Chapter 5 Principles of Refrigeration 89 Copyright by Goodheart-Willcox Co., Inc. refrigerant. Th e receiver-drier also contains desiccant to absorb any moisture that has entered the refrigeration sys- tem. Some receiver-driers contain a sight glass. Filter Some systems have a fi lter installed in the line between the condenser and fl ow restrictor. Th e fi lter traps metal and rubber particles to keep them from damaging other refrig- eration system parts. A typical fi lter is shown in Figure 5-15. Aft ermarket fi lters are sometimes installed when the system has suff ered a major compressor failure, and there is no economical way to remove all the debris from the refrig- eration system. Aft ermarket fi lters are also installed in the liquid line between the condenser and fl ow restrictor. Sight Glass On older R-12 systems a sight glass was always used, Figure 5-16. Th e technician could get a general idea of the amount of refrigerant in the system by checking for bubbles in the sight glass. A few bubbles meant the sys tem was slightly low. Many bubbles or foam meant the system was severely low. If no bubbles were present, the system was either full or completely empty. Th e sight glass was always installed between the condenser and the fl ow restrictor. Some sight glasses were installed at the top of the receiver-drier, while others were in the line to the evaporator. Cycling clutch type refrigeration systems do not have a sight glass. Sight glasses are sometimes installed in R-134a systems, but they should not be used to check the state of charge. Bubbles will appear in the sight glass of an R-134a system, even when the system is properly charged. Flow Restrictor Th e fl ow restrictor is installed at the entrance to the evaporator. It reduces the fl ow of refrigerant into the evapo- rator, lowering evaporator pressure. As mentioned earlier, the system is oft en identifi ed by the type of fl ow restrictor. Th ere are two kinds of refrigerant fl ow restrictors. Fixed Orifi ce Tube Th e fi xed orifi ce is used on many modern refrigeration systems. Th e fi xed orifi ce is a non-adjustable opening, usu- ally installed in the tubing leading into the evaporator. See Figure 5-17. Th e word orifi ce means opening. Th e orifi ce allows refrigerant to enter the evaporator at a controlled rate. Th e size of this hole is not adjustable. Evaporator pres- sure is controlled by controlling the amount of refrigerant drawn into the compressor. Some systems using orifi ce tubes are called cycling clutch orifi ce tube (CCOT), fi xed orifi ce tube (FOT), Ford orifi ce tube cycling clutch (FOTCC), variable displacement orifi ce tube (VDOT), thermostatic switch systems, pressure switch systems, or accumulator systems. Expansion Valve A fl ow restrictor with a variable opening is called an expansion valve. An expansion valve is sometimes called a thermostatic expansion valve (TXV). Th e expansion valve has a small shaft with a large end. Th e large end section can partially block or seal off the opening. Th e needle is attached Figure 5-15. When a refrigeration system becomes contami- nated due to catastrophic compressor failure, a filter should be installed between the condenser and the flow restrictor. Figure 5-17. The opening of an orifice tube flow restrictor does not change. Varying the compressor output controls refrigerant flow. The orifice is usually located at the inlet tube of the evaporator. Figure 5-16. This sight glass is located in the receiver-drier. Other sight glasses are located in the line between the receiver-drier and the expansion valve, or in the expansion valve assembly.