Mobile Air Conditioning

1
Mobile Air Conditioning

• Basic A/C Operation

2
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

3
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

4
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

5
Low Side Components

• Expansion device
• Evaporator
• Accumulator (if equipped)

6
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

7
TXV System
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.
Animation A/C Systems, TXV
8
OT System
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
9
Thermal Expansion Valves, TXVs
The 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.
Internally Balanced
Externally Balanced
Block Valve
Animation TXV Operation
10
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

11
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.
12
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

13
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.
14
Accumulators

• Accumulators 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.

15
Refrigerant Charge Levels

• Low Refrigerant Levels
• Allow refrigerant to vaporize before evaporator
• Prevent proper heat transfer in evaporator
• Causes low system pressures

16
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

17
Evaporator Icing Controls

• Cycling Clutch Systems
• Evaporator Pressure Controls
• Variable Displacement Compressors

18
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

19
Evaporator Pressure Controls

• Devices used to control evaporator pressure
• STVs (suction throttling valves)
• POAs (pilot-operated absolute)
• EPRs (evaporator pressure regulator)
• Usually mounted in evaporator outlet or
  compressor inlet
• Used to restrict refrigerant flow to compressor

20
Variable Displacement Compressors

• Provide smooth compressor operation
• Maintain constant evaporator temperature
• Reduces compressor load on engine when system
  cooling load is low

21
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

22
High Side Components

• High begins at compressor and ends at expansion
  device
• Compressor
• Condenser
• Receiver-drier (if equipped)

23
Compressors
There 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
24
Compressor Operation
Out/Discharge High Pressure, about 200 psi
High Temperature, above ambient
In/Suction Low Pressure, about 30 psi
Low Temperature, close to freezing
The compressor increases the refrigerant pressure
about five to ten times. This increases the
temperature so heat can leave the refrigerant in
the condenser.
25
Piston Compressors
Reed Valve Plate
Piston Connecting Rod Crankshaft
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.
Shaft Seal
Animation Piston Compressor
26
Scotch Yoke Compressors
Discharge Reed Suction Reed
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
Animation Scotch Yoke Compressor
27
Scroll Compressors
Orbiting Scroll
Fixed Scroll
The orbiting scroll is driven by the crankshaft
and moves in a small circular orbit. The fixed
scroll remains stationary
Shaft Seal
Clutch Assembly
Animation Scroll Compressor
28
Swash Plate Compressors
Pistons
Reed Plate
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,
Swash Plate
Shaft Seal
Clutch Assembly
Animation Swash Plate Compressor
29
Vane Compressors
Rotor Vane
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.
Shaft Seal
Discharge Reed
Animation Vane Compressor
30
Wobble Plate Compressors
Piston
Wobble Plate Bearing Angle/Drive Plate
Animation Wobble Plate Compressor
31
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.
32
Variable Displacement Wobble Plate Compressors
Low Angle Minimum Stroke
The evaporator pressure has dropped, and the
control valve has increased crankcase pressure
High Angle, Maximum Stroke
Normal operation when cooling is required.
Crankcase pressure is low.
Control Valve
33
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.
34
Condenser Types
Condensers 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.
35
Serpentine Condenser
Refrigerant flows from the upper inlet to the
bottom outlet through two tubes. These tubes wind
back and forth though the condenser.
36
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.
37
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.

38
Receiver Dryers
Barb Connections, Note Sight Glass
Male Flare Connections

• 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.

Male O-ring Connections, Note Switch
39
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

40
Pressure Release
Pressure Relief Valve
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.
Fuse Plug