Cool Conversion
Performing an R-134A air conditioning retrofit
By Joe Escobar July 2001
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.
R-134A Conversion
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.
Operational Check
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.
Recovery
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.
Compressor replacement
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.
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.
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.
ASE Training
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
The Source
Additional resources....
National Institute for Automotive Service Excellence
13505 Dulles Technology Drive
Suite 2
Herndon, VA 20171-3421
www.asecert.org
Steinbach & Associates, Inc.
2224 Carolinda Drive
Waco, TX 76710
(254) 752-6773
Aerospace Products International
3778 Distriplex Drive North
Memphis, TN 38118
(800) 950-0111
AC 43-16A Aviation Maintenance Alerts, June 2001 Issue