Flame Sensors Dry Up Processing Problems
The corn dryers selected for installation were built by Grain Systems Inc., Assumption, IL, a company with which Monsanto had previously worked. Each Grain Systems corn dryer consists of an Airstream Series 2000 vaneaxial heater mounted on a centrifugal fan. The fan blows heated air into a plenum connected to several positions, each capable of holding three boxes of seed to be dried. With each heater capable of 500,000 BTUs, the system is designed to dry the corn at about 95oF (35oC). If it is dried at too high a temperature, the corn germ will die and destroy the seed.
The heaters from Grain Systems contain computerized components, including digital readout of heater functions, ignition system to monitor the burner and view window for observation. They are designed for accurate control provided there is at least an eight degree differential between the outside ambient air and the drying temperature setpoint for the corn. In the open air of Hawaii, however, the temperature can sometimes reach 95oF (35oC), which makes control more difficult. To solve this problem the burners were modified so that they could be turned down, making possible a control differential of about 3oF. However, this creates a difficult situation for the computerized control monitoring the flame's presence.
Initially, the flame sensor used at Haleakala Farm was a flame rod inserted into the burner flame. Flame rods work on the principle of flame rectification, which requires the flame rod to be in contact with the flame and the base of the flame to be in contact with a ground surface. The surface should be approximately four times the area touching the flame rod. The flame-control circuit then applies approximately 120 V to the flame rod. This voltage, when in contact with a flame, causes a current to flow through the flame to the ground surface. If the ground surface is properly grounded and the proper ratio of area exists between the ground and the flame rod, then a small direct current will occur through the flame and will be detected by the flame-control circuit. Many things can affect the sensitivity and reliability of this method. Flame size, ground surface area, good ground connections and corrosion on the flame rod are potential problems.
During high ambient conditions when the flame in the heater was low, the sensitivity of the flame-rod sensor was inadequate to sense the flame, resulting in burner shutdown. Another problem was the high humidity, which often was present during the early morning hours on Maui. At such high humidity, the flame rod often would become moist or wet. This in turn would cause the rod's high voltage to short out, prevent the control from seeing any flame signal and thus shut down the dryer. In some cases, the operator would have to repeatedly attempt to light the burner until the flame rod was sufficiently dried out.
The fact that the rod was in constant contact with the flame resulted in carbon buildup and corrosion covering the surface of the sensing rod. This prevented the current flow necessary from the flame rod to prove the flame's presence. Again, the dryer shut down. In addition, the flame was small, making it hard to reach with the flame-rod tip. This resulted in many artistic bends and shapes of the flame rod in an effort to reach the flame. It was not long before another solution was being sought.
Ultraviolet Sensors Offer a SolutionUltraviolet sensors have long been on the market. Because a gas flame emits ultraviolet radiation, an ultraviolet sensor appeared to be a logical choice. However, because the heater was intended to operate continuously 24-hr a day, seven days a week, a self-checking type of sensor was needed. Should the sensor fail internally and give a false flame indication, the sensor's internal circuitry would detect that fault and immediately shut down the system.
Monsanto had tried other ultraviolet self-checking sensors in the past. The ones tried were not sensitive enough or were not reliable in shutting down the dryer in a loss-of-flame situation. Tom Dunn, manager of research equipment at Monsanto, found an answer to these problems at GN Electronics, Machesney Park, IL.
GN developed a flame sensor with an internal microcomputer called the Quanta-Flame 5002. It is an ultraviolet sensor with a variety of available outputs that works on the principle of detecting ultraviolet radiation emitted from a flame. The Quanta-Flame's internal microcomputer automatically adjusts the sensor's sensitivity to allow for small or weak flames. In addition, it employs a self-check mechanism that checks the integrity of the detection tube every 10 sec. With the Quanta-Flame scanner in hand, Dunn flew to Hawaii to retrofit the grain dryers.
The flame rods were removed and a hole was drilled into the heater housing to enable the Quanta-Flame to have a sight opening to the flame. The unit was designed to sense the ultraviolet radiation from the small flame within the heater. Although its internal microcomputer adjusted the sensitivity to detect the flame's presence, Dunn believed the flame signal strength could be improved by enlarging a hole in the burner cone to give the Quanta-Flame a better view of the flame.