Sprinkler Ice Plugs
Automatic fire sprinkler systems that are used in cold storage freezer installations present unique problems that require specialized design considerations regarding corrosion and potential ice plug obstructions.
Historically, fire sprinkler designs for freezers have involved the removal of water content from the pressure maintenance air to prevent ice plugs at the point of penetration of the dry pipe main into the freezer. The chemistry and physics of the corrosion reaction at freezing temperatures are very different from installations at room temperature and above. We focus on eliminating sprinkler ice plugs.
The removal of water from the compressed air stream used for pressure maintenance in dry and preaction fire sprinkler systems for freezer service has historically been accomplished by using a regenerative desiccant type air dryer in conjunction with the air compressor. Using this approach, the air entering the freezer would have a lower dew point than the temperature within the freezer which should prevent condensation and freezing of moisture in the pipeline. Ice plugs in sprinkler mains have the potential to greatly restrict or completely obstruct water flow into the freezer.
The FM Global Property Loss Prevention Data Sheet 8-29 entitled Refrigerated Storage suggests that the dew point target of the pressure maintenance air for the dry or preaction fire sprinkler system should be 20°F below the temperature inside of the freezer that is being protected. This reduced dew point would achieve a relative humidity of 30% for the air within the piping at a temperature within the freezer.
Using Nitrogen to Eliminate Sprinkler Ice Plugs
A nitrogen generator provides several advantages over traditional air compressor/dryer configurations:
- The 98% nitrogen gas produced using a nitrogen generator has a much lower dewpoint of -70°F to -90°F than the dry air produced by a dryer at about -40°F.
- Nitrogen generators eliminate oxygen corrosion (less than 2% oxygen) that occurs in supply mains outside of the cold storage area compared to the 21% oxygen supplied by dryers.
- Nitrogen generators require less maintenance and cost less to operate because they run less frequently than a regenerative desiccant dryer which must constantly use dry air or heat to regenerate the desiccant bed.
- Nitrogen generators can supply a significant volume of supervisory gas to multiple dry pipe sprinkler systems, minimizing the equipment needed at large facilities.
- Nitrogen generators cost approximately the same as regenerative desiccant dryers for equivalent size dry and preaction system installations.
Case Study: Nitrogen Generator vs Dry Air Pac in Cold Storage Facility
In the fall of 2017, a cold storage facility in the food service industry commissioned a local fire sprinkler contractor to replace its legacy dry air pac system, which was repeatedly producing ice plugs in the air supply line.
The contractor elected to replace two dry air pacs with one ECS nitrogen generator. This led to a complete elimination of ice plugs and corrosion in the system.
For more information and details, download our full case study below.