Heat Detectors

There are certain fire-protection applications where smoke detectors are not suitable, such as non-life-safety installations where the environment has too many airborne particulates due to excessive steam, moisture, dust, humidity or temperature, such as in attics, garages, warehouses, storage facilities, elevator machine rooms and electrical closets. Smoke detectors applied in those environments are cost prohibitive from a maintenance standpoint.

Electronic heat detectors use a thermistor as the primary heat sensing device. A thermistor is a component that changes resistance with temperature. Typically, electronic heat detectors have additional features, such as LED indicators that blink to indicate proper standby operation.

Mechanical heat detectors are bi-metallic or pneumatic. Bi-metallic heat detectors have a strip consisting of two dissimilar metals. When the strip is heated, the metal distorts and closes a contact. These detectors tend to be very inexpensive.

Pneumatic heat detectors, on the other hand, use an air chamber that is sealed with a moveable diaphragm. When the air inside the sealed chamber gets warm, the chamber expands and distorts the diaphragm. This, in effect, puts pressure on a set of contacts, which makes an electrical connection. Pneumatic detectors are often used in harsh environments because they can be sealed against corrosive elements.

In general, mechanical heat detectors are less expensive than electronic heat detectors; however, because they have a fixed temperature, they are not restorable after a field test.

Fixed-temperature heat detectors are designed to alarm at a particular temperature. Because of thermal lag, however, if the rate of temperature rise is fast, the detector may actually alarm when the room temperature is higher than the set point. Furthermore, these detectors are not restorable after a field test. The alarm is destroyed.

A rate-of-rise component is sometimes added to a fixed-temperature design. This way, when either the fixed temperature or a pre-set temperature increase rate is exceeded, the detector will alarm. Heat detectors with a rate-of-rise feature tend to produce a higher level of protection in many applications, but should be used with caution. One should verify that the intended environment does not have naturally rapid temperature rises that exceed the detector’s trip point. This may be the case in an attic, for example. Additionally, these detectors are restorable. They reset to a non-alarm condition after a field test.

Spot type detectors essentially have their “detection mechanism” at one location. That is, the sensing element is in one physical location. Contrast that with linear heat detection, where the sensing element is spread out over a large physical area.

In linear heat detection, a special multicore wire or cable is utilized. The cable has two conductors that are fitted with an insulating jacket with a specific melting point. That melting point corresponds to the detection scheme’s fixed temperature set point. When the temperature increases enough, the insulating jacket separating the conductors melts and they come into contact with one another, shorting out. This short can be detected. These linear heat detection cables can be hundreds of feet in length, making them ideal for conveyor or cable tray applications.

In a slowly developing fire, this form of detector responds when the temperature of the air surrounding the detector reaches a predetermined level. In a rapidly developing fire, the detector anticipates the air temperature reaching the operating point, accelerating the operation of the detector. This produces a fixed-temperature detector with virtually no thermal lag.