By Drew Arnberg
It's time to get ready for heater season again. Everyone has a list of items to be checked to get their Janitrol heater "tuned-up" for the cold weather. Each list may differ slightly, yet collectively these "pre-season checks" represent the 100-Hour Inspection in the Janitrol Overhaul and Maintenance Manual (24E25-1). Using the manual as a guideline helps ensure sustained operations on the ground and in flight.
The following short overview will focus on useful inspection criteria. We will also highlight some of the design features of the Business and Utility class of heaters from the Janitrol Aviation Heater product line. This class of heaters (the B-series) is one of the more common types of combustion heaters in the general aviation industry. The unique design characteristics of the Janitrol combustion heater result in safe, dependable and long-lived performance. This reputation began with the product line's introduction during WW II and continues up until the present.
The "Whirling Flame" Principle
The main design feature at the heart of the Janitrol heater's reputation is the "whirling flame" principle of combustion. Air for combustion is ducted from an outside scoop or blower and enters the sealed combustion tube via the combustion air inlet. It is introduced into the combustion area at a tangent to the inner surface of the tube that sets up a spinning or whirling motion. Simultaneously, liquid fuel flows under pressure from a pump and regulator to the fuel nozzle where it creates a fine spray that is mixed with the whirling combustion air to make a combustible mixture. The fuel/air mixture is ignited by a high voltage spark plug. The resulting "whirling flame" resembles a tightly coiled spring and maximizes the heat production in the short combustion tube.
Combustion gases travel the length of the combustion tube, double back over the outside of the combustion tube, bypass the vent airflow through a crossover to an outer heat transfer area, and then make one last trip down the length of the heater to exit at the exhaust. The heat transfer process is completed as ventilation air is ducted from an air scoop or blower (separate from the combustion air) through the ventilating air passages between the combustion tube assembly's layered walls. This vent air enters at the combustion head end of the heater, exits downstream and is ducted to the aircraft's cabin.
Temperature control in the cabin is maintained by the adjustment of a duct switch. Besides the duct switch inputs, temperature control in the heater includes a cycling switch and an overheat switch. The cycling switch maintains optimum combustion temperatures by controlling the fuel solenoid. The manual reset overheat switch disables the entire heater system when an overheat condition occurs.
This explanation is old news for most pilots and technicians, but it serves to introduce important features of the Janitrol design. The whirling flame is very stable throughout all attitudes and air velocities. The tight coil ensures continuous re-ignition as the flame whirls around itself. This ensures that the combustion process is complete. To aid in complete combustion, Janitrol uses an "excess oxygen" fuel/air ratio. The excess air is supplied to ensure not only complete primary combustion, but also complete secondary reactions. Within the heated chamber, the secondary oxidation of carbon monoxide yields carbon dioxide. In other words, with proper combustion air and proper fuel pressures, the exhaust will have miniscule amounts of the deadly carbon monoxide.
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