FIRE DETECTORS-2

POINT TYPE HEAT DETECTORS

The FSIB introduces the second in a series on types of detectors available for the many and varied applications one might face.

 

Heat detectors

Heat detectors are a valuable detector in the fire detection designer’s armoury. They come in various forms including conventional, addressable, linear and heat probes. For this edition we are concentrating on point types of heat detection. Conventional and addressable operate in the same manner.

 

 

 

Heat detector types.

There are two main types of heat detectors on the market Rate of Rise heat detector or Fixed Temperature detector. Both being available as conventional or addressable types.

If you order a heat detector from your supplier, he will in most cases issue you with a rate of rise heat detector.

 

Rate of Rise Detector

Rate of rise detectors are designed to detect a fire as the temperature increases, but they also have a fixed upper limit at which the detector will go into alarm if the rate of temperature increase has been too slow to trigger the detection earlier.

A rate of rise detector monitors a gradual build-up of heat in the protected space i.e.: 15oC / minute.

Should the temperature in the room rise at a rate of 15oC in one minute or faster the detector will

operate.

 

Fixed temperature Detector

Fixed heat detectors only change to the alarm state at their preset temperature.

 

Applications

Heat detectors are installed to monitor for high temperatures at the ceiling/roof level. They are normally used where smoky, steamy, or dusty environments can be experienced.

SANS 10139 also recommends the use of heat detectors for flaming fire where little smoke will be experienced.

Typical applications would be alcohol stores, kitchens, parking garages, electrical substations, oil stores.

 

How do they work?

The rate of rise detector operates by using a matched pair of thermistors to sense heat. One thermistor is exposed to the ambient temperature, the other is sealed. In normal conditions, the two thermistors register similar temperatures, but, on the development of a fire, the temperature recorded by the exposed thermistor will increase rapidly, resulting in an imbalance of the thermistors and causing the detector to change to the alarm state.

Addressable detectors use a single thermistor to sense the air temperature at the detector position.

As temperatures rise the fire panel monitors the change until it reaches the preset parameters and then raises the alarm.

The fixed temperature detectors use a single thermistor set at a specific temperature. When the temperature surrounding the detector reaches this level, it will raise the alarm.

There are a range of detectors monitoring for temperatures between 25o to 90oC.

 

Where should we not use it?

The standards do not recommend using heat detectors to protect corridors due to its late alarm compared to a smoke detector.

Heat detectors should not be installed in bedrooms, hospitals, clean rooms, IT environments or for actuating gas protection systems.

 

Where must it be installed?

It must always be installed at the highest point where the heat will always accumulate.

This can be any ceiling or roof and must be installed within 150mm of the roof or ceiling.

The detector can cover an area up to 56m2 but must be within 3.8m of the walls and spaced 7.5m apart.

  

Have you suffered false alarms?

The heat detector is less prone to false alarms than smoke detectors. If the device itself goes faulty you will receive an alarm at the fire panel.

Although false alarms are rare, heat detectors can be subject to nuisance alarms especially if positioned near ovens or furnaces.

 

Conclusion

A good detector for dirty. dusty or humid environments, commonly used in kitchens and garages.

As the heat detector is recognised as slow to operate, designers should only choose a heat detector if there are no other suitable alternatives.