KOLEJ KOMUNITI
KEMENTERIAN PENGAJIAN TINGGI
SIJIL MODULAR KEBANGSAAN : SERVIS PENYAMAN UDARA KENDERAAN
TAJUK : The Bus Air Conditioning Refrigeration Cycle
· Based on a signal from the thermostat, by means of an inter connecting electrical system, the electromagnetic clutch on the compressor engages.
· Once engaged, the compressor then circulates refrigerant through the system through inter connecting hoses.
· Refrigerant, existing as a gas, and containing the heat from the passenger compartment, is pumped by the compressor, under high pressure, into the condenser coil.
· The refrigerant undergoes a change-of-state, from a gas to a liquid, through a process called condensation.
· During condensation, the hot gas rejects its heat load to the outside air, which was transferred from the passenger compartment, into the refrigerant, by the evaporator.
· The refrigerant now exists as a cool liquid, which passes through the filter drier, which removing moisture and impurities, and then the sight glass, which enables visual inspection of the refrigerant.
· The cool liquid is then pumped to the evaporator where an expansion valve meters the refrigerant into the evaporator coil.
· Fans pull passenger compartment air through a filter, which removes particulate matter, then passes the cleaned air through the evaporator coil.
· The refrigerant undergoes a change of pressure from high to low, and a corresponding change-of-state from a liquid to a gas, through a process called evaporation.
· During evaporation, the heat contained in the air in the passenger compartment is absorbed by the gaseous refrigerant.
· A warm air passes through the evaporator coil, moisture condenses, and is collected and drained to the exterior of the vehicle,
· The hot gas is then suctioned back to the compressor and pumped to the condenser.
Thermostat
· A temperature sensitive device, which when activated, signals the electromagnetic compressor clutch to engage.
· Located in the return air of the evaporator
Inner Connecting Electrical
· Connects air conditioning components through the use of electrical wiring to control system operation
· Electrical System Control Types (General)
1. Basic
2. Microprocessor
3. Multiplex
Electromagnetic Clutch
A key component of the compressor that controls its operation
Refrigerant
· A material, which possesses high heat transfer capabilities.
· Acts as the medium for heat transfer, which facilitates the movement of heat from the passenger compartment to the outside air.
· Refrigerant, under varying pressures, exists in different states, and performs different heat absorbing functions
1. Under low pressure, refrigerant exists as a gas which can absorb heat
2. Under high pressure, refrigerant exists as a liquid which can reject heat.
· The heat transfer properties exhibited when refrigeration changes state is the foundation of the refrigeration cycle.
Inner Connecting Hoses
· Connects air conditioning components by means of hoses and fittings in order to transport refrigerant throughout the system
· Hose / Fitting Types
1. Beadlock
2. Barb
3. O-Ring
4. Aeroquip (EZ Clip)
5. TA Clip-Loc (Burgaflex)
6. Reusable ORS
Condenser
· Rejects heat, which was transferred to the refrigerant, by the evaporator, from the passenger compartment of the vehicle. As heat is rejected, gas refrigerant condenses to a liquid.
· Generally located outside the vehicle (skirt or roof)
· Systems can have one or more condensers
· Condenser Types
§ Typically located in the front coupled with a driver's area system only
§ Most common in the smallest / lightest duty vehicles
§ Uses the vehicle fan for airflow
Poor system performance can
§ result if engine overheats (see Skirt Mounted Condensers if this is an issue)
§ Typically the least expensive
§ Most widely utilized condenser type
§ Uses electric fans for air movement to maximize heat rejection
§ Easy to service
§ Close proximity to the ground and environmental elements can sometimes lead to short life span (see Roof Mounted Condensers if this is an issue)
§ Cleaner and cooler environment than the skirt mounted condenser
§ Overhead clearance could be an issue
§ Typically more expensive
§ A combination system that combines evaporator(s) and condenser(s) within one module
§ Most common in transit bus applications
§ Modules be roof mounted or rear mounted
§ Typically most expensive
Filter Drier
· Located within the condenser
· Removes moisture and particulate matter from refrigerant within the liquid line.
· Moisture in the refrigerant of an air conditioning system causes system contamination and eventually could lead to total system failure.
Sight Glass
· Located within the condenser
Allows for visualization of the filtered liquid refrigerant to assess the level of moisture and/or particle contamination within the system
Expansion Valve
· Acts as a "spray nozzle" to meter refrigerant into the evaporator coil from the liquid line.
· High pressure created from compressor operation is lowered by the expansion valve.
Evaporator
· Transfers heat, contained in the passenger compartment air, into the refrigerant, which is circulated by the compressor, through the evaporator coil. During this transfer process, the air is also filtered and dehumidified.
· Generally located inside the interior of the vehicle
· Systems can have one or more evaporators
· Evaporator Types
§ Air is directly discharged from the evaporator
§ Most common on small to mid-sized shuttle buses and all sizes of school buses
§ Most efficient
§ Air flow / direction cannot be controlled by passengers
§ Typically the least expensive
§ Air is directed throughout the vehicle through ducts with slots or adjustable louvers
§ Most common on larger buses and over-the-road / tour operations.
§ Gentler air flow than free blow evaporators
§ Various Ducting Styles
1. Corner Ducting
2. Overhead Parcel Rack Ducting
3. Advertisement Ducting
4. Transit Lighted Ducting
§ Typically more expensive
