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 airconditioning system.
• List the five major components of a heavyduty 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 airconditioning 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 airconditioning system malfunctions.
• Outline the advantages of connecting airconditioning 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 airconditioning 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.
How air is “conditioned” in the
cab.
PRINCIPLES OF
REFRIGERATION
Air-conditioning and refrigeration systems
manage basic thermodynamic principles to
produce a more comfortable climate within an
enclosed area. To understand how an airconditioning 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.
Absolute Heat, Heat Movement,
and Measurement
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 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.
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.
CHLOROFLUOROCARBON
REFRIGERANTS
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.
Temperature/pressure relationship
of refrigerant
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 identifier
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).
THE REFRIGERATION CYCLE
AIR-CONDITIONING SYSTEM
COMPONENTS
Refrigerant Flow Cycle
Piston-Type Compressors
Cutaway view of a piston-type
compressor
Rotary Vane Compressor
Cutaway view of a typical rotary
vane compressor
Swashplate-Type Compressors
Compressor Drives
CONDENSER
RECEIVER/DRYER
Binary Switch
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.
Fusible Plug
Accumulator
THERMOSTATIC EXPANSION
VALVE/ ORIFICE TUBE
Thermostatic Expansion Valve
(TXV)
Orifice Expansion Tube
EVAPORATOR
THERMOSTAT (EVAPORATOR
FREEZE PROTECTION DEVICE)
REFRIGERATION LINES, HOSES,
AND COUPLERS
Hose clamp shell crimp fittings will not
seal the new style barrier hose.
New hoses are designed to
prevent refrigerant leakage.
Special Crimping Tools
SIGHT GLASS
IN-LINE DRYER
Located between the receiver/dryer and TXV
or tube, an in-line dryer absorbs any moisture
that gets by the receiver/dryer. It also helps to
prevent TXV or tube freeze-up.
MUFFLER
The muffler is usually located on the
discharge side of the compressor. It also may
be located on the suction side. Its function is
to reduce the characteristic pumping noise
produced by the compressor.
BLOWER MOTOR AND FAN
ENGINE COOLING FAN
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.
SAFETY PRECAUTIONS
Refrigerant lines are under some pressure
even when the system is not running. This
means that they should not be disconnected
until the refrigerant has been discharged to a
refrigerant recovery unit. Refrigerants are
safe when handled properly.
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.
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 should
never be 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.
Refrigerant Storage
Because of its very low boiling point,
refrigerant has to be stored under pressure in
containers. These containers should never be
exposed to temperatures higher than 125°F.
Refrigerant cans should not be left in direct
sunlight.
PERFORMANCE TESTING AN
AIR CONDITIONING SYSTEM
Because there are differences in each OEM
system, this is just a general set of guidelines,
and the best approach to testing and
troubleshooting a system is to use the OEM
service manual.
Cooling Check
Odor Diagnosis
A/C systems may produce a musty odor, usually
at startup in hot weather. This type of odor is
usually temporary, and, providing it disappears
after a short period of operation, should not be of
concern. If it persists, it can be an indication of
microbial growth on the evaporator core. This
requires the removal of the evaporator core and
its thorough cleaning with a disinfectant. Some
other odor diagnoses are shown in Table 35–1
page 23.
LEAK TESTING A SYSTEM
CAUTION!
Never pressure or leak test R-134a service
equipment or vehicle A/C systems with
compressed air. Some mixtures of air and R134a have been shown to be combustible at
elevated pressures.
Shop Talk
Because of their sensitivity, electronic-type
detectors must be correctly calibrated before
each use to detect the lowest permissible
leak rate of the component being checked.
Trace the refrigerant system in a continuous
path so that no potential leaks are missed. If
a leak is found, always continue to test the
remainder of the system.
Shop Talk (Cont.)
At each area checked, 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. Slower and closer movement of
the probe greatly improves the likelihood of
finding a leak. It also helps to place the probe
lower than a suspected leak because the
refrigerant is heavier than air.
MANIFOLD GAUGE SETS
R-12
R-134a
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 lowside pressures.
Service Valves
Schrader Type
Shop Talk
In R-12 systems, the Schrader fitting on the
high pressure side is smaller than the lowpressure side, and special adapters are
necessary to connect the high-side service
hose into the system. The difference in fitting
sizes is to prevent mixing up the high and
low-pressure sides of the system when
attaching he gauge set.
Shop Talk
The different size fittings were introduced to
prevent disposable 1 lb cans of refrigerant from
being connected to the high side of the system,
causing them to explode. After disconnecting the
gauge lines, check the valve areas to be sure that
the service valves are correctly seated and that
the Schrader valves are not leaking. In R-134a
systems, the size as well as the type of
connection is different; that is, the larger
connection is used for the high side.
MANIFOLD SERVICE HOST SETS
Vacuum Pump
Rotary Vane-Type Vacuum Pump
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.
Thermistor Vacuum Gauge
The electronic thermistor vacuum gauge is
designed to work with the vacuum pump to
measure the last, most critical inch of Hg
vacuum during evacuation. It constantly
monitors and visually indicates the vacuum
level so that a technician will know for sure
when a system is entirely free of air and
moisture.
AIR CONDITIONING SERVICE
PROCEDURES
•
•
•
•
•
System recovering and recycling
System flushing
Compressor oil level checks
Evacuation
System recharging
CHARGING CYLINDER OR
CHARGING STATION
Typical Refrigerant Stations
Discharge and Suction Service Valves.
Recovery Hookup
PURGING
CHECKING COMPRESSOR OIL
LEVEL
EVACUATING THE SYSTEM
TOP-OFF RECHARGE OF
AN A/C SYSTEM
Weighing a Refrigerant Canister
COMMON AIR
CONDITIONING PROBLEMS
Performance Testing
Troubleshooting Flow Chart
CONVERTING AN R-12 SYSTEM
TO AN R-134A SYSTEM
In theory, R-12 should not be available today,
meaning that every retrofit should have taken
place. In reality R-12 is still readily available.
This means that there are trucks running R12 systems. It also means that shops
continue to the process of retrofitting systems
to comply with the MACS standards. Some of
the systems are being aftermarket retrofit to
blend systems.
AIRFLOW
LIQUID-COOLED HEATING
SYSTEM
HEATER CORE
A typical heater core is designed and
constructed very much like a miniature
engine cooling radiator. It features inlet and
outlet tanks connected by headers to a heat
exchanger core. The heater core tank, tubes,
and fins can become clogged over time by
rust, scale, and mineral deposits circulated by
the coolant.
HEATER CONTROL VALVE
A heater control valve, also known as a
coolant flow valve, controls the flow of coolant
into the heater core from the engine. In a
closed position, the valve allows no flow of
hot coolant to the heater core, allowing it to
remain cool.
THERMOSTAT
The thermostat that helps to regulate the
engine coolant temperature plays a role in
providing the cab with heat. A malfunctioning
thermostat can cause the engine to overheat
or not reach normal operating temperature,
and either of these conditions will impact the
cab heater performance.
COMBINATION HVAC CORE
Cable-Controlled Coolant Valve
Control Panel and Airflow
Sleeper Compartment Heater
and Air Conditioner
Supplementary Coolant Heater
Auxiliary Power Units (APUs)
ELECTRONICALLY MANAGED
CLIMATE CONTROL
Most heavy-duty trucks use some level of
electronic controls to manage cab climate.
The terms used to describe these control
systems are air-conditioning protection and
diagnostics system (APADS) and airconditioning protection unit (ACPU).
COMPUTER-CONTROLLED
A/C COMPONENTS
System components include the A/C control
module, which performs logic processing of
the system inputs and switches the outputs. It
also includes an input and output circuit. The
A/C control module masters A/C operation
and tracks diagnostic data. It is usually
located somewhere in the engine
compartment rather than in the vehicle cab.
A/C Clutch Coil
A/C Control Logic
BLUECOOL HEAT
EXCHANGER
BLUECOOL OPERATION
BlueCool uses a pair of aluminum coil circuits
surrounded by a graphite matrix storage core
contained in insulation. One coil circuit carries
R-134a refrigerant gas and the other a 50/50
mixture of ethylene glycol (EG) and water.
The system uses an icebox principle. The
graphite matrix absorbs water in liquid both
liquid and frozen states. The storage coil is
chilled down to subzero temperatures while
the truck is operated on the road.
Summary
• Heavy-duty heating and air-conditioning (A/C)
systems are designed primarily to keep the
cab comfortable despite the outside weather.
• Liquids absorb heat when changed from a
liquid state to a gas.
• Gases release heat when changed from a
gaseous state to a liquid.
Summary (cont.)
• Heat always moves from a hotter area to a
cooler area.
• The temperature at which a liquid changes
state to a gas depends on the pressure acting
on it.
• Refrigerants are required to have a low boil
point and to change quickly from a liquid state
to a gas state and back again.
Summary (cont.)
• The basic refrigeration cycle consists of
compression, condensation, expansion, and
evaporation.
• Compression heats up refrigerant gas.
Condensation changes the state of the
refrigerant from a gas to a liquid. Expansion
reduces the pressure of the liquid refrigerant.
Evaporation changes the refrigerant from a
liquid state to a gas.
Summary (cont.)
• There are two types of refrigerant lines:
suction lines and discharge lines.
• A/C systems are sensitive to moisture and
dirt. Clean working conditions are extremely
important.
• The electrical system must be checked
periodically to prevent the truck A/C system
from failing unexpectedly.
Summary (cont.)
• Some trucks use electronic climate and A/C
controls. These systems can either be standalone or connected to the chassis data bus.
• Current A/C system controllers are networked
to the chassis data bus. This enables them to
communicate with other MIDs to optimize
system operation.
Summary (cont.)
• When the A/C controller is networked to the
chassis data bus, the system is accessed by
the J1939 connector. This allows the system
to be read by any software that can interpret
MIDs, parameter identifiers (PIDs),
subsystem identifiers (SIDs) and FMIs. To
perform operations other than simply read the
system, proprietary software is usually
required.
Summary (cont.)
• The A/C electronics can respond to a request
off the data bus to put the A/C compressor
into dropout mode. An example would be a
temporary request for full power from the
engine electronics, when it would be
desirable to eliminate as many parasitic loads
as possible.
Summary (cont.)
• BlueCool uses an icebox principle to cool
truck bunks during engine shutdown at zero
fuel consumption and with no emissions.
Any Questions ?