Q39. What is the most important thing you should know before you perform a diagnosis on a system problem?

Q40. A refrigerant leak appears in what way at the point of the leak?

Q41. What is the most common cause of leaks on automotive air-conditioning systems?

Q42. For a refrigerant to be a suitable flushing agent, it must remain in what state during flushing operations?

Q43. Which part of an automotive air-conditioning system is replaced because it is impossible to clean?

Q44. A type IV certification is also known as what type of certification?

Q45. Who approves organizations to certify technicians?

DUCTWORK

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 ducts - conditioned 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 duct.

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 types - single return, multiple return (fig. 7-42), or combination of the two systems.

CONSTRUCTION

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 construction and fabrication methods in the Steelworker, volume 2. It details design and fabrication of steel ductwork.