150 Auto Heating and Air Conditioning Copyright by Goodheart-Willcox Co., Inc. Th e desiccant used in R-134a systems is called XH-7 desic- cant. Desiccants used with R-12 are sometimes called XH-5 desiccants. Th e technician must use the right type of desic- cant. R-134a and R-12 desiccants are not interchangeable. XH-9 desiccant can be used with R-134a or R-12 refrigerants. Th is desiccant is usually used in aft ermar- ket receiver-driers and accumulators especially those designed as universal replacements for original equipment units. One diff erence between XH-9 and XH-7 desiccant is XH-9 contains additional silica gel. Some original equip- ment manufacturers do not recommend the use of an XH-9 desiccant in R-134a systems. XH-7 and XH-9 desic- cants are compatible with R-1234yf refrigerants. Pressure Relief Devices Some R-12 systems contained one of two devices to pre- vent excessive pressure on the high side of the system. Th e pressure relief valve is a spring-loaded valve that remains closed during normal operation. If the system pressure becomes too high, the spring pressure will be overcome and the valve will open to relieve pressure. Aft er enough refrig- erant has been released, the lowered pressure will not be enough to overcome spring pressure and the valve will close. Figure 9-14 shows the location of a pressure relief valve on a late-model compressor. Th e fusible plug resembles a pipe plug with a center sec- tion made of a material soft er and more heat sensitive than any other part of the refrigeration system. Th is plug may also be referred to as a melting bolt. During normal system operation, the fusible plug has no eff ect. If the temperature and pressure become too high, the fusible plug will soft en and blow out. Th is removes system pressure and prevents damage to the compressor and other system components. Once the fusible plug blows, it must be replaced and the system evacuated and recharged. Both pressure relief valves and fusible plugs are no longer used because of the potential to discharge refrigerant to the atmosphere. Some R-12 systems with pressure relief valves may also contain a high pressure switch. Th is switch is also referred to as a refrigerant containment switch, and cuts off the compressor before the pressure reaches the relief valve opening point. If the system is not equipped with one, a high pressure switch must be installed if the system is retrofi tted to accept a diff erent refrigerant. Note: Early R-12 systems did not use a high pressure switch. Figure 9-14. Pressure relief valves are usually installed on the rear of the compressor body. Relief valves are not used on all systems. (Chrysler) High pressure relief valve Figure 9-13. This illustration shows the internal parts of a typical accumulator. The desiccant removed any moisture from the refrigerant. (Jack Klasey) Desiccant