Should one travel along the conveying pipes of an exhaust plant, one would meet swarms of sparks burning out along their way to or dying out in the bin. Many people think it hard to believe, since bins do not catch fire everyday and there are even bins which never did. In fact most sparks bum out while running, because particles are small and subject to the strong action of the air flow dragging them through the pipe. Only the biggest ones reach the bin, where they either fall into the heap and are covered by the material (dust and chips) coming in, or are dragged towards the upper section of the bin, against the filter grids. This second case generates a constant wear of the filter grids, since burning particles cause a lot a small holes affecting the filter effectiveness. It is difficult to estimate the percentage of sparks able to set the bin on fire; several factors can influence the process, i.e. wood, moisture degree, particles size, length of the conveying pipes, what the spark finds in the bin and how it falls in it. A lot of sparks reach bins without causing any damage, while sometimes one single spark is enough to produce a disaster. All woodworking machines produce sparks, but some are more dangerous than others. Here is a short list. The multiple circular sawing machine is the most dangerous one. It consists of several coaxial rotating blades subject to heavy working loads. When the blades meet an inclusion (nail, stone) or a particularly hard knot, they can loose one or more teeth. The resulting overheating burns the inclusions and the chips. Even without harmful inclusions, when teeth are worn out and the cut is no longer clean the overheated blade burns the wood while cutting it. The sanding machine. It looks as the most innocuous operation, while it is among the most dangerous ones. It generates fine and dry wood dust, susceptible to catch fire in presence of the smallest spark. The work piece often includes metal staples generating swarms of sparks. The wheel. It is employed in profiling operations. It is a continuous source of sparks, so that it is often equipped with a separate exhaust line passing through a safety cyclone. Otherwise it is served by the general exhaust plant. The chipper is the machine starting the chipboard production process. A series of rotating cutters transform logs into chips. However, inclusions (nails, hooks, etc.) are often hidden in the logs, causing breakage of the cutters and the following overheating and setting on fire of the chips. The chipping machine used to destroy wood wastes destined to be burnt in the boiler plant: it often meets metal parts (bolts, screws, nails, metal sheets) which become red-hot and set fire to wood. The drier is used to dry chips in the manufacturing of chipboard panels. Because of a possible malfunctioning, the burner sets the chips on fire, instead of only drying them. It should be mentioned that wood catches fire at about 380°C, while temperature inside the drier reaches approximately 350°C. The margin of safety is therefore very thin.
HOW AND WHERE THE DETECTOR WORKS In order to avoid that bins and working lines served by the exhaust plant are set on fire, the Control Logic, a Milan based firm, supplies special devices able to detect and extinguish all kind of sparks in due time, before they reach their destination. The time available is very short indeed, since particles speed inside the conveyor is approx. 30 m/s and pipes can also be very short, sometimes less than 10 m. At 30 m/s speed, 10 m mean a third of a second. Within this lapse of time the spark detector must intervene, operate a water jet or close a lock-gate. Most of the time is absorbed by the automatic jet sprayer or the electro-pneumatic lock. The first device must open an electrovalve and afterwards accelerate water to form a jet. The latter must operate a compressed air cylinder and then turn the lock in the off position. But the short time available does not represent a difficulty for the detector, because its intervention time is very short, i.e. a few hundredths of a second. Three are the elements challenging the spark detector: the time taken by the spark to transit in front of the observing window, the spark size and the pipe diameter. The window is about 100 mm wide. Travelling at 30 m/s the spark remains in front of the window only for a few thousandths of a second. The size of the burning particle can be very small, only few millimetres. The distance of the spark from the detector has therefore a strong influence on its capability to detect it. Sparks passing nearby can be very small, while those passing far away should be larger. Such a distance is connected with the pipe diameter, usually ranging from 200 mm to 1 m. In pipes with diameters up to 50 cm, only one device is usually installed. In fact in this case the maximum distance between a spark and the detector is 50 cm. Pipes larger than 50 cm are usually equipped with two devices facing one another. In this case too the maximum distance between sparks and the device is 50 cm, since each detector surveys one half of the pipe. The use of two opposing detectors removes any "shadow" zones located at the window sides, out of the observing cone of the detector.
WORKING PRINCIPLE The spark detector does not work like a human eye. In fact it is not stimulated by the light but by the infrared radiation, thus detecting sparks which could not be seen by the eye. Indeed many of the sparks normally detected by the device would be invisible for a human observer. Burning particles emit red light and infra-red thermal radiation, the latter in a quantity 100 times larger. In addition the spark detector is sensitive only to moving burning bodies. This feature makes the device suitable for installation also in well-lighted open spaces, to survey belt and helicoidally conveyors. To grant an easy check of its efficiency, the detector is equipped with a "test" device. A remote control button permits to simulate the passage of a spark in order to check the efficiency of either the water jet or the lock-gate. The more sophisticated systems are equipped with an automatic control of all main functions, operating an alarm signal if a spark has not been extinguished or stopped, if a detector is out of order or if there is a leakage in the water jet system.
THE DETECTOR AS A SAVING FACTOR Everybody knows that the fire rate in the woodworking industry (sawmills, manufacturers of furniture, chipboard panels, fibreboard, etc.) is very high, resulting in expensive insurance premiums, amounting to the double of the figures paid by the metalworking industry. Unfortunately, in case of fire, the insurance often does not cover all damages. To all this it should be added that insurance companies are often open to grant remarkable discounts when the woodworking plant is equipped with spark detectors.