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Chapter 35. Heavy-duty Heating, Ventilation, and Air Conditioning Systems. Objectives (1 of 4). Understand the basic theory of heavy-duty truck air conditioning systems. Outline the requirements of the Clean Air Act that apply to a heavy-duty truck air conditioning system.
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Chapter 35 Heavy-duty Heating, Ventilation, and Air Conditioning Systems
Objectives (1 of 4) • Understand the basic theory of heavy-duty truck air conditioning systems. • Outline the requirements of the Clean Air Act that apply to a heavy-duty truck air conditioning system. • List the five major components of a heavy-duty air conditioning system and describe how each works in the operation of the system. • Explain how the thermostatic expansion valve or orifice tube controls the flow of refrigerant to the evaporator.
Objectives (2 of 4) • Identify the refrigerants used in heavy-duty truck air conditioning systems. • Describe the function of the main components in a typical heavy-duty air conditioning system. • Recognize the environmental and personal safety precautions that must be observed when working on air conditioning systems. • Identify air conditioning testing and service equipment.
Objectives (3 of 4) • Test an air conditioning system for refrigerant leaks. • Outline the procedure required to service a heavy-duty air conditioning system. • Perform some simple diagnosis of air conditioning system malfunctions. • Outline the advantages of connecting air conditioning management electronics to the chassis data bus and explain how to access the system.
Objectives (4 of 4) • Explain how a truck cab ventilation system operates. • Describe the role a liquid-cooled heating system plays in a truck cab heating system. • Describe some types of auxiliary heating and power units.
Principles of Refrigeration • States of matter • Absolute heat, heat movement, and measurement • Pressure and heat • Latent heat
Summary of Air Conditioning Principles • Heat always moves from a warmer area to a cooler area. • When liquids are heated and evaporate to a vapor state, heat is absorbed. • When a gas condenses from a vapor to a liquid state, heat is released.
Air Conditioning System Components • Compressor • Condenser • Receiver/dryer or accumulator • Expansion valve or orifice tube • Evaporator
Shop Talk • Refrigerant containers for R-12 and R-134a are color coded. • R-12 containers are white, and R- 34a containers are light blue and clearly marked. • In addition, R-134a containers use 1/2-inch 16 acme threads, which cannot be connected to an R-12 gauge set or recovery machine.
The A/C Compressor • Piston-type compressors • Rotary vane Compressor • Swashplate-type compressors
Shop Talk • Remember that just one drop of water added to refrigerant will lead to corrosion and refrigerant breakdown. • Corrosive hydrochloric acid can be produced in the older R-12 system when trace drops of water are added. • Also, the smallest amounts of air in the refrigerant system can start chemical reactions that result in system malfunctions.
Caution • Refrigerant should never come into contact with skin or eyes. • Liquid refrigerant, when exposed to the air, quickly evaporates and will almost instantly freeze skin or eye tissue. • Serious injury or blindness could result.
First Aid • Refrigerant splashed in the eyes should be treated as follows: • Flush eyes with cold water to raise the temperature above freezing point. • Avoid rubbing eyes. • If available, apply an antiseptic mineral oil to the affected area. • This will form a protective film over the eyeball to reduce the possibility of infection. Then rinse with a weak boric acid solution. • Call an eye specialist or doctor immediately and receive medical treatment as soon as possible.
Caution • Avoid working in areas where refrigerant may come into contact with an open flame or any burning material, such as a cigarette. • When it contacts extreme heat, refrigerant breaks down into poisonous phosgene gas, which, if breathed, causes severe respiratory irritation.
Caution • Under current federal Clean Air Act regulations, refrigerants must be recovered and recycled by all users to protect the environment and never released into the atmosphere. • Under these regulations, service facilities not having the required recovery and recycling equipment and properly trained and certified personnel are not permitted to perform any refrigeration service work.
Odor Diagnosis • See Table 35-1 on page 114 of the textbook.
Caution • Never pressure or leak test R-134a service equipment or vehicle air conditioning systems with compressed air. • Some mixtures of air and R-134a have been shown to be combustible at elevated pressures.
Shop Talk • Electronic-type detectors must be correctly calibrated before each use to detect the lowest permissible leak rate. • Trace the refrigerant system in a continuous path. • If a leak is found, always continue to test the remainder of the system. • The probe should be moved around the location at a rate no more than 1 to 2 inches/second and no more than 1/4 inch from the surface completely around the position. • It also helps to place the probe lower than a suspected leak because the refrigerant is heavier than air.
Caution • Never open the high-side hand valve with the system operating and a refrigerant source at the center hose connection. • This will cause refrigerant to exit the A/C system under high pressure into the source container, which could cause it to burst. • The only time both hand valves should be open is when evacuating the system.
Shop Talk • Atmospheric pressure reduces as altitude increases. • The gauge pressure must be corrected using an OEM correction chart when checking system low-side pressures.
Shop Talk • A vacuum pump is unable to remove moisture in liquid state from a system. • It lowers the system pressure and, therefore, the boiling point of liquid moisture. • It then removes the vaporized moisture. • This is why it is necessary to pull a vacuum on a system for an extended period to ensure that no moisture is left in it.
Air Conditioning Service Procedures • System recovering and recycling • System flushing • Compressor oil level checks • Evacuation • System recharging
Shop Talk • The efficient operation of the air conditioning system greatly depends on the correct amount of refrigerant in the system. • A low charge will result in inadequate cooling under high heat loads, because of a lack of reserve refrigerant, and can cause the clutch cycling switch to cycle faster than normal. • An overcharge can cause inadequate cooling because of a high liquid refrigerant level in the condenser. • Refrigerant controls will not operate properly, and compressor damage can result. • In general, an overcharge of refrigerant will cause higher-than-normal gauge readings.
Caution • Accumulator systems do not have a sight glass. • This is because bubbles are always present in the liquid line, even with a full charge. • If the refrigerant is added until the bubbles are gone, serious damage and injury can result from overcharging the system.
