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Like many who drive Interstate 15 where Davis and Salt Lake counties converge, Antonio Salmero had always wondered about the "white puffs of smoke" rising from the stacks at Chevron's refinery.

On Saturday, Salmero and others discovered the answer — it's water vapor— during a unique refinery tour offered to the public.

More than 750 residents from all over the Wasatch Front signed up for the free bus tours to see the workings of the refinery, which is usually off limits to the public.

At times the 40-minute ride seemed other-worldly as the buses navigated between giant steel structures and massive tanks — connected with miles and miles of pipes. Workers were all but invisible, doing their jobs deep inside the plant away from public view.

While no one thinks twice about putting gas in their car, "what happens at a refinery is a bit of a mystery," said Clark Moss, one of several dozen Chevron employees that led the tours, answering questions and explaining the chemical process used to change crude oil into gasoline, jet fuel and diesel.

"What's the white smoke?" was one of the most asked questions of the day.

Moss said steam is used in the cooling process, much like an evaporative "swamp" cooler in many homes. While there is steam all year long, it is more obvious in the fall and winter when temperatures are cooler, just like your breath when you go outside.

And what about the occasional flares?

Those occur when things get too hot and pressure needs to be released, Moss said. But because of pollution concerns, refineries have worked hard to limit flares, finding ways to store the vapors rather than release them.

Many years ago, the Chevron refinery gave public tours but they were discontinued. Greg Gabel, the current refinery manager, said he wanted to bring them back as a way to "share what we are doing" with the public, especially in the areas of safety and protecting the environment.

The refineries that dot southern Davis County have become a focal point in the ongoing debate over air quality along the Wasatch Front.

While it had nothing to do with the refinery, Gabel said many Utahns also still connect the Chevron name with the two separate leaks near Red Butte Gardens. In 2011, the pipeline spilled 54,600 gallons of crude oil in Salt Lake City's eastern foothills last year and the company has since spent tens of millions of dollars on cleanup.

Incidents like those, create questions about how crude oil is shipped in and how its processed, said Gabel.

Duane Withers, who took a tour with his wife Brandi and their two grandchildren, said he was most surprised at the refinery's capacity and "how far products are shipped."

The Chevron refinery can turn up to 50,000 barrels of crude oil a day into fuel. Some of the crude oil is trucked in from the Uinta Basin but most is transferred via pipeline from various parts of the U.S. and Canada.

Once refined, most of the gasoline is used in Utah, but a portion is shipped to parts of Idaho, eastern Washington and Las Vegan, said Gable.

Elmer Scott of Lehi took the tour and brought along his 90-year old father, Vernon , who had worked in a gas plant in Bakersfield, Calif., decades ago.

"It's great to show people what you're doing," he said. "It looks like they are trying to do a good job. But I'm one of those people who believes we need gas in the future. I don't think we can just rely on solar power."

Facts about Chevron's Salt Lake Refinery

The refinery opened in 1948 and has been operating continuously — 24 hours a day 365 days a year — ever since.

The refinery has the capacity to turn up to 50,000 barrels of crude oil a day into gasoline for cars as well as jet fuel and diesel.

The refinery currently employs about 340 people, but another 400 contractors work on the site daily.

The refinery makes additional income by selling byproducts such as sulfur for fertilizers; and petroleum coke, a black, coal-like substance used in the making of aluminum and steel.

The refinery has it own fire station and trained firefighters, experts in handling petroleum fire.

The refinery has a fleet of bicycles that employees use to get from one area to another.

Since 1992, the refinery has reduced its emissions 90 percent, from more than 7,000 tons per year to 1,000 tons per year.