Transcript Mobile Air Conditioning - Educypedia, The educational

Mobile Air Conditioning
Basic A/C Operation
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System Components
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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
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2 primary systems used to control
evaporator pressure and temperature:
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TXV systems (thermal or thermostatic
expansion valve)
OT systems (orifice tube)
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Fixed
Variable
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Low Side Operation
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Goal is to provide constant evaporator
temperature of 32 degrees
Refrigerants have low boiling points
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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
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Expansion device
Evaporator
Accumulator (if equipped)
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Expansion Devices
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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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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
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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
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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.
Animation: TXV Operation
Internally
Balanced
Externally
Balanced
Block Valve
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Thermal Expansion Valves, TXVs
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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
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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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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.
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Refrigerant Charge Levels
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Low Refrigerant Levels:
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Allow refrigerant to vaporize before
evaporator
Prevent proper heat transfer in evaporator
Causes low system pressures
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Refrigerant Charge Levels
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High Refrigerant Levels:
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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
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Cycling Clutch Systems
Evaporator Pressure Controls
Variable Displacement Compressors
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Cycling Clutch Systems
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Pressure control
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Switch mounted on low side
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Accumulator
Suction line
Cycles compressor on at 42-49 psi
Cycles compressor off at 22-28 psi
Temperature control
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Thermistor senses evaporator temperature
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Evaporator Pressure Controls
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Devices used to control evaporator pressure
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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
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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
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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
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High begins at compressor and ends at
expansion device
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Compressor
Condenser
Receiver-drier (if equipped)
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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
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Compressor Operation
In/Suction:
Low Pressure,
about 30 psi &
Low Temperature,
close to freezing
Out/Discharge:
High Pressure,
about 200 psi &
High Temperature,
above ambient
The compressor increases the
refrigerant pressure about five
to ten times. This increases the
temperature so heat can leave
the refrigerant in the condenser.
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Piston Compressors
Piston
This two-cylinder
Connecting
compressor uses a
Rod
crankshaft to move the
pistons up and down. Crankshaft
Refrigerant flow is
controlled by the suction
and discharge reeds in
the valve plate.
Animation: Piston Compressor
Reed Valve Plate
Shaft Seal
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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
Animation: Scotch Yoke Compressor
Yoke
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Scroll Compressors
The orbiting scroll is
driven by the crankshaft
and moves in a small
circular orbit. The fixed
scroll remains stationary
Fixed
Scroll
Orbiting
Scroll
Shaft Seal
Clutch Assembly
Animation: Scroll Compressor
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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
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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.
Discharge
Reed
Animation: Vane Compressor
Shaft Seal
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Wobble Plate Compressors
Piston
Wobble Plate Bearing Angle/Drive Plate
Animation: Wobble Plate Compressor
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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
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
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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 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.
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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
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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:
Barb
Connections,
Note Sight
Glass
Male Flare
Connections
•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.
Male O-ring
Connections,
Note Switch
•provide a location for switch mounting.
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High Pressure Controls
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High pressure cutoff
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Switches designed to open compressor
clutch circuit at high pressures
High pressure release
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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
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