Conditioning the Air
- By Ronnie Rittenberry
- May 01, 2005
From football fields to foundries, fog fans offer a solution for coping with the harshest heat conditions.
The fog comes
on little cat feet.
It sits looking
over harbor and city
on silent haunches
and then moves on.
--Carl Sandburg, "Fog" (1916)
A lesser-known fact about the Pulitzer Prize-winning Sandburg is that he knew at least as much about heat as he did fog. The one-time fireman would probably not have been surprised by today's use of industrial fogging systems to prevent heat stress, but had he seen the systems in action, witnessing the high-powered injection of atomized water particles into the atmosphere of a hot work environment, he might well have modified his famous metaphor, for the fog propelled from these machines at up to 50 miles per hour conjures more the muted roar of a lion than the little feet of a cat.
If you've visited an amusement park or outdoor fair in recent years, you've likely encountered with relief the misting stations that are something like the distant cousins of these fogging systems. Heat is by far the number one source of illness and injury at theme parks, largely because visitors often aren't acclimated to the extreme temperatures or aware until it's too late how vulnerable their bodies are to the conditions. Strategically placed misting stations combat such conditions by offering walk-through cool-down opportunities.
Hit and Mist
Like fogging systems, the misting stations rely on evaporative cooling to work, spraying fine mist into the atmosphere where it flash evaporates, removing heat. Every gram of water that evaporates absorbs about 540 calories of heat from the air, making for cooler temperatures in the immediate area of the stations; if the mist contacts an individual's skin, that same amount of heat is removed directly from the body.
This is, of course, a welcome effect on high-heat days at the park, but it typically does not offer the kind of conditions under which you could or would want to work. The difference between mist and fog is all in the size of the water droplet. Mist droplets are large enough to settle out in a few seconds and will wet the surfaces on which they land. Fog droplets are almost invisibly small (10 microns or smaller) and will remain suspended for several minutes, causing no wetness when properly applied. Aside from their respective droplet outputs, the difference between misting stations and fogging systems has primarily to do with pressure.
"It's different kinds of pressure, different kinds of atomization, different sizes of nozzles," said Mark Finewood, vice president of sales and marketing for Blythewood, S.C.-based Patterson Fan Inc. Although some fogging systems operate without nozzles, dispersing water via centrifugal force right out of a fan's blades, most systems function through use of nozzles situated in rings on the face of a fan. Whereas misting nozzles generally operate at standard domestic water pressures of 45 to 65 pounds per square inch (psi), fogging systems incorporate high-pressure pumps and impeller-driven nozzles that reach up to 1,000 psi and blast the fog out at up to 22,000 cubic feet per minute. Higher-end systems employ concentric rings of the nozzles that serve to stage the fog, allowing an increase or decrease in fog output as humidity levels change.
"When a fan's velocity grabs a droplet that's already atomized and then smacks it with that much air, it just kind of shatters it, and it evaporates," Finewood said. "People try to make it more complicated than it is. It's not rocket science, but it is an effective way to remove heat out of the air. The key is air exchange. If a building has sufficient air exchange, proper ventilation, then fog fans will cool the environment, guaranteed."
Fog on the Field
Ventilation is not the issue outdoors, but, often, having an airstream that is cooled and blowing over the body is very much an issue. This is why the National Football League has become one of fog fans' more visible proponents, with fog systems set up near sideline benches almost as ubiquitously as barrels of thirst quenchers. Jim Anderson, head athletic trainer for the St. Louis Rams, said heat stress prevention is a primary consideration for the league as a whole and its use of fog fans, along with education about effective hydration, is part and parcel of players' on-the-job safety.
"We're just trying to create a cooler environment for our players however we can," Anderson said. "Once our schedule comes out in May, I look down it and see where games with potential heat problems are, and I'll usually order [fog fans] just to have them available, because you never know what kind of temperatures you're going to get. You know, when you go to Arizona or down to Houston, Tampa, Miami, or some of those other outdoor stadiums in the South, it's nice to have equipment like that."
This article originally appeared in the May 2005 issue of Occupational Health & Safety.