The Logic Behind The Magic. “Smoke and Heat Detectors"

There are two types of smoke detectors, each having different applications

Ionization Smoke Detectors detect the presence of smoke by means of a small ionization chamber and a source of ionizing radiation. Ionization Smoke Detectors are generally better at detecting smaller smoke particles such as that produced by rapidly burning fires, but are less sensitive to the larger smoke particles generated by smouldering fires, etc.

Photoelectric (Optical Point) Smoke Detectors detect the presence of smoke by means of a small light source and a separate photoelectric light sensitive cell housed within a sensing chamber. Photoelectric Smoke Detectors are generally more sensitive to larger smoke particles such as that generated by dense smoke or smouldering fires, but are less sensitive to small particles of smoke found in rapidly burning fires.

While, Heat detectors is usually installed in enclosed equipment spaces with high densities of equipment where flame detector methods may be restricted. 

Heat detectors is usually be used in hydrocarbon areas as a back up to flame detectors where good flame detector coverage cannot be achieved.

Heat detectors will be positioned at appropriate points inside enclosures where smoke detection would be rendered ineffective or unsuitable (e.g. generator enclosure).

These type of detector usually have 4 signals and status that are shown in the control room,

1. Faulty Status
2. Smoke and Heat alarm
3. Exec.Action Trip
4. Inhibit Status

The logic are:

1. If the  detector is in the faulty status (open loop or short). Exec. Action trip command and alarm will be deactivated until the reset command is activated.
2. If the detector is in the inhibit status. None of the other signals will be executed until the reset command is activated.
3. If the alarm set point detected. Smoke and heat alarm and exection trip will be activated until the reset command is activated.

Sample of the logic is shown below: