Although fuel spray nozzles are an integral part of the  fuel system, their design is closely related to the type of  combustion chamber in which they are installed. The fuel  nozzles inject fuel into the combustion area in a highly  atomized, precisely patterned spray so that burning is  completed evenly, in the shortest possible time, and in the  smallest possible space. It is very important that the fuel be  evenly distributed and well centered in the flame area within  the liners. This is to preclude the formation of any hot spots  in the combustion chambers and to prevent the flame burning  through the liner. Fuel nozzle types vary considerably between engines,  although for the most part fuel is sprayed into the combustion  area under pressure through small orifices in the nozzles. The  two types of fuel nozzles generally used are the simplex and  the duplex configurations. The duplex nozzle usually requires  a dual manifold and a pressurizing valve or flow divider for  dividing primary and secondary (main) fuel flow, but the  simplex nozzle requires only a single manifold for proper  fuel delivery. The most widely used filters are the 200-mesh and the  35-mesh micron filters. They are used in fuel pumps, fuel  controls, and between the fuel pump and fuel control where  removal of micronic particles is needed. These filters, usually  made of fine-mesh steel wire, are a series of layers of wire. The wafer screen type of filter has a replaceable element,  which is made of layers of screen disks of bronze, brass,  steel, or similar material.  [Figure 2-63]  This type of filter  is capable of removing micronic particles. It also has the  strength to withstand high pressure. The fuel nozzles can be constructed to be installed in various  ways. The two methods used quite frequently are: 2-45 Figure 2-63.  Wafer screen filter. Figure 2-62.  Aircraft fuel filter.  1.  External mounting wherein a mounting pad is provided  for attachment of the nozzles to the case or the inlet  air elbow, with the nozzle near the dome; or 2.  Internal mounting at the liner dome, in which the  chamber cover must be removed for replacement or  maintenance of the nozzle. Fuel Spray Nozzles and Fuel Manifolds Filter element Outlet Baffle Relief valve Drain cock Inlet Filter head Filter sump Spacer O-ring Filter disk Human hair is about 100 microns in diameter 25,400 microns = 1 inch