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What are the different types of watermist nozzles for fire protection?

Open Nozzles

Open nozzles are those which are not closed by an integrated thermal sealing element. Release of water into the pipe network will result in discharge from all the nozzles connected to that network (unless sections of the network are zoned by valves). Given there is no heat-sensitive element, the system activation is achieved either by a detection system or manually. Open nozzles are typically used on local applications where several nozzles are aimed at the specific fire risk being protected.

Automatic Nozzles

The heat-sensitive bulb is designed to break when the air around it reaches a pre-determined temperature. As the bulb fragments fall away, this allows a valve to open by water pressure allowing the nozzle to discharge the specified mist droplets.  This temperature should be a function of the highest expected ambient temperature.  It also requires the nozzle to be placed on the highest point in the volume as the bulb needs to be exposed to the hot smoke layer which will rise towards the ceiling.

Because only the nozzles exposed to hot air will activate, not all nozzles in a compartment will necessarily activate, only those in the vicinity of the fire.  However, an assumed number of nozzles must be considered to activate as a function of the occupancy/risk and size of the area protected. This is the Assumed Maximum Area of Operation (AMAO) is a key performance requirement, specified in standards, for the correct operation of a water mist system by ensuring there will be enough water flow and pressure for the likely number of nozzles activated. 

Electronically Controlled Nozzles

A nozzle in which the release of water is made automatically by electronic means instead of by mechanical means. The electronic control allows for a predetermined number of nozzles to operate, controlled by electronic means, ensuring consistent activation for the defined AMAO.  It also allows the nozzle to be activated by other means than a mechanical heat-sensitive bulb, such as a smoke, heat or flame detector. The use of electronic nozzles seeks to allow for quicker activation and/or avoid nozzle skipping, which can occur with mechanical nozzles.

Read more - What are electronically controlled water mist nozzles for fire protection?