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Heat Detection Heat detectors are the oldest type of automatic fire detection device. They began development of automatic sprinklers in the 1860s and have continued to the present with proliferation of various types of devices. A sprinkler can be considered a combined fire detection and extingushing device. Heat detectors that only initiate an alarm and have have no extinguishing function are still in use. Although they have the lowest false alarm rate of all automatic fire detector devices, they also are the slowest in fire detecting. A heat detector is best situated for fire detection in a small confined space where rapidly building high-output fires are expected, in areas where ambient conditions would not allow the use of other fire detection devices, or when speed of detection is not prime consideration. Heat detectors are generally located on or near the ceiling and respond to the convected thermal energy of a fire. They respond either when the detecting element reaches a predetermined fixed temperature or to a specified rate of temperature change. In general, heat detectors are designed to operate when heat causes a prescribed change in a physical or electrical property of a material or gas. Heat detectors can be sub-devided by their operating principles Fixed-Temperature Heat Detectors ___________________________________________________________________ Fixed Temperature Heat Detector Fixed-temperature heat detectors are designed to alarm when the temperature of the operating elements reaches a specific point. The air temperature at the time of alarm is usually considerably higher than the rated temperature because it takes timne for the air to raise the temperature of the operating element to its set point. This condition is called thermal lag. Fixed-temperature heat detectors are available to cover a wide range range of operating temperatures - from about 135'F (57'C) and higher. Higher temperatures detectors are also necessary so that detection can be provided in areas normally subject to high ambient temperatures, or in areas zoned so that only detectors in the immediate fire area operate.
___________________________________________________________________ One effect that flaming fire has on the surrounding area is to rapidly increase air temperature in the space above the fire. Fixed-temperature heat detectors woll not initiate an alarm until the air temperature near the ceiling exceeds the design operating point. The rate-of-rise detector , however, will function when the rate of tem,perature increase exceeds a predetermined value, typiclly around 12 to 15'F (7 to 8'C) per minute. Rate-of-rise detectors are designed to compensate for the normal changes in ambient temperature that are expected under non-fire conditions. 1) line-type 2) spot-type
___________________________________________________________________ A rate compensation detector is a device that responds when the temperature of the surrounding air reaches a predetermined level, regardless of the rate of temperature rise. A typical element is a spot-type detector with a tubular casing of metal that tends to expand lengthwise as it is heated, and an associated contact mechanism that will close at a certain point in the elongation. A second metallic element inside the tube exerts an opposing force on the contacts, tending to hold them open. The forces are balanced so that, with slow rate of temperature rise, there is more time for heat tp penetrate to the inner element. This inhibits contact closure until the total device has been heated to its rated temperature level. However, with fast rate of temperature rise, there is less time for heat to penetrate to the inner element. The element therefore exerts less of an inhibiting effect, so contact closure is obtained when the total device has been heated to a lower level. This compensates for thermal lag. Thermal detectors using expanding metals are also automaticallly self-restoring after operation, when the ambient temperature drops to some point below the set point.
___________________________________________________________________ Combination detectors comtain more than one element which responds to fire. These detectors may be designed to respond from either element, or from the combined partial or complete response of both elements. An example of the former is a heat detector that operates on both the rate-of-raise and fixed-temperature principles. Its advantage is that the rate-of-rise element will respond quickly to rapidly developing fire, while the fixed-temperature element will respond to a slowly developing fire when the detecting element reaches its set point temperature. The most common combination detector uses a vented air chamber and a flexible diaphragm for the rate-of-rise function, while the fixed-temperature element is usually leaf-spring restrained by an eutectic metal FIGURE. When the fixed-temperature element reaches its design operating temperature,, the eutectic metal fuses and releases the spring, which closes the contact.
Electronic Spot-type Thermal sensor A thermoelectric effect detector is a device that utilizes a sensing element consisting of one or more thermistors, which produce a change in electrical resistance in response to an increase in temperature FIGURE. This resistance change is monitored by a associated electronic circuitry, and the detector responds when the resistance changes at an abnormal rate (rate-of-rise type) or when the resistance reaches a specific value (fixed-temperature type). Rate-of-rise detectors of this type use two thermistors. One is exposed to changes in atmospheric temperatures. When the temperature rapidly changes as in fire situation, the temperature of the exposed thermistor increases faster than the temperature of the unexposed reference thermistor, generating a net change in resistance causing the detector to go into alarm condition. Most rate-of-rise detectors are designed with fixed-temperature backup feature so that, should the temperature rise be slower than 15'F per min, the detector will operate when the exposed thermistor has reached a predetermined fixed temperature.
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