CSLB C-20 HVAC ContractorCooling and RefrigerationMedium
A technician is charging a system using R-407C, a zeotropic refrigerant blend. Why must this refrigerant be added to the system as a liquid rather than a vapor?
- AR-407C exists only in liquid form at room temperature and has no vapor phase
- BCharging as a vapor could cause fractionation, changing the blend's composition and altering system performance
- CLiquid charging is simply faster and more convenient for the technician
- DVapor charging of any refrigerant is prohibited under federal law
Show answer & explanationAnswer & explanation
Correct answer: B. Charging as a vapor could cause fractionation, changing the blend's composition and altering system performance
R-407C is a zeotropic blend composed of multiple refrigerant components with different boiling points, creating temperature glide. If charged as a vapor, the more volatile components boil off first, changing the blend ratio (fractionation) and shifting the refrigerant's performance characteristics. Charging as a liquid preserves the intended blend composition.
Why the other options are wrong
- A. R-407C has both liquid and vapor phases like other refrigerants; it is not liquid-only at room temperature.
- C. Speed is not the technical reason; it's about preserving blend composition.
- D. There is no blanket federal law against vapor charging; the issue is specific to zeotropic blends.
Zeotropic Blend Fractionation
Zeotropic refrigerant blends (like R-407C) contain components with different boiling points and must be charged as a liquid to prevent the mixture from separating (fractionating) during charging.
- Zeotropic blends exhibit temperature glide
- Vapor charging removes lighter components first, altering blend ratio
- Near-azeotropic blends like R-410A have minimal glide but manufacturers still recommend liquid charging
Memory trick: Blends stay true only charged in liquid brew.