These types of sprinklers are used in Museums where a sprinkler system could cause major damage were it to go off by accident or in heavy commercial and industrial applications like large freezers and refrigerated warehouses where the entire building is the freezer or refrigerator .
In these cases a conventional dry pipe system is not enough because just knocking a head off with a fork lift will cause the freezer to flood . Thats where Preaction systems fit the bill. Dry air is still used in pipes but only for sensing purposes not to hold water back like a dry system . Water in a Preaction system is allowed to flow by an electric releasing solenoid .
With Preaction systems 2 or 3 actions must take place before they activate and flow water . In a freezer situation both the sprinkler head would have to activate and special heat sensitive Protectowire sense a fire and melt together or heat detectors activate to allow the water to flow .
Typical wet system stand pipes on left Preaction system pipes on right |
Unlike wet and dry systems which operate by themselves with out assistance of a control panel Preaction systems require the use of special releasing panels as you see in picture below manufactured by Potter Signal.
Potter releasing panels with Silent knight fire system modules below monitoring relays for water flow ,trouble ,supervisory functions |
You do not see many Preaction systems as dry Systems are preferred due to the costs associated with Preaction systems initial cost but in the end Preaction system are more cost effective. I have only seen and worked on a handful of Preaction Systems VS Dry Pipe Systems .Because of cost issues.
Special compressor which insures as completely as possible dry air enters Preaction pipes |
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