Heat Transfer Fluids: A Driving Force of the Asphalt Industry

In the summer of 1970, my first summer job was working on a paving crew.

Back then, the equipment, and the labor used for layering the prep, the screenings, and the asphalt surfacing, was much less specialized than it is today.  We were laying country roads, and an occasional driveway, in rural Chester County, Pennsylvania.  The crew consisted of a foreman, a crew leader, 2 or three drivers  and equipment operators, and around ten laborers.  There was no project engineer as such.  The owner of the company occasionally showed up (he had several working crews at the time) and grabbed a shovel himself.  At 14, I was the youngest, and smallest, and pretty much the least of them, in terms of responsibility and capability. Certainly in terms of experience.  This was a rough-edged, but good humored bunch, and included all sizes, races and ages.

When a stretch of road was prepped and ready, and a dump truck showed up full of hot black asphalt mix, everybody grabbed a tool and pitched in.  It was a controlled, cooperative frenzy to properly, carefully tilt the dump bed, deposit part of the load, shovel, rake and smooth the mix, then steamroll it and move along to the next section.  For twenty minutes, we’d sweat in the summer heat.  Then, until the next load arrived, the pace slowed while screenings were raked and other prep was done, and the fellows chafed each other about their weekend conquests down the shore in Wildwood while chugging ice water from the water jugs the foreman brought along.  If the saltiness of the language was lightened for a 14 year old, it was still a pretty spicy stew.  Sometimes they’d send me off to clean the shovels of the encrusted asphalt cement, with kerosene.

Most Americans don’t think about the roads they are riding on while driving from point A to point B. What they may not realize is that asphalt is literally paving the way for almost every single one of us to get where we need to go. Remarkably, of the 2.4 million miles of paved roads throughout the U.S., 2.3 million of them are paved with hot mix asphalt (HMA).

As of 2009, there were 3,900 asphalt plants producing 360 million tons of HMA, valued at $24 billion.  It’s an industry that’s huge and imperative—over the next 50 years, it’s estimated that it will cost $185 billion to maintain our country’s aging infrastructure, and HMA is going to be a very large part of it. As such an important aspect of our lives, these 3,900 asphalt plants in operation need to be functioning at their best at all times—any delay can be detrimental.

I didn’t know it at the time of course, but around the time when I had my first summer job, hot-oil systems, which indirectly heat varied equipment at asphalt plants, were rapidly replacing inefficient and emissive direct-fired heating, and helping enable plants to lengthen the viable storage time of prepared hot-mix asphalt.  Nowadays, virtually every plant has a hot oil system which heats the asphalt cement—hundreds of thousands of tons of it across North America. Using low-cost oils can cause long-term, serious problems to a system, as well as delays. Such multi-purpose oils are not designed to perform the continuous heating functions HMA plants require. Engineered heat transfer fluids, on the other hand, are specifically designed for continuous high-temperature systems, and will not break down the way multi-purpose lubricating or hydraulic oils can.

This is an industry where calculations, limitations and specifications have become increasingly important.  The practical limit for distance from the plant to the job is around 50 miles, because the insulated trucks will only keep the mix hot and workable for so long. This is why those 3900 asphalt plants are literally peppered all across the country. Which means that those average Americans moving from point A to point B have seen  asphalt plants hundreds of times, and may in fact see them every day without knowing it.  Asphalt plants have a distinctive look with a few telltale visible characteristics; pyramid-like piles of gravel (the aggregate) a slanted conveyor to move the aggregate, and tall cylindrical structures which are either asphalt cement tanks or storage silos.  In 1970, when I worked briefly on a paving crew, you could also see the smoke from the plant’s stacks.  These days, emissions are very well controlled and regulated.

Drawing silhoette of asphalt plant with silos, heater, piles of aggregate

As anyone in the industry knows, this is a seasonal business—in cold weather climates, operation and paving runs from the spring through the late fall, as paving can’t efficiently be done below 40 degrees. This off-season is a great time to maintain the heat transfer fluids and keep them working optimally whereas, during the season, time is of the essence. Keeping a program of routine checks, including a fluid analysis, cleaning equipment, checking insulation, and practicing shut down procedures will ensure that come spring, everything is working perfectly.

Chemical analysis of the heat transfer fluid (usually referred to as “the hot oil” in this industry) is particularly important as the cold season approaches.  If a hot-oil system has been running continuously for several months, and the fluid has significantly degraded due to oxidation or overheating, the heat transfer fluid could actually solidify when the system is finally shut down.   And dismantling a hot oil system is an expensive way to change the oil.  If a cooled sample of hot oil won’t pour, proceed with caution; keep the circuit hot until you consult with the heater or fluid manufacturer.

When our nation’s entire road transportation system depends on the performance of HMA plants, the right kind of heating is essential.

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