Heat Detectors According to NFPA 72 - 2019 edition

Heat Detectors 

Heat Detector according to NFPA 72

Detector Definition according to NFPA72:

A device suitable for connection to a circuit that has a sensor that responds to a physical stimulus such as gas, heat, or smoke.

Heat Detector Definition according to NFPA72:

A fire detector that detects either abnormally high temperature or rate-of-temperature rise, or both.

Advantages of Heat Detectors:

• Heat detectors are very reliable and have the lowest false alarm rate of all automatic fire detectors.

Applications of Heat Detectors:

• They are best suited for fire detection in small confined spaces where rapidly building high-heat-output fires are expected, in areas where ambient conditions would not allow the use of other fire detection devices, or where very early warning of fire is not required.

How do Heat Detectors work?!

• Heat detectors respond primarily to the convected thermal energy of a fire.
• They operate either when the detecting element reaches a predetermined fixed temperature or when a specified rate of temperature change occurs.

What are the required approvals for Heat Detectors?!

• Heat sensing fire detectors shall be listed in accordance with applicable standards such as ANSI/UL 521 

Types of Heat Detectors:

1- Fixed Temperature Heat Detectors: 

• Fixed-temperature heat detectors are designed to alarm when the temperature of the operating element reaches a specified point.
• There are different types of fixed temperature heat detectors such as fusible element type, continuous line type, and bimetallic type.

2- Rate-of-Rise (ROR) Heat Detectors:

• A device that responds when the temperature rises at a rate exceeding a predetermined value.
• Why do we use ROR heat detectors?
  One effect that a flaming fire has on the surrounding area is to rapidly increase air temperature in the space above the fire, Fixed-temperature heat detectors will 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 temperature increase exceeds a predetermined value, typically around (7 to 8°C) per minute. Rate-of-rise detectors are designed to compensate for the normal changes in ambient temperature (less than [6.7°C] per minute) that are expected under nonfire conditions.
• There are two types of ROR heat detectors: Line type, and spot type.

3- Rate Compensation Heat Detectors: 

• A rate compensation detector is a device that responds when the temperature of the surrounding air reaches a predetermined level, with the rate of temperature rise having minimal effect on response (low thermal lag). This makes them rather sensitive fixed-temperature detectors.

4- Combination Fixed Temperature / Rate of Rise detectors:

• Combination fixed-temperature/rate-of-rise detectors contain more than one element that responds to a fire. 
• These detectors may be designed to respond from either element or from the combined partial or complete response of both elements. 
• Its advantage is that the rate-of-rise element will respond quickly to a rapidly developing fire, while the fixed-temperature element will respond to a slowly developing fire when the detecting element reaches its set point temperature.

5- Electronic Spot-Type Heat Detectors:

• 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. This resistance change is monitored by 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).

Temperature Classification and Color Code for Heat Detectors:

Temperature Classification and Color Code for Heat Detectors

References:

1- NFPA 72 - 2019 edition (National Fire Alarm and Signaling Code®)

2- Fire Protection Handbook 20th edition.

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