Refrigerants, Antifreezes, and Cooling Liquids Information
A primary refrigerant uses a thermodynamic or adiabatic process (evaporation-condensation cycles) to remove heat and cool a region. The ideal fluid has favorable thermodynamic properties, is noncorrosive, nonflammable, nontoxic, stable, environmentally safe, and economical to produce. The trade-offs between these physical properties and environmental considerations play a large role in the selection of refrigerants.
Refrigerant Designation "R-"Number
The designation and safety classification of primary refrigerants is according to American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 34. ASHRAE assigns an R-number to chemical refrigerants which are determined systematically as follows:
Pure Fluorocarbons (FC)
Refrigerants composed of pure fluorocarbons (FC) are identified by the alpha character "R-"followed by the corresponding HFC-number. The HFC-number describes the number of fluorine atoms, hydrogen atoms, and carbon atoms in the molecular formula. The rightmost value represents the number of fluorine atoms, to the left is the number of hydrogen atoms plus 1, and two places to the left is the number of carbon atoms less one. (e.g. R-134 represents Tetrafluoroethane CHF2-CHF2)
FC refrigerants include other haloalkane compounds such as chlorofluorocarbon (CFC), which is a chlorinated fluorocarbon; hydrofluorocarbon (HFC), a hydrogenated fluorocarbon; and hydrochlorofluorocarbon (HCFC), which is a CFC substitute that is more environmentally friendly, since not all of the hydrogen has been replaced by chlorine or fluorine atoms.
(Miscellaneous organic compounds are also designated by their HFC-number)
Isomers are chemical compounds with the same HFC number, but different chemical structures. They are designated by a lower case letter. (e.g. R-134a represents CH2F-CF3). If more than one isomer exists they are assigned lower case alpha letters increasing in order with respect to the increase in asymmetry. (e.g. R-xxx, R-xxxa, Rxxxb, ect.)
Refrigerant blends are designated by accepted ASHRAE nomenclature. They include zeotropic mixtures as well as azeotropic mixtures. Blends having the same pure components with different compositions are distinguished by an upper case letter. (e.g. 404A)
Zeotropic mixtures, designated by R-4xxx, are mixtures never have the same vapor phase and liquid phase composition. This is due to the vapor-liquid equilibrium state of the various components.
Azeotropic mixtures, designated by R-5xxx, are also referred to as constant boiling mixtures. They may not be separated by fractional distillation as boiling produces a vapor phase has the same proportion of constituents as the liquid phase.
Inorganic compounds are designated by the R-7xx series. The numerical code is formed by adding the sum of the relative molecular mass of the components to 700. (e.g. Ammonia (molecular mass 17) = R717)
Refrigerant Safety Classification
The designation and safety classification of standards is according to ASHRAE Standard 34. It is based on a two place alphanumeric code describing the flammability and toxicity as follows:
Secondary refrigerants, such as antifreeze, provide cooling solely through heat transfer. They are a heat transfer medium that dissipates heat through the use of a heat exchanger. Fluid medium used as secondary refrigerants include water, air, hydrocarbons, ammonia, and carbon dioxide. Antifreeze additives are used in aqueous systems where corrosion or freezing is a concern.
Circulating coolants are a type of secondary refrigerant used in industrial processes. The single phase heat transfer fluids may be composed of either synthetic or non-synthetic fluids
Non-synthetic products consist of petroleum or mineral oils and may include water. Mineral oil and petroleum based fluids are used as a coolant in mechanical gears and assemblies as they provide lubrication.
Synthetic products include ester and diester compounds, polyglycols and water-glycol fluids, fluorocarbons, and silicone-based greases and oils. They are formulated from alkaline organic and inorganic compounds and used in diluted form with concentrations ranging from 3% to 10%. Synthetic products are more expensive than non-synthetic fluids, but provide better fire resistance and cooling performance.
Refrigerants, antifreezes, and cooling liquids are used in a wide variety of industries, including automotive, marine, food and beverage, medical, and aerospace. Typically applications include refrigeration systems, cooling of combustion engines, and process cooling for equipment such as gears, drives, pistons, and compressors.
Compressor based refrigeration systems use the thermodynamic properties or a refrigerant to cool a region. The adiabatic process uses a compressor, condenser, expansion valve, and evaporator to complete the cycle.
Refrigerants in the vapor state are compressed and fed into a condenser.
- The condenser acts as a heat exchanger, dissipating heat, while refrigerant condensate collects forming a liquid phase.
- The expansion valve is then used to depressurize the fluid evaporating the liquid phase forming a super cooled vapor in the evaporator coil.
The evaporator draws in heat from the region to be cooled before the refrigerant is cycled back through the closed system.
Refrigeration Cycle Video Animation:
Video Credit: AZVocationalTraining
Combustion Engine Coolants
Antifreeze, a secondary refrigerant, is a solution of glycol-based chemicals in water and can withstand temperatures below 0 degrees Celsius. Antifreeze is used to cool the engine block in combustion engines. It is circulated into a cooling chamber where it makes contact with and cools the engine block. A thermostat, thermally actuated valve, opens when the temperature of the antifreeze exceeds the threshold temperature where it is fed into a radiator coil. The radiator cools the fluid before it is pumped back through the system.
Process coolants are heat transfer fluids used to dissipate heat away from a device or system. There are a wide range of applications for process coolants including machining operations, nuclear reactors, electrical transformers, mechanical drives, and food processing. In most cases the heat transfer fluids dissipates heat in order to prevent undesired temperature induced effects. Heat is dissipated either by a heat sink or are otherwise circulated through a heat exchanger via a heat pump.
Many refrigerants, antifreezes, and cooling liquids based on fluorocarbons are being phased out in many refrigeration applications because of their effect on the environment. CFCs are damaging to the ozone; HCFCs and HFCs are safer for the ozone, but also contribute to greenhouse gases.
Anhydrous ammonia is widely used as a refrigerant in compressor based refrigeration systems in the food and beverage industry. It has also been used as a replacement for CFC's, however the environmental benefits of using ammonia must be weighed against the higher safety risks. It is a toxic gas under ambient conditions and when pressurized risk of exposure increases. The Occupational Safety and Health Administration (OSHA) Permissible Exposure Level (PEL) is 50 parts per million (ppm) and the National Institute of Occupational Safety and Health (NIOSH) has established an Immediately Dangerous to Life and Health (IDLH) level of 300 (ppm).
Hydrocarbon refrigerants are non-toxic, have zero ODP (Ozone Depletion Potential), and very low GWP (Global Warming Potential). The flammability concerns are outweighed by their low environmental impact and excellent thermodynamic properties.
BRE IP18/00 - This standard gives preliminary guidance on how to overcome problems associated with assumptions made for safe locations for ammonia refrigeration systems.
ISO 817 - This International Standard provides an unambiguous system for assigning designations to refrigerants. It also establishes a system for assigning a safety classification to refrigerants based on toxicity and flammability data, and provides a means of determining the refrigerant concentration limit. Tables listing the refrigerant designations, safety classifications and the refrigerant concentration limits are included based on data made available.