Refrigerant and the Ozone layer : Climate change
Leakage of Refrigerant gas is reported to damage the ozone layer
(search Montreal protocol and Kyoto protocol about
climate change).
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 and pressure of refrigerant gas flowing through the fluid circuit of the
air conditioning system. 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. Commercial
vehicle transport refrigeration systems may use alternative refrigerants.
A gradual loss of refrigerant gas can occur over time. The only effective cure for a refrigerant leak is a repair or component renewal.
Air conditioning systems which have lost the refrigerant gas charge may require leak testing with Nitrogen gas and subsequent 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
raise pressure and 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.
Commercial vehicle refrigeration systems may be powered by a diesel engine or electric motor.
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 (blue guage line) and High side (red guage line)
refrigerant circuit of the air conditioner system to aid fault diagnosis.
The lubrication oil (POE is recommended for R134a systems) for the air conditioning refrigeration 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 and seizure.
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 passing through the condenser changes state to
liquid refrigerant by condensation.
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.
Due to its location near the front of the vehicle the condenser is often 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,
nsilica 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 , receiver dryer gel pack
collapses, or ice build up occurs.
Evaporator (low pressure side) :
function:
Transfer heat from inside the passenger compartment or loadspace to refrigerant . Purify and dehumidify air.
The pressurised liquid refrigerant passes via the metering device into the evaporator and changes state
(refrigerant has a low boiling point) 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 blown air passing through the evaporator fins and out of the
ventilation ducts providing a cool breeze inside the vehicle.
Inside the evaporator 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 (sometimes noticed as water under the vehicle).
The evaporator can be a source of bad odours inside the vehicle if a build up of surface 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.
The switch unit may be combined as a trinary switch.
Moisture and the Air conditioning system
Moisture is a main enemy of the air conditioning system components 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
( aka 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 use of 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 a blower motor has stopped working it may be as simple as a blown fuse, faulty switch, or failed motor unit.
Alternatively it 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 fan speed,
and switch the air conditioning compressor electromagnetic clutch on or off, enabling the preset temperature to be maintained.
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 may well be fitted with a higher output alternator
than standard to cope with extra power requirements.
Climate control systems are often linked to engine management 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 require diagnosis with the aid of
electronic vehicle diagnostic equipment.
Aftermarket equipment is available for most vehicles.
Thermal comfort systems for ambulances, minibuses, coaches, Hgv trucks ,
motorhomes, marine craft, Municipal vehicles .
Roof Mount Air conditioning units for retrofit on tractors , combines and plant.
Considerations when specifying an add on mobile air conditioning system to
vehicles:
BTU Rating :
British Thermal Unit Rating : This figure defines the cooling capacity of the
air conditioner in btu / hour.
Glass area of cab
: Solar radiation from sunlight will cause heat gain in the passenger
compartment. Some energy will be reflected by the glass, some will be absorbed
and transferred to the passenger compartment by convection.
Passenger Capacity
: Body heat generated by passengers will increase cabin temperature.
Onboard Equipment
: Mobile kitchen equipment , Computers, electric motors etc can generate
additional heat.
Loadspace :
Determines the volume of air to be cooled .
Climate temperatures
in area where vehicle will primarily be used.
Filtration :
Pollen filters are fitted as standard to most oe vehicle air conditioning
systems.
Crop sprayers with air conditioning fitted may require additional carbon
filtration to remove chemical odour from the atmosphere entering the cabin *
see hse guidance notes
relating to COSHH.
Power Supply : Many retrofit vehicle air conditioners are powered
by 110v or 240v electric motors which require special power supply equipment
(inverters) to operate from standard 12v dc vehicle electrics. Auxiliary
batteries , battery management system, split charging device and uprated
alternator may also be required.
Vibration
: Air conditioning equipment used on industrial machinery can be
subjected to increased vibration levels which may have warranty implications.
Noise Level : The
noise level generated by air conditioning or refrigeration compressors, engines
, motors and fans can be a consideration particularly where equipment will be in
use at night.
Mounting Position
: Roof mounted air con units may increase vehicle height . Mobile air
conditioner may form one integrated unit or may be a split system with
components mounted at different areas of the vehicle.
Ducting : Air
distribution by ducting to required areas.
Serviceability :
Type of refrigerant , service points fitted for regas, type of compressor fitted
. Some packaged systems may be fitted with a electrically driven hermetically
sealed compressor requiring low maintenance. A seperate engine driven compressor
requiring scheduled servicing may be fitted to split ac systems.
Hot water handwash units are also available for fitment to service vehicles
We welcome enquiries.
West Mercia Mechanical Services,
10 Arrow Road,
Telford.TF5 0LF.
United Kingdom.
Telephone: 01952 247007
Mobile: 07703 558610
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