Chapter 5 Principles of Refrigeration 93 Copyright by Goodheart-Willcox Co., Inc. valve to perform the vapor separation once done by the receiver-drier. Pressure Decrease Phase—Flow Restrictor Th e liquid refrigerant tries to enter the evaporator but is stopped by the fl ow restrictor. Th e action of a fl ow restrictor is similar to the nozzle of a water hose. It sprays the liquid refrigerant in a sheet or mist of atomized particles. Breaking the refrigerant into small particles helps it to vaporize easily. Th e fl ow restrictor also works with the compressor to control the evaporator pressure. Th e compressor is always trying to draw refrigerant from the evaporator. Th e restric- tor controls evaporator pressure by controlling the amount of refrigerant entering the evaporator. Since the pressure is low, the refrigerant in the evaporator vaporizes, beginning the cycle again. Refrigeration System Variations Th e refrigeration cycles of most air conditioning refrig- eration systems follow the fundamental principles explained in the above sections. Some newer air conditioning refrig- eration systems, however, are variations on this basic design. Th e following systems use the primary refrigeration system components such as condensers, compressors, and fl ow restrictors. Most systems probably have some parts, such as accumulators and receiver-driers, not shown on the simpli- fi ed fi gures that follow. Refer to the fi gures as you read the following paragraphs. 1234yf Safety System Th ere is a slight chance of fi re if 1234yf refrigerant con- tacts high temperature engine parts aft er a collision. Some vehicle manufacturers are not comfortable with the fl am- mability of 1234yf, and have designed their refrigeration systems to ensure that no 1234yf enters the passenger com- partment. Th ese systems use a specialized heat exchanger in place of the conventional evaporator. In the heat exchanger the evaporating refrigerant absorbs heat from a circulat- ing liquid. Various liquids can be used, including engine coolant. Th e cooled liquid is then pumped to the cooler in the passenger compartment. A fan blows across the cooler, removing heat from the entering air. Figure 5-21 is a simple diagram of a heat exchanger in a 1234yf air conditioner. Note that there are two fl uid paths in this system: conven- tional refrigerant fl ow and liquid coolant fl ow. Heat Pump System All-electric vehicles have no cooling system. Th ey can- not, therefore, provide hot coolant for heating the passenger compartment during cold weather. Hybrid vehicle engines run under certain conditions only and cannot provide a reliable source of heated coolant. To heat the passenger com- partment of these vehicles, the air conditioner refrigeration system is modifi ed into a heat pump, similar to heat pumps used in stationary buildings. A heat pump contains an evap- orator, condenser, compressor, and refrigerant restriction. As with most stationary heat pumps, the vehicle heat pump usually has an electric grid to provide extra heat in very cold weather. Th e heat pump also has an extra valve called a reversing valve. Th e reversing valve changes the direction of refrigerant fl ow, reversing the functions of the evaporator and condenser. Th e compressor always rotates in the same direction and fl ow through the system is controlled entirely by the reversing valve. See Figure 5-22. For simplicity, the reversing valve shown in the following fi gures is a rotary type. Linear types are also used. ENGINE COMPARTMENT PASSENGER COMPARTMENT Fan Heat exchanger Expansion valve or fixed orifice Blower Cooler Condenser Compressor Liquid pump Low-pressure liquid Low-pressure gas High-pressure liquid High-pressure gas Liquid coolant Figure 5-21. The 1234yf safety system has two separate heat transfer paths. The refrigeration system is a conventional type that transfers heat to the outside air through a condenser. The main difference is that, instead of passenger compartment air being cooled directly by the refrigerant, heat is first removed from coolant, which then removes heat from incoming air.