1. HVAC Lesson A/C Systems

2. 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)

3. Principles of nature necessary for A/C to work

4. 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

5. 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

6. 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

7. Ice 2?
If ice is at 32F adding just enough heat to change it into water will create water at 32F

8. 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

9. 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

10. 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

11. 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

12. Air pressure at sea-level is _______ psi
Pressures below atmospheric are called a ____________
Vacuum is most commonly measured in inches of ___________ or ”HG

13. 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

14. 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

15. 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)

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

17. 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

18. 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

19. 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

20. 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

21. Components Compressor Condenser Evaporator Restriction
Accumulator / receiver
Dryers / desiccants

22. WELCOME BACK J

23. 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)

24. 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

25. 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

26. 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

27. 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

28. 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

29. Clutch control devices
Low side low pressure switches
High side low pressure switches
High side high pressure switches
Ambient air temperature switch
Computers

30. 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)

31. 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

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

33. 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

34. SKIP NEXT 3 SLIDES

35. 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

36. Rotary compressors
Works like a rotary engine

37. Scroll compressors
Smooth and easy to operate

38. 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

39. 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

40. 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

41. 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

42. 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

43. Sight glass Just a window into the high side liquid line
Used for diagnosis of system

44. 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

45. 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

46. END IT NOW DUMMY