Automatic fire sprinkler systems that are used in cold storage freezer installations present unique problems that require special considerations when designing. Corrosion and potential ice plugs are perhaps the two biggest issues when designing a freezer fire sprinkler system.
Historically, fire sprinkler designs for freezers have involved the removal of water content from the pressure maintenance gas to prevent trapped water and ice plugs accumulating at the point of penetration into the freezer.
In this article, we'll talk about what experts need to know about corrosion and freezers.
Are Cold Storage Freezers Prone To Corrosion?
The whole system must be considered when talking about corrosion in cold storage.
1. The actual freezers
There are two significant physical attributes that affect oxygen corrosion in freezers:
First, for oxygen corrosion of steel piping to occur, liquid water must be present. Under freezing conditions there is no liquid water present so oxygen corrosion can not occur.
Second, all chemical reactions speed up at elevated temperatures. For every 18oF (10oC) increase in temperature, the rate of any chemical reaction doubles. The opposite is true for decreasing temperatures.
Since there is no liquid water present, and freezer temperatures are at or below 32oF, it would suggest that corrosion inside freezer is virtually impossible.
2. the Riser Rooms
Though oxygen corrosion does not occur inside the freezers, we still must consider parts of the system that are exposed to normal operating conditions.
The riser rooms of freezer sprinkler systems are often located on the outside of the building surrounded by uninsulated walls. Often at times these rooms can reach temperatures approaching 100oF in mid-summer.
Loading docks/coolers where the temperature is above 32oF are areas also prone to oxygen corrosion.
In these areas we have liquid water present and temperatures are above 32oF promoting the chemical reaction of oxygen corrosion.
Ice Plugs and Pressure Maintenance
Water removal from the compressed air stream used for pressure maintenance in freezer sprinkler systems has historically happened using regenerative desiccant air dryers in conjunction with compressed air. This process gives the air entering the freezer a lower dew point than the freezer's temperature, eliminating the chance of condensation. Moisture from the condensation process could potentially lead to an ice plug and significantly restrict or completely block 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 sprinkler systems should be 20oF (11.1oC) 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 in the freezer sprinkler system. In doing this, the “dried” air should not form ice plugs when penetrating the freezer.
Conventional Air Compressors and Ice Plugs
Conventional air compressors offer no protection against corrosion and, most importantly, ice plugs. The compressed air (oxygen) that is delivered to the fire sprinkler system is saturated with water which condenses within the system piping, promoting corrosion and ice plugs.
An alternate to conventional air compressors is the use of regenerative desiccant air dryers. A desiccant air dryer essentially “dries” the air from an air compressor by sending it through desiccant material that absorbs moisture from the air stream. The typical dewpoint of pressure maintenance air from a desiccant air dryer is -40oF.
Even though the use of a desiccant air dryer reduces the moisture content of the compressed air stream, it offers no protection from oxygen corrosion. The air stream from an air dryer contains 21% oxygen which is the primary cause of corrosion in fire sprinkler systems.
Get the Right Equipment for the Job
Nitrogen generators like the PGEN models offered by Engineered Corrosion Solutions have a proven track record in cold storage applications. Read more about how a large warehouse facility used a nitrogen generator to eliminate ice buildup.