Principles of heat transfer
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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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PRINCIPLES OF HEAT TRANSFER

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Principles of heat transfer

PRINCIPLES OF HEAT TRANSFER


Sources of heat

SOURCES OF HEAT


Principle one

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

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

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

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

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

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

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

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

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

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

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

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

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

  • 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

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

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 affects boiling point

PRESSURE AFFECTS BOILING POINT


Pressure increase

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

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

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


That s all folks

That’s all folks!!!


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