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Mobile Air Conditioning

Basic A/C Operation

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System Components All systems contain six major components: Compressor to circulate the refrigerant Condenser to transfer heat to ambient air Receiver dryer or accumulator to store reserve

refrigerant, contain the desiccant, and filter refrigerant

Expansion device to control amount of refrigerant entering evaporator

Evaporator to transfer heat from in-vehicle air to the refrigerant

Lines and hoses to connect these parts together

And the refrigerant

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Automotive A/C Systems 2 primary systems used to control

evaporator pressure and temperature: TXV systems (thermal or thermostatic

expansion valve) OT systems (orifice tube)

Fixed Variable

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Low Side Operation Goal is to provide constant

evaporator temperature of 32 degrees

Refrigerants have low boiling points When liquid boils, it absorbs large

amounts of heat Amount of heat absorbed in evaporator

is proportional to amount of refrigerant boiled

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Low Side Components Expansion device Evaporator Accumulator (if equipped)

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Expansion Devices The expansion device separates the high side

from the low side and provides a restriction for the compressor to pump against.

There are two styles of expansion devices: The TXV can open or close to change flow. It

is controlled by the superheat spring, thermal bulb that senses evaporator outlet temperature, and evaporator pressure

Most OTs have a fixed diameter orifice

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A TXV controls the refrigerant flow from the high pressure side to the evaporator. A receiver dryer is mounted in the liquid line of all TXV systems.

TXV System

Animation: A/C Systems, TXV

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An OT controls the refrigerant flow from the high pressure side to the evaporator. An accumulator is mounted in the suction line of all OT systems.

Animation: A/C Systems, OT

OT System

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Thermal Expansion Valves, TXVsThe three major types of expansion valves:

Internally balanced TXVs are the most common.

Externally balanced TXVs are used on some larger evaporators.

Block valves route the refrigerant leaving the evaporator past the thermal sensing diaphragm so a thermal bulb is not needed.

Animation: TXV Operation

Internally Balanced

Externally Balanced

Block Valve

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Thermal Expansion Valves, TXVs

Variable valve that can change size of opening in response to system load

Opens or closes depending on evaporator pressure and temperature

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Orifice Tubes, OTs

The OT used in a modern vehicle is a tubular, plastic device with a small metal tube inside. The color of the OT is used to determine the diameter of the tube. A plastic filter screen is used to trap debris that might plug the tube.

Some older General Motors vehicles used an OT that resembled a brass fuel filter.

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Orifice Tubes, OTs Fixed diameter orifice Simple and cheap to produce Cannot respond or change

according to evaporator temperature

System requires accumulator to prevent liquid refrigerant from reaching compressor

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Evaporator Operation

Hot, liquid refrigerant flows through the expansion device in the low side to become a fine mist.

Refrigerant boils or evaporates to become a gas inside the evaporator.

The boiling refrigerant absorbs heat from the air during this change of state.

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AccumulatorsAccumulators are used in the suction line of all OT systems.

The accumulator:

•separates liquid refrigerant so only gas flows to the compressor.

•Allows oil in the bottom of the accumulator to return to the compressor.

•provides storage for a refrigerant reserve.

•contains the desiccant bag for water removal.

•provides a place to mount low pressure switches and sensors.

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Refrigerant Charge Levels Low Refrigerant Levels:

Allow refrigerant to vaporize before evaporator

Prevent proper heat transfer in evaporator

Causes low system pressures

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Refrigerant Charge Levels High Refrigerant Levels:

Prevent refrigerant expansion in evaporator

Can cause extremely high system pressures

Prevents heat transfer because of high pressure in evaporator

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Evaporator Icing Controls Cycling Clutch Systems Evaporator Pressure Controls Variable Displacement

Compressors

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Cycling Clutch Systems Pressure control

Switch mounted on low side Accumulator Suction line

Cycles compressor on at 42-49 psi Cycles compressor off at 22-28 psi

Temperature control Thermistor senses evaporator

temperature

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Evaporator Pressure Controls Devices used to control evaporator

pressure STV’s (suction throttling valves) POA’s (pilot-operated absolute) EPR’s (evaporator pressure regulator)

Usually mounted in evaporator outlet or compressor inlet

Used to restrict refrigerant flow to compressor

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Variable Displacement Compressors

Provide smooth compressor operation

Maintain constant evaporator temperature

Reduces compressor load on engine when system cooling load is low

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High Side Operation Takes low pressure vapor from

evaporator and returns high pressure liquid to expansion device

Must increase vapor temperature above ambient temperature for heat transfer to occur resulting in change of state from vapor to liquid

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High Side Components High begins at compressor and

ends at expansion device Compressor Condenser Receiver-drier (if equipped)

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CompressorsThere is a large variety of compressors. Some of variations are:

The compressor manufacturer

Piston, vane, or scroll type

The piston and cylinder arrangement

How the compressor is mounted

Style and position of ports

Type and number of drive belts

Compressor displacement

Fixed or variable displacement

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Compressor Operation

The compressor increases the refrigerant pressure about five to ten times. This increases the temperature so heat can leave the refrigerant in the condenser.

Out/Discharge: High Pressure, about 200 psi & High Temperature, above ambient

In/Suction: Low Pressure, about 30 psi & Low Temperature, close to freezing

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Piston Compressors

Animation: Piston Compressor

This two-cylinder compressor uses a crankshaft to move the pistons up and down. Refrigerant flow is controlled by the suction and discharge reeds in the valve plate.

Reed Valve Plate

Piston

Connecting Rod

Crankshaft Shaft Seal

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Scotch Yoke Compressors

Animation: Scotch Yoke Compressor

A Scotch yoke compressor has two pairs of pistons that are driven by a slider block on the crankshaft. The pistons are connected by a yoke.

Pistons Yoke

Discharge Reed

Suction Reed

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Scroll Compressors

Animation: Scroll Compressor

Fixed Scroll

Orbiting Scroll

Shaft Seal

Clutch Assembly

The orbiting scroll is driven by the crankshaft and moves in a small circular orbit. The fixed scroll remains stationary

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Swash Plate Compressors

Animation: Swash Plate Compressor

Pistons

Swash Plate

Shaft Seal

Clutch Assembly

The swash plate is mounted at an angle onto the drive shaft. It drives three double-ended pistons. Two sets of reeds control the refrigerant flow in and out of the cylinders,

Reed Plate

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Vane Compressors

Animation: Vane Compressor

Rotor Vane

Shaft SealDischarg

e Reed

The rotor is driven by the clutch and driveshaft. The vanes move in and out of the rotor to follow the outer wall to pump refrigerant.

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Wobble Plate Compressors

Animation: Wobble Plate Compressor

Piston

Wobble Plate Bearing Angle/Drive Plate

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Wobble Plate Compressors

The wobble plate does not rotate; it just wobbles, being driven by the angled drive plate that does rotate. Variable displacement, wobble plate compressors can change the angle of the drive plate, and this changes piston stroke and compressor displacement. Most wobble plate compressors have 5 to 7 pistons.

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Variable Displacement Wobble Plate Compressors

The evaporator pressure has dropped, and the control valve has increased crankcase pressure

Normal operation when cooling is required. Crankcase pressure is low.

Low Angle Minimum Stroke

High Angle, Maximum Stroke

Control Valve

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Condenser Operation

Hot, high pressure gas is pumped from the compressor to enter the condenser. The gas gives up its heat to the air passing through the condenser. Removing heat from the hot gas causes it to change state and become liquid.

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Condenser TypesCondensers A and C are round tube, serpentine condensers.

Condenser B is an oval/flat tube, serpentine condenser.

Condenser D is an oval/flat tube, parallel flow condenser.

Flat tube condensers are more efficient.

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Serpentine Condenser

Refrigerant flows from the upper inlet to the bottom outlet through two tubes. These tubes wind back and forth though the condenser.

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Parallel Flow Condenser

Refrigerant flows from the upper inlet to the bottom outlet through groups of parallel tubes. Some carry refrigerant from the right to the left, and others move it back to the right side.

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Heat Exchangers Condensers have to move heat from the refrigerant to

the air. Evaporators must move heat from air to the refrigerant. Both require a lot of contact area for both air and

refrigerant. Both require free movement of air and refrigerant.

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Receiver Dryers

A receiver dryer is mounted in the liquid line of a TXV system. It is used to:

•to store a reserve of refrigerant.

•hold the desiccant bag that removes water from the refrigerant.

•filter the refrigerant and remove debris particles.

•provide a sight glass so refrigerant flow can be observed.

•provide a location for switch mounting.

Barb Connections, Note Sight Glass

Male Flare Connections

Male O-ring Connections, Note Switch

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High Pressure Controls High pressure cutoff

Switches designed to open compressor clutch circuit at high pressures

High pressure release Designed to release refrigerant at

high pressures

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Pressure Release

A/C systems can include a pressure release valve that is usually mounted at the compressor or a fuse plug mounted on the receiver dryer. The relief valve can open at a preset pressure and then reclose. The center of the fuse plug melts to let pressure escape.

Pressure Relief Valve

Fuse Plug