HVAC Lesson A/C Systems
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)
Principles of nature necessary for A/C to work
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
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
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
Ice 2?
If ice is at 32F adding just enough heat to change it into water will create water at 32F
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
Latent heat
Latent heat is most commonly measured in Btu’s – Example; it takes 970 added Btu’s to turn
one pound of water into one pound of steam
– Example; 970 Btu’s must be removed from one pound of steam to create one pound of water
Can be measured in calories
Boiling points
Raising the pressure on a material will cause it’s 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
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
Air pressure at sea-level is _______ psi
Pressures below atmospheric are called a ____________
Vacuum is most commonly measured in inches of ___________ or ”HG
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
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
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)
Different types of refrigerant in use today is R12 and R134a
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
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
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
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
Components
Compressor Condenser Evaporator Restriction Accumulator / receiver Dryers / desiccants
WELCOME BACK
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)
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
TXV
Variable restriction before evaporator inlet
Will sense evaporator temps or evaporator’s 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
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
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
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
Clutch control devices
Low side low pressure switches High side low pressure switches High side high pressure switches Ambient air temperature switch Computers
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)
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
Compressor
End of the low side and beginning of the high side
Nothing more than a belt driven pump
Operate similar internally to engines
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
SKIP NEXT 3 SLIDES
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 it’s internal displacement causing varying amounts of suction
Rotary compressors
Works like a rotary engine
Scroll compressors
Smooth and easy to operate
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
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
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
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
Receiver/dryer
Some systems that don’t 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
Sight glass
Just a window into the high side liquid line
Used for diagnosis of system
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
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
END IT NOW DUMMY