What is the most important thing you should
know before you perform a diagnosis on a system
A refrigerant leak appears in what way at the
point of the leak?
What is the most common cause of leaks on
automotive air-conditioning systems?
For a refrigerant to be a suitable flushing agent,
it must remain in what state during flushing
Which part of an automotive air-conditioning
system is replaced because it is impossible to
A type IV certification is also known as what type
Who approves organizations to certify
Learning Objective: Understand the basic types
of ductwork systems and the components of those
systems for distribution of conditioned air.
Distributed air must be clean, provide the proper
amount of ventilation, and absorb enough heat to cool
the conditioned spaces. To deliver air to the
conditioned space, air carriers are required, which are
called ducts. Ducts work on the principle of air
pressure difference. If a pressure difference exists, air
will flow from an area of high pressure to an area of low
pressure. The larger this difference, the faster the air
will flow to the low-pressure area.
CLASSIFICATION OF DUCTS
There are three common classifications of
ductsconditioned air ducts, recirculating-air ducts,
and fresh-air ducts. Conditioned air ducts carry
conditioned air from the air conditioner and distribute
it to the conditioned area. Recirculating air ducts take
air from the conditioned space and distribute it back
into the air conditioner system. Fresh air ducts bring
fresh air into the air-conditioning system from outside
the conditioned space.
Ducts commonly used for carrying air are of a
round, square, or rectangular shape. The most efficient
duct is a round duct, based on the volume of air handled
per perimeter distance. In other words, less material is
needed for the same capacity as a square or rectangular
Square or rectangular duct fits better to building
construction. It fits above ceilings and into walls and is
much easier to install between joists and studs.
TYPES OF DUCT SYSTEMS
There are several types of supply duct systems (fig.
7-42) that deliver air to room(s) and then return the air
from the room(s) to the cooling (evaporator) system.
These supply systems can be grouped into four types:
1. Individual round pipe system
2. Extended plenum system
3. Reducing trunk system
4. Combination (of two or more systems)
Return air systems are normally of three
typessingle return, multiple return (fig. 7-42), or
combination of the two systems.
Ducts may be made of metal, wood, ceramic, and
plastic. Most commonly used is sheet steel coated with
zinc (galvanized steel). Sheet metal brakes and
forming machines are used in fabricating ducts.
Elbows and other connections, such as branches, are
designed using geometric principles. Some types of
duct connections used in constructing duct systems are
shown in figure 7-43.
Sheet metal ducts expand and contract as they heat
and cool. Fabric joints are often used to absorb this
movement. Fabric joints should also be used where the
duct connects to the air conditioner. Many ducts are
insulated to lower noise and reduce heat transfer. The
insulation can be on the inside or the outside of the
duct. Adhesives or metal clips are commonly used to
fasten the insulation to the duct. As we are only briefly
discussing construction here, you can find construc-
tion and fabrication methods in the Steelworker,
volume 2. It details design and fabrication of steel