How can you handle extreme under-roof temperatures that degrade employee productivity on the plant floor? The effect of stratified heat is compounded in summer. As outside temperatures approach and exceed 90°F, indoor temperatures in unconditioned spaces frequently hit triple digits at floor level. So, you install fans and encourage everyone to take frequent breaks and drink plenty of water – necessary steps to safeguard employee health and safety. Still, your employees and coworkers grumble about the conditions.
Indoor temperatures rise for several reasons. You have employees and machinery producing heat, but a substantial portion of the problem is from another source: the large heat collector above your head, your roof. Often, roof temperatures exceed 165°F, and under-roof temperatures can easily exceed 120°F, depending on the "U" value of the roof's insulation.
How can you eliminate your roof as a heat source? We know that evaporating a gallon of water absorbs 8,652 BTUs of heat. Thus, evaporating about 1.5 gallons of water per hour will absorb the equivalent of one ton of air conditioning.
By applying water in a thin film, just enough to wet the roof, then allowing it to evaporate, the roof can be cooled to within 10°F to 12°F of the ambient wet bulb temperature. With a 78°F wet bulb temperature, this means the roof temperature can be dropped to a relatively cool 86°F to 88°F, resulting in under-roof temperatures of 82°F to 84°F, virtually eliminating the solar roof load.
Weis Markets, Inc. installed a water-spray system to convert the roof on its 170,000 sq. ft. Milton, Pa. distribution center into a large cooling panel. The Solar Shield evaporative roof cooling System, from Patterson Roof Cooling (www.pattersonroofcooling.com), uses piping and spray nozzles set up in grids or zones to distribute water on the roof. Through a series of timers and sensors, mist cools the roof as temperatures reach preset trigger points.
The system thoroughly wets the roof surface without puddling or runoff. In many cases, wastewater can be used as long as it’s relatively clean. Systems generally use about 80 gallons of water per 1,000 sq. ft. of roof per 10-hour day. Thus, a 170,000-sq.-ft. building would evaporate approximately 14,000 gallons of water per day. At an average cost of $1 per 1,000 gallons for city water, the total operating cost of the system would be about $14 per day.
Indoors, the cooler air falls, displacing hotter air to where, with proper ventilation, it can be exhausted from the plant. This reduces under-roof temperatures and brings working conditions to tolerable, even comfortable, levels.
The cooler floor-level conditions also allow humans to more efficiently cool themselves. This is particularly effective when the reduced temperatures are enhanced with high-velocity fans. On its third-level pick module, Weis Markets installed fans supplied by Patterson through its sister company, Patterson Fan.
Hybrid fans, an innovation introduced by Patterson Fan this year, use a combination of motor and fan efficiency to reduce fan energy consumption by 16% or more, a significant savings.
For Weis Markets’ non-air-conditioned distribution center, the result is the difference between safe, reasonably comfortable workspaces and an environment conducive to fatigue, heat stress or potentially worse conditions. In this case, temperature reductions were significant enough for employees to continuously work on the third level.
Air-conditioned plants have the added incentive of lowering electric power bills and extending the lives of air conditioning systems. Realistic energy demand reductions of 30% to 40% may be achieved. Both air-conditioned and unconditioned facilities may extend the lives of their roofs by maintaining a lower, more consistent temperature level.
Roof cooling systems are often considered in the South because of the extreme heat and the long summers. This application in Pennsylvania demonstrates the effectiveness of the systems in northern states.