Refrigerant Retrofit Decision on a Chiller
A 300 kW air-cooled chiller runs on R-22 and has developed a slow leak at a flare joint. The owner wants to 'top it up and keep going' because a replacement is expensive. A contractor proposes dropping in an R-22 'replacement blend' without changing the mineral oil or the expansion valve, claiming it is 'a direct drop-in'. The system has a liquid-line filter drier, a thermostatic expansion valve set for R-22, and a low-pressure cut-out. Ambient design is 35 °C; condensing pressure is already near the high-pressure trip on hot days.
1. Evaluate the 'top up and keep going' and 'direct drop-in' proposals against good practice.
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Both proposals are unsound:
- 'Top up and keep going' ignores that R-22 is an ozone-depleting HCFC subject to phase-out and leak-repair obligations. A known leak must be located and repaired, not masked by topping up. Repeated top-ups also hide the leak rate and risk running the system low on charge.
- 'Direct drop-in' is a marketing claim, not an engineering one. Most R-22 replacement blends are not true drop-ins: they have different glide, different capacity, different mass flow, and often require a different lubricant (e.g. POE instead of mineral oil) and different seals/elastomers.
- Leaving the R-22 TXV and mineral oil in place with a blend can cause poor superheat control, oil return problems and reduced capacity.
- The system is already near its high-pressure trip at 35 °C ambient; a blend with different pressure/temperature characteristics could push it over on hot days.
Correct approach: repair the leak, pressure-test, evacuate and dehydrate, then decide retrofit vs replacement on a proper engineering basis.
2. Describe the engineering checks you would complete before any refrigerant change, and the conditions under which you would refuse the retrofit.
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Checks before change:
- Repair and leak-test the flare joint; confirm the system holds under test.
- Identify the existing oil, TXV, seals and any pressure switches; obtain the blend manufacturer's retrofit guidance.
- Compare the blend's glide, capacity and mass flow with R-22; check whether a TXV/orifice change and oil change are required.
- Check condenser capacity against the new refrigerant at 35 °C design ambient and confirm the high-pressure trip has margin.
- Check the low-pressure cut-out setting against the new charge and expected suction pressures.
- Confirm component compatibility (elastomers, drier type) and that the new refrigerant is legally permitted.
Refuse the retrofit if:
- The blend requires an oil change the owner will not fund, or the compressor is not approved for the new lubricant.
- The condenser cannot reject the load at design ambient without tripping.
- The TXV and controls cannot be re-set for the new refrigerant.
- The leak cannot be repaired first. In those cases recommend replacement or a properly engineered conversion with new components.