Heat detectors respond to the thermal energy signature from a fire and are generally located on or near the ceiling. They respond when the detecting element reaches either a predetermined fixed temperature or a specified rate of temperature rise occurs. Knowing the difference between types of detectors is very important. Periodic tests must be made of all detectors. Applying a safe source of heat can test restorable detectors, while non-restorable detectors must be tested mechanically or electrically. It is important to know which types of heat detectors are installed so that tests can be made on all restorable heat detectors, but not on the fusible elements of non-restorable detectors.
Fixed-Temperature Heat Detectors:
These detectors initiate an alarm when the detecting element reaches a predetermined fixed temperature. Because of inherent thermal lag, when the detector actually operates, the temperature of the air surrounding the detector has always extended considerably higher then the set point of the detector.
One form of a spot-type fixed temperature detector uses a fusible element made from a eutectic metal alloy that melts rapidly at a predetermined temperature (commonly 135°F). Automatic sprinklers, fire dampers and door fusible links commonly use a similar material. The operation of the detector destroys either the entire unit (or at least the operating element) which the person who maintains the system must replace. Another form of spot-type fixed-temperature heat detector uses a bimetallic element. After operating, the bimetallic type automatically restores when the temperature falls to a point below the set point of the detector.
Fixed-Temperature Heat Detectors features :
- Relatively inexpensive compared to other types of systems
- Can be slowest to activate
- Activate when heated to temperature for which rated
- Activate by one or more of three mechanisms
- Fusible device
- Frangible bulb
- Continuous line detector
- Operate on assumption that temperature in room will increase faster from fire than from normal atmospheric heating
Fixed-temperature heat detectors are designed to alarm when the temperature of the operating elements reaches a specific point. The air temperature at the time of alarm is usually considerably higher than the rated temperature because it takes time for the air to raise the temperature of the operating element to its set point. This condition is called thermal lag. Fixed-temperature heat detectors are available to cover a wide range of operating temperatures – from about 135’F (57’C) and higher. Higher temperatures detectors are also necessary so that detection can be provided in areas normally subject to high ambient temperatures, or in areas zoned so that only detectors in the immediate fire area operate
Rate-of-Rise-Compensated Fixed Temperature Detector:
In a slowly developing fire, this form of de-tector responds when the temperature of the air surrounding the detector reaches a prede-termined level. In a rapidly developing fire, the detector anticipates the air temperature
reaching the operating point, and accelerates the operation of the detector. This produces a fixed temperature detector with virtually no thermal lag
Rate-of-Rise Detector:
A rate-of-rise detector will operate when the rate of temperature increases from a fire exceeds a predetermined level, typically around 5°F in twenty seconds or 15°F per minute. Small, normal changes in ambient temperature that can be expected under non-fire conditions will not operate the detector. These detectors are available as both line- type or spot-type detectors, and are restor-able
- Operate on assumption that temperature in room will increase faster from fire than from normal atmospheric heating
- Designed to initiate signal when rise in temperature exceeds 12° to 15°F (-11°C to -9°C) in one minute
- Can be initiated at room temperature far below that required for initiating fixed-temperature device
- Reliable, not subject to false activations
- Pneumatic rate-of-rise spot detector
Linear Heat Detector:
For some applications, the use of a linear heat detector is an option to consider. These may be installed in head to reach areas, or areas that are subject to high heat. The detector is contained within a cable which when exposed to heat that is greater than its rating, will short circuit, causing an alarm.
Spacing of heat detectors
Spot type heat detector spacing ratings are based on detector installation on a flat, smooth ceiling that is 10 feet (3 m) high. The listed spacing equates detector operation with the opening of a standard sprinkler head within 2 minutes (+/- 10 seconds) located 10 feet (3 m) from the same fire. Spot type detector spacing is shown in the figure below. Detector coverage is typically represented as a square because most structures have flat sidewalls. Actual detector coverage is a circle
whose radius is 0.7 times the listed spacing. Since all of the area within the detector’s circle of coverage is suitable for detecting a fire, the shape and dimensions of the detector coverage “square” in the figure below may be modified. Note that, although the coverage “square” is now a “rectangle,” the coverage area remains within the overall detector circle of coverage.
A spot type heat detector will cover all points located within 0.7 times the listed spacing. The listed spacing for heat detectors is S = 50 ft. (15.3 m).
Ceiling height and construction
When heat detectors are installed on other than flat, smooth ceilings or at ceiling heights greater than 10 ft. (3 m), spacing adjustments must be made. The table below lists the reduction in listed spacing that must be applied when detectors are mounted on ceilings higher than 10 ft. (3 m). This reduced spacing yields the equivalent response of detectors located on a 10 ft. (3 m) ceiling.