R-22 to R-407C Retrofit: Glide, Oil, and Charging Method
A supermarket medium-temperature display case runs on R-22 with mineral oil and a TXV. The owner wants to convert to R-407C rather than replace the case. The system holds 18 lb of R-22 and uses a semi-hermetic reciprocating compressor, copper tubing, a suction filter, and a replaceable-core liquid-line drier. Design is 25°F evaporator and 110°F condensing, with 10°F superheat at the evaporator outlet. After recovery, the technician plans to drain the mineral oil, add POE, replace the liquid-line drier, evacuate to 500 microns, and charge 18 lb of R-407C as a vapor through the suction service valve. The site PT chart shows that at 70 psig the bubble point is 38°F and the dew point is 47°F. A test run gives 70 psig suction and 56°F suction-line temperature at the evaporator outlet. The technician sets superheat to 10°F using the 38°F bubble-point value.
1. Calculate the correct superheat and the error if 10°F is set from the bubble point. What damage can result?
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At 70 psig, the evaporator superheat is measured from the dew point for a zeotropic blend: 56°F - 47°F = 9°F. That is close to the 10°F target.
If the technician uses the bubble point: target line temperature = 38°F + 10°F = 48°F. The actual dew-point superheat at 48°F would be 48°F - 47°F = 1°F. That is flooding, not 10°F superheat. Consequences include liquid returning to the compressor, oil dilution, high oil carryover, and possible slugging or compressor damage.
Use dew point for superheat and bubble point for subcooling. Also take the pressure at the bulb location or correct for suction-line pressure drop; using compressor suction pressure can bias the result.
2. Critique the retrofit plan: oil handling, driers, charging method, and omitted steps.
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The plan has three serious errors and several omissions.
Errors:
- Charging R-407C as a vapor. R-407C is zeotropic with about 9°F glide; vapor charging fractionates the blend and leaves a charge with the wrong composition. Charge liquid by weight.
- Treating a mineral-oil drain as an oil change. Residual mineral oil remains in the compressor, traps, and lines; POE will gradually clean it out, so plan a follow-up oil and drier change.
- Assuming the existing TXV and compressor are approved without checking. Capacity, mass flow, and valve trim may differ.
Omissions:
- Use fresh, sealed POE and minimize exposure to air; POE is hygroscopic.
- Replace the liquid-line drier and consider a suction filter for cleanup; change driers after initial operation.
- Pressure-test, evacuate with a micron gauge, leak-check with an HFC-rated detector, label the system, and verify superheat/subcooling and compressor amps after conversion.
3. After charging correctly, subcooling is 2°F and the sight glass bubbles. What is your next step?
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Low subcooling plus bubbles points to undercharge or a restriction, but first confirm full load and measure subcooling at the condenser outlet with pressure and temperature at the same point. For R-407C, use the bubble point for liquid subcooling. If subcooling is 2°F against a target of 8-12°F, add liquid refrigerant in small weighed increments, allowing stabilization between additions. Do not top off with vapor. If subcooling does not rise and bubbles persist, look for a restriction (drier, solenoid, TXV screen) or a measurement error. Bubbles alone are not a charge indicator on a blend.