Usage:
Caterpillar Machines Equipped With Air Conditioning
Reference: The Mobile Air Conditioning Society Worldwide, Inc., P.O. Box 100, East Greenville, PA 18041, "MACS Service Reports", March 1996
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Effective July 1, 1992 regulations prohibit the venting of any refrigerant into the atmosphere. Refer to Caterpillar publications SENR3334-01, SENR5664 and NEDG5065-01 for the proper procedure, equipment and tools to reclaim refrigerant from any Caterpillar machine.
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Always wear goggles when working on air conditioning systems. The system is under pressure at all times, engine running or not. Escaping refrigerant R-12 (C C12F2Dichlorodifluoromethane) and refrigerant R-134a can cause freezing of human flesh. Do not smoke while working on air conditioning. Inhaling refrigerant R-12 or R-134a through any smoking material, although not toxic or flammable, can cause violent illness. Also heat must never be applied to a charged system. See Air Conditioning And Heating Service Manual SENR3334-01 and SENR5664, for more information on removal and installation of lines and refrigerant from the system. |
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Contaminated air conditioning refrigerant is becoming a more common source of poor air conditioning system performance. In CFC-12 based systems there are no Caterpillar approved alternative refrigerants other than completely converting to a HFC-134a based system. In the CFC-12 based systems some of the sources of contamination are unapproved refrigerants such as R-22, and some flammable refrigerants. Another source of contamination is from air leaking into the system and from air introduced through the use of recycled CFC-12. These contaminants can be detected with the use of a refrigerant identifier such as the 138-6629 Identifier, the 139-2610 Identifier, or the 139-2611 Identifier. The maximum allowable amount of NCG (non-condensable gases) in an CFC-12 system is 1.5 percent.
Refer to Chart 1 and Chart 2 to see the effects of contaminates on the pressure/temperature ratios within the system.
Chart 1. This MACS chart shows how pressures rise from under 634 kpa (92 psi) at 29.4°C (85°F) with pure CFC-12, up to 703 kpa (102 psi) with about 1.5% air contamination, the maximum tolerable amount of NCG (non-condensable gases, basically air). Next, the chart shows how the CFC-12 pressures rise from contamination with 2% HFC-134a or 2% HCFC-22, then 5% HFC-134a or 5% HCFC-22, and finally with 10% HFC-134a or 10% HCFC-22.
Chart 2. This MACS chart shows how pure HFC-134a compares with the same types of contamination, in this case with CFC-12 and HCFC-22, also at 29.4°C (85°F). The pressure of pure HFC-134a is just over 648 kpa (94 psi). With about 2% contamination by air (NCG), the pressure rises to 689 kpa (100 psi). Next, the chart shows how HFC-134a pressures rise from contamination with 2% CFC-12 or 2% HCFC-22, then 5% CFC-12 or 5% HCFC-22, and finally with 10% CFC or 10% HCFC-22.
As shown in Chart 1 and Chart 2, contaminated refrigerant can generate abnormal pressure readings. Refrigerant temperature/pressure charts indicate the pressure of a pure refrigerant for a specific temperature (see Chart 3 and Chart 4). When an air conditioning system is turned off, temperature and pressure will equalize throughout the system. If the pressure readings do not match the data on the chart, the refrigerant may be contaminated.
CFC-12 based systems may become contaminated with air by the use of recycled refrigerant. Some of the air in a tank of recycled CFC-12 lies on top of the liquid, however, some of the air is trapped within the liquid (similar to the process of carbonation). Air contamination can create noise in the compressor, this can lead to the false assumption of a failing compressor. The noise becomes a factor when the amount of air in the system approaches 6 percent.
According to the referenced article, some tanks of recycled CFC-12 have pressure readings of up to 1516 kpa (220 psi) compared to 427 kpa (62 psi) readings for tanks of virgin CFC-12. These high pressure readings can be a direct result of air contamination. In one instance cited in the referenced MACS article, a tank of CFC-12 had a pressure reading of 896 kpa (130 psi) with 27 percent air by weight, that contained 0.83 L (28 ounces) of refrigerant.
Some of the sources of the air trapped within the liquid refrigerant are as follows. Most recovery/recycling equipment is designed to empty the system until a slight vacuum is developed which triggers an automatic shutoff. A leak in either the cooling system or in the recovery/recycling system may introduce enough air to prevent the automatic shutoff from functioning, this could allow contamination of the refrigerant. In an experiment performed for the MACS article, a recovery/recycling machine with a leaking service hose coupling seal was left running for approximately 15 minutes. The tank pressure rose to 1722 kpa (250 psi). Other sources of air contamination result from failure to evacuate the system properly and failure of the recovery/recycling equipment air purge.
Improper evacuation of air from the refrigerant system can lead to air contamination. Even when all leaks have been eliminated from the refrigerant system, if the vacuum pump will not create a vacuum below 25 inches, the refrigerant system may retain 3 to 4 percent air. According to the MACS article, most vacuum gauges are approximate instruments which may not have the accuracy required. The use of electronic vacuum gauges is recommended, such as the 9U-6060 and the 9U-6061 Electronic Vacuum Gauges.
Bypassing the automatic air purge function of recovery/recycling machines can lead to air contamination. If the machine has a manual air purge, failure to use it may also lead to air contamination. The automatic air purge cycle of the recovery/recycling machine should not be terminated even if there is low scale reading on the vacuum gauge.
If the refrigerant system has not been properly purged and is contaminated with air, pressure readings may be adversely effected. If a high pressure reading is taken, bleeding the system down to a pressure close to the ambient temperature may actually introduce more air into the refrigerant. The release of the refrigerant vapor can lower the temperature of the tank causing the pressure to temporarily drop. The lower temperature induced in the coolant can allow it to absorb more air. As cited in the MACS article, a tank with 27 percent air was purged over the course of an hour at 18.3°C (65°F). The pressure dropped from 827 kpa (120 psi) to 510 kpa (74 psi), however, when checked with an identifier the air content was still 27 percent. Bleeding the system down is not recommended.
Recommendations for controlling and eliminating air contamination are as follows. Read and follow all instructions pertaining to the recovery/recycling equipment, particularly the section dealing with manual air purge, which is based on gauge pressure versus temperature of the air next to the tank. Considering that the air purge function uses the temperature of the air next to the tank, it is good practice to place the machine in a shaded part of the shop in order to isolate it from sources of excessive heat or cold. A temperature gauge should also be placed close by the machine. If the recovery/recycling machine is equipped with an automatic or semi-automatic air purge, do not bypass it. Inspect all service hoses connections, seals, and couplings to make sure they are working properly. According to the MACS article, installing a premium low side pressure gauge, calibrated in one p.s.i. increments, is recommended. The premium gauge will insure the accuracy required when taking readings. If the manufacturer's manual is not available, refer to the MACS Certification Training Manual that contains maximum pressure/temperature charts for CFC-12 and HFC-134a refrigerant. If the MACS Certification Training Manual is not available, refer to Chart 3 and Chart 4.
