. Two technicians are discussing retrofitting CFC-12 systems to HFC-134a standards. Technician A says that the process doesn’t take long but that mineral oil must be added before the retrofit is complete. Technician B says that 50 ounces of CFC-12 should be replaced with 50 ounces of HFC-134a. Who is correct? A. Neither Technician A nor B B. Both Technicians A and B C. Technician B D. Technician A
- Two technicians are discussing retrofitting CFC-12 systems to HFC-134a standards. Technician A says that the process doesn’t take long but that mineral oil must be added before the retrofit is complete. Technician B says that 50 ounces of CFC-12 should be replaced with 50 ounces of HFC-134a. Who is correct? A. Neither Technician A nor B B. Both Technicians A and B C. Technician B D. Technician A
The Correct Answer and Explanation is:
Correct Answer: A. Neither Technician A nor B
Explanation:
When retrofitting a CFC-12 (R-12) air conditioning or refrigeration system to use HFC-134a (R-134a), several technical considerations must be followed to ensure system compatibility, efficiency, and longevity. Neither Technician A nor Technician B provides accurate information.
Why Technician A is Incorrect:
Technician A claims that mineral oil must be added before completing the retrofit. This is incorrect. Mineral oil, commonly used in R-12 systems, is not compatible with HFC-134a. R-134a requires polyalkylene glycol (PAG) oil or ester-based (POE) oil, which are chemically compatible with HFC-134a and help ensure proper lubrication in the system. If mineral oil is left in the system, it may not mix well with R-134a, leading to poor oil return and potential compressor damage.
During a retrofit, mineral oil must be drained and replaced with the appropriate synthetic lubricant. Sometimes, a residual amount of mineral oil can remain if a POE oil is used, as POE is somewhat miscible with mineral oil. However, adding more mineral oil, as Technician A suggests, is contrary to best practices.
Why Technician B is Incorrect:
Technician B suggests that 50 ounces of R-12 should be replaced with 50 ounces of R-134a. This is also incorrect. R-134a has different thermodynamic properties and density compared to R-12, meaning it operates under different pressure-temperature relationships and does not require a 1:1 replacement by weight.
When retrofitting, the system typically needs less R-134a than the original R-12 charge. The correct amount of R-134a should be determined based on system performance (usually around 75–85% of the original R-12 charge by weight), and charging should be done by superheat/subcooling or pressures, not just by weight.
Conclusion:
Successful retrofitting requires replacing the incompatible mineral oil with a synthetic alternative and properly adjusting the refrigerant charge. As neither technician provided accurate information, the correct answer is A: Neither Technician A nor B.
