PRINCIPLES OF HEAT TRANSFER

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PRINCIPLES OF HEAT TRANSFER
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SOURCES OF HEAT
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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
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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

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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.

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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)

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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
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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
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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.

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PRINCIPLE SIX

• When a vapor is compressed, its temperature and
  pressure will increase even though heat has not
  been added

pg.. 10 fig 10
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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)

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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)

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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)

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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.

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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

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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
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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)

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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
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PRESSURE AFFECTS BOILING POINT
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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

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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

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Temperature and Pressure Relationship of
Refrigerant R-12

• R-12 has a close relationship of its 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
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Thats all folks!!!