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Heavy Duty Heating, Ventilation and Air Conditioning Systems. Ken Collins 704-330-4183 kennth.collins@cpcc.edu. Objectives. Understand the basic theory of heavy-duty truck air-conditioning systems.
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Heavy Duty Heating, Ventilation and Air Conditioning Systems Ken Collins 704-330-4183 kennth.collins@cpcc.edu
Objectives 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.
(Objectives cont.) Explain how the thermostatic expansion valve or orifice tube controls the flow of refrigerant to the evaporator. 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.
(Objectives cont.) Recognize the environmental and personal safety precautions that must be observed when working on air-conditioning systems. Identify air-conditioning testing and service equipment. Test an air-conditioning system for refrigerant leaks.
(Objectives cont.) 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 cont.) 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.
INTRODUCTION Heating, ventilation, and air-conditioning (HVAC) systems are designed to keep the cab, or cab and bunk, in a truck at a comfortable temperature. The air conditioning system also helps clean (condition) the air in the cab by removing dust, pollen, smoke, and moisture.
THERMODYNAMICS To understand air-conditioning theory, it is important for the service technician to have a basic understanding of how heat behaves. The behavior of heat is a branch of physics known as thermodynamics. An air-conditioning system uses some very basic thermodynamic principles to remove heat from the cab of a truck and dissipate or lose it to the atmosphere outside the truck.
PRINCIPLES OFREFRIGERATION Air-conditioning and refrigeration systems manage basic thermodynamic principles to produce a more comfortable climate within an enclosed area. To understand how an air-conditioning system works, we first have to know something about the states of matter, heat flow, and something called latent heat.
STATES OF MATTER • Solid water is known as ice. • Liquid water is known as water. • Vaporized water is known as steam. The temperature of the water determines which of these three states it is in.
Pressure and Heat The temperature at which a liquid boils depends on the pressure acting on the liquid. Decreasing the pressure lowers the boil point. Increasing the pressure raises the boil point.
Latent Heat Whenever a substance changes state, it either releases or consumes heat energy. Latent heat is the amount of heat necessary to change a substance from one state to another.
SUMMARY OF AIR CONDITIONINGPRINCIPLES 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.
Typical heavy-duty truck heating and an air-conditioning system
REFRIGERANT The function of a refrigerant is to absorb heat from the air in the cab and transfer it to the atmosphere outside the cab. The refrigerants used in truck A/C systems are industrially manufactured chemicals of some complexity. Refrigerants in mobile systems are controversial and constantly undergoing change.
CHLOROFLUOROCARBONREFRIGERANTS Until 1995, the common refrigerant used in a truck A/C system was known as R-12. This is classified as a chlorofluorocarbon (CFC). R-12 refrigerant boils at −22°F (−30°C, and for years, it was considered an ideal mobile vehicle A/C refrigerant. However, because it is a substance of some toxicity as well as ozone depleting, its use has in theory become strictly controlled under the guidelines of the federal Clean Air Act.
HYDROCHLOROFLUOROCARBONS(HCFCS) HCFC 134a – used in automobile and truck cab HVAC systems HCFC 22 – used in trailer reefer systems HCFC 40a – used in trailer reefer systems
Ozone Depletion Depletion of the ozone layer has occurred at a rate of around 4 percent per decade since the mid-1970s and is beyond the scope of this textbook. A simple explanation requires understanding a little about ozone. Ozone in the upper atmosphere filters out some of the harmful sunlight directed toward Earth. Chemically, ozone (O3) is triatomic oxygen; that is, three oxygen atoms bond to form an O3 molecule.
Future Refrigerants Europe currently requires trailer reefers to be equipped with electrical standby for use whenever a vehicle is parked so that CO2 is not produced while running an onboard reefer engine. To minimize refrigerant losses, flared fittings are being phased out in favor of brazed unions, reducing the incidence of leaks. Also under consideration are the use of hybrids and semi-hermetic compressors.
Refrigerant Oils HCFCs require the use of two refrigerant specific oils: • Polyalkylene glycol (known as PAG) • Polyolester (known as POE or ester)
REFRIGERANT CONTAMINATION R-134a systems look similar to R-12 systems. However, refrigerants should never be mixed in a system; to emphasize this fact, it has been made illegal. Guidelines issued by the Environmental Protection Agency (EPA) specify that all refrigerants must be approved under the Significant New Alternatives Policy (SNAP). Only R-134a meets current SNAP guidelines for mobile refrigerants, but a number of other mobile refrigerants are under research.
Shop Talk Refrigerant containers for R-12 and R-134a are color-coded. R-12 containers are white and R-134a containers are light blue and clearly marked. In addition, R-134a containers use ½-inch 16 acme threads, which cannot be connected to an R-12 gauge set or recovery machine.
REFRIGERANT RECOVERY The Clean Air Act, passed in 1992, has resulted in major changes in industrial, domestic, and vehicle A/C systems.
AIR-CONDITIONING CERTIFICATION Three training programs in particular are recognized by the federal government: the Mobile Air Conditioning Society (MACS), the International Mobile Air Conditioning Association (IMACA), and the National Institute of Automotive Service Excellence (ASE).
Trinary Switch The trinary switch is mounted on the receiver/dryer in a similar fashion to the binary switch. The trinary pressure switch performs three functions to monitor and control pressure inside the A/C system. The low- and high-range functions are the same as on the binary switch system.
Pressure Relief Valve A pressure relief valve is also often located on the receiver/dryer. It provides an added high-pressure relief feature if a failure were to develop in the high pressure cut-off switch. The pressure relief valve is designed to pop off when the refrigerant pressure exceeds a preset maximum safe pressure value. Pop off pressure can often be as high as 400 psi.
