Alternative one-component refrigerants

R717 refrigerant. Chemical formula NH₃ (ammonia). Related to HFC group. Among "natural" refrigerants, R717 holds one of the first places as an alternative to R22 and R502. Production of ammonia all over the world reaches 120 million tons, and only small portion of it (up to 5%) is used in the refrigeration equipment.

Ammonia does not deplete ozone layer (ODP = 0) and does not directly contribute to increase of greenhouse effect (GWP = 0). Gas with sharp rank smell harmful for the human body. Tolerance concentration in the air is 0,02 mg/dm³, which corresponds to its inclusion volume fraction 0,0028%. In combination with the air at volume fraction 16...26,8% and availability of the open fire ammonia is explosive. Ignition temperature with air is 651 ^oС.

Ammonia vapors are lighter that the air, it is well soluble in water (one unit of water can resolve 700 units of ammonia which excludes moisture freezing in the system). Ammonia hardly resolves mineral oils. It does not effect ferrous metals, aluminum and phosphorous bronze, but at the presence of moisture it destroys non-ferrous metals (zinc, copper and its alloys). Moisture mass proportion in ammonia should not exceed 0,2%.

According to thermodynamic qualities, ammonia is one of the best refrigerants: as to bulk cold-productivity it considerably exceeds R12, R11, R22 and R502, has higher heat transfer coefficient which allows to use pipes of smaller diameter under assigned cold-productivity in the heat transfer apparatus. Characteristics of R717 refrigerant on the saturation line are given in Appendix 6. Due to ammonia sharp smell, leakage in the refrigeration system can be easily detected by the personnel. Due to these very reasons, R717 found its wide use in large refrigeration facilities. R717 refrigerant has low cost.

One of the disadvantages f ammonia is higher value of adiabatic line (1,31) compared to R22 (1,18) and R12 (1,14), which causes considerable increase of discharge temperature. In connection with this, they present strict requirements to thermal stability of refrigeration oils used in combination with ammonia during a long period of time at operation of a refrigeration facility. Condenser should have developed heat-transfer area which results in increase of its steel intensity.

Characteristics of R717 refrigerant, related to HFC group, as well as of some refrigerants of CFC and HCFC groups on the saturation line are given in the table.

Ammonia has extremely high value of heat of vaporization and, as a consequence, comparatively small mass consumption of the circulating refrigerant (13... 15% compared to R22). It is a profitable quality for large refrigeration facilities but it makes regulation of ammonia delivery to the evaporator under low power difficult.

Additional difficulties while refrigeration equipment production are caused by ammonia high activity towards copper and copper alloys; that's why, pipelines, heat-exchangers and accessories are made of steel. Due to ammonia high toxicity and combustibility, welded connections are thoroughly controlled. Due to high conductivity of R717, creation of half-hermetic and hermetic compressors is impeded. At the same time, for industrial refrigeration facilities with the power more than 20 kWt, ammonia is the best alternative.

Many heat pumps are operated on ammonia. Thus, in Norway there's operating a heat pump with the power of 200 kWt. In the system there's circulating about 30kg of ammonia, there are used half-hermetic compressor and plate heat-exchange apparatus. Control system of ammonia leakage and effective ventilation are provided.

Ammonia water coolants-chillers with plate apparatus of "Alfa-Laval" company, developed by "York Refrigeration" company, are characterized by minimum quantity of a filled refrigerant. Thus, in high-temperature refrigerants HTLS, refrigerant filling constitutes 3,7...8,5 kg with cold-productivity correspondingly 60... 140 kWt and consumed power 14...35 kWt, in mid-temperature ones MTLS - 3,1...6 kg with cold-productivity 32...63 kWt and consumed power 12...24 kWt, in low-temperature ones LTLS - 2,6...4,5 kg with cold-productivity 9... 19 kWt and consumed power 7...17 kWt.

In heat pumps of HPLS type the refrigerant filling is 3...6,7 kg at cold-productivity correspondingly 37...87 kWt and consumed power 12...31 kWt.

Considered refrigerants and heat pumps have not big overall dimensions (length - 1170mm, width - 800mm, height - 1550mm).

Using of ammonia in small refrigerating machines for commercial facilities is expected.

Oils used at the moment are not resolved in ammonia; that's why, it is necessary to insert into the refrigeration machine model oil eliminators, which increases its cost. Recently there is carried out intensive research on development of the oil soluble in ammonia and creation of refrigeration equipment with the "dry" evaporator. Oil solubility in ammonia excludes formation of the oil skin on heat-exchange surfaces which increases heat transfer coefficient from 2700 up to 9100В tons/(m²*К).

Progress achieved recently in development of refrigerating oils soluble in ammonia R717 can fundamentally change tendencies in development of refrigerating machine-building.

R744 refrigerant. Chemical formula СО₂ (carbon dioxide). Related to HFC group. Cheap non-toxic incombustible and ecologically clean (ODP = 0, GWP= 1) substance. The cost of carbon dioxide is 100...120 times lower than R134a.

Carbon dioxide has low critical temperature (31 ^oС), comparatively high triple point temperature (-56 ^oС), big pressures in triple point (more than 0,5 МPа) and critical pressure (7,39 МPа). It can serve as an alternative refrigerant. It is contained in the atmosphere and biosphere of the Earth, has the following advantages: low price, simple servicing, compatibility with mineral oils, insulating and structural materials. At the same time, while using carbon dioxide it is required water cooling of the refrigerating machine condenser, increases steel intensity of the refrigerating facility (compared to steel intensity of facilities operating on haloid-derivative refrigerants). High critical pressure has also positive aspect connected with low level of compression and as a consequence, the compressor's effectiveness becomes considerable. There exist possible perspectives of using carbon dioxide in low temperature two=spool facilities and air-conditioning systems in automobiles and trains. It is also suggested to use in domestic refrigerators and heat pumps.

R728 refrigerant. Chemical formula N₂. Related to HFC group. Liquid nitrogen is used as cryogenic refrigerating substance in some countries (England, USA, etc.). Under the atmosphere pressure , boiling temperature of nitrogen is -196 ^oС, and specific heat of vaporization is 199 кJoule/kg. Non-toxic and ecologically clean (ODP = О, GWP = 0) refrigerant. Cryogenic method of cooling with liquid nitrogen presupposes its single use. This method is realized in the machineless running system where actuating agent does not perform closed cyclic process.

In connection with discovery in Russia of considerable reserves (about 340 billion m³) of underground gases with high content of nitrogen, cost of natural nitrogen becomes lower compared to nitrogen derived using the method of air liquation and separation, which will allow to apply in industrial scale machineless method of cooling in the machines for quick freezing of food products. In order to increase the level of using low-temperature potential of gasiform nitrogen, specialists have suggested the system of mobile cold supply.

R290 refrigerant. Chemical formula С₃Н₈ (propane). Related to HFC group. Ozone depletion potential ODP = 0, global warming potential GWP = 3. It is characterized by low cost and it is non-toxic. While using this refrigerant, there is no problem with selection of structural materials for the parts of compressor, condenser and evaporator. Propane is easily resolved in mineral oils. Boiling point under the atmospheric pressure is -42,1 ^oС. The advantage of propane is also low temperature at the exit out of the compressor. However, propane as a refrigerant has two fundamental disadvantages. First, it is fire risky; second, size of the compressor should be bigger than while using of R22 of the assigned cold-productivity in the refrigeration machine.

In industrial refrigeration facilities, propane has been used for many years already. During recent years, more and more often it is suggested to use propane in refrigeration transport facilities.

In Germany in 1994 there were manufactured more than 1000 domestic refrigerators on propane, isobutane and their blends. Similar refrigerators are being manufactured in China, Brazil, Argentina, India, Turkey and Chile. According to estimations of developers of this equipment, refrigerating factor at using of hydrocarbons is almost the same (+(-)1%), as while operating on R12. Only small alterations in the compressor design are required. There are used the same mineral oils, the same insulation, the same sealing materials, pipes of the same diameter, the procedure of servicing is almost unchanged. Discharge temperature becomes lower than while operating on R22 or R502. Propane can be immediately charged into the system where there was ozone-depleting refrigerant before. As the research has shown, in this case up to 10% of cold-productivity is lost if in the system there was R22 before, and 15% is lost, if R502. A number of specialists believe that this reduction could be also prevented by adding polypropylene to propane.

In the USA it is prohibited to use hydrocarbons in domestic refrigerators. American Environment Protection Agency is foreseeing up to 30 000 fires per year in case of their use.

In New Zealand hydrocarbons are allowed to use in commercial refrigeration equipment.

While locating commercial refrigeration equipment operating on propane in public places, it is necessary to follow safety rules. In case of exceeding of assigned charging norms (more than 2,5 kg of R290), refrigeration equipment should be installed in a separate, specially equipped place which increases capital outlays.

Propane is used also in heat pumps. In Lillehammer (Norway) a heat pump is operating on propane with power of 45 kWt with half-hermetic compressor and plate heat-exchangers. In the system of a heat pump propane mass is a little more than 1 kg; equipment is placed in a separate building. According to specialists' opinion, control of fire risk is possible.

R600a refrigerant. Chemical formula С4Н10 (isobutane). In comparison with R12 and R134a refrigerants, isobutane has considerable ecological advantages. This natural gas does not deplete ozone layer (ODP = 0) and does not cause greenhouse effect (GWP = 0,001). Mass of the refrigerant circulating in the refrigeration aggregate at butane using, is considerably reduced (approximately by 30%). Specific mass of isobutane is twice bigger than specific mass of the air - gaseous R600a is spread along the ground. Isobutane is easily soluble in mineral oil, it has higher refrigerating factor that R12 and it reduces energy consumption. Physical qualities of R600a compared to R12 and R134a refrigerants are given in the table.

Isobutane is combustible [refrigerant of the 3rd class (It/DIN 8975)], easily inflammable and explosive, but only in combination with wax at the refrigerant volume fraction 1,3...8,5%. Lower limit of explosive risk (1,3%) corresponds to 31 gm of R600a on 1 m³ of the air; upper limit (8,5%) - 205 gm of R600a on 1 m³ of the air. Inflammation temperature is 460 ^oС.

At the moment, Italian and German companies used R600a in domestic refrigeration equipment. In particular, companies Necci compressori" and "Zanussi" of the international concern "Electrolux compressors" manufacture compressors operating on isobutane. Refrigeration aggregates with R600a are characterized by lower level of noise due to low pressure in the refrigerant operating contour.

The use of isobutane in the existing refrigeration equipment is connected with the necessity of substitution of the compressors with compressors of bigger productivity because according to specific volumetric refrigerating effect, R600a considerably exceeds R12 refrigerant (almost twice).

R125 refrigerant. Chemical formula СНР₂СР₃. Related to HFC group, it does not contain chlorine. Ozone depletion potential ODP = 0, global warming potential GWP = 860. Boiling point at the atmospheric pressure is -48,1 ^oС (Appendices 7, 18). The refrigerant is reccomended to use in a clear type or as a component of alternative blends to substitute R22, R502 and R12. R125 refrigerant is fire safe. As to energy performance and heat-transfer coefficient, it plays back to R22 and R502 refrigerants. Compared to R502, it has more abrupt curve characterizing dependence of saturated vapors pressure on temperature, low critical temperature and not high specific temperature of heat-vapor buildup which causes the necessity of increase of the compression level. In connection with this, possibilities of R125 use in the refrigeration equipment using condensers with air cooling are extremely limited.

At the same time, R125 has lower (compared to R22 and R502) discharge temperature and high mass flow under low suction pressures. Piston refrigeration compressors operating on R125 are characterized by optimal cylinder charging, and as a consequence, they have big volume efficiency.

R134a refrigerant. Chemical formula CF₃CFH₂ (tetrafluoroethane). R134a molecule has smaller size than R12 molecule which makes danger of leakage more considerable. Related to HFC group. Ozone depletion potential ODP = 0, global warming potential GWP = 1300. Physical qualities of R134a are given in table, and characteristics on the saturation line - in Appendix 8.

R134a refrigerant is non-toxic and does not flare up within the whole range of operational temperatures. However, in case of air ingress into the system and compression there can be formed combustible blends. You shouldn't mix R134a with R12 because there is formed azeotrope blend of high pressure with mass proportions of the components 50 and 50%. Saturated vapor pressure of this refrigerant is a bit higher than that of R12 (correspondingly 1,16 and 1,08 МPа at 45 ^oС). R134a vapor is decomposed under the influence of flame with formation of poisonous and irritating contextures, such as fluorohydrogen.

According to classification ASHRAE, this product is related to class А1. In middle-temperature equipment (boiling point -7 ^oС and higher), R134a has operation performance close to R12.

R134a is characterized by not high discharge temperature (it is approximately 8...10^oС lower than for R12) and not high values of saturated vapors pressure.

In refrigeration facilities operating under boiling point lower than -15 ^oС, energy data of R134a are worse than those of R12 ( specific volumetric refrigerating effect is 6% lower at -18 ^oС), and refrigerating factor. In such facilities it is purposeful to use refrigerants with lower normal boiling point or compressor with increased time volume circumscribed by pistons.

In middle temperature refrigeration facilities and air-conditioning systems, refrigerating factor of R134a is equal to the factor for R12 or higher than that.

In high-temperature refrigeration facilities, specific cold-productivity when operating on R134a is also a bit higher (6% more at tо = t₀ ^oС), than that of R12. Ranges of using R134a refrigerant are shown in Fig., and dependence of cold-productivity and refrigerating factor on boiling point is shown further on the figure.

Due to considerable global warming potential GWP, it is recommended to use R134a in hermetic refrigerating systems. Contribution of R134a to greenhouse effect is 1300 times as bigger as that of СО₂. Thus, air venting of R134a one filling from a domestic refrigerator (about 140 gm) corresponds to ejection of 170 kg of СО₂. In Europe about 448 gm of СО₂ is formed at production of 1 kWt/h of power, that is, this ejection corresponds to production of 350 kWt*h of power.

For operation with R134a refrigerant, only polyester refrigeration oils characterized by increased hygroscopic property are recommended.

R134a is widely used all over the world as a main substitute of R12 fir refrigeration equipment operating within middle-temperature range. It is used in automobile air-conditioners, domestic refrigerators, commercial refrigeration middle-temperature equipment, industrial facilities, air-conditioning systems in buildings and industrial areas, as well as on refrigeration transport. The refrigerant can be used also for retrophite of the equipment operating at lower temperatures. However, in this case, if the compressor is not substituted, refrigeration system can have reduced cold-productivity.

R134a is compatible with a number of sealing materials, in particular, with pads made of such materials as "Buna-N", "Khailalon 48", "Neopren", "Nordel". As the analysis carried out by company "Du Pont" showed, change of mass and linear swelling of such materials used in domestic refrigeration equipment as phenoplast and polyamide chocks, textolite, paronite and polyethylene terephtalate skins, in combination of SUVA R134a with polyester oil "Castrol SW100" at 100 ^oС during 2 weeks have been insignificant.

Analysis of foreign publications and the results of the research of native specialists indicate that substitution of R12 for R134a, which has high global warming potential GWP, in refrigeration compressors is connected with solution of a number of technical tasks basic of which are:

• Improvement of volume and energy performance of hermetic compressors;
• Increasing of chemical constancy of enamel wires of the hermetic compressor electric motor;
• Increasing of dehumidifying ability of filter-dehydrators due to high hygroscopic property of R134a system - synthetic oil.

All this should result in considerable increase of the cost of refrigeration equipment. At the same time, in water-cooling plants with screw and centrifugal compressors the use of R134a has certain perspectives.

R143a refrigerant. Chemical formula CF₃-СН₃ (trifluoroethane) Related to HFC group.

R143a has ozone depletion potential ODP = 0 and comparatively high global warming potential GWP = 1000, non-toxic and fire-safe, it does not interact with structural and packing materials. Three hydrogen atoms in R143a molecule contribute to good solubility in mineral oils. Specific heat of vaporization is 19,88 kJoule/mole at normal boiling point which is a bit higher than for R125 (18,82 kJoule/mole). Discharge temperature is lower than that of R12, R22 and R502. As energy analysis has showed, energy effectiveness of two-step cycle with R143a is close to the effectiveness of the cycle with R502, lower than that of R22, and higher than that of R125. R143a refrigerant belongs to composition of multi-component alternative blends suggested for substitution of R12, R22 and R502.

R32 refrigerant. Chemical formula CF₂H₂ (difluoromethane). Related to HFC group. R32 characteristics are given in Appendix 9. R32 has ozone depletion potential ODP = О and low global warming potential GWP = 220, in comparison with R125 and R143a. Non-toxic, fire risky. It has high specific heat of vaporization 20,37 kJoule/mole at normal boiling point and abrupt dependence of saturated vapors pressure on the temperature and, as a result, R32 is characterized by high discharge temperature, which is the highest of all alternative refrigerants, except ammonia. R32 is soluble in polyester oils.

When used in refrigeration facilities, R32 is characterized by high cold-productivity and energy effectiveness, but it is slightly inferior to R22 and R717. High level of R32 compression causes necessity of considerable alteration in the refrigeration facility design at retrophite, and as a consequence, it results in increasing of its metal intensity and cost. Therefore, R32 is recommended to use mainly as a component of alternative working blend. Due to small size of R32 molecule compared to molecules of ethane row refrigerants , R32 selective leakage through looseness in the refrigerating system is possible, which can change the composition of multi-component working blend.