Top view Side view Filter pads Desiccant From condenser In Out To expansion  valve Stand tube Sight glass 16-47 Figure 16-76.  A receiver dryer acts as reservoir and filter in a vapor  cycle system. Bubbles viewed in the sight glass indicate the system  is low on refrigerant and needs to be serviced.  Figure 16-75.  A small can of R134a refrigerant used in vapor cycle  air conditioning systems. Expansion Valve Receiver Dryer  Occasionally, vapor may find its way into the receiver dryer,  such as when the gaseous refrigerant does not completely  change state to a liquid in the condenser. A stand tube is  used to remove refrigerant from the receiver dryer. It runs  to the bottom of the unit to ensure liquid is withdrawn and  forwarded to the expansion valve. At the top of the stand  tube, a sight glass allows the technician to see the refrigerant.  Liquid refrigerant from the condenser flows into the receiver  dryer. Inside, it passes through filters and a desiccant  material. The filters remove any foreign particles that  might be in the system. The desiccant captures any water  in the refrigerant. Water in the refrigerant causes two major  problems. First, the refrigerant and water combine to form  an acid. If left in contact with the inside of the components  and tubing, the acid deteriorates the materials from which  these are made. The second problem with water is that it  could form ice and block the flow of refrigerant around the  system, rendering it inoperative. Ice is particularly a problem  if it forms at the orifice in the expansion valve, which is the  coldest point in the cycle. refrigerants boil violently at typical atmospheric temperatures  and pressure. They rapidly absorb heat energy from all  surrounding matter. If a drop lands on skin, it freezes,  resulting in a burn. Similar tissue damage can result if a drop  gets in one’s eye. Gloves and other skin protection, as well as  safety goggles, are required when working with refrigerant.  The receiver dryer acts as the reservoir of the vapor cycle  system. It is located downstream of the condenser and  upstream of the expansion valve. When it is very hot, more  refrigerant is used by the system than when temperatures are  moderate. Extra refrigerant is stored in the receiver dryer for  this purpose.  Refrigerant exits the receiver dryer and flows to the expansion  valve. The thermostatic expansion valve has an adjustable  orifice through which the correct amount of refrigerant is  When enough refrigerant is present in the system, liquid flows  in the sight glass. If low on refrigerant, any vapor present in  the receiver dryer may be sucked up the stand tube causing  bubbles to be visible in the sight glass. Therefore, bubbles in  the sight glass indicate that the system needs to have more  refrigerant added.  [Figure 16-76]