Chapter 9 Evaporators, Condensers, Accumulators, and Receiver-Driers 149 Copyright by Goodheart-Willcox Co., Inc. vaporized refrigerant rises to the top of the receiver-drier, only liquid refrigerant goes to the expansion valve. Th is allows the expansion valve and evaporator to operate with peak effi ciency. A small screen fi lter at the pickup tube catches any particles of dirt or metal that do not settle to the bottom of the tank. Receiver-driers on R-12 systems had a sight glass located at the top of the tank, Figure 9-12. Th e sight glass gave the technician a way to make a rough determination of the amount of refrigerant in the system. Th e technician could check for bubbles in the sight glass aft er the system had been operating for a few minutes. A slightly low system would cause a few bubbles in the sight glass. A system that was very low would have many bubbles or foam in the glass. No bubbles meant the system was either full or completely empty. Sometimes the sight glass was located at another place in the liquid line between the receiver-drier and the expansion valve. Some older vehicles manufactured shortly aft er the switch to R-134a have a sight glass. Th ese sight glasses should be ignored when servicing the system. Th e receiver-drier may have a pressure switch. Th e function of this switch is to cut off the compressor when the high side pressure becomes excessive. Once the pressure returns to normal, the switch resets and the compressor begins operating again. A dual function switch is used on some receiver-driers. Th e dual function switch will cut off the compressor if the pressure is too high or too low. Desiccants Receiver-driers and accumulators use a desiccant to remove moisture and a fi lter to catch debris. Th e desiccant bag is located at the bottom of the tank. In this position, almost all of the liquid refrigerant will contact it before entering the pickup tube. Desiccants hold water by one of two methods, absorp- tion and adsorption. Absorption means moisture enters the desiccant and is held there. Adsorption means moisture sticks to the surface of the desiccant. Most modern desic- cant used with R-134a refrigerant works by absorption and adsorption. Desiccant bags are made of nylon mesh or some other fabric that will not break down under normal operating conditions. Th e desiccant bag is usually placed in the tube loop directly above the oil return hole. Figure 9-13 shows the relationship of the desiccant inside an accumulator. In most systems, only part of the refrigerant passes through the desiccant as it circulates through the system. Allowing the refrigerant to bypass the desiccant bag keeps moisture from being pulled out of the desiccant if it is com- pletely saturated (fi lled with water). Bypassing the desiccant also allows the refrigerant to continue fl owing if the desic- cant bag becomes restricted. Most desiccant bags are sealed in the receiver-drier or accumulator. If the desiccant becomes saturated, the entire unit must be replaced. A few desiccant bags can be replaced by removing an end cap from the receiver-drier and replac- ing the bag. Desiccant Types In the past, all desiccants were made of silica gel. Silica gel is a combination of silicon, carbon, and sulfur that absorbs moisture. Silica gel granules look like large sand particles. In addition to silica gel, a modern desiccant may contain calcium compounds and activated carbon to absorb moisture. It also contains various polymers (plastic compounds) that hold moisture by adsorption. Some poly- mers have microscopic openings that allow R-134a to pass but trap the larger water molecules. Th is polymer design is called a molecular sieve. Th e desiccant mixture is pressed into small pellets. Th e pellets resist disintegration and allow maximum refrigerant fl ow through the desiccant bag. Figure 9-12. A sight glass was sometimes installed at the top of the receiver-drier. Other refrigeration systems had the sight glass installed elsewhere. Figure 9-11. The pressure cycling switch is often installed on the accumulator. When low side pressure becomes lower than the set point, the switch contacts open, de-energizing the compressor clutch. (General Motors) Accumulator O-ring seal Pressure cycling switch