98 Modern Refrigeration and Air Conditioning Copyright Goodheart-Willcox Co., Inc. reduced pressure. The metering device maintains this lower pressure in the evaporator. There are several types of metering devices: • Fixed orifice. • Capillary tube. • Thermostatic expansion valve (TXV). • Automatic expansion valve (AXV). • Electronic expansion valve (EEV). • Low-side float (LSF). • High-side float (HSF). These types of metering devices and their charac- teristics are explained in Chapter 20, Metering Devices. 6.6 Evaporating In compression refrigeration systems, evaporation is the process by which refrigeration takes place. As mentioned earlier in this chapter, evaporation occurs when a liquid absorbs heat and becomes a gas or vapor. In a refrigeration system, an evaporator is the compo- nent that aids a refrigerant in heat absorption. Warm air passes through the fins of an evapo- rator. These fins add to the surface area of the evap- orator tubing, increasing the transfer of heat from the air to the tubing. Heat is absorbed by the liquid refrigerant, causing it to evaporate. The refriger- ant that had entered the evaporator as a liquid now changes to a vapor. Refrigerants with low boiling points are preferred, as the temperature difference between the air and the refrigerant is required for heat to flow from hot to cold. The temperature of the air flowing over the evapora- tor becomes increasingly colder, which cools the con- ditioned area. 6.6.1 Evaporator An evaporator is a heat-exchanging device that absorbs heat into its refrigerant. When a refrigera- tion system is running, liquid refrigerant entering the evaporator from the metering device is suddenly under low pressure. The tremendous pressure drop between the metering device and evaporator low- ers the boiling temperature of the liquid refriger- ant. It is so low that some liquid refrigerant entering the evaporator immediately boils into vapor and absorbs heat. As the rest of the liquid refrigerant travels through the evaporator, it boils into vapor by absorbing heat from the air around the evaporator. This absorption of heat is how the refrigeration of the conditioned space begins. Evaporators can be either natural draft or forced draft (forced air). With a natural-draft evaporator, air naturally moves by the evaporator due to changes in temperature and pressure. Remember that hot air naturally rises. With a forced-draft evaporator, air is blown around the evaporator by a fan, Figure 6-12. Danfoss Sealed Unit Parts Co., Inc. Figure 6-11. Two types of metering devices are shown here. A—A thermostatic expansion valve has a sensing bulb that reacts to temperature and can modify the flow of refrigerant through the valve. B—A capillary tube, which is commonly used in domestic refrigerators, has a narrow passageway that restricts the flow of refrigerant. Sensing bulb Inlet Outlet A B