HVAC Lesson A/C Systems

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HVAC Lesson

A/C Systems
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Purpose of A/C system bb

• To remove heat from the passenger compartment and
  dissipate it to atmosphere
• To remove moisture from the air in the passenger
  compartment (lowers humidity)

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Principles of nature necessary for A/C to work
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Heat

• Heat will always flow from a warmer object to a
  colder one
• Nature is will try to balance out heat - heat is
  tangible not cold
• All materials contain some heat, even ice
• All heat is not gone until -460F

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Matter - does it????

• All matter has three states
• Solid
• Liquid
• Gas
• The molecules that make up the material are the
  same in every state --- it is the same material

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Suckin it up

• When any material changes state it absorbs or
  gives off heat
• When water reaches 212F it will not change into
  steam unless more heat is added
• The water can not get hotter than 212F as heat is
  added
• Additional heat will be absorbed and cause it to
  change to steam faster

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Ice 2?

• If ice is at 32F adding just enough heat to
  change it into water will create water at 32F

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Latent heat

• Heat that is added or removed from a material to
  cause it to change state is called latent heat
• Latent heat can not be measured with a
  thermometer and can not be felt by your skin

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Latent heat

• Latent heat is most commonly measured in Btus
• Example it takes 970 added Btus to turn one
  pound of water into one pound of steam
• Example 970 Btus must be removed from one pound
  of steam to create one pound of water
• Can be measured in calories

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Boiling points

• Raising the pressure on a material will cause
  its boiling point to raise
• Lowering pressure will cause boiling point to
  drop
• Some materials such as A/C refrigerants will boil
  at less than room temperature

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Vapor pressure

• If a liquid that has a lower boiling temp than
  ambient temp is in a closed container, it will
  boil until the expanding vapor creates enough
  pressure to raise the boiling point above ambient
  temp
• Pressure on R12 at room temperature will change
  with heat

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Air pressure at sea-level is _______ psi

• Pressures below atmospheric are called a
  ____________
• Vacuum is most commonly measured in inches of
  ___________ or HG

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Cooling of passenger compartment is performed by
absorbing the heat from the air

• The heat is absorbed in the evaporator
• The heat is absorbed as latent heat

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The dissipating of heat is performed by
transferring the heat to the outside air

• The heat is dissipated in the condenser
• The heat given off was stored as latent heat
• The latent heat is carried in the refrigerant

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Refrigerant stages

• Refrigerant will go through four stages as it
  circulates through the A/C system
• Refrigerant will be a low pressure liquid with a
  low boiling point
• It will then boil into a gas in the passenger
  compartment and absorb heat in the process
  (evaporator)
• As it is a gas, it will then be charged to a high
  pressure with a high boiling point
• It will then dissipate the latent heat and
  condense back to a liquid (condenser)

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Different types of refrigerant in use today is
R12 and R134a
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R12

• R12 is a CFC and is responsible for ozone layer
  damage
• R12 is no longer manufactured legally after Jan
  1, 1996
• R12 is a bad, bad thing

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R134a

• R134a is a HFC (hydrofluorocarbon) and will not
  damage the ozone layer - as much
• R134a is the only refrigerant sold in new
  vehicles today

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Comparisons

• Pressure vs. temperature is somewhat different
  between the two gasses
• R12 actually works better in the automotive
  setting
• Commonly purchased in 30lb and 50lb containers
• R12 is stored in white containers
• R134a is stored in light blue containers

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A/C system basics

• The A/C system is divided into two parts low
  pressure and high pressure
• Components in the A/C system work to create low
  and high sides, or they support heat transfer, or
  they support the other components

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Components

• Compressor
• Condenser
• Evaporator
• Restriction
• Accumulator / receiver
• Dryers / desiccants

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WELCOME BACKJ
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Low side - where the heat is absorbed

• Goal of the low side is to maintain as low as
  temperature as possible without falling below 32F
• If temperature gets too low, condensed moisture
  will freeze on evaporator
• Low side begins at the restriction
• TXV (thermal expansion valve)
• OT (orifice tube)

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Evaporator

• Has controlled flow of refrigerant through core
• Refrigerant should enter as a liquid and boil
  about ½ of the way through the core with mostly
  gas exiting
• Will condense moisture on fins
• Moisture will drain off requiring the need for
  evaporator drains
• Wet evaporators will trap dirt and small
  particles in the air moving across it

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TXV

• Variable restriction before evaporator inlet
• Will sense evaporator temps or evaporators
  pressure and temps
• Will use a sensing bulb held closely to
  evaporator outlet line to control TXV
• It will try to maintain pressures as low as
  possible without evaporator freezing

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STV or POA

• Some TXV systems will also have a STV (suction
  throttling valve) or a POA (pilot operated
  absolute valve)
• These valve are at the evaporator outlet line to
  prevent evaporator freezing due to abnormally low
  pressures in the evaporator
• They monitor pressures of evaporator and restrict
  refrigerant flow to compressor

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OT (orifice tube)

• A fixed orifice before evaporator to cause a
  pressure drop for evaporator
• Will use no variable valves
• Will use a cycling compressor clutch system
  (turning compressor on and off) or a variable
  displacement compressor to prevent evaporator
  freeze up and oil starvation

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Compressor clutches

• Used to connect and disconnect compressor with
  spinning belt
• Uses stationary winding to create magnetic field
• Field acts on armature mounted to front of
  compressor
• Armature is pulled in causing engagement with
  pulley

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Clutch control devices

• Low side low pressure switches
• High side low pressure switches
• High side high pressure switches
• Ambient air temperature switch
• Computers

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Cycling compressor clutch system

• This system uses a pressure switch in the low
  side
• If system pressures fall too low, the compressor
  turns off allowing low side pressures to rise
• The pressure switch is set to open the electrical
  circuit to the clutch when pressures fall to the
  point of evaporator freeze up or loss of oil
  movement (28-35psi for R12)

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Accumulators

• Mounted after evaporators
• Used to store any liquid refrigerant that makes
  it through evaporator
• Used on most cycling clutch / orifice tube
  systems
• Usually contains a desiccant

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Compressor

• End of the low side and beginning of the high
  side
• Nothing more than a belt driven pump
• Operate similar internally to engines

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Compressor pressure relief valves

• Will be used in the event of dangerously high
  pressures
• If valve opens will make loud popping noise and
  will vent off refrigerant to atmosphere

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SKIP NEXT 3 SLIDES
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Piston compressors

• Piston operation and arrangement will vary
• Inline or V arrangement with crankshaft
• Radial with yoke
• Axial with swash plate
• Coaxial with wobble plate
• Coaxial with variable wobble plate
• By internally sensing refrigerant pressures, the
  compressor will vary its internal displacement
  causing varying amounts of suction

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Rotary compressors

• Works like a rotary engine

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Scroll compressors

• Smooth and easy to operate

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Compressor lubrication

• Oil is carried throughout system by suspension in
  refrigerant
• Some older compressors had oil sumps to store oil
• R12 systems use a specific mineral oil
• R134a systems use either a PAG (polyalkylene
  glycol) oil or an ester oil
• Lubricants can not be intermixed
• Retrofit problem of mixing oils

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High side- where the heat is dissipated

• Goal of the high side is to condense the high
  pressure gas back to a liquid while giving off
  the latent heat stored
• High side starts with the compressor supplying
  high pressure gas

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Condenser

• Acts like a radiator to give off heat and
  condense the refrigerant back to a liquid
• Will be made up of tubes and a core similar to a
  radiator
• Can be affected by an excessively hot radiator

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Condesnser air flow

• Will have some means of air flow across fins to
  dissipate heat AND ensure acceptable pressures
• Commonly use electric fans ran off of a high side
  pressure switch

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Receiver/dryer

• Some systems that dont use an accumulator will
  have a receiver/dryer after the condenser
• Will store any remaining gas
• Filters liquid for debris
• Contains desiccant which all refrigerant must
  pass through

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Sight glass

• Just a window into the high side liquid line
• Used for diagnosis of system

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Hoses and lines

• Most connections must be flexible to allow for
  movement of components
• Most common type of rubber hose in use today is a
  5 - 7 layer barrier hose
• Barrier hoses have a layer of nylon inside to
  keep refrigerant loss low
• Barrier hoses necessary for R134a

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Hoses and lines

• Hose and line fittings will normally be sealed
  with an o-ring
• Orings now commonly made from neoprene to be
  compatible with R134a
• Fords have spring locks but most are threaded
  fittings

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END IT NOW DUMMY