Chapter 9 Evaporators, Condensers, Accumulators, and Receiver-Driers 145 Copyright by Goodheart-Willcox Co., Inc. Plate and Fin Evaporator Instead of a tube surrounded by fi ns, the plate and fi n evaporator is a set of fl at aluminum plates between two alu- minum end tanks. Fins are installed between the plates. Th e fl at tube design reduces resistance to airfl ow while allowing more contact between refrigerant and air. On early plate and fi n evaporators, the refrigerant entered at the top of the evaporator and passed through a header that distributed equal amounts of refrigerant through each fi n. Th is design caused oil to drop out of the refrigerant (along with some liquid refrigerant) and collect at the bottom of the evaporator. A separate oil return line, sometimes called a bleed line or a liquid return line, was needed. Figure 9-2 is an illustration of a plate and fi n evapo- rator with an oil return line. Later model plate and fi n evaporators are multiple-pass types. Th ey are similar in appearance to the earlier models but can be identifi ed by the lack of an external oil return line, Figure 9-3. As a general rule, multiple-pass plate and fi n evaporators are used with cycling clutch systems, while older plate and fi n evaporators are used on systems with expansion valves. Th ere are, however, exceptions to this rule depending on the manufacturer. Serpentine Evaporator Th e serpentine evaporator is a single-pass evapora- tor. It consists of a fl at tube coiled to allow refrigerant to pass through the entire tube. Fins similar to the ones used on other evaporator designs are attached to the tube. Th e shape of the serpentine evaporator tube resembles a serpent, Figure 9-4. Parts Attached to the Evaporator The orifice tube or expansion valve is installed at the inlet of the evaporator. The fitting is either a com- pression type using an O-ring or a spring lock fitting. When an expansion valve is used, the sensing bulb will be located at the outlet tube of the evaporator. The bulb will be placed in a pocket made into the tube or clamped to the tube exterior and covered with insulation. Figure 9-5 illustrates the two sensing bulb attachment methods. Some type of evaporator temperature control is oft en used to prevent icing. Th ey are either installed on the evaporator outlet or in the evaporator fi ns. Th is may be Figure 9-1. This figure shows a simple tube and fin evaporator. The fins conduct heat from the air passing through the case to the tubes, where it is absorbed by the refrigerant. Figure 9-3. This plate and fin evaporator is designed so refrigerant makes several passes through the fins. It has no oil return line. Figure 9-2. This plate and fin evaporator distributes equal amounts of refrigerant through each fin. This type can be recognized by its use of an oil return line. (Ford) Oil return line