The Eight Steps of Retrofitting HFC Refrigerants
There are specific skills a service technician needs to have to do such retrofits safely and efficiently. The article that follows illustrates the procedures based on the use of HFC-438A (marketed by DuPont™ as ISCEON® MO99™ and referenced as such in the story) which can be used in place of R-22 in direct expansion residential and commercial air conditioning and medium- and low-temperature refrigeration.
Review DuPont ISCEON MO99™ retrofit guidelines including the information on safety and use of proper personal protective equipment when handling refrigerants (available at www.isceon.com).
Check baseline performance of the R-22 system. Collect system performance data while the old refrigerant is in the system. Check for correct refrigerant charge and operating conditions. The baseline data of temperatures and pressures at various points in the system (evaporator, condenser, compressor suction and discharge, superheat, and subcool) at normal operating conditions will be useful when optimizing operation of the system with MO99 refrigerant.
Remove R-22 refrigerant from the system and collect it in a recovery cylinder using a recovery device capable of pulling 10-15 inches Hg vacuum (50-67 kPa absolute). If the recommended charge size for the system is not known, weigh the amount of R-22 removed, as this will be helpful information in Step 7 when charging the system. Break the vacuum with dry nitrogen.
Replace the old filter drier. It is routine practice to replace the filter drier during system maintenance. Filter driers compatible with HFC refrigerants are compatible with MO99.
While the system is empty, check and replace any critical elastomeric seals that may be near the end of their serviceable life. Even if they were not previously leaking, the change of swell characteristics when changing to any new refrigerant (e.g., R-22 to any HFC refrigerant) and the general disturbance to the system may cause worn seals to leak after retrofit. Components commonly affected are Schrader core seals, liquid level receiver gaskets, solenoid valves, ball valves, flange seals, and some shaft seals on open-drive compressors, but all external seals in contact with the refrigerant should be viewed as a potential leak source post-retrofit. Field experience has shown that the older the system, the greater the likelihood of seal and gasket leaks. It is recommended to change any system-critical seals (e.g., those which require removal of the refrigerant charge to allow seal replacement, e.g., liquid receiver, condenser system) as a matter of course and to have spare seals for other components available during the retrofit should any seal failure occur. Details on minimizing system retrofit leaks by changing out critical elastomeric seals are outlined in a training video available at http://youtu.be/5M30Rq-1OiI. A rigorous leak check regime pre- and post-retrofit will minimize any refrigerant losses.
Pull vacuum on the system following typical service practices. To remove air or other non-condensable gases and any residual moisture from the system, evacuate the system to near-full vacuum (29.9 inches Hg vacuum [500 microns] or less than 10 kPa), isolate the vacuum pump from the system and observe the vacuum reading. If the system does not maintain vacuum, it is an indication that there might be a leak. Pressurize the system with nitrogen, taking care not to exceed the system design maximum pressure and check for leaks. Do not use mixtures of air and refrigerant under pressure to check for leaks; these mixtures can be combustible. After leak checking with nitrogen remove residual nitrogen using a vacuum pump.
Charge system with MO99 and check system performance. MO99 (R-438A), like other 400-series refrigerant blends, is a near-azeotrope. Therefore, the vapor composition in the refrigerant cylinder is different from the liquid composition. For this reason, MO99 should be transferred from the container in the liquid phase during system charging. (If the cylinder does not have a valve with a dip tube, invert the cylinder so that the valve is underneath the cylinder.) The proper cylinder position for liquid removal is often indicated by arrows on the cylinder and cylinder box. Once liquid is removed from the cylinder, the refrigerant can be allowed to enter the refrigeration system as liquid or vapor as desired. Use the manifold gauges or a throttling valve to flash the liquid to vapor if required. WARNING: Do not charge liquid refrigerant into the compressor. This will cause serious irreversible damage. In general, refrigeration systems will require about the same weight of R-438A refrigerant as the original R-22 charge. The optimum charge will vary depending on the system design and operating conditions. The initial charge for systems without a liquid receiver should be approximately 85 percent of the standard charge for R-22. After start-up and adjustment, the final charge amount will usually be 95 percent of the original R-22 charge. For systems with a liquid refrigerant receiver, charge the system to the normal refrigerant level in the receiver.
Start-up system and adjust charge size and let conditions stabilize. If the system is undercharged (as indicated by the level of superheat at the evaporator exit, or by the amount of subcool at the condenser exit) add more MO99 in small amounts (still by transferring liquid from the charging cylinder) until the system conditions reach the desired level. See the pressure-temperature charts to compare pressures and temperatures in order to calculate superheat or subcooling for the refrigerant.
If present, sight glasses in the liquid line can be used in most cases as a guide to system charge, but correct system charge must be determined by measuring system operating conditions (discharge and suction pressures, suction line temperature, compressor motor amps, superheat, etc.).
Attempting to charge until the sight glass is “full” (bubble-free) may result in overcharging the refrigerant. For any of the 400-series refrigerant blends, it is reasonable to expect to see some bubbles in the sight glass that are entrained in the liquid, not flashing off, even once the system is properly filled.
Ensuring that the correct evaporator exit and compressor suction superheat is set is very important for reliable system operation. Experience has shown that superheat (at the compressor inlet) for MO99 should be the same as for the refrigerant being replaced.
Monitor oil levels. When compressor sight glasses are present, during initial operation of the system, monitor the level of oil in the compressor (or compressor oil management system) to verify that oil is returning to the compressor in an adequate manner.
Thoroughly leak-check the system. It is possible that refrigerant leakage can occur during or immediately after a retrofit. Experience has shown that some leaks will not appear until after the new refrigerant has been charged to the system. Pay particular attention to Schrader valve core seals, solenoid valves, and ball valve stems on the liquid high-pressure side.
Label the system to clearly and permanently show the refrigerant and oil(s) present in the system.
Publication date: 3/18/2013