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Air Conditioning and Heating Basics by Jim Clark Hot Rod Library
What: Temperature control systems in a modern car or truck can be quite elaborate, having components that cycle between heating and cooling in response to the settings input via the system controls. Both heating and cooling are accomplished by blowing air across or through a finned smaller version of the typical car radiator.
How: Most rod builders understand the relatively simple process used in a typical automotive heater system. Hot water from the cars radiator is routed through the heater core. Air blown across the heater core absorbs heat and is distributed throughout the vehicle via ducting. Controlling the flow of hot water through the heater core and fan speed regulates temperature within the vehicle.
How: Cooling the air inside the vehicle is more of a challenge though. Heat must be extracted from the air inside the vehicle and expelled outside of the vehicle. In a typical A/C system this basic process involves the use of three main components: the evaporator, the compressor, and the condenser. In the evaporator the refrigerant boils and changes into a vapor, absorbing heat as it does so. A pump, or compressor, puts pressure on the refrigerant so it can get rid of its heat. A condenser outside the passenger compartment helps discharge the heat into the surrounding air.
A more detailed explanation of how this typical system functions makes the simplified explanation of how the system works clearer. The evaporator always has a set of coils or a finned radiator core through which the air to be cooled passes. The liquid refrigerant boils inside the evaporator. As the refrigerant boils it absorbs heat changing it to a vapor. Piping that vapor outside the passenger compartment carries that heat with it.
After the vapor has left the evaporator the heat it contains has to be removed. Removing that heat lets the refrigerant condense into a liquid so that it can be used over again in the heat removal process. That vapor coming out of the evaporator is actually very cold. Liquid refrigerant boils at temperatures well below freezing and vapors arising from it are only slightly warmer, even though they do contain some absorbed heat. Consequently, heat can’t be removed from subfreezing vapors by cooling them in air temperatures ranging from 60 to 100 degrees. Heat refuses to flow from a cold object to a warmer object.
However, a pump can squeeze that heat-laden vapor into a smaller space. Compressing the vapor concentrates the heat it contains making it hotter without adding any more heat. This makes it possible to cool it in relatively warm air.
The compressor in an air conditioning system is responsible for circulating the refrigerant through the system, but has two other functions to perform. It exerts pressure making the vapor hot enough to cool off in warm air and raises the refrigerants pressure above the condensing point at the surrounding air so that it will condense.
Refrigerant leaving the compressor is still a vapor, though now quite hot and ready to give up the heat it absorbed in the evaporator. Sending the refrigerant vapor through a radiator-like component known as a condenser is the easiest way to discharge this heat. The condenser is a simple radiator-like device that condenses the high-pressure, high-temperature refrigerant vapor discharged by the compressor into a high-pressure liquid refrigerant. Subjecting the high-pressure, high-temperature liquid refrigerant to the cooler metal surfaces of the condenser causes this to occur. This due to fundamental physical laws—heat travels from a warmer to a cooler surface, and when heat is removed from a vapor, liquid is produced.
After the refrigerant has condensed into a liquid, it is ready to pipe back to the evaporator where it boils absorbing heat and starting the cycle over again.
Regulating the flow of vapor and liquid through the cycle is controlled by a thermal expansion valve or orifice and the cycling of the compressor in response to temperature settings. Other components are included in the system to remove moisture and regulate the distribution of air within the vehicle. These will vary depending on the design of the system.
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