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What is Rate of Rise Heat Detector?

Wed, Feb 15, 2023

A heat detector is a fire alarm device designed to respond when the convective heat energy of a fire increases the temperature of a thermal element. The thermal mass and electrical conductivity of the element regulate the rate of heat flow into the element. All thermal detectors have this thermal hysteresis.

There are two main operating classifications for heat detectors, "rate of rise" and "fixed temperature".

Temperature detector When the temperature of the surrounding environment rises faster than a certain speed, it will trigger a fire alarm. The air in the air chamber expands and closes the contacts that generate the alarm.

Rate-of-Rise Thermal Detectors

Rate of rise temperature detectors measure the rate at which air temperature changes during a fire. Alarms can respond more quickly by measuring temperature changes than by measuring temperature levels within the space.

Rate-of-rise detectors measure changes in room temperature by using a differential pressure switch. The switch contains an air chamber that separates the air from the ambient space through a flexible diaphragm.

As the air in the ambient space changes temperature, the air pressure increases, creating a pressure differential across the diaphragm.

The air chamber is constructed with a calibrated leak so that normal temperature and pressure fluctuations in the room space accommodate across the diaphragm and do not cause the alarm contact to close.

During a fire, the air temperature rises faster than normal, causing the diaphragm to heat up on one side of the chamber. The leak cannot be compensated, so the diaphragm moves and closes the detector contacts.

Advantages of rate-of-rise heat detector:

  • The response speed is faster than the constant temperature detector
  • Unaffected by dusty or dirty environments
  • More reliable than smoke detectors
  • Cheaper than smoke detectors
  • Minimal maintenance

Disadvantages of rate-of-rise heat detector:

  • Slower to respond than smoke detectors
  • Combustion products cannot be detected
  • Only suitable for protecting property
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