Spark-Spray Ignition; Duct Switch; Combustion Air Blower And Pressure Switch; Heater Safety Limit Switch - Piper Navajo Chieftain PA-31-350 Service Manual

NAVAJO CHIEFTAIN SERVICE MANUAL
For the overhaul and complete disassembly of the heater and its components, refer to paragraph 13-37
of this manual. A wiring diagram of the heater Electrical System Installation will be found in Section XI of
this manual.
13-6. DESCRIPTION OF HEATER AND BASIC COMPONENTS.
13-7. SPARK-SPRAY IGNITION. (Refer to Figure 13-3.)
The controlled atomized spray from a specially designed spray nozzle, coupled with high-voltage spark
plug ignition, insures instant firing and continuous burning under all flight conditions.
Heat is produced by burning a fuel-air mixture in the combustion chamber of the heater. Aviation
gasoline is injected into the combustion chamber through the spray nozzle. The resulting cone-shaped fuel
spray mixes with combustion air and is ignited by a spark from the spark plug. Electric current for ignition
is supplied by an ignition unit which converts 28 volts to high-voltage, oscillating current to provide a
continuous spark across the spark plug gap. A shielded, high voltage lead connects the ignition assembly to.
the spark plug. Combustion air enters the combustion chamber tangent to its surface and imparts a whirling
or spinning action to the air. This produces a whirling flame that is stable and sustains combustion under
the most adverse conditions because it is whirled around itself many times. Therefore. ignition is
continuous and the combustion process is self-piloting. The burning gases travel the length of the
combustion tube, flow around the outside of the inner tube, pass through crossover passages into an outer
radiating area. then travel the length of this surface and out the exhaust.
Ventilating air passes through the heater between the jacket and combustion tube assembly outer
surface and through an inner passage in the assembly. Consequently, ventilating air comes into contact with
two or more heated, cylindrical surfaces.
13-8 DUCT SWITCH. (Refer to Figure 13-5.)
This switch is installed in the ventilating air duct downstream from the heater to sense the ventilating
air outlet temperature. To select the desired cabin temperature. the switch max be adjusted manually from
a high of 250 F 10 downward through a range of 146 F. The switch has a differential of 15 F 5 at an\
given setting.
13-9. COMBUSTION AIR BLOWER AND PRESSURE SWITCH.
This centrifugal-type blower supplies air to the combustion chamber of the heater. Performance of the
combustion air blower is assisted by the use of ram air during flight. A differential pressure switch cuts off
the heater operation whenever the fuel-air mixture is not appropriate for efficient heater operation. This
switch is set to activate at 0.5 inches of water
.03 inches.
13-10. VENTALATING AIR BLOWER.
This blower is attached to the inlet end of the heater assembly and provides a source of ventilating air
through the heater. Ram air from the ventilating air intake scoop is used during flight.
13-11 HEATER SAFETY LIMIT SWITCH. Located in the heater is a heat limit switch which acts as a
safety device to render the heater system inoperative if a malfunction should occur causing excessively high
temperatures. This control is located in the downstream end of the vent jacket. with the reset button on the
heater shroud. It is reached only through the access hole in the right side of the nose section to insure that
the malfunction causing the overheat condition is corrected prior to future heater operation
Revised: 10/15/82
HEATING AND VENTILATING SYSTEM
4B9