How Fire Sprinklers Work
The typical
sprinkler head consists of a plug held in place by a trigger mechanism.
The most common type of trigger is a glass ampule filled with a glycerin-based
liquid that expands when heated
A less commonly used type of trigger consists of two metal
plates held together by a solder point. When the solder melts, two spring
arms pull the plates apart, releasing the plug.
This
liquid is designed to expand and break the tube at a certain temperature.
The most common are designed to break at 155 degrees. In the average
sized room, a 5mm diameter ampule will usually break in about one to
one and a half minutes from contact with a heat source. Ampules as thin
as 1mm are manufactured for a faster response time.
The
plug is forced out by the pressurized water behind it and deflected away by a beveled edge. The water sprays over the deflector plate which is
designed to distribute it in an even pattern. Water will continue to flow until the main valve is shut off
How an Uncontrolled Fire Spreads 
Smoke and toxic gases rise from the source of the fire. They spread quickly along the ceiling and heat the air in the room.
The current of hot air forces a curtain of deadly 
gases down the walls, making escape more difficult. In a few minutes the air will
become so hot that the entire contents of the room will ignite. This is known as flashover and usually occurs between 1000 and 1500 degrees. No-one,
and nothing, survives flashover.
How a Sprinkler System puts the Fire Out
Even a small smoldering fire acts like a heat engine
as it steadily increases the air temperature directly above it. The hot air fans out across the ceiling, heating up the nearest sprinkler
head.
As soon as the trigger mechanism is heated to the required
temperature, it trips and the water is released. 
The immediate cooling
of the heat source usually prevents other sprinkler heads from activating. Often, one or two sprinkler heads are enough to control a fire.
