The refrigerant R134a is the chemical compound tetrafluoroethane comprising of two atoms of carbon, two atoms of hydrogen and four atoms of fluorine. Its chemical formula is CF3CH2F. The molecular weight of refrigerant R134a is 133.4 and its boiling point is -15.1 degree F.
Refrigerant R134a is a hydrofluorocarbon (HFC) that has zero potential to cause the depletion of the ozone layer and very little greenhouse effect. R134a is the nonflammable and non-explosive, has toxicity within limits and good chemical stability. It has somewhat high affinity for the moisture. The overall physical and thermodynamic properties of refrigerant R134a closely resemble with that of refrigerant R12. Due to all the above factors, R134a is considered to be an excellent replacement for R12 refrigerant.
R134a as Replacement for R12
Refrigerant R12 is the most widely used of all the refrigerants for different refrigeration and air conditioning applications. It is indeed very tough that any refrigerant will be able to replace this highly versatile refrigerant in different operating conditions. However, R134a has been able to replace R12 successfully in number of applications. Let us see how R134a compares well with R12.
1) Power required per ton of refrigeration: For the evaporator temperatures of -7 degree C and above the isentropic discharge temperature from the compressor for both the refrigerants is same. The total horsepower required per ton of refrigeration is also same. For the temperatures below -7 degree C, if R12 is replaced by R134a there will be significant loss of the refrigerating effect, and in such cases it is advisable to use the blends of refrigerants as the replacement instead of using R134a.
2) Low temperature application: For the instance where the saturation temperature is -15.08 degree F at the standard barometric pressure, and if the evaporator temperature is below 0 degree F, the pressure in the evaporator will be still well above the vacuum Thus the refrigerant R134a can be used for the low temperature applications also without the need to produce vacuum in low pressure side of the refrigeration system.
3) Heat transfer coefficients: The heat transfer coefficients for refrigerant R134a are higher than R12 in different conditions depending on the temperature. If the refrigerants exist in single liquid phase the heat transfer coefficient of R134a is higher by 27 to 37% and if they are in gaseous phase it is higher by 37 to 45%. If the refrigerants exist in two phases, liquid and gaseous, the heat transfer coefficient for R134a is higher by 28 to 34% in the evaporator and 35 to 41% in the condenser.
4) Miscibility of R134a with oil: Oil is used as the lubricant in the compressor of the refrigeration and the air conditioning system. When refrigerant is sucked and discharged by the compressor, it picks up some oil particles. It is important that the refrigerant is miscible with the oil so that it can be easily separated from it. Refrigerant R12 is highly miscible with mineral oil, while R134a is miscible with the synthetic oil or lubricant. Thus when the refrigeration equipment is being altered for the use of R134a instead of R12, all the mineral oil should be removed from the system and it should be replaced with ester based synthetic oil.
5) Refrigerating effect per pound of refrigerant: The refrigerating effect produced by one pound of R134a is about 22% more than that produced by R12. Thus the mass flow rate of the R134a required per ton of refrigeration is about 18% lesser than R12. This means for the given capacity of the refrigeration system the amount of R134a required is 18% less than if R12 was used. This means that in all the equipment where R12 is being replaced by R134a, the amount of refrigerant to be charge should be lesser than R12. However, the specific volume of R134a is somewhat more than R12, thus for the same amount of the refrigerant, the volume occupied by R134a is more than R12. Hence the increase in the refrigerating effect of R134a is offset by increase in its specific volume. Thus though R134a charged in the retrofitted systems should be about 5 to 10% lesser than R12, but the volume occupied by it is almost the same as R12, thus the compressor of the same capacity is required.
R12 to R134a Conversion
The process of retrofitting R12 system with R134a is quite an easy process. First of all, complete R12 should be removed from the system and recovered in the container. Then all the lubricating oil from the system should be removed and the maximum amount of oil allowed to be remained inside the system is 5% of the total amount of oil present in the system. The mineral oil should be replaced with ester based synthetic oil. The drier and the oil filter also should be replaced. The amount of R134a required in the system is about 90 to 95% of R12. Labels should be placed in the systems that have been retrofitted with R134a describing the new refrigerant and the lubricating oil.
- Book: Principles of Refrigeration by Roy J. Dossat, fourth edition, Prentice Hall