Ice from infiltrating humid outdoor air is an ongoing problem that increases annual maintenance costs and liability insurance coverage by tens of thousands of dollars for frozen food warehouses. These commercial facilities traditionally rely on heated high-speed freezer entrance doors to address ice and snow accumulation when humid outdoor air mixes with the freezer's -10°F conditions - and in dry climates, the system works.

In Florida, however, where there's an ASHRAE (American Society of Heating, Refrigerating, and Air Conditioning Engineers) 0.4% design dew point of 79°F and a humidity ratio of 151 grains/lb, heated freezer entrance doors can't eliminate ice build-ups.

Mechanical engineer Gary Evans, manager of HVAC sales at manufacturer's representative/distributor, System Components Corp., Ocala, FL, teamed up with HVAC contractor Bentzel Mechanical, St. Petersburg, FL, to solve incessant ice buildups at a refrigerated warehouse in Bradenton, FL. The duo used a combination of desiccant equipment and fabric duct air distribution as an energy-saving, maintenance-cutting supplement to heated freezer entrance doors for the 320,000 sq ft facility.

Bentzel Mechanical installed a Model F-30 FreezAire® desiccant dehumidifier (400 lbs/hr of moisture-removing capacity) by Munters -- North American Dehumidification Division, Amesbury, MA. Meanwhile, Evans designed the air distribution system featuring custom engineered fabric duct manufactured by FabricAir® Inc., Louisville, KY.

Using a desiccant system and industrial-grade fabric duct to distribute dehumidified air is a unique solution to this age-old problem. However, the projected two-year payback on the $130,000 project and the concept's efficiency are now getting the attention of other Florida refrigerated warehouse facility operators. The payback comes from an 80% reduction in costly semi-daily defrost cycles for each of the 140,000 sq ft freezer/cooler's 21 evaporator coils and dozens of rooftop condensing coils. Thanks to the new system, the Bradenton plant has reduced the ammonia refrigeration system's defrost cycles to only once per week. The energy saved from reduced defrost cycles was initially estimated by Evans and verified by a Florida Power and Light energy audit.

Not included in the payback calculations are an estimated additional $50,000 annual savings for ice removal on the 40 x 360 ft trucking dock that was previously performed by in-house maintenance people, according to Evans. Ice had regularly accumulated on the floor and ceilings as far as 20 ft from each of the four 14 x 8 ft automatic entry freezer door thresholds.

"With the new desiccant and fabric ductwork system, we reduced relative humidity from worse case scenarios of 90% to a constant everyday 40% RH," said Evans.

The five-year-old facility saved an additional $90,000 by not installing a supplementary heated vestibule system estimated at $240,000.

"Heated vestibules don't completely eliminate the ice like a desiccant system can," added Evans. "Granted, a desiccant system uses natural gas that raises our utility costs, but that expenditure is saved with a reduced electrical load needed from limited defrost cycles in a less humid atmosphere. Heated vestibules would not offer this advantage."

Bentzel Mechanical installed the 10,000 cfm desiccant system and subcontracted fabrication of the sheet metal supply and return to Express Metals, St. Petersburg. The supply duct connects the rooftop desiccant unit to the 140 ft long run of 42 in. diameter fabric duct that runs just above the four freezer doors. The project also includes 180 feet of 40 x 24 in. metal return duct with 12 grills that are spread evenly along the 23 truck dock openings.

Typically fabric duct distributes air through fabric permeability and/or linear diffusers. In the Bradenton facility's case however, the permeability was designed by FabricAir's custom engineering department to control static pressure. Above each freezer door, an 8 ft long section of fabric duct has 120 plastic, narrow-throw air nozzles that bathe the floor and door hardware with dry air.

The Bradenton plant manager also likes fabric duct's aesthetic appearance and easy maintenance. FabricAir used custom coloring to match the firm's corporate blue color. And fabric duct simply springs back to its original shape when bumped by objects such as forklifts or pallets, whereas metal duct is permanently damaged. In the inevitable event of irreversible forklift damage, the plant's maintenance department has replacement sections in stock that can be installed in less than one hour, whereas a damaged metal duct would require a system shutdown and repairs by a sheet metal contractor.

The maintenance department also plans to clean the duct routinely, because disassembling, cleaning, and replacing can be accomplished with in-house maintenance staff in an estimated 4-hour period.

"This is our second experience with fabric duct and we're now considering it in an ice rink application," said Doug Peregoy, Bentzel senior vice president. "I think fabric duct has a real future here in the U.S. because it can be installed in 80% less time and is cheaper than metal duct."