Retrofit from R-22 to R-407C in a Cold Room
A cold room uses an R-22 condensing unit with a nominal capacity of 8 kW. The system has a charge of 10 kg R-22. The owner wants to retrofit to R-407C due to R-22 phase-out. The existing system has a thermostatic expansion valve (TEV) designed for R-22, and the evaporator is a direct expansion coil. The compressor is a hermetic reciprocating type with mineral oil. The system has a liquid line filter drier and a suction accumulator. The retrofit is planned during a scheduled shutdown of 3 days. You need to ensure compatibility and performance.
1. What are the key considerations for retrofitting this system to R-407C, and what steps would you take to ensure a successful conversion?
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R-407C is a zeotropic blend with a temperature glide of about 5-7 K, so you must consider the glide when charging and setting superheat. The existing TEV may not be suitable; R-407C requires a different TEV or adjustment because of different mass flow and pressure-temperature relationship. The mineral oil is not compatible with R-407C; you must change to a polyolester (POE) oil. This requires flushing the system to remove mineral oil, replacing the filter drier with a compatible one (often a solid core with POE compatible desiccant), and possibly replacing seals and gaskets. The suction accumulator may need resizing? Usually not. The compressor may need a new oil charge. You must recover the R-22 properly and not mix refrigerants. After flushing and oil change, evacuate to a deep vacuum (below 500 microns) to remove moisture. Charge R-407C as a liquid to avoid fractionation. Adjust the TEV to achieve proper superheat (typically 5-8 K) and check subcooling. Monitor pressures and temperatures; R-407C has different pressures than R-22 (slightly higher). Capacity may be slightly reduced. Ensure all labels are updated and logbook records the retrofit.
2. How would you determine the correct charge amount for R-407C and verify system performance after retrofit?
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The charge amount cannot be directly calculated from R-22 charge due to different densities and glide. Start with about 90-95% of the original R-22 charge by weight as a rough estimate, but better to charge by subcooling and superheat. For R-407C, charge as liquid. Use the manufacturer's guidelines if available. After initial charge, measure subcooling at the condenser outlet (target 5-10 K) and superheat at evaporator outlet (target 5-8 K). Adjust charge until subcooling is within range. Verify performance by checking evaporating and condensing pressures, compressor current, and room temperature pull-down. Also check oil level and ensure no leaks. Record all data in the logbook.