HFC 134a Application in Car Air Conditioning
HFC 134a is an environmentally friendly refrigerant widely used in car air conditioners. With the popularity of family cars, the consumption proportion of HFC 134a used in car air conditioners in car refrigerants has rapidly expanded.
Common refrigerants used in car air conditioners include CFC-12, HFC 134A, R-404A, R-407C, etc.
- CFC-12 (Freon-12), scientific name dichlorodifluoromethane, is a medium-pressure and medium-temperature refrigerant, almost non-toxic, non-flammable, non-explosive, and very safe. However, R-12 has been banned as a CFC refrigerant;
- HFC 134A (tetrafluoromethane): an environmentally friendly refrigerant recognized and recommended by most countries in the world;
- R-404A: mainly used in pure electric heating and cooling passenger car air conditioners, a medium temperature (medium pressure) refrigerant;
- R-407C: often used in refrigerated truck air conditioners, a mixed refrigerant.
Why is HFC 134a widely used in automotive air conditioners?
For a long time, due to its many excellent properties, CFC-12 has been used as a satisfactory refrigerant for automotive air conditioners. However, since the 1980s, people have gradually realized the destructive effect of freon substances on the ozone layer, so a series of conventions have been formulated to eliminate freon-based refrigerants within a time limit and seek new alternative refrigerants.
New substitutes are required to meet the following chemical properties: stable, inert:
- Health, safety and environmental considerations: non-toxic, non-flammable; low ODP value (preferably 0) and low GWP value; can be degraded in the environment, that is, has a short atmospheric life; does not reduce the energy efficiency of the equipment;
- Thermophysical properties: suitable critical point and boiling point, low freezing point; low molar vapor heat capacity, low viscosity, high thermal conductivity, low surface tension;
- miscible with lubricating oil in a large temperature range; high vapor dielectric constant (high electrical insulation); compatible with refrigeration equipment materials; easy to detect leaks;
- Have a mature synthesis process and have been industrialized, and the product price is acceptable to the application industry.
HFC 134a is a refrigerant that basically meets the above requirements and is also a more suitable substitute for automotive air-conditioning refrigerants. At present, various automotive air-conditioning industries in the world generally use HFC 134a to replace CFC-12 technology.
HFC 134a Technical requirements to replace CFC-12
1. Modification of automobile air conditioning technology using HFC 134a
In automobile air conditioning systems filled with HFC 134a, the compression ratio increases due to the decrease in evaporation pressure and the increase in condensation pressure, and the increase in compression ratio causes the gas transmission coefficient to decrease, resulting in a decrease in compressor displacement. The decrease in displacement causes the HFC 134a system flow rate to be lower than that of CFC-12.
In order to prevent the performance of automobile air conditioning systems using HFC 134a refrigerant from decreasing and to overcome the adverse effects of poor system matching performance and decreased condenser heat exchange performance caused by the difference in thermodynamic properties between HFC 134a and CFC-12, it is necessary to modify and improve the CFC-12 automobile air conditioning system when using HFC 134a.
2. Replacement of refrigeration lubricating oil
There is usually no pre-oil separator in the automobile air conditioning system. The lubricating oil brought out by the compressor exhaust circulates in the system with the refrigerant and then returns to the compressor, which requires that the lubricating oil and the refrigerant have good mutual solubility within the working temperature range to ensure good oil return.
HFC 134a and the mineral oil used in the CFC-12 system are not miscible. To solve this problem, some foreign chemical companies have developed two types of synthetic lubricating oils that are miscible with HFC 134a: PAG (Polyalkylene Glycols) and POE (PolyolEsters). Compared with traditional mineral oils, both have strong water absorption, and PAG has stronger water absorption. Because PAG oil has better low-temperature lubricity, it is widely used by major automobile air conditioning manufacturers.
3. Modification of the compressor
Higher exhaust pressure and lower suction pressure increase the compression ratio, reduce volumetric efficiency, reduce displacement, and lead to a decrease in cooling capacity. Therefore, the compressor needs to be improved, mainly in the following two aspects.
1) Increase the compression ratio: The compression ratio directly affects the volumetric efficiency and the required compression work. The higher the compression ratio, the lower the volumetric efficiency of the compressor. Therefore, the structure needs to be changed to increase the displacement.
2) Improve the sealing requirements: The increase in the suction and exhaust pressure difference, especially the increase in the exhaust pressure, will increase the compressor load and correspondingly increase the sealing requirements.
4. Replace rubber seals and hoses
HFC 134a refrigerant will cause the hoses and gaskets originally used for CFC-12 to change in material, causing refrigerant leakage; at the same time, the molecular volume of HFC 134a is smaller than that of CFC-12, and its permeability is stronger than that of CFC-12, so technical improvements should be made to the rubber hoses and sealing materials.
5. Improvement of the condenser
In automotive air-conditioning systems with limited space, high-efficiency condensers should be used as much as possible, such as parallel flow condensers, to improve heat exchange performance. In addition, the condenser fan air volume and fin heat exchange area can also be appropriately increased.
6. Evaporator improvement
Due to the decrease in system cooling capacity, the use of advanced laminated (also known as plate-fin) structure evaporators will help improve the performance of HFC 134a refrigeration systems.
7. Modification of thermal expansion valves
According to the matching requirements of HFC 134a automobile air-conditioning systems, adjust the static superheat of the thermal expansion valve to obtain the best matching effect.
8. Requirements for system cleanliness
HFC 134a and PAG, POE-based lubricants, have put forward higher requirements for the cleanliness of the refrigeration system. The pollutants in the air-conditioning system are mainly mineral oil, inorganic or organic matter, water, etc.
Therefore, in the production and maintenance of equipment, it is necessary to avoid the use of mineral oil and chlorine-containing raw materials, such as prohibiting the use of mineral oil to lubricate parts and the use of chlorine-containing solvents (such as trichloroethylene, tetrachloroethylene, methyl chloroform, etc.) to clean the system and parts.
9. HFC-134a recovery
Although HFC 134a has no destructive effect on the ozone layer, it is still a greenhouse-effect substance. Its GWP value is 1300 (CO equivalent); that is, emitting 1t HFC 134a is equivalent to emitting 1300t CO.
In order to protect the earth’s ecological environment, in addition to reducing system leakage as much as possible, the refrigerant remaining in the system should be carefully recovered when repairing the air-conditioning system. The recovery operation can use a dedicated HFC 134a recovery machine, which includes a gas compressor, a dryer, an acid remover and a liquid storage tank. The recovered refrigerant must be inspected and qualified by a professional department before it can be reused.
In the absence of a dedicated recovery machine, the recovery operation can be carried out with a small gas compressor. The collected refrigerant can be sent to a designated unit for purification and regeneration.
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