can-type combustion chambers consists of an outer case or  housing, within which there is a perforated stainless steel  (highly heat resistant) combustion chamber liner or inner  liner.  [Figure 1-53]  The outer case is removed to facilitate  liner replacement. Older engines with several combustion cans had each can  with interconnector (flame propagation) tube, which was a  necessary part of the can-type combustion chambers. Since  each can is a separate burner operating independently of the  other cans, there must be some way to spread combustion  during the initial starting operation. This is accomplished  by interconnecting all the chambers. As the flame is started  by the spark igniter plugs in two of the lower chambers, it  passes through the tubes and ignites the combustible mixture  in the adjacent chamber and continues until all the chambers  are burning. The flame tubes vary in construction details from one  engine to another, although the basic components are almost  identical.  [Figure 1-54]  The spark igniters previously  mentioned are normally two in number and are located in  two of the can-type combustion chambers. Another very important requirement in the construction of  combustion chambers is providing the means for draining  unburned fuel. This drainage prevents gum deposits in the fuel  manifold, nozzles, and combustion chambers. These deposits  are caused by the residue left when the fuel evaporates.  Probably most important is the danger of afterfire if the fuel  is allowed to accumulate after shutdown. If the fuel is not  drained, a great possibility exists that, at the next starting  attempt, the excess fuel in the combustion chamber will ignite  and exhaust gas temperature will exceed safe operating limits. The liners of the can-type combustors have perforations  of various sizes and shapes, each hole having a specific  purpose and effect on flame propagation within the liner.  [Figure 1-52]  The air entering the combustion chamber  is divided by the proper holes, louvers, and slots into two  main streams—primary and secondary air. The primary or  combustion air is directed inside the liner at the front end,  where it mixes with the fuel and is burned. Secondary or  cooling air passes between the outer casing and the liner and  joins the combustion gases through larger holes toward the  rear of the liner, cooling the combustion gases from about  3,500 °F to near 1,500 °F. To aid in atomization of the fuel,  holes are provided around the fuel nozzle in the dome or  inlet end of the can-type combustor liner. Louvers are also  provided along the axial length of the liners to direct a cooling  Figure 1-54. Interconnecting flame tubes for can-type combustion chambers. Figure 1-53. Inside view of a combustion chamber liner.  Elbow Elbow Ferrule Seal Seal Interconnecter tube Extension tube Interconnecter tube 1-45