PRINCIPLES OF HEAT TRANSFER

SOURCES OF HEAT

PRINCIPLE ONE

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

pg. 41 C

PRINCIPLE TWO

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

pg. 37 C

EVAPORATION

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.

pg. 38C

PRINCIPLE THREE

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)

pg. 41 C

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

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)

pg. 43C

RADIATION

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)

pg. 42C

CONDUCTION

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)

Pg 42C

SPECIFIC HEAT

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.

Pg 40C

SENSIBLE HEAT

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

Pg 36C

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 AFFECTS BOILING POINT

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

That’s all folks!!!