MIRROR: G K BHAT
When fire takes place or likely to take place there are various changes takes place in the environment of surrounding area due to certain reasons .Mainly emission of smoke, change in ambient temperature, light flash ect. Each one of the indication helps us to detect the fire or possible fire. In our previous topics we discussed the technique of detecting the fire through smoke. In this topic let us discuss about heat detectors and technique.
Combustion is essentially an exothermic, gas-phase chemical reaction. Gaseous fuels combust by breaking bonds in the fuel molecules, forming other chemical species and releasing thermal energy. For solid or liquid fuels, Soren of the thermal energy is needed to produce the phase change to a gas before the actual combustion takes place. This required energy is the heat of gasification. The net remaining energy then goes to increase the temperature of the gases and air leaving the combustion zone. This hot gas rises due to buoyancy to the ceiling and spreads radially outward in a ceiling jet. The temperature and velocity of this ceiling jet govern the heat transfer rate to thermally activated detectors located on the ceiling.
Here are two methods for detecting fire from the presence of heat;
- Fixed temperature heat detectors operate when the ambient temperature increases sufficiently to predetermined level where the heat detector will operate; or
- A rate-of-rise heat detector operates when the ambient temperature increases over time equal to or greater than the rate of change the detector was manufactured to operate.
To detect the heat, there are various technologies are used. Among these technologies, mainly about four types of detectors are generally used and marketed. Over the years there has been continuous development in the technology to detect heat. These technologies can be broken down into four main types.
Electromechanical – a very old technology
Electromechanical – As the name suggests electromechanical heat detectors operate due to a combination of mechanical movement creating an electrical circuit. There are four fundamental types of electromechanical fixed-temperature heat detector;
- The thermostat comprises a bi-metal strip, with one end fixed into position and the other end free to move depending to the change in its temperature. The bi-metal strip also forms one part of an electrical circuit. When the temperature increases the bi-metal strip completes an electrical circuit to actuate an alarm. This type of detectors generally not used nowadays.
- The second, and most common type of fixed temperature heat detector is a fusible link comprising a eutectic alloy. This type of heat detector was the most common heat detector found between the years 1970 and 2000. An eutectic alloy is mixture of two or more metals whose melting point at a lower temperature than the individual metal. When the ambient temperature increases to the eutectic temperature, the alloy changes state from a solid to a liquid, like solder. This enables a spring held under pressure to release and make an electrical circuit to actuate an alarm. However this type of technique is extensively used to operate fire doors, fire dampers ect .
- The third type may be found in very old fire systems and includes a length of heat sensitive wire strung between two points. This type of heat detector is a very old method of heat detection and is no longer used. The detector comprises a length of heat sensitive wire with one point fixed into position, and the second point draped over a pulley wheel. At the end of the wire is a weight to maintain the tension on the wire. As the ambient temperature increases the wire expands and the weight drops down. This system is calibrated so that at a predetermined fixed temperature the weight drops to a point where it can create an electrical circuit and actuate an alarm.
- The fourth type of electromechanical fixed-temperature heat detector is a distributed heat detector (also known as a line-type heat detector) that comprises a twisted pair of electrical conductors separated from each other by a heat sensitive insulator, and enclosed in an protective sheath. When exposed to heat the insulator changes physical state from a hard solid to a molten state enabling the twisted conductors to create an electrical circuit to actuate an alarm.
Another technique is used in this type of fire detector is Opto-mechanical
Opto-mechanical – This type of heat detector is a modern variation of the electromechanical line-type heat detector and could be classified as an opto-mechanical distributed heat. Opto-mechanical heat detectors contain one or more fibre optic cables separated by a heat sensitive insulator and protected by an outer sheath. A focused light signal is passed through the fibre optic cable. When exposed to heat, the heat sensitive insulator changes state from a solid to a molten state which has the effect of degradation or discontinuation of the focused light signal. The signal change is monitored by a device which then would actuate an alarm.
Even Opto- mechanical system is not a popularly used system. We will discuss in our next topic the system used nowadays.
G K Bhat