Fire detection system in aircraft

What are the common fire detection methods followed in aircraft? Explain them with neat diagrams.

Thermal Switch:

Thermal switches are heat –sensitive units that complete electrical circuits at a certain temperature. They are connected in parallel with each other. If the temperature rises above a set value in any one section of the circuit, the thermal switch will close the electrical circuit and gives indication to the pilot through indication lights.

Thermo-couple systems:

A thermocouple gives the rate of temperature rise.

A Thermocouple is the junction of two dissimilar metals which generates a small electric current that varies according to the temperature of the junction. For this reason it does not require an external power source. The dissimilar metals can be constantan and iron, Alumel and Chromel, or some other combination of metals or alloys which will produce the required results. The complete thermocouple circuit consists of the ‘cold’ junction, the ‘hot’ junction, electric leads (made from the same material as the thermocouple), and a galvanometer-type indicating instrument as illustrated below.

Constantan is chosen as one of the metals because its resistance is affected very little by changes in temperature. When the hot junction is at higher or lower temperature than the cold junction, a current will flow through the circuit and instrument. The value of current is depending on the difference in temperature between the two junctions.

Continuous loop systems:

A continuous loop detector or sensing system permits more complete coverage of a fire hazard area than any type of detectors. There is no rate of heat rise sensitivity in a continuous loop system. the three commonly used continuous loop systems are given in an ellobratic manner.

Kidde System

In the kidde continuous loop system, two wires are inserted in a special ceramic core within a inconel tube. One of the two wires in the kidde sensing system is welded to the case at each end and acts as an internal ground. 

The second wire is a hot lead (above ground potential) that provides a current signal when ceramic core material changes its resistance with change in temperature. If any fire is near this instrument causes the ceramic core resistance to drop and thereby wire will conduct more current than usual, from that we can find the fire.

Fenwal System:

It uses a single wire surrounded by a continuous string of ceramic beads in an inconel tube. 

The beads in the Fenwal detector are wetted with a eutectic salt which posses the characteristics of suddenly lowering its electrical resistance as the sensing element reaches its alarm temperature.

Systron-Donner

Physics: If the volume of a gas is held constant, its pressure increases as temperature increases; thus the helium between the two tubing walls exerts a pressure proportional to the average absolute temperature along the entire length of the tube. One end of the tube is connected to a small chamber containing a metal diaphragm switch. 

One side of the diaphragm is therefore exposed to the sensor tube pressure and one to the ambient pressure. When system pressure exceeds some predetermined value, diaphragm will activate the electrical contacts which in-turn activates the alarm. When the temperature is reduced to the normal levels after action has been taken to suppress the fire, the gas absorption material reabsorbs the gas and pressure drops, thus opening the switch and cutting off the alarm.