HVAC Lesson A/C Systems. Purpose of A/C system bb u To remove heat from the passenger compartment and dissipate it to atmosphere u To remove moisture.

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