# HVAC Lesson A/C Systems

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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 J

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 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