Introduction to the properties of common refrigerants

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A refrigerant, also known as a cooling medium, is the working substance in a refrigeration cycle. Heat is transferred through the phase changes of the refrigerant; it absorbs heat when vaporizing in the evaporator and releases heat when condensing in the condenser. There are over 80 substances that can be used as refrigerants, with ammonia, Freon types, water, and a few hydrocarbons being the most commonly used. In September 1987, a specialized international conference was held in Montreal, Canada, where the Montreal Protocol on Substances that Deplete the Ozone Layer was signed. It came into effect on January 1, 1989, imposing restrictions on the production of CFCs such as R11, R12, R113, R114, R115, R502, and R22. In June 1990, the second meeting of the parties to the protocol was held in London, where restrictions were imposed on the production of all CFCs, carbon tetrachloride (CCL4), and methyl chloroform (C2H3CL3). Developed parties among the signatories were required to completely cease the production of these substances by the year 2000, while developing parties could delay this deadline until 2010. In addition, a control schedule for the transitional substance HCFC after 2020 was proposed. R123 and R134a in HCFCs are alternatives to R12 and R22. Thermodynamic requirement 1: At atmospheric pressure, the evaporation temperature (boiling point) ts of the refrigerant must be low. This is a very important performance metric. The lower the ts, the lower temperature can be achieved, and at a certain evaporation temperature to, its evaporation pressure Po can be made higher than atmospheric pressure. To prevent air from entering the refrigeration system, making leaks easier to detect. 2 It is required that the condensation pressure Pc of the refrigerant at normal temperatures be as low as possible, in order to avoid excessive strength requirements for equipment such as compressors, condensers, and exhaust pipes that operate under high pressure. Furthermore, excessively high condensing pressure can also lead to refrigerant leakage and increase the power consumption. 3 For large piston compressors, it is necessary to have as high a cooling capacity per unit volume of refrigerant, qv, as possible, so as to reduce the size of the compressor and the amount of refrigerant that needs to circulate ； For small or micro compressors, the cooling capacity per unit volume can be lower ； For small centrifugal compressors as well, it is required that the refrigerant qv be low, in order to expand the range of applications for such compressors and to avoid the difficulties associated with manufacturing impellers of small size. 4 The critical temperature of the refrigerant should be higher, and the condensation temperature should be lower. The level of the critical temperature determines whether the refrigerant can be liquefied at normal temperatures or in the range of moderate low temperatures. 5 The condensation temperature is the lower limit of the range in which a refrigerant can be used; the lower the condensation temperature, the wider the range of applications for that refrigerant. Refrigerant, Molecular formula, Molecular weight u, Normal evaporation temperature ts(°C), Freezing point tf(°C), Critical temperature tkp(°C), Critical pressure PKP (absolute pressure), Adiabatic index K. Water (R718): H2O, 18.02, +100, ±0, +374.1, 225.6, 1.33. Ammonia (R717): NH3, 17.03, -33.4, -77.7, +132.4, 115.2, 1.31. R11: CFCl3, 137.39, +23.7, -111, +198, 44.6, 1.17. R12: CF2Cl2, 120.92, -29.8, -155, +111.5, 40.86, 1.15. R13: CF3Cl, 104.47, -81.5, -180, +28.8, 39.4. R22: CHF2Cl, 88.48, -40.8, -180, +96, 50.3, 1.19. R115: C2F5Cl, 154.48, -38, -106, +80, 33. Physicochemical requirements: 1. The viscosity of the refrigerant should be as low as possible in order to reduce flow resistance in pipes and enhance the heat transfer efficiency of heat exchange equipment. 2 The thermal conductivity of the refrigerant should be high to improve the efficiency of heat exchange equipment and reduce the heat transfer area. 3 Intermiscibility of refrigerant and oil: The property of a refrigerant to dissolve in lubricating oil should be analyzed from two aspects. If the refrigerant and lubricating oil can mix with each other freely, the advantage is that the lubricating oil can penetrate together with the refrigerant into all the components of the compressor, thereby creating favorable conditions for lubricating the machinery ； Moreover, an oil film is not likely to form on the heat exchange surfaces of the evaporator and condenser, thereby preventing heat transfer. Its disadvantage is that it causes an excessive amount of oil to be carried away by the compressor, and it can raise the evaporation temperature in the evaporator. Refrigerants that are partially or slightly soluble in oil have the advantage of requiring less oil to be carried away by the compressor, resulting in a more stable evaporation temperature in the evaporator. Its drawback is that an oil film that is difficult to remove forms on the heat exchange surfaces of the evaporator and condenser, affecting heat transfer. Effects of different categories of soluble refrigerants: 1. Insoluble: NH3, CO2, R13, R14, R15, SO2 – No effect. 2. Slightly soluble (intermingle in the compressor crankcase and condenser, but decompose in the evaporator): R22, R114, R152, R502 – Reduces the viscosity of lubricating oil when dissolved. 3. Completely soluble: R11, R12, R21, R113, hydrocarbons, CH3CI, R500 – Lowers the viscosity and freezing point of lubricating oil, causes paraffins in the oil to settle, and increases the evaporation temperature. 4. Should have a certain water-absorbing capacity to prevent the formation of “ice plugs” in the refrigeration system, which could disrupt normal operation. 5 It should have chemical stability: it should not burn or explode, and it should not decompose or deteriorate during use. At the same time, the refrigerant itself, or when mixed with oils, water, etc., should not have a significant corrosive effect on metals, and it should cause little swelling of sealing materials. Safety requirements: Since refrigerants may leak during operation, it is required that the working substance be harmless to human health, non-toxic, and non-irritating. Classification of refrigerants 1. The refrigerants widely used in compression-type refrigerators are ammonia, Freon, and hydrocarbons. Based on their chemical composition, refrigerants can be divided into five categories: inorganic compound refrigerants, Freons, saturated hydrocarbon refrigerants, unsaturated hydrocarbon refrigerants, and azeotropic mixture refrigerants. Based on the condensation pressure, refrigerants can be divided into three categories: high-temperature (low-pressure) refrigerants, medium-temperature (medium-pressure) refrigerants, and low-temperature (high-pressure) refrigerants. 2 Inorganic compound refrigerants: These types of refrigerants have been in use for a long time, such as ammonia (NH3), water (H2O), air, carbon dioxide (CO2), and sulfur dioxide (SO2). For inorganic compound refrigerants, the international designation consists of the letter R followed by three digits; the first digit is “7” and the last two digits represent the molecular weight. Such as R718... etc. 3 Freon (halocarbon refrigerants): Freon is a general term for derivatives of saturated hydrocarbons in which all or part of the hydrogen atoms are replaced by chlorine (Cl), fluorine (F), and bromine (Br). International standards use “R” as the code for such refrigerants, such as R22... etc. 4 Saturated hydrocarbons: These refrigerants mainly include methane, ethane, propane, butane, and cyclic organic compounds. Like Freon, these refrigerants are designated with the letter “R”; they are flammable and explosive, and pose a low level of safety. Such as R50, R170, R290... etc. 5 Unsaturated hydrocarbon refrigerants: These refrigerants mainly include ethylene (C2H4), propylene (C3H6) and their halogenated derivatives; the number following R in these compounds is usually “1”, such as R113, R1150, etc. 6 Azeotropic mixture refrigerants: These are refrigerants that consist of two or more different refrigerants mixed together in specific proportions to form an azeotic mixture. Such refrigerants maintain a constant evaporation temperature at a given pressure, with the composition of their gas or liquid phase remaining unchanged; however, their thermodynamic properties differ from those of the individual components before mixing. Azeotropic mixtures can be used to improve the properties of refrigerants. Such as R500, R502..., etc. 7 High-temperature, medium-temperature, and low-temperature refrigerants: They are classified based on the standard evaporation temperature of the refrigerant and its condensation pressure at room temperature. Refrigerant, Operating Temperature Range, Compressor Type, Applications, Notes: R717 (Ammonia): Medium and low temperatures; Piston and centrifugal types; Used for refrigeration and ice production in general refrigeration applications. R11: High temperatures; Centrifugal type; Used for air conditioning. R12: High, medium, and low temperatures; Piston, rotary, and centrifugal types; Used for refrigeration and air conditioning; High temperature range: 10–0°C. R13: Ultra-low temperatures; Piston and rotary types; Used for ultra-low temperature applications. R22: High, medium, and low temperatures; Piston, rotary, and centrifugal types; Used for air conditioning, refrigeration, and low-temperature applications; Medium temperature range: 0–20°C. R114: High temperatures; Piston type; Used for special air conditioning applications; Low temperature range: -20–60°C. R500: High and medium temperatures; Piston, rotary, and centrifugal types; Used for air conditioning and refrigeration; Ultra-low temperature range: -60–120°C. R502: High, medium, and low temperatures; Piston and rotary types; Used for air conditioning, refrigeration, and low-temperature applications. Characteristics of ammonia (R717): Ammonia (R717, NH3) is one of the medium-temperature refrigerants; its evaporation temperature is -33.4°C, and its operating range is from +5°C to -70°C. When the temperature of the cooling water reaches 30°C, the working pressure in the condenser generally does not exceed 1.5 MPa. 2 Ammonia has a high critical temperature (tkr=132°C). Ammonia has a high latent heat of vaporization, at atmospheric pressure it is 1164 KJ/Kg, and it also provides a high cooling capacity per unit volume, allowing ammonia compressors to be of smaller size. 3 Pure ammonia has no adverse effect on lubricating oils, but the presence of water reduces the lubricating effect of refrigerant oils. 4 Pure ammonia has no corrosive effect on steel, but when ammonia contains moisture, it will corrode copper and copper alloys (except phosphor bronze); therefore, copper and copper alloys are not used for pipes and valves in ammonia refrigeration systems. 5 Ammonia vapor is colorless and has a strong, pungent odor. Ammonia is highly toxic to the human body; when ammonia liquid splashes on the skin, it can cause frostbite. An explosion can occur when the volume of ammonia vapor in the air reaches 0.5-0.6%. Therefore, the ammonia concentration in the air inside the machine room must not exceed 0.02 mg/L. 6 Ammonia does not burn easily at room temperature, but when heated to 350°C, it decomposes into nitrogen and hydrogen gas; the hydrogen gas can explode when mixed with oxygen in the air. Properties of Freon 1: Freon is a transparent, odorless, non-toxic refrigerant that is not flammable or explosive, and it has chemical stability. Fluorine-based refrigerants with different chemical compositions and structures exhibit significant variations in their thermodynamic properties, allowing them to be used in high-temperature, medium-temperature, and low-temperature refrigerators to meet the requirements of various cooling temperatures. 2. Freon has low solubility in water; when moisture enters a refrigeration system, acidic substances are formed, which can easily cause \"ice blockages\" in low-temperature systems, blocking the throttle valves or pipes. Additionally, to prevent fluorocarbons from reacting with natural rubber, the device should use NBR as gaskets or seals. 3 Commonly used Freon refrigerants include R12, R22, R502, and R1341a, as other types of refrigerants have now been discontinued or banned. No explanation is given here. 4. Freon 12 (CF2Cl2, R12): This is one of the most widely used freon refrigerants, and it is extensively employed in medium and small-sized food storage facilities, household refrigerators, as well as in cooling systems for water and road transport. R12 has good thermodynamic properties, with a low refrigeration pressure; the pressure using air cooling or natural condensation is approximately 0.8–1.2 KPa. The standard evaporation temperature of R12 is -29°C; it is a medium-temperature refrigerant. When used in medium and small piston compressors, it can achieve low temperatures of -70°C. Large centrifugal compressors can achieve a low temperature of -80°C. In recent years, R134a has become the replacement refrigerant for refrigerators. 5. Freon 22 (CHF2Cl, R22): This is one of the most widely used freon refrigerants, primarily employed in household air conditioners and low-temperature refrigerators. The thermodynamic properties of R22 are similar to those of ammonia. The standard vaporization temperature is -40.8°C, and the condensation pressure is usually no more than 1.6 MPa. R22 is non-flammable and non-explosive, making it safer and more reliable than ammonia in use. The specific capacity of R22 is about 60% higher than that of R12; its cooling capacity per unit volume and saturation pressure at low temperatures are both higher than those of R12 and ammonia. In recent years, R134a has been widely used as a substitute for refrigerants in large air-conditioning chiller units. 6 Freon 502 (R502): R502 is an azeotropic mixture composed of R12 and R22 in proportions of 51.2% and 48.8%, respectively. Compared to R115 and R22, R502 has better thermodynamic properties and is more suitable for low temperatures. The standard evaporation temperature of R502 is -45.6°C, and its normal operating pressure is similar to that of R22. Under the same operating conditions, its cooling capacity per unit volume is higher than that of R22, but its exhaust temperature is lower than that of R22. R502 is used in fully enclosed, semi-enclosed, or certain medium and small refrigeration units, with an evaporation temperature that can drop as low as -55°C. R502 is widely used in refrigerators. 7 Freon 134a (C2H2F4, R134a): It is a relatively new type of refrigerant, with an evaporation temperature of -26.5°C. Its main thermodynamic properties are similar to those of R12; it does not destroy the ozone layer in the atmosphere, and it is an environmentally friendly refrigerant that has been promoted in recent years, but it does cause the greenhouse effect. It is a relatively ideal R12 replacement refrigerant. 8 The relationship between Freon and water: Freon and water are almost completely immiscible with each other, having an extremely low solubility for water. The moisture that enters the device from the low-temperature side is in the form of water vapor; it is compressed together with the Freon vapor and enters the condenser, where it condenses into liquid water. This water, in the form of droplets, mixes with the Freon liquid. At the expansion valve, it freezes into ice due to the low temperature, blocking the valve and preventing the refrigeration system from functioning properly. Moisture can also cause Freon to hydrolyze, producing acid and leading to a \"copper plating\" phenomenon within the refrigeration system. 9 The relationship between Freon and lubricating oil: Generally, it is soluble in refrigeration oil, but at high temperatures, Freon breaks down from the refrigeration oil. That is why large chillers are equipped with heaters in their oil tanks to maintain a certain temperature and prevent the dissolution of Freon. Refrigerant replacement: Refrigerants include R12, R22, R134a, R152a, R600a, H-01, RH, H, R404, R401, as well as mixed refrigerants of R152a and R22. Commonly used refrigerants are R12, R22, R134a, R152a, and R600a; R12 can generally be used as well. R22 can be replaced by R152a, H-01, RH, H, and R404. Mixed refrigerants of R152a and R22 can also be replaced by R12. R404: A mixed refrigerant of R152a and R22, which can be replaced with R22. R12 and R22 generally cannot be substituted for each other. Common refrigerants for refrigerators include R12, R134, R600, and the R152/R22 azeotrope. The common refrigerant for air conditioners is R22, while a newer type of refrigerant is R404. R12 can be used as a substitute for R12 in refrigerator repairs. When replacing a R134 system, it is necessary to replace the compressor system as well, mainly due to differences in the refrigeration oil; therefore, the pipelines need to be cleaned. Air conditioner repair – designed for R404; manufacturers do not allow R22 to be used as a substitute.