144 Auto Heating and Air Conditioning Copyright by Goodheart-Willcox Co., Inc. Th e evaporator and condenser are used on every refrig- eration system. Almost all refrigeration systems have either an accumulator or a receiver-drier. Th e evaporator and con- denser work with the compressor and fl ow control device to cause the refrigerant to change state. Th e accumulator and receiver-drier are used to store, separate, and remove any moisture from the refrigerant. Th is chapter will cover the design and operation of these parts. It also explains how they can aff ect the operation of other refrigeration system parts. Studying this chapter will enable you to better diagnose and service these components when you encounter them in the service chapters. Evaporators Chapter 5 discussed how refrigerant changes state from liquid to vapor by absorbing heat from the evaporator. Refrigerant fl ow through the evaporator can be downward, upward, or sideways, depending on the design. Air fl ows through the entire evaporator, usually in as straight a line as possible. To keep air from fl owing around the evaporator, it is usually a close fi t in the evaporator case, with a rubber or foam seal at the top. Th e following sections cover the types of evaporators, how evaporators are constructed, and other parts that may be attached to them. Note: Some vans and trucks may have two evaporators. Each evaporator on these systems is made and operates in the same way as a single evaporator. Evaporator Design and Construction Th e basic design of an evaporator allows the air enter- ing the passenger compartment to fl ow through it with little resistance. Th e contact between the evaporator metal and the air transfers (exchanges) heat in the air to the evaporator, which is then absorbed by the refrigerant. For this reason, the evaporator is oft en called a heat exchanger. Th e evapora- tor must allow maximum airfl ow while being as compact as possible. Moisture collected on the surface of the evaporator must be channeled to the bottom of the evaporator case. Th e lowest point at the bottom of the evaporator case has a drain, usually with a rubber fl ap or neck that allows water to drip out. Th e fl ap seals against the entry of dirt, insects, and outside air. Evaporator tubes and fi ns are usually constructed from aluminum or aluminum alloy. Aluminum transfers heat very well and makes an excellent heat exchanger. Almost all original equipment tubing is made of alu- minum. Some replacement evaporators may use copper tubing. Aluminum tubing provides better heat transfer than copper. However, leaks in copper tubing can be more easily repaired. Fins are attached to the coils to aid heat transfer into the tubing. Fins may be soldered in place, but most are simply pressed tightly over the tubing. The fins are made of aluminum sheets so they are as thin and flat as pos- sible. The use of thin flat fins allows maximum airflow and heat transfer. The fins may be bent into V or accor- dion shapes to maximize the contact between the fin metal and the moving air. Bending the fins also makes them stronger, and they will be less likely to be bent out of shape by debris. Types of Evaporators Th ere are three refrigerant evaporator designs. Th e use of each depends on the manufacturer and the space and airfl ow limitations of the system. Manufacturers oft en use more than one type of evaporator. All evaporators can be classifi ed as either single-pass or multiple-pass. Single- and Multiple-Pass Evaporators Many evaporators use one tube, in multiple rows. Others are long thin designs to fi t in the available space of a small car. Some evaporators have headers that direct the refrigerant through several sets of tubes at the same time. Evaporators may use one tube throughout to move the refrigerant. Th is type is known as a single-pass evaporator. Single-pass evaporators are made from a single piece of tub- ing folded into a series of coils. Single-pass evaporators are oft en used in smaller vehicles, where underdash space is at a premium. Some evaporators are designed so refrigerant enters through the top and passes through three or four of the plates before being redirected upward through the next set of plates. Th is design is called a multiple-pass evaporator. Th e multiple-pass design keeps refrigerant velocity high, preventing oil from dropping out and collecting in the bot- tom of the evaporator. Tube and Fin Evaporator Th e tube and fi n evaporator is usually a single-pass evaporator. Th e coils are formed to reduce evaporator size as much as possible without causing restriction to the refriger- ant fl ow. Figure 9-1 illustrates a tube and fin evaporator. Tube and fin evaporators are made in many shapes and sizes. Some tube and fin evaporators are double or even triple row designs with sets of coils arranged in several rows. Refrigerant flowing through the coils absorbs the heat transferred from the air by way of the fins and tubing. If the evaporator and pressure controls are working properly, very little liquid refrigerant will be left to exit the evaporator. During hot weather, the refrigerant will be superheated. As was explained in Chapter 5, superheat is any extra heat added after the refrigerant changes to a vapor.