Heat Detectors - Principle of Operation

The third instalment of the Principle of Fire Safety series looks at heat detectors, one of the four methods of detecting fire through the the by-products of combustion. This is an area of fire safety that has been rapidly evolving over the last 20 years.

Research and development has improved well established detection technologies and provided an array of new technologies to improve fire detection while also being less susceptible to the causes of false alarms.

Before we go any further it's important to lay the foundation for what is fire; fire also known as combustion is a sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the by-products of combustion being; heat, smoke & electromagnetic radiation (light). Personally I think a illustration explains this chemical reaction in terms easier to understand.

It is also important to recognise that smoke is an aerosol or mixture of particulates suspended in air that comprises a collection of airborne solids, liquid particulates and gases emitted when a material undergoes combustion.

This is important because each of the four detection technologies are designed to respond to one of the three by-products of combustion. There are four principal methods for detecting fire explored in this article including; Heat, Smoke, Flame and Combustion detectors.

Heat Detectors

There are two methods for detecting fire from the presence of heat;

  1. Fixed temperature heat detectors operate when the ambient temperature increases sufficiently to predetermined level where the heat detector will operate; or
  2. 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.

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

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;

  1. Not used in modern fire safety systems, 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.
  2. 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.
  3. 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.
  4. 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.

Optomechanical

Optomechanical - This type of heat detector is a modern variation of the electromechanical line-type heat detector and could be classified as an optomechanical distributed heat. Optomechanical 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.

Electropneumatic

Electropneumatic - The principal for an electropneumatic rate-of-rise heat detector was first patented in 1941 and has been subject to ongoing product development since that time. Electropneumatic heat detectors comprise a controlled vented chamber containing a diaphragm that moves due to a pressure differential according to the rate of change of the ambient temperature. When the ambient temperature changes faster than the calibrated rate which the vent has been designed to release, the diaphragm moves sufficiently to create an electrical circuit to indicate an alarm. The main benefit of electropneumatic heat detectors is that they operate at a range of temperatures because they respond to the rate of change in temperature, not at a fixed temperature only.

Electronic (Thermistor)

Electronic (Thermistor) - The most common form of heat detector used throughout Australia is electronic device with a thermistor acting as the heat sensitive element. A thermistor is a type of resistor whose resistance changes significantly according to temperature. These detectors can operate as a fixed-temperature and a rate-of-rise device or both depending on their intended design. These detectors may also include features to reduce the possibility of deceptive phenomena causing false alarms.

There are five classifications of heat detector established for use, they are classified according to their fixed temperature operation or rate-of-rise (if fitted).

Type A (white dot)
normal temperature duty, incorporating both fixed-temperature (58ºC - 88ºC) and a rate-of-rise actuation
Type B (blue dot)
normal temperature duty, incorporating fixed temperature (58ºC - 88ºC) actuation only.
Type C (green dot)
High temperature duty, incorporating both fixed-temperature (88ºC - 132ºC) and a rate-of-rise actuation.
Type D (red dot)
High temperature duty, incorporating fixed temperature (88ºC - 132ºC) actuation only.
Type E (yellow dot)
Special purpose fixed temperature. This type of heat detector is selected when the purpose of protection cannot be satisfied by types A to D

In the near future, these classification Types will be replaced by the ISO standard (AS 7240.5) heat detectors with a wider range of classes.

While heat detectors are a very reliable form of fire detection, they are not normally recommended as a life safety device. Heat detectors are often selected to detect the presence of fire where there is the potential for deceptive phenomena from one of the other by-products of combustion.

The next part in our series on the Principles of Fire Safety Series will focus on Smoke, Flame and Gas Detectors.

About the author

Russ Porteous is the CEO and one of the founders of Maintenance Essentials, he has over 22 years experience in the installation and maintenance of fire and essential safety measures. Russ is an active contributor to a variety of Australian Standards including AS1851 for the Maintenance of Fire Protection Systems and Equipment. Russ speaks regularly at conferences and trade shows as a subject matter expert. You can follow Russ online via Twitter @rport or his popular fire safety blog.

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