The newly designed extinguishing medium is watermist with a droplet diameter of about 100 u.m. The large surface area due to the small droplet diameter enables a very fast vaporization of the generated watermist. The ability to extinguish the explosion is based generally on the following two criteria:

(1) the evaporation of the watermist at the centre of the flame displaces only there locally the air and thereby the 21 % share of oxygen since the volume of the water expands at the transition from liquid to gaseous state by a factor of 1600. This leads to a very rapid quenching, whereby the concentration of oxygen in the remaining space stays unchanged

(2) at the evaporation of the water droplets, heat is drained out of the ambient air in a magnitude of 2.2 MJ/kg water. This cooling not only affects the centre of the flame, but also the whole compartment. This retaining of heat, which is missed in the gases, reduces the danger of back flash.

Spectronix Automatic Fire and Explosion Detection and Suppression System (AFEDSS)

Development

The Automatic Fire and Explosion Detection and Suppression System (AFEDSS) was developed by Spectronix to meet the requirements of the Israel Defence Force following their experiences in the 1973 Middle East war. Many Israeli tank crews were killed or injured when their vehicles caught fire, following penetration of the tank armour by High Explosive Anti-Tank (HEAT) and Kinetic Energy (KE) weapons.

The main Israel Defence Force requirements were to increase crew and vehicle survivability and to provide for an add-on modular design, enabling interchangeability and commonality between vehicles.

Following extensive trials, series production of the AFEDSS commenced in 1981 and the Israel Defence Force now installs it on all new production Merkava MBTs. It has been retrofitted to earlier production Merkavas as well as Centurion, M48 and M60A1/ M60A3 MBTs. It is probable that the AFEDSS is fitted to the specialised APCs developed by Israel based on converted MBT chassis.

crew compartments which is covered in detail in a separate entry. It has an ergonomic display which provides the current status of the system at all times, as well as a manual hand release for emergency use.

The MFS temperature sensors are based on Pt 500 thin film elements and, together with the MFS central processing unit, can be adapted to fit any engine compartment. This is achieved by setting the temperature signature of each sensor via the software of the MFS Central Unit.

The bottle type MFS extinguishant contains the nitrogen. Depending on the type of vehicle, the quantity of nitrogen required will generally be two to three times higher than halon. The bottle type containers are available in various sizes to suit different vehicle applications.

The manufacturer states that this system has a number of advantages, including the measurement of the actual temperature at several points in the engine compartment by temperature sensors.

The system consists of the Central Unit EXWA, sensor station EXWA and the extinguishant container EXWA.

The Central Unit EXWA is a microprocessor controlled central unit, which is also used for the MFS engine compartment fire detection and suppression system covered in a separate entry. This has a display panel giving the current status of the system, as well as a manual hand release which can quickly activate the system if required.

The sensor station EXWA consists of two optoelectronic detectors which operate in two different spectral ranges. High energetic impacts and explosions are recognised within 5 ms, while small hydrocarbon fires can provide reaction times of 10 to 40 ms.

The extinguishant container EXWA is available in various sizes depending on the size of the vehicle. The container eliminates the influences of light and oxygen and so allowing the water to be stored for many years.

The main advantages of this system are:

(1) extreme fast suppression by water mist can only be performed by using a propellant charge and an excellent sensor system/processing unit

(2) in comparison to the extinguishing gases, the excellent function at suppression of hydrocarbon combustions, which proceed like an explosion, will be caused through the enormous cooling at evaporation of the mist droplet. The danger of a reflash, for example at hot fragments, will be minimised

(3) the charge watermist produces no toxic and no corrosive substances at the evaporation

(4) the EXWA is without pressure during standard operation and therefore no additional source of danger is caused during fire, thus the maintenance and testing effort is minimal

(5) watermist is the most environmentally friendly agent, requiring neither disposal, nor associated expenses.