Basics of Heating and Air Conditioning

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Basics of Heating and Air Conditioning

• Heat energy
• Heat measurement
• Heat movement
• States of matter
• Latent and sensible heat
• Boiling points
• Pressures- gauge and absolute
• Refrigerants

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Heat Is Energy
Heat, like other forms of energy, cannot be
created nor destroyed. However, we have the
ability to move it from one place to another.
Moving heat is more efficient than using energy
to produce heat.
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Heat Transfer
Mobile heating, ventilation, and air
conditioning, MVAC systems are concerned with
moving heat to or from the interior of a vehicle.
Heat transfer involves moving something that we
can feel but cannot see.
Understanding the principles of heat transfer
helps a technician understand what happens within
an A/C system.
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Heat Measurements, Temperature
We are all familiar with temperature. This is a
measurement of heat intensity. It is measured
using a thermometer. Two different scales are
commonly used Celsuis (C)
which is used in most of the World and
Fahrenheit (F) which is commonly used in the
United States.
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Heat Measurements, Quantity
Heat quantity is also measured using two scales
Calories (c) by most of the World and British
Thermal Units (BTU) in the United States. One
BTU will increase the temperature of one pound of
water by one degree F. One c will increase the
temperature of one g of water by one degree C.
Burning a large wooden match produces about 1 BTU
of heat.
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Intensity and Quantity
A MHVAC technician is concerned with heat
intensity when he or she is testing or repairing
a HVAC system to make sure it is operating
correctly. A vehicle design engineer is
concerned with heat quantity while designing a
system to make sure it is the correct size to
provide adequate heat transfer. It must move
enough heat to keep the interior of a vehicle
warm in cold climates and cool in very hot areas.
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Heat Measurement

• Humidity the amount of water vapor suspended in
  the air
• Warm air can hold much more water vapor than cold
  air
• Humid cold air feels much colder than cold dry
  air
• Humid hot air slows down evaporation of
  perspiration, which makes the air feel hotter

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Heat Movement
Heat always travels from Hot to Cold.
In this example, heat within the engine will move
from the very hot combustion chamber to the
cooler coolant. After the coolant is moved to the
radiator, the heat will move to the cooler air
passing through it.
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Heat Transfer Rate
Heat transfer rate is determined by the
temperature difference between the hot and cold
items. A minor difference (top) will have a slow
rate. A large difference will cause a much faster
rate of heat exchange.
Animation Heat Transfer
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Heat Movement

• 3 ways heat can move
• Conduction
• Convection
• Radiation

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Heat Conduction
Heat transfer through a material, from one
molecule to the next is called conduction. The
heat entering this metal rod is flowing to the
ends.
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Convection
When we move heat by circulating a medium like
air or water, we call the process convection.
The coolant circulating in the cooling system
transfers heat from the engine to the radiator
and the warm or cool air circulating through the
vehicle are both examples of convection.
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Radiation
The heat transfer from the Sun to the Earth is an
example of radiation. Heat is transferred, but it
does not warm the medium (space) through which it
passes. Radiant heat will warm the interior and
exterior of a vehicle that is parked in the sun.
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Adding Heat
When we add heat, we will warm up the liquid, in
this case it is water. We are converting the
energy in the fuel to fire and transferring this
heat to the water. We would have to remove heat
if we wanted to cool the water.
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States of Matter
Solid Ice Below 32 F Liquid
Water
Water is a molecule formed from one hydrogen atom
and two oxygen atoms. So is ice and steam. Ice is
normally a solid at cold temperatures, and steam
is the high temperature, gas form of water. They
are all the same molecule, but they have a
tighter or looser molecular bond between the
atoms. It takes energy to change this bond.
Water H2O
Water H2O
Gas Steam, Above 212 F
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Sensible and Latent Heat

• Sensible heat causes a change in temperature
• Latent heat causes a change of state but no
  change in temperature

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Sensible and Latent Heat
If we add heat to water, the temperature will
increase, and this is called sensible heat.
Sensible heat causes a change in temperature. If
we add heat to ice at 32o F or to water at 212o
F, the temperature will not increase. The added
heat will melt some of the ice or boil some of
the water. This is called latent heat. Latent
heat causes a change in state but not a change in
temperature. This heat energy changes the
molecular bond within the molecule.
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Latent Heat of Evaporation Condensation
A rather large amount of heat, 979 BTUs, is
required to change one pound of water into one
pound of steam with no temperature increase. This
change of state is used as the refrigerant boils
when it enters the evaporator and condenses when
it passes through the vehicles condenser.
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Latent Heat of Vaporization

• It takes 180 Btus to raise the temperature of 1
  lb. of water from 32 deg. to 212 deg.
• It takes 970 Btus to boil or vaporize 1 lb. of
  water at 212 deg.

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Latent Heat of Vaporization

• In an automotive A/C system, the refrigerant
  changes state and absorbs heat in the evaporator
  and releases heat as it changes state again in
  the condenser
• Evaporator liquid changing to gas
• Condenser gas changing to liquid

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Boiling Points

• Boiling points can be increased or decreased by
  raising or lowering the pressure on the liquid
• Increased pressureincreased boiling point
• Decreased pressuredecreased boiling point

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Boiling Points and Pressure
Atmospheric Pressure
10 psi
Boiling Point 212 Degrees F
Boiling Point 242 Degrees F
The boiling point will increase if we raise the
pressure on a liquid. In this case, the boiling
point will increase about 3o F for each psi. A 10
pound pressure will increase the boiling point
about 3 X 10 or 30o F. The condensing point of a
vapor is the same as the boiling point.
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Boiling Points

• Why are A/C systems evacuated prior to recharging?

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Boiling Points

• At 29 of vacuum, the boiling point of water is
  76 deg. F

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Boiling Points

• The condensing point of a gas is the same as the
  boiling point
• Heat is removed from a gas at 212 deg. to cause
  the gas to condense to a liquid
• Raising the pressure of a gas allows the gas to
  condense at temperatures above the normal boiling
  point

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Boiling Points

• Critical temperature the maximum point at which
  a gas can be liquefied by raising the pressure
• Critical pressure the pressure that is necessary
  to liquefy a gas at critical temperature

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Boiling Points

• Superheat the temperature of a vapor above its
  boiling point
• Saturated vapor a liquid and a gas inside a
  closed chamber

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Pressures

• Pressure a certain amount of force exerted on an
  area
• Pressure at sea level is 14.7 psi
• Vacuum pressures below atmospheric pressure
• Vacuum is measured on a gauge calibrated in
  inches of mercury
• 29.92Hg. is a perfect vacuum and represents no
  pressure (0 psi)

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Pressures

• A deep or high vacuum is close to a perfect
  vacuum and measured in microns of mercury
• Gauge pressures gauges with zero points at
  atmospheric pressure (psig)
• Absolute pressures gauges with zero points at an
  absolute vacuum (psia)

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Refrigerants

• The working fluid of an A/C system
• First developed by DuPont under the brand name of
  
• Refrigerants must have a very low boiling point-
  below 32 deg.
• Refrigerants must be able to mix with oil and
  remain stable

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Refrigerants

• Two main types in automotive applications
• R-12 CFC-12
• R-134a HFC-134a
• R-12 contains chlorine molecules which are
  thought to be harmful to the environment
• R-134a is very similar to R-12 but does not
  contain chlorine

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Refrigerants

• R-134a weighs about 90 of R-12
• R-134a is not compatible with mineral oil
• R-134a systems use polyalkaline glycol (PAG) oils
• R-12 systems converted to R-134a can use ester
  oil as a lubricant

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Refrigerants

• R-134a is not compatible with desiccants used in
  R-12 systems
• Refrigerants must not be mixed
• Some refrigerant blends contain flammable
  chemicals and cannot be recycled in a shop
• Abnormally high operating pressures can be caused
  by blend refrigerants

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Refrigerant
R-134a has a boiling point of 15o F. When it is
released into the vehicles evaporator it will
boil immediately and absorb heat. R-134a will
absorb almost 78 BTUs of heat for each pound that
boils.
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Refrigerant

• R134a has a critical temperature of 214 deg. F
  and a critical pressure of 589 psi
• This is the upper limits of the high pressure
  side of an A/C system using R134a

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Refrigerant Oils

• R-12 systems use mineral oil
• R-134a systems use PAG oils
• Viscosities are chosen by manufacturer
• R-12 systems converted to R-134a use ester oil