventilation, but no measures different or beyond those for other refrigerants such as CO2. The non-miscibility of mineral oil and ammonia requires specific design precautions for oil separation and recirculation during system planning and construction. In the past, the incompatibility of copper and ammonia led to the use of “open“ systems in which the motor and compressor were connected by a shaft. Motor, shaft and compressor need to be aligned, the shaft seal requires some maintenance and must be replaced regularly after a certain period of operation. Dry operation and aging can lead to leaks, which are immediately noticed (also in the further neighbourhood) by the acrid smell of ammonia.

To overcome the apprehension about ammonia and to facilitate the changeover to this refrigerant, SRM, the Swedish pioneer of screw compressors since 1910, has developed a complete semi-hermetic compressor series, which should appeal especially to market participants willing to convert. The product was launched in the market in 2017 the first time and by now the range is completed.