Electronic Detectors Spot Fire in the Incipient State

Electronic Detectors Spot Fire in the Incipient State

1970S CHALLENGING YEARS

Early water, chemical vapors released by smoldering fuel activate alarm

Early warning fire detection, though once only a dream, is now a reality through modern electronics. This dream was evident as far back as 15th century Europe when lookouts, furnished by the guild of musicians, were placed day and night in city towers to sound a trumpet when a fire was spotted.

Fire towers were still in use, when around 1850 in Boston a newly invented telegraph was installed to connect each tower directly to a fire station. Electricity now was used to help give an early fire warning. Later it would help give early fire detection.

This need for early detection became evident upon examining the three phases of fire fighting. The first phase concerns commencement of the fire until detection and alarm. The second phase lasts from when the fire department receives the alarm until it arrives at the fire. Extinguishment is the third phase of fire fighting. Although only this last phase is concerned with the actual putting out of the fire, success in preventing grave fire losses depends heavily on the time factors of phases one and two. Phase two, the traveling time of the fire department, has a relatively constant time factor. Therefore, phase one, the detection and alarm, becomes the critical one for effectively minimizing fire losses.

Before electronics can be applied to detecting a fire, an understanding of a fire’s four stages of development is necessary. The first is the incipient stage. Here invisible products of combustion are given off, yet no visible smoke, flame or appreciable heat is apparent. In stage two, combustion products now are concentrated enough to become visible as smoke, but flame and appreciable heat are still not present. The flame stage is the third stage. Though a recognizable fire exists, an appreciable amount of heat still is not present, although it follows almost immediately. Finally, comes the rapidly expanding air and uncontrollable heat of the fourth stage, which combines to form a dangerous and costly fullfledged fire. Until the turn of the century, a fire generally developed into this fourth stage before it was detected and an alarm could be given. Before firemen were given a chance, the fire had already caused extensive damage to life and property.

With the advent of the sprinkler system around 1900, the thermal fire detection system was born. A fusible metal which melted upon being heated by the fire was widely used at this time. Later, the bimetallic strip came into existence. Then many variations of these were developed, along with the capability of direct electrical connection to the firehouse. Even though the speed of notifying the fire department was appreciably reduced through this direct connection, nevertheless all these thermal elements operated only during the advanced heat stage; and in too many instances, the fire was quite large by the time the fire department arrived.

In the history of applying electricity to the detection and alarm phase of fire fighting, so far only the “alarm” time factor had been effectively shortened. It was possible to notify the fire department almost instantaneously, but only after the fire had developed into its final stage of destructive heat and hazardous toxic gases and smoke. With the application of electronics to fire detection came the capability of detecting fire before the final heat stage.

Types of defectors

In the application of electronics to fire detection, detectors operating on new principles detect the earlier stage of fire development. The third or flame stage, is detected with the flame detector. The photoelectric type device detects the second, or smoke, stage. Ionization and combustion vapor detectors sense the fires in the incipient stage.

The flame detector, which operates during the fire’s third stage of development, generally comes in two types, the infrared flame detector and the ultraviolet flame detector.

The infrared flame detector responds to that energy just below the visible light band on the electromagnetic spectrum. It uses a lead sulfide photo-conductive cell to pick up the energy radiated by the flame. A filter lens in the cover permits only infrared energy to pass. To discriminate between the infrared energy from flames and that of other light sources striking the detector an electronic filter circuit is used to limit response to energy varying in frequency from 6 to 30 cps. This frequency corresponds to the flickering of flames.

The relationship between time and heat in the four stages of fire. 1. Incipient stage-invisible products of combustion given off. No visible smoke, flame or appreciable heat yet present. 2. Smoldering stage—combustion products now visible as smoke. Flame or appreciable heat still not present. 3. Flame stage—actual fire now exists. Appreciable heat still not present, but follows almost instantaneously. 4. Heat stage—uncontrolled heat and rapidly expanding air now complete the dangerous combination. Hazards include: 5. major hazard; 6. moderate hazard; 7. no hazard.

The ultraviolet flame detector responds to that energy just above the visible light band on the electromagnetic spectrum. It detects with a gasfilled glass envelope containing two symmetrical electrodes—one negatively charged, the other positive. Ultraviolet radiation in wave lengths of 1900 to 2900 angstrom units releases electrons from the negative electrode. With a sufficient amount of radiation, the released electrons ionize the gas in the tube, permitting current flow between the electrodes. The current flow actuates the alarm.

Photoelectric detectors

The second, or smoke stage, of a fire can be detected with any one of three basic types of photoelectric detectors. All of them operate after combustion products from the fire accumulate in sufficient quantities to be visible as smoke.

The beam-type photoelectric detector employs a transmitting unit containing a light source and a receiving unit containing the photo-sensitive element. These units are usually mounted near the ceiling on opposite ends of the protected area. The light beam from the source is focused upon the receiving units’ photo-sensitive element and when smoke interrupts this beam, the alarm is tripped. A time delay is usually built in to prevent alarming when the beam is intermittently interrupted by passing ladders, etc. This device is usually adjusted so that 2 to 4 percent obscuration of light per foot will activate the detector.

The refractive, or spot, type photoelectric detector is a small ceilingmounted unit containing a light source, a smoke chamber, a light catcher, and a photo-sensitive element. A beam of light is projected across the smoke chamber into the light catcher. During ambient conditions, this light catcher absorbs all the energy and no light reaches the photo-sensitive element. When smoke enters the chambers, light is reflected by these visible combustion particles in all directions. Some of this reflected light reaches the photo-sensitive cell that is at a right angle to the light source. Obscuration of 2 to 4 percent per foot will activate this detector.

The air-sampling photoelectric detector draws air from the protected area to a central point. Here the air is passed between the light source and the photo-sensitive element. If 2 to 4 percent obscuration per foot exists, the detector will alarm.

Incipient fire detectors

Two principles of operation are used to detect the first or incipient stage of the fire. These detectors operate at the soonest possible moment to give firemen the most amount of time to reach the fire before it becomes dangerous and costly.

In one detection principle, the detector responds to rapid changes in relative humidity. Two of the invisible products of combustion are water and chemical vapors which cause a rapid rise in the moisture content of the air. The detector unit has two nickel grids etched on high-resistance vycore glass and which have high voltage applied to them. This glass acts like a blotter to water vapor. One grid, the compensator, maintains the balance of an electronic bridge circuit under normal atmospheric changes. The second, the detector grid, is exposed to die atmosphere to react to rapid changes of moisture caused by combustion products. The moisture, absorbed by the blotter-like glass, being of less resistance to current flow, causes a grid current to develop. This current flow, amplified, actuates the alarm-initiating relay.

Infrared flame detector, with lenses and cathode tube.Spot-type smoke detector uses a photoelectric cell.Ionization chamber detects invisible combustion products of an incipient fire.

In the other principle used to detect the invisible combustion products of an incipient fire, air is ionized and made electrically conductive by means of alpha particles emitted from a minute radioactive source in the detector ionization chamber. Voltage across this chamber causes minute electrical current flow. Invisible products of combustion entering this chamber also become ionized and also conduct. However, due to their much larger size and mass, their movement across the chamber is slower, and the amount of current flow is appreciably reduced. This reduced current increases the voltage on the electrode of a cold cathode tube, causing it to fire and activate relays for the alarms.

These are the developments in electronic fire detection. In the past, a fire was of considerable size before detection could be accomplished by visual or thermal means. Now a fire can be detected at the flame, smoke and even the incipient stage. In the past, a fire was thought of as raging flames and intense heat. But practically all these large fires were once incipient fires. Needed now is a revision of the old definition in the minds of men.

This new recognition will then open the door of the future for early warning fire detection. Early detection means a fire can be attacked sooner, controlled sooner, extinguished sooner. Early warning systems can actuate extinguishing systems, close smoke barriers, start exhaust fans, shut down equipment, sound the alarm before the toxic gases reach a critical level and give firemen time to extinguish the fire when it is small—before it becomes a serious danger to life and property.

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