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Refrigerant and the Ozone layer : Climate
change
Refrigerant gas leak possibly damages the
ozone layer (search Montreal protocol and Kyoto
protocol about climate change on Google) and there is strict legislation
regarding recovery and recycling of refrigerant
from vehicle airconditioning systems.
Air Conditioning System Pressure Temperature
Relationship of Refrigerant Gases:
Vehicle air conditioning systems rely on change
of temperature of refrigerant gas flowing
through the fluid circuit of the air con system
as the pressure of the refrigerant changes. As
gas pressure increases ,temperature also
increases and vice versa.
Basic vehicle air conditioner components and
function
Refrigerant
The refrigerant is the fluid of
the air conditioning system which transports
heat. The air
conditioning system will not function without
refrigerant. Most vehicle air conditioning
systems use R134a as the refrigerant.
A gradual loss of refrigerant gas can occur over time.
The only effective cure for a refrigerant leak
is a repair.
Air conditioning systems which have lost the
refrigerant gas charge will require leak
testing with Nitrogen gas and repair before a
new refrigerant gas charge can be undertaken.
Persons handling refrigerant must be
qualified as a refrigerant handler and meet the
requirements of EC307/2008 from 4th July 2010.
Compressor
The
heart of the air conditioning system is the
compressor which acts as a pump to move the
flow of refrigerant around the system. Most
vehicle air conditioning compressors are engine driven via the auxiliary
drive belt and an electromagnetic clutch
pulley. The compression of refrigerant gas
causes an increase of gas pressure and
temperature at the outlet of the compressor.
The compressor can only compress gas. If liquid
enters the compressor component damage will
result.
Air
conditioning systems have a Low pressure side
(suction side of compressor) and a High
pressure side (output side of compressor).
Manifold Pressure gauges can be attached to
service ports fitted to Low side and High side
refrigerant circuit of the air conditioner
system to aid fault diagnosis.
The lubrication oil (POE is recommended for
R134a systems) for the air conditioning
refrigerant compressor is injected in to the
refrigerant circuit during service.
A refrigerant leak may
cause loss of this lubrication oil and
eventually result in compressor damage.
Condenser (high pressure side)function :
Transfer heat from refrigerant.
The
Condenser is usually located near to the front
of the vehicle just in front of the coolant
radiator.
High temperature ,high pressure
refrigerant gas changes state to
refrigerant liquid by condensation as cooling
air is blown across the radiator fins of the
condenser by the radiator fans or condenser fan. Refrigerant
flow direction is from top inlet as gas to
bottom outlet as liquid. The Condenser is prone
to stonechip and impact damage which may cause
leaks.
Suction accumulator
A suction accumulator
may be fitted at the evaporator outlet
(compressor inlet side) to remove liquid from
the refrigeration circuit to prevent compressor
damage. Suction Accumulator devices are fitted
to systems with orifice tube type metering
devices.
Receiver dryer: function: To remove
moisture and particle contamination (metallic
particles,rust,silica gel, dirt and debris) from refrigerant circuit
A receiver dryer
containing a desiccant gel pack to absorb moisture may be
fitted after the compressor outlet (usually
located between condenser and evaporator) . Receiver filter dryers are
fitted to systems with thermal expansion valve. During operation , moisture
in the air conditioning circuit is removed by
the filter dryer. Filter dryer units are a
service item requiring periodic renewal as
saturation occurs. Vehicle manufacturers
recommend the receiver dryer is renewed every 2
to 3 years or 24000 miles or if the system has
been opened to air for more than two hours (eg
system dismantled). Standard vehicle aircon systems will have
either a receiver dryer or an accumulator.
Refrigerant passing around the air conditioner
circuit flows through two heat exchangers ,
namely the evaporator radiator usually located
behind the dashboard, and the condenser
radiator located at the front of the engine bay
in front of the coolant radiator. Transfer of
heat occurs as air is blown through each heat
exchanger radiator fins by fans.
Metering device : function to regulate flow
of refrigerant
Depending on system type,
either a thermal expansion valve or orifice tube
restricts the quantity of liquid refrigerant
entering the evaporator. This is the point
where blockage can occur particularly if a
compressor fails or ice build up occurs.
Evaporator (low pressure side) : function:
Transfer heat to refrigerant . Purify and dehumidify air.The
pressurised liquid refrigerant passes via the
metering device into the evaporator and changes
state to a low pressure vapour.
Air enters the
vehicle cabin via a pollen filter which
initially removes some dust particles . The air
is then blown through the fins of the
evaporator by the blower fan. Heat transfer
occurs as heat is removed from the air as it
passes the evaporator fins and out of the
ventilation ducts providing a cool breeze
inside the vehicle. Heat transfer occurs
between the evaporator coils and the
refrigerant causing the refrigerant to boil
into a vapour as it flows away from the
evaporator .
The evaporator also acts as
a dehumidifier and cleaner as moisture in the
air which passes through the evaporator fins
forms condensation on the external surface
before exiting via drain tubes. The
evaporator can be a source of bad odours inside
the vehicle of a build up of
contamination occurs
Pressure Switches
High pressure switch acts to control
condenser electrical fan and compressor
electromagnetic clutch operation. Low pressure
switch operates to prevent compressor operation
in cases where a refrigerant leak may have
occurred resulting in loss of system
lubrication fluid which could cause compressor
seizure.
Moisture in the Ac system
The main enemy of the air conditioning system
components is moisture because; Moisture can form ice and
cause refrigerant circuit components to block. Moisture
within the refrigerant circuit can
mix with oil to form acids which corrode the
air conditioner system components.
Moisture accumulating on the surface of the
evaporator and within air vents can breed
bacteria causing smells inside the vehicle (
Sick car syndrome)
Moisture can enter the refrigerant system if a
leak occurs allowing air ingress or if the
system has been opened.
Removal of Moisture during air conditioning
service
Moisture is removed from the air conditioner
system prior to adding refrigerant by using a
vacuum pump to deep vacuum the air con system
causing moisture to boil off as a vapour.
Moisture is also absorbed by the PAO or PAG oil
which should be drained and replenished. The
filter dryer should also be renewed.
Electrical fault diagnostics:
If the
blower motor has stopped working it may be as simple as a blown fuse or faulty switch, or could be more complex in
a vehicle fitted with climate control and air conditioning.
A typical climate controlled vehicle has a drivers control panel linked to a climate control computer
with associated electrical circuits. The driver sets
the desired temperature , then the computer processes other inputs from various temperature sensors and pressure switches
located in the cabin and engine compartments of the vehicle and provides outputs to control air vent flaps, blower motor speed,
radiator fans,
and switch the air conditioning compressor electromagnetic clutch on or off, enabling the preset temperature to be maintained.
Even heat from the sun can affect the cabin temperature of the vehicle as it shines on the windscreens so a solar sensor may also be fitted.
The extra load demand introduced as a vehicle air conditioning system operates means the vehicle will be fitted with a
heavier duty alternator than standard to cope with extra power requirements.
Climate control systems are often linked to engine control systems so idle speed is increased when the air conditioning
is switched in, or when the vehicle is accelerating at wide open throttle (WOT) the air conditioning system compressor
may be
temporarily disengaged.
Faults with computerised climate control
systems may be diagnosed with the aid of
electronic vehicle diagnostic equipment.
Aftermarket equipment is available for most vehicles.
Typical uses for stand alone heaters :
Night heaters in trucks.
Thermal comfort systems for ambulances, minibuses,coaches,marine craft,etc.
Air conditioning units for retrofit on tractors and plant.
Equipment such as crop sprayers may require additional carbon filtration to remove chemical from the atmosphere.
Hot water handwash units are also available for fitment to service vehicles
We are pleased to answer any enquiries.
West Mercia Mechanical Services,
10 Arrow Road,
Telford.TF5 0LF.
United Kingdom.
Telephone: 01952 247007
Mobile: 07703 558610
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