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PRINCIPLES OF HEAT TRANSFER. SOURCES OF HEAT. PRINCIPLE ONE. pg. 41 C. Heat ALWAYS flows from hot to cold when objects are in contact or connected by a good heat conductor. The rate of heat transfer will increase as the difference in temp between the two objects increases. pg.. 6 fig 2.
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PRINCIPLE ONE pg. 41 C • Heat ALWAYS flows from hot to cold when objects are in contact or connected by a good heat conductor. • The rate of heat transfer will increase as the difference in temp between the two objects increases pg.. 6 fig 2
PRINCIPLE TWO pg. 37 C • Cold objects have less heat than hot objects of the same mass • To make a object colder, remove heat To make is hotter, add heat • The mass of the object remains the same regardless of the heat content
EVAPORATION pg. 38C • The process of moisture becoming a vapor(molecules escaping from the surface of the liquid) • As moisture vaporizes from a warm surface, it removes heat and lowers the temperature of the surface. • The warmer the substance the quicker it will evaporate.
PRINCIPLE THREE pg. 41 C • Everything is composed of matter • All matter exists in one of three states: solid, liquid or vapor. • LATENT HEAT OF VAPORIZATION: When matter changes from liquid to vapor or vice versa, it absorbs or releases a relatively large amount of heat without a change in temperature.(970 Btu)
BRITISH THERMAL UNIT • BTU is a heat quantity measure • BTU is the quantity of heat needed to raise the temperature of 1 lb. of water one degree Fahrenheit. • Vaporization: Will absorb more than five times amount of heat pg.. 7 fig 5
PRINCIPLE FOUR • CONDENSATION When a vapor is cooled below its dew point, it becomes a liquid. (boiling point in reverse) • When vapor condenses, releases five times as much heat pg.. 8 fig 6
PRINCIPLE FIVE • Changing the pressure on a liquid or a vapor changes the boiling point. • Each lb. of pressure above atmospheric pressure, raises the boiling point about three degrees Fahrenheit.
PRINCIPLE SIX • When a vapor is compressed, its temperature and pressure will increase even though heat has not been added pg.. 10 fig 10
CONVECTION pg. 43C • Occurs only in liquids, gases or vapors • The transfer of heat by the circulation of a liquid or a vapor (like cooling system) • Heat flows from a hot surface to a surface containing less heat. • Heat rises. (Like on a stove)
RADIATION pg. 42C • The process that moves heat from a heat source to an object by means of heat rays without the medium becoming hot. • Works on the principle that heat moves from a hot surface to a surface with less heat. • Does not require air movement or anything in between the source and component. (Like rays of the sun)
CONDUCTION Pg 42C • Heat is transferred through a solid and gets the solid hot. (molecules get hot than they in turn give motion to nearby molecules and they get hot too) • Different solids conduct different amounts of heat in a specific time. (copper vs. glass)
SPECIFIC HEAT Pg 40C • The amount of heat that must be absorbed by a certain material if it is to undergo a temperature change of 1 degree Fahrenheit • Materials will absorb, emit and exchange heat at different rates. It takes different amounts of heat energy (Btu's) to make a temperature change of the material.
SENSIBLE HEAT Pg 36C • Any heat that can be felt (with your senses) and can be measured with a thermometer. • Like ambient air. You “feel” the change in temperature which makes you feel cold or feel hot. Even a few degrees
PRESSURE • Pressure: A force exerted per unit of surface area. • Atmospheric Pressure: 21% Oxygen 78% Nitrogen 1% other gases • Atmospheric pressure is 14.696 psia fig 6.1
PRESSURE MEASUREMENT • Service Manuals refer to pressure when using A/C gauges as: psig (pounds per square inch gauge) • A/C Gauges are calibrated to compensate for atmospheric pressure. • Pressures below atmospheric are called vacuum and measured in inches of mercury (in Hg)
ATMOSPHERIC PRESSURE • At sea level where atmospheric pressure is 14.7 PSI, the boiling point of water is 212 degrees Fahrenheit • At any point higher than sea level the atmospheric pressure is lower and so is the boiling point of water. • Boiling point of H20 decreases by 1.1 degrees F for every 1000 foot in altitude. page 36 fig 6-3 7th edition
Pressure Increase • A Pressure increase also raises the boiling point of water. • For every 1 PSI of pressure increase, the boiling point raises 2.53 degrees Fahrenheit
Result of controlling Pressure • If water boils at a higher temperature when pressure is applied and at a lower temperature when the pressure is reduced, it is obvious that the temperature can be controlled by controlling the pressure. • This is the basic theory of physics that determines and controls the temperature conditions of air conditioning systems
Temperature and Pressure Relationship of Refrigerant R-12 • R-12 has a close relationship of it’s pressure and temperature on the Fahrenheit scale and pressure scale (of the refrigerant itself) • 20 degrees F/psig to 80 degrees F/psig • The objective of automotive a/c is to allow the evaporator to reach its coldest point without icing. Page 44 fig 8-3 7th edition