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Thermodynamics
One BTU (British Thermal Unit) will change the temperature of one pound of water one degree (approximately the amount of heat produced by one kitchen match burned to it's entirety). Heat will always travel from the warmer object to the cooler object. This movement cannot be stopped, only slowed down (insulation is used to slow the movement of heat). Heat is transferred by convection, conduction or radiation. The air conditioning process uses conduction and convection to transfer heat (a minimum amount of heat is transferred by radiation). There are two types of transferred heat, Sensible and Latent. Sensible heat causes a change in temperature. As the heat is transferred, the warmer object gets cooler and the cooler object gets warmer. Heat will transfer until both objects are at the same temperature. Latent heat causes a change in state. As when water in the liquid form is heated to the boiling point (saturated state) and changes to steam, latent heat is transferred. One BTU Of sensible heat changes one pound of water one degree. At sea level, latent heat of evaporation is 970 BTU's and latent heat of freezing is 144 BTU's for one pound of water. The greatest amount of heat (latent heat) is absorbed or released by water when it is in it's saturated state. In the evaporator, latent heat is absorbed into the refrigerant as it changes from a liquid to a vapor. In the condenser, the refrigerant changes from a vapor to a liquid as it releases latent heat. In each component sensible heat is also transferred, but the greatest heat transfer is in the latent state. A thorough understanding of the "pressure / temperature" relationship is critical for trouble-shooting the air conditioning system. Refrigerant at a given pressure will always be at a determined temperature. Charts are published to provide the information to show the exact temperature of a given refrigerant at given pressure. Service gauges allow access to the system’s operating pressure. This pressure tells the service technician the temperature of both the evaporator and condenser under proper conditions. Pressure and temperature measurements determine if the coils are at the proper temperature to allow heat to move into the evaporator and out
of the condenser.
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