Performing an R-134A air conditioning retrofit
By Joe Escobar
What about the older R-12 systems?
With R-12 no longer being produced, the law of supply and demand has resulted in ever-rising prices for the available supplies. It is this price escalation that is forcing aircraft owners to consider retrofits. Scott Steinbach, owner of Waco, Texas-based Steinbach and Associates – a developer of STCs for R-134A conversions (marketed as ConversionAir™) explains: "At this time, there is not a mad rush of owners wishing to convert a normally operating system. Many times, they are opting for a conversion either because the system is not working efficiently, or because of the expense of R-12."
What type of conversion to choose?
Following the manufacturing ban on R-12 and other CFCs, many replacement refrigerants popped up in the marketplace. Although R-134A is the most popular choice for replacement refrigerant, there are other options available. When considering a replacement refrigerant, remember that there is no such thing as a "drop-in" replacement. The EPA requires that the original R-12 must be removed from the system before charging with any alternative refrigerant. This procedure ensures that no cross-contamination occurs. Refrigerants mixed within an air conditioning system could adversely affect performance and may damage the system.
The following information is based on an R-134A retrofit performed by Gary Drapella of JAG Aviation and Scott Steinbach on a Lear 55 owned by American Jet International at the Million Air facility in Houston, Texas. It describes some of the tasks involved with performing the conversion. As with any maintenance task, be sure to follow the specific manufacturers’ procedures.
Before any disassembly is done, a good operational check is in order. Run the system and check for operating pressures. Air circulation should be checked as well as temperature drop. Operate system through all modes including manual and auto as applicable. Note all discrepancies for corrective action.
After operational checks, the R-12 can be recovered. Only an approved recovery/recycling service unit should be used. Remember that all refrigerants have unique service fittings and matching servicing lines. Although this is a way to prevent cross-contamination, it is possible to have incorrect fittings or service lines installed. Care must be taken at all times when working with refrigerants.
After disconnecting the aircraft battery, the recovery cart should be connected to the service ports either on the compressor or on the aircraft plumbing. Ideally, the recovery unit should have a vacuum pump capable of obtaining a pressure on 30-in. Hg vacuum. Evacuate the system for a minimum of 30 minutes after the system has reached a gauge pressure of 28-in. Hg vacuum. Note that altitude will affect obtainable vacuums.
Flush out that system
Clean the system by using a flush rig method. This method is designed to provide a quick, easy, and thorough means of cleaning the air conditioning system. The flush accomplishes two objectives: First, the system is cleaned of all contaminants and foreign material. Second, all mineral oil that was used with the R-12 is removed. The mineral-based oils and lubricants used with R-12 are not compatible with R-134A, so not performing a flush will seriously affect the performance of the system.
Before the flush process can begin, all restricting devices need to be removed and replaced with through-line fittings. These include the receiver dryer and expansion valve. The compressor should also be removed. The flush unit lines should then be connected to the open tubing connections.
Flush the system using recommended procedures. Usually, a dual flush is recommended. In a dual flush, the system is flushed in one direction for about 20 minutes, then the lines are reversed and the system is flushed in the other direction for an additional 20 minutes. This helps to free any debris or contaminants that may be lodged in certain areas.
In the case of this retrofit, the flush was accomplished using an ether-based flushing fluid. Once the system went through the dual flush, a final flush was accomplished using isopropyl alcohol. After the alcohol flush, it was purged using low pressure, dry filtered shop air until all remaining fluids were expelled. A final purge was performed using nitrogen. When performing the dry nitrogen purge, the regulated nitrogen was monitored so that it did not exceed 250 psig. Like the fluid flush, the nitrogen purge should be accomplished in both directions until the exiting gas is clear of fluids. After purging, clean all areas of any residual oil and FOD using isopropyl alcohol.
If accessibility permits, the evaporator should be removed for a more thorough flush. By performing the flush off the aircraft, the unit can be rotated into different positions to ensure that no contaminants or debris are trapped inside. This also allows for a good cleaning and straightening out of the fins. This small amount of extra work can reap large benefits by providing a much more efficient system operation at the completion of the retrofit.
During an R-134A retrofit, it is a good practice to change the compressor.
"The compressor is the weak point in the system," Steinbach says. "As explained earlier, the usual candidates for a retrofit are aircraft that are having air conditioning system problems. Couple this with the fact that compressors manufactured prior to 1993 are not R-134A compatible, and it just makes good sense to go ahead and replace the compressor during the retrofit."
The compressor provided with the conversion STC is R-134A compatible. The fittings need to be swapped over using seco seals as applicable. In addition, the high-pressure switch needs to be swapped using R-134A compatible packings provided. All associated mounting structure and components are swapped over as well.
R-134A molecules are smaller than R-12 molecules. It is this smaller molecular size that can cause leakage problems in some flexible hoses. In general, if the hoses are still flexible and show no signs of deterioration, they will probably be just fine; however, just because the line appears good doesn’t mean it won’t leak once the system is serviced. It is a good practice to go ahead and replace flexible hoses if possible with R-134A compatible hoses. This can save many headaches in the long run.
The through-fittings that were used for the flush process are removed for the installation of components. The new compressor is installed per manufacturer’s installation procedures. The new servicing ports are installed. The new expansion valve is installed as well as the evaporator (if removed). One note on the expansion valve: If it has a thermal bulb, it needs to be mounted on the side of the line (not on the top or the bottom) at an angle so that a proper reading can be taken (see illustration below). If it is placed at the top or bottom, inaccurate readings will lead to inefficient system operation. Once all other components are installed, the R-134A compatible receiver/dryer is installed. The receiver/dryer should be installed last so as to prevent moisture from saturating the desiccant in the filter.
The system can now be serviced with the required amount of refrigerant lubricant (ester-based oil). As with the receiver/dryer, the oil is hydroscopic in nature — drawing moisture out of the air. For this reason, the oil should be kept sealed until just prior to servicing, and the system should be closed and a vacuum pulled as quickly after servicing as possible.
Pulling a vacuum
Once all items are reinstalled and the system is serviced with oil, a vacuum pump should be hooked up to the system in order to pull a vacuum. Pulling a vacuum serves two functions. First, it removes moisture from the system. Second, it helps to determine if any leaks are present. The system should be evacuated for a minimum of 30 minutes after it has reached a pressure of 28-in. Hg vacuum or less. The valves should then be closed and the vacuum pump turned off. If any excessive leaks are noted, investigate and correct before proceeding further.
Oh, the pressure
The next step in the process that many technicians are not aware of is performing a leak check with a pressurized system. In this step, the system is pressurized using dry nitrogen. This is another way to find system leaks.
"Sometimes, you can have a system that will hold a tight vacuum for hours, but then find many leaks once there is pressure in the system," Steinbach says.
Pressure drop is a good indicator of leaks. However, keep in mind that temperature variation can also cause small changes in pressure. You can find leaks by using a soapy water mixture or by using a halogen leak detector. To help locate hard-to-find leaks, add a small amount of R-134A to the system.
After any leaks found are corrected, the system can be serviced. Once again, evacuate the system for a minimum of 30 minutes after it has reached a pressure of
28-in. Hg vacuum or less. Using a servicing cart, charge the system to the specified amount. Once the system is serviced, it should be run to check operation, as it may require additional servicing according to gauge or system performance.
Completing the paperwork
Conversion of vapor cycle air conditioning systems is considered a major alteration to the aircraft type design. Whether the task was performed using a Supplemental Type Certificate (STC) or an FAA Field Approval, the paperwork needs to be completed before the aircraft can be approved for return to service. The June 2001 issue of the FAA Aviation Maintenance Alerts discusses this in General Aviation Air Condition Refrigerant Conversions.
These are the basic issues concerning R-134A conversions. If choosing to perform a retrofit, be sure to do your homework and choose the best product for your needs. This cool conversion can provide years of trouble-free service and an environmentally-friendly alternative to R-12.
The National Institute for Automotive Service Excellence (ASE) provides training for technicians wishing to service air conditioning systems. The technician receives a Refrigerant Recovery and Recycling Review and Quiz booklet discussing CFC issues as well as good servicing practices. Once done with the reading, the technician takes an open-book test and sends it in. If at least 24 out of the 30 questions are answered correctly, he receives credentials from ASE certifying completion of the training. Booklets can be requested by contacting ASE at the address included in The Source at the end of the article. This course has recently been made available online at www.asecert.org
National Institute for Automotive Service Excellence
13505 Dulles Technology Drive
Herndon, VA 20171-3421
Steinbach & Associates, Inc.
2224 Carolinda Drive
Waco, TX 76710
Aerospace Products International
3778 Distriplex Drive North
Memphis, TN 38118
AC 43-16A Aviation Maintenance Alerts, June 2001 Issue