Read and translate the Text 5A.

                             Text 5 A. NATURAL REFRIGERANTS.

Refrigerants and air conditioning play important roles in modern life. They not only provide comfortable and healthy living environments, but also come to be regarded as necessities for surviving severe weather and preserving food. Unfortunately, accelerated technical development and economic growth in much of the world during the last century have produced severe environmental problems, forcing us to acknowledge that though these technological advances may contribute to human comfort, they also can threaten the environment through ozone depletion and global warming. Aside from cost reduction, these concerns are the biggest driving forces for technical innovation in the field of refrigeration and air conditioning.

Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) – used as working refrigerants in refrigerators and air conditioners as well as blowing agents in foams – are now being regulated because of their contribution to ozone depletion. Hydrofluorocarbons (HFCs) could be useful as short- and midterm replacements, but may ultimately not be suitable, owing to their global-warming potential. Accordingly, a long-term solution will require the use of natural refrigerants. The refrigerator and automotive air-conditioning industries have already begun to address the challenges of replacing HCFCs, including R-2, and eventually, HFCs on a global level.

The search for a new and environmentally benign refrigerants to replace the existing CFCs and HCFCs has led to the introduction of HFCs. However, HFCs have a much higher global-warming potential and higher costs than natural refrigerants. These concerns have spurred calls for the investigation of alternatives to HFCs. Some environmentalists would like the refrigeration industry to bypass HFCs and employ natural refrigerants as soon as possible.

Natural refrigerants are working fluids based on molecules that occur in nature. Examples are such substances as air, water, ammonia, hydrocarbons, and carbon dioxide. Nevertheless, the actual fluids used in refrigeration systems may very well be synthesized and will not necessary be extracted from nature. Ammonia, for instance, is synthesized in large quantities, and hydrocarbons undergo an extensive chemical processing procedure. Still, the cost of these fluids is much lower than that of HFC refrigerants, and they do not affect the environment in an unknown way. Also, the amount of fluid produced is negligible compared with the amount available in nature.

Studies of natural refrigerants are already underway. Air, water, ammonia, hydrocarbons, and carbon dioxide have a low or zero direct global-warming potential and zero ozone-depletion potential.

9. Read and translate the Text 5B.             

           Text 5B. VARIETIES OF NATURAL REFRIGERANTS   

Although natural refrigerants were used extensively in the early years of refrigeration technology, a number of technical and safety challenges caused them to be readily abandoned when CFCs became available. These challenges still exist today for air, water, ammonia, hydrocarbons, and carbon dioxide.

Air is used extensively as a refrigerant in aircraft cooling. Its advantages are that open systems require fewer heat exchangers, aircraft have compressed air available already, and systems tend to be low in weight. Its efficiency, however, is quite poor. Nonetheless, German railways have installed air cycles in the latest generation of high-speed trains, because of weight concerns and, most importantly, because maintenance time is very short. There is no cumbersome and time-consuming refrigerant reclamation and no system evacuation is required.

Water has the potential to be a very efficient refrigerant, but because it requires operation in a deep vacuum, its vapor density is quite low. This leads to costly large- volume vacuum tanks that must house all the machinery, such as heat exchangers and compressors. Furthermore, water’s pressure ratio is very high, imposing additional challenges for compressors that must operate in a deep vacuum.

The only applications where water is used as a refrigerant on a commercial basis are in large-capacity lithium-bromide water-absorption chillers. Over the last decade, a few large-scale water-vapor compression systems have been used commercially. One such system provides cooling to the LEGO factory in Denmark, another to a mine in South Africa. In both cases, open systems are employed, and the need for heat exchangers is eliminated by using direct- contact heat exchanger. Thus, the chilled water that circulates through the facility is also used as the refrigerant. Although various research projects are underway worldwide, a demanding compressor technology, the need for vacuum pumps, and degassing remain great challenges in terms of cost and design.

Ammonia is also a very good refrigerant and is used to a significant extent in large warehouses. Ammonia is toxic and, under certain limited conditions, flammable and even explosive. However, with its intense, pungent odor, it is a self-alarming refrigerant. Ammonia has emerged as a refrigerant for water chillers in Europe. These units are entirely self-contained, including a gas-tight cabinet that houses the entire unit and a water tank to dissolve any ammonia in case of a leak. These measures, to be sure, increase costs considerably.

Hydrocarbons are excellent refrigerants, but they are also flammable and explosive. In North America, any flammability risk is unacceptable, but some countries in Europe have less-stringent liability laws. Since the mid-1990s, virtually all refrigerator production in Germany has used hydrocarbons as the working fluid. Some heat pump manufacturers whose systems are installed entirely outdoors have followed suit, and some commercial installations have recently become publicly known. Nevertheless, the danger of fire remains an overriding concern.

Carbon dioxide is a refrigerant that operates at very high pressures in a transcritical cycle for most operating conditions. Thus, the refrigerant condenser of a conventional refrigeration system serves now as a cooler for supercritical fluid. Only after the expansion process is liquid carbon dioxide available to provide cooling capacity through evaporation. Because of the nature of the transcritical cycle, the efficiency of carbon dioxide is quite poor. However, this is its only disadvantage. All other characteristics of carbon dioxide are very favorable. It is environmentally safe, has low toxicity, and allows for extremely compact systems. The vapor pressure of CO₂ is approximately seven times higher than that of R-22. Moreover, the supercritical CO₂ has a higher density than subcritical fluids, so there is potential to reduce the size of hardware.

10. Read and translate the Text 5C.