Ch58 Principles of heating and Air Conditioning Systems
7. Technician A says refrigerant HFO-1234yf is currently used in vehicle ac systems. Technician B says refrigerant R-143a is used in vehicle ac systems. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B
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9. Technician A says pressure-cycling switch is used in CCOT (orifice tube) systems and is located on the accumulator. Technician B says it is not possible to remove and replace the switch without having to discharge the system. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B
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5. How many cylinders does a rotary compressor typically have that is used in a highway tractor? A. 2-4 B. 4-8 C. 5-10 D. 6-12
C. 5-10, p1673
4. What would be the equivalent pressure in "psig" units be for a 3-psia measured at sea level? A. 0 psig B. 3 psig C. -3 psig D. 17.7 psig
D. 17.7 psig, p1668
Latent heat of vaporization
The process of adding heat energy to matter to produce a change of state.
Superheat
The temperature differential between the refrigerant vapor at the evaporator outlet and the refrigerant vapor at the evaporator inlet.
Common compressor controls include
The thermostatic switch, low-pressure cutout switch, pressure-cycling switch, trinity switch, high-pressure cutout switch, high-pressure fan switch, and ambient temperature switch.
Convection
The transfer of heat through a gas.
PSIA
The units of pressure, in pounds per square inch, using sea level as a reference. Atmospheric pressure at sea level = 0 psia.
7. Which of the following statements is correct concerning the TXV expansion device? A. The TXV expansion device lowers the pressure of the warm, high-pressure liquid refrigerant entering the evaporator and allows the refrigerant to expand to greater volume. B. An orifice in the TXV creates a pressure differential forming the dividing point in the low side of the ac system. C. The high-side pressure and low-side temperature regulate the valve operation. D. It must be combined with a high-side pressure switch to regulate evaporator temperature.
A. , p1675
2. Which of the following functions of an ac system is directly related to safe vehicle operation? A. It reduced driver fatigue B. It cleans and filters air. C. It keeps the operator and passengers comfortable. D. It is automatically activated in defrost mode.
A. It reduces driver fatigue., p1664
8. What is the advantage of a rotary type variable displacement ac compressor? A. It regulates evaporator pressure. B. It has fewer moving parts. C. It uses less energy. D. It can pump more refrigerant than other types of compressors.
C. , p1673
1. Which of the following is the most recent category or type of ac refrigerant? A. Freon B. R-134a C. HFO D. HCHC
C. HFO, p1687
Accumulators are used in
CCOT or fixed-orifice tube (FOT) systems.
Most recently developed is HFO1234yf, an HFO class of refrigerant, and is a
Close drop-in replacement for the R-134a refrigerant with almost no negative environmental impacts.
The condenser typically consists of
Coiled tubing mounted in a series of thin cooling fins and is mounted directly in front of the radiator, where it can receive full airflow created by forward vehicle motion and the engine fan.
Freon, a brand trademark, is a
Colorless, odorless, nonflammable, and noncorrosive gas or liquid chlorofluorocarbon (CFC).
Rotary piston compressors have
Cylindrical-shaped housings and enclose multiple pistons. The primary advantage of rotary compressors is their minimization of noise, vibration, and harshness (NVH).
The APADS/ ac protection unit is an
Electronic, microcontroller-based device that operates both ac controls and the diagnostic system.
The air conditioning compressor is an
Engine-driven pump that increases refrigerant pressure and circulates refrigerant through the the system.
Refrigerants are manufactured from
Hydrogen, fluorine, chlorine, and carbon molecules. Two categories of refrigerant compositions are used today: hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs).
The APADS module receives
Inputs from the vehicle data link, the evaporator thermostatic switch, and high- and low-pressure system switches.
Hydrofluoro-olefin (HFO)
Is the most recently developed, near drop-in replacement for the R134a refrigerant. It's chemical name is HFO1234yf.
Ac systems need lubricant to
Reduce friction between moving surfaces and bearings inside the compressor and thermostatic expansion valves, and to prevent any corrosive substances from attacking the internal system components.
The thermostatic expansion valve controls
Refrigerant flow by the action of a spring-loaded control valve. Evaporator temperature and pressure ate two factors that change the size of the restriction opening.
Modern mobile air conditioning has
Safety benefits. A comfortable vehicle environment reduces driver fatigue and distractions and can increase driver productivity.
The orifice tube is a
Simple, plastic tube with a calibrated brass orifice used on current CCOT systems to produce refrigerant pressure drop in the evaporator.
Latent heat of fusion
The process of removing heat energy to matter to effect a change of state.
Latent heat
The quantity of heat required to produce a change of state from a solid to a liquid or a liquid to a gas.
HVAC system
The system in the vehicle responsible for heating and cooling the air.
Superheat
The temperature differential between the refrigerant vapor at the evaporator outlet and the refrigerant vapor at he evaporator inlet.
Radiant heat transfer
The transfer fo heat through a medium, such as a gas or vacuum, which does not cause the medium itself to heat.
Radiant heat transfer
The transfer of heat through a medium, such as a gas or vacuum, which does not cause the medium itself to heat.
Conduction
The transfer of heat through a solid item, such as a body panel or evaporator.
Calorie
The unit of energy that reflects the amount of energy required to raise the temperature of 1 gram of water by 1 degree C.
PSIG
The units of pressure, in pounds per square inch, expressed relative to atmospheric pressure. Atmospheric pressure at sea level = 14.7 psig.
Rotary piston compressors
HVAC compressors that use cylindrical-shaped housings and enclose multiple pistons to minimize of Lise, vibration and harshness.
Rotary piston compressors
HVAC compressors that use cylindrical-shaped housings and enclose multiple pistons to minimize of noise, vibration, and harshness.
The evaporator is a
Heat exchanger located inside the cab or passenger compartment containing the chamber where refrigerant boils and converts to a vapor.
Sensible heat
Heat that can be sensed or felt.
Trinary switch
An ac system switch with 3 sets of internal contacts to protect against low pressure, cutout in case of high pressure, and turn the engine fan on and off.
APADS/ac protection unit (ACPU)
An electronic microcontroller based device that operates both ac and diagnostic systems.
Thermostatic expansion valve (TXV)
An expansion device used in commercial vehicle ac systems.
Heater core
An in-cab heat exchanger that regulates heating by circulating engine coolant.
6. Consider a truck with 36,000 BTU cooling capacity. How many tons of ac does this system have? A. 1 ton B. 2.2 tons C. 3 tons D. 4 tons
B. , p1667
The greater the heat energy in a substance, the
Faster its molecules can vibrate (heat = speed).
The concepts of gas temperature and pressure are
Related.
1. Technician A says heat energy in an ac system travels from cold to hot. Technician B says ac system's condenser removes heat from the cab and transfers it to the atmosphere via the evaporator. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B
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10. Technician A says ac control module receives information from he vehicle data link for chassis air pressure in the air reservoirs to pretext the ac system components. Technician B says the battery voltage is critical to protect ac system components. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B
10.
2. Technician A says reducing refrigerant pressure causes the refrigerant to absorb heat. Technician B says reducing refrigerant pressure causes it to evaporate. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B
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3. Technician A says increasing refrigerant pressure causes its temperature to increase. Technician B says increasing refrigerant pressure causes it to vaporize. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B
3
Chlorofluorocarbons (CFCs)
A chlorine-based compound of a fluorine hydrocarbon compound.
Condenser
A component of HVAC system that transfers heat from the refrigerant to the atmosphere.
Evaporator freezing
A condition in which excess refrigerant floods the evaporator reducing its temperature to below the freezing point of water.
Cycling clutch orifice tube (CCOT)
A fixed-orifice tube for evaporator pressure control obtained by cycling the compressor clutch on and off.
Freon
A refrigerant produced by reacting carbon tetrachloride, commonly used as "carb-cleaner," with fluorine gas.
Receiver-dryer
A storage reservoir for refrigerant that also absorbs moisture from the ac system.
10. Which of the following devices senses evaporator pressure to control the cycling of the ac clutch in a Navistar CCOT system? A. Thermistors B. Pressure transducer and high-pressure switch. C. Cab thermostatic switch D. CAN controls
A. , p1680, 81, 82
3. Which measurement of heat best describes the amount required to produce a change of state? A. Latent B. Sensible C. Radiant D. Convection
A. Latent, p1666
Boiling points of liquids are
Affected by the pressure exerted upon them.
Saturated temperature refers to the temperature at which a refrigerant changes
From a liquid to vapor or vapor to liquid at a given pressure.
Unlike a CCOT system, which meters refrigerant at a fixed rate, the TXV
Meters refrigerant into the evaporator at a variable rate.
Condensation
Moisture that collects on cooler surfaces as a result of hot vapors coming into contact with the cooler surface.
The APADS control system is composed
Of an electronic control module, two smart pressure switches, and a conventional evaporator thermostat.
Too frequent cycling of the engine fan can cause
Premature wear of the fan's clutch friction discs.
Venting of refrigerant to the atmosphere is
Prohibited by law.
The most common refrigerant used in automotive ac systems,
R-134a, has a GWP of 1200, as it contains no chlorine and has no impact on the ozone layer.
Polyalkylene glycol (PAG)
Synthetic oil used in all R-134a systems.
Polyalphaolefin (PAO)
Synthetic refrigerant oil used in ac systems.
British Thermal Unit (BTU)
The amount of energy required to heat or cool 1 pound of water 1 degree F.
Specific heat
The amount of heat a substance must absorb to undergo a temperature change of 1 degree F.
A calorie is
The basic unit for measuring the quantity of heat energy.
Evaporator
The cold surface of the ac system heat exchanger that absorbs heat from a ab or vehicle interior and transfers that heat to the atmosphere through a condenser.
The TXV expansion device lowers
The pressure of the warm high-pressure liquid refrigerant entering the evaporator and allows the refrigerant to expand to greater volume.
Three basic type of ac compressors can be
Used to build pressure exceeding 350 psi: piston, rotary vane, and scroll. Piston compressors are almost universally used in commercial vehicles.
Mobile HD ac compressors are usually
belt-driven front the engine's crankshaft and have an electromagnetic on/off clutch.
Air conditioning and refrigeration systems operate in a
closed-loop where the refrigerant is used over and over as it circulates through the system.
The lower the boiling point of a refrigerant, the
colder the evaporator temperature is.
An expansion device restricts the flow of
high-pressure, warm liquid refrigerant line and is designed to lower the pressure of refrigerant entering the evaporator.
A refrigerant with a high specific heat loses
its ability to refrigerate on a weight basis. Similarly, a refrigerant with a high latent heat capacity absorbs more heat per unit of weight. Combining a refrigerant with high latent heat and low specific heat increases effectiveness of a refrigerant on a per-lb basis.
Automotive air conditioning systems use two fluids:
refrigerant (to absorb heat) and refrigeration oil (to lubricate the compressor and expansion valves).
The primary function of the receiver-dryer is to serve as a
storage reservoir for refrigerant. A second function is to absorb any moisture in the air conditioning system.
Conduction, convection, and radiation are
3 types of heat transfer.
4. Technician A says ac systems transfer cold from the atmosphere and into the passenger compartment. Technician B says ac systems dehumidify and clean air passing through the evaporator. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B
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5. Technician A says outside air flows over the fins of the condenser transferring heat from the refrigerant, causing it to convert from a gas to a liquid. Technician B says the condenser compressor pressure helps convert hot gas to a liquid in the condenser. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B
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6. Technician A says the thermostatic expansion valve controls refrigerant flow through the action of a variable orifice control valve. Technician B says approximately a 5 degree F temperature difference between evaporator inlet and outlet is required to open the valve. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B
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8. Technician A says low-pressure cut out switch is connected in series with the compressor clutch electrical circuit. Technician B says when a low refrigerant charge is sensed, the switch opens and prevents compressor operation. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B
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Defroster
A HVAC air circuit that operates to clear the windows by blowing dehumidifier air into the cab.
Global warming potential (GWP)
A measure of a refrigerant's contribution to global warming over 100 years for a given mass compared to the same mass of carbon dioxide.
Heat is measured in
A number of different ways; most commonly sensible heat is measured with a thermometer.
9. How is lubricating oil circulated in an ac system? A. Refrigerant dissolves oil like a solvent and circulates oil. B. The compressor pumps oil. C. It is stored in each component needing oil for lubrication. D. The velocity of refrigerant gas carries oil.
A., p1688