to protect the fuel control from contaminants that could come  from the low pressure pump. The three most common types of filters in use are the micron  filter, the wafer screen filter, and the plain screen mesh filter.  The individual use of each of these filters is dictated by the  filtering treatment required at a particular location. The  micron filter has the greatest filtering action of any present- day filter type and, as the name implies, is rated in microns.  [Figure 2-62]  (A micron is one thousandth of 1 millimeter.)  The porous cellulose material frequently used in construction  of the filter cartridges is capable of removing foreign matter  measuring from 10–25 microns. The minute openings make  this type of filter susceptible to clogging; therefore, a bypass  valve is a necessary safety factor. One method of regulating fuel temperature is to use a fuel  heater which operates as a heat exchanger to warm the fuel.  The heater can use engine bleed air or engine lubricating oil  as a source of heat. The bleed air type is called an air-to-liquid  exchanger and the oil type is known as a liquid-to-liquid  heat exchanger. The function of a fuel heater is to protect  the engine fuel system from ice formation. However, should  ice form in the filter, the heater can also be used to thaw  ice on the fuel screen to allow fuel to flow freely again. On  most installations, the fuel filter is fitted with a pressure-drop  warning switch, which illuminates a warning light on the  cockpit instrument panel. If ice begins to collect on the filter  surface, the pressure across the filter slowly decreases. When  the pressure reaches a predetermined value, the warning light  alerts the flight deck personnel.  Fuel deicing systems are designed to be used intermittently.  The control of the system may be manual, by a switch in the  cockpit, or automatic, using a thermostatic sensing element  in the fuel heater to open or close the air or oil shutoff valve.  A fuel heater system is shown in  Figure 2-61 . In a FADEC  system, the computer controls the fuel temperature by sensing  the fuel temperature and heating it as needed. Since the micron filter does such a thorough job of removing  foreign matter, it is especially valuable between the fuel  tank and engine. The cellulose material also absorbs water,  preventing it from passing through the pumps. If water does  seep through the filter, which happens occasionally when  filter elements become saturated with water, the water can and  does quickly damage the working elements of the fuel pump  and control units, since these elements depend solely on the  fuel for their lubrication. To reduce water damage to pumps  and control units, periodic servicing and replacement of filter  elements is imperative. Daily draining of fuel tank sumps and  low-pressure filters eliminates much filter trouble and undue  maintenance of pumps and fuel control units. A low-pressure filter is installed between the supply tanks and  the engine fuel system to protect the engine-driven fuel pump  and various control devices. An additional high-pressure fuel  filter is installed between the fuel pump and the fuel control  Fuel Filters Figure 2-61.  Fuel heater. 2-44 Cooling fins Fuel inlet Air baffle Fuel tubes Fuel temperature sensor Air inlet Air outlet Fuel outlet Air shutoff valve