94 Auto Heating and Air Conditioning Copyright by Goodheart-Willcox Co., Inc. Factors That Affect System Performance Until now we have simply referred to conditioning warm humid air passing through the evaporator. However, the amount of heat and humidity present in the air, the amount of sunlight, and other factors have a major eff ect on the effi ciency of a vehicle’s refrigeration system, and there- fore the cooling power of the air conditioner. Temperature Th e heat load is the total eff ect of heat and humidity of the surrounding, or ambient air. Th e higher the temperature of the air entering the evaporator, the harder the refrigerant in the evaporator must work to remove the heat. Also, the hotter the outside air passing through the condenser, the harder it is for the refrigerant in the condenser to give up its heat. Humidity Th e eff ect of humidity on refrigeration system operation is oft en overlooked. Water vapor is turned into liquid water as it passes through the evaporator. Changing state from vapor to liquid causes the water to release latent heat. Th is heat, which is considerable, must be absorbed by the refrigerant. Relative humidity is the amount of moisture in the air compared to the maximum amount of moisture the air could hold at its current temperature. Warm air can hold more water vapor than cooler air. For example, on a typi- cal summer morning, the relative humidity might be 40% at 70°F (21°C). Later in the day, the relative humidity may change to 20% at 90°F (32°C). At both temperatures the air has the same amount of water vapor, but the amount of water the air could hold has increased with temperature. Sunlight Earlier in this chapter we discussed heat transfer by radiation. When the sun is directly overhead, infrared radi- ation is striking the vehicle directly, causing a considerable heating eff ect. Th is is referred to as sunlight load or simply sunload. When the sun is low in the sky, the infrared radia- tion is striking the vehicle at an angle, and heating is less. Overcast skies reduce, but do not totally eliminate the eff ect of infrared radiation. At night, the infrared radiation from the sun is not a factor. All of these factors change the heat load and greatly aff ect the operation of the refrigeration system. As an example, an air conditioner can produce 45°F (7°C) air when the outside temperature is 80°F (27°C) with low sun- light and humidity. However, it may be able to lower the temperature only to 60°F (15°C) when the outside air is at 95°F (35°C) with high humidity and the sun overhead. Figure 5-22. A reversing valve controls the heat pump’s mode of operation. A—The heat pump reversing valve is positioned so the system functions as an air conditioner. Refrigerant is changed to a gas in the evaporator, absorbing heat. The refrigerant changes to a liquid in the condenser, giving up heat. B—The reversing valve has moved to close off one set of ports and open another. This movement reverses flow through the system. The system is now drawing heat from the outside air and delivering it to the passenger compartment. Fan Blower Evaporator Expansion valve or fixed orifice Condenser Compressor Low-pressure liquid Low-pressure gas High-pressure liquid High-pressure gas ENGINE COMPARTMENT A Rotary reversing valve PASSENGER COMPARTMENT Fan Blower Evaporator Expansion valve or fixed orifice Condenser Compressor ENGINE COMPARTMENT B Rotary reversing valve PASSENGER COMPARTMENT