Over the years I’ve found myself at numerous small paving organizations training a new plant operator to replace one who had either had an accident or had simply quit. In nearly every instance their asphalt plant operator was one of their key people—one the company thought would be around forever. Management simply never foresaw a need to cross-train someone for his job. When things went wrong, they found themselves scrambling for a solution to a dilemma which could easily cost them a substantial amount of money. For this reason, an in-house training regimen has become essential for even the smallest of paving companies.
What it comes down to is that we, as an industry, must do something to attract new blood. We must get into the minds of our young and show them that they can have a rewarding career in asphalt. Kids, by nature, are fascinated by large machinery. We need to capitalize on that. Maybe we should have youth-oriented websites for paving companies, or paving associations. It seems clear that the industry needs to do something.
Making the big smaller
In some areas of our country larger paving companies seem to be buying up their smaller counterparts at a rapid rate. As a result of this trend, larger asphalt plants are being operated in realms they were not designed for.
An example of this would be a 10-foot CMI drummer I put into Burley, Idaho. This plant, nominally rated at 550tph, was destined to make 1,000 tons or less per day for most of it’s year. It needed to be able to run as efficiently at 175tph as it did at 550tph. This is very difficult to do, due to flight veiling considerations. Simply put, at 550tph the flights are full and carry material all the way around the drum yielding an even and dense veil. When that material is cut back to 200tph, the flights empty part way around the drum. This leaves a hole in the veil on one side of the drum which allows superheated air direct access to the baghouse causing it to overheat.
I corrected the problem on this plant with some modifications to the drum. Some aftermarket manufacturers have been experimenting with variable speed drums. As the production rate goes down, the speed of the drum increases, carrying more material farther around the drum and closing up that hole in the veil. They use a variable frequency drive to vary the drum’s drive motor speed. Unfortunately, this is a bit expensive, but with the current trend of downward electronic prices I suspect that we will see plants leaving the factory with this option.
Bagging the dust
Innovations over the next few years will probably relate to plant efficiency and cost effectiveness. Baghouse dust control is one area in need of improvement. Thinking back on my travels around the country working on various asphalt plants this last summer, one recurrent theme stands out in my mind: Inconsistent sample results relating to material passing the 80 through 200 screens. I’ve encountered this at more than a few facilities in the North, South, East and West. Some of the plants were easy to fix. But after eliminating the obvious problems, like poor quality control of the aggregates, operational protocols and calibration issues, I was left with a core of plants that still exhibited inconsistencies in their sample results. This problem was predominantly evident on plants with pulse-jet baghouses that used an automatic pulsing card. What I discovered is that when the baghouse is being pulsed a set rate of fines were removed, when it was not being pulsed none was removed.
A brief explanation of baghouse operations should help to understand the problem. As air is pulled through the baghouse, the main fan generates a certain amount of suction. As that same air is pulled through the bags, there is a certain amount of restriction. The fan suction is measured by a gauge, such as a photohelic meter in the control room. The suction also is measured at the inlet to the baghouse and the two readings are compared. The resultant difference is called the "pressure differential." When the plant is running, this air is drawn through the drum and carries with it a certain amount of fines. These fines collect on the bags and increase the pressure differential. The higher the pressure differential, the less air that can be drawn through the baghouse. As the air volume falls, so does our production rate since the burner requires air to make heat. To correct this problem we pulse a jet of air through our bags to clean them off. This operation is handled by a pulse card located on the side of the baghouse or in the control room. By using two adjustable pointers on the photohelic, the plant operator can adjust the operational "pressure differential" as the plant runs. The low pressure differential is typically set at around 4 in., while the high pressure differential is set around 6 in. As the plant is running, the photohelic will automatically begin pulsing the bags when the pressure reaches 6 in. and stop when it falls to 4 in.
What happens is that while the baghouse is being pulsed, the dust that collects in the baghouse is put back into the mixing drum by augers or a blower. When the baghouse is not being pulsed, no fines are being added to the mixing process. It doesn’t take an expert to see that this process causes the gradation of your hot-mix to be continually changing. In essence, when the pulse card is not calling for cleaning action the baghouse is storing the fines drawn off of a large amount of aggregate, and when the card again calls for pulsing action the baghouse dumps an elevated amount of dust in a relatively small tonnage. This, of course, would lead to inconsistent sample results and, more importantly, inconsistent mix.
To correct this problem at these plants I added a "manual" pulsing mode so that we could set the baghouse up to be pulsed any time the plant is running, regardless of the pressure differential. This way, a constant amount of baghouse dust is being added to the mixing process. I used the manual settings on the pulse card to set the pressure differential to 4 in. The downside of this is that the plant operator must adjust the pulse card himself to assure the pressure differential stays in the range he wants. This "inconvenience" is far outweighed by the improvement in dust control and improved mix consistency.
Several companies are now working on a fully automated, variable pulse card to only pulse the bags as needed. These cards would sense the amount of pressure drop, compare it to the rate of production and vary the intensity of the pulse in conjunction with the frequency of the pulse to match the plant production requirements.
In the future
What’s in our immediate future? I think baghouses will become smaller and more efficient through the use of either elliptical or pleated bags that look like the air cleaner in your car and have far more surface area in the same diameter and length of bag. A 47,000 ACFM baghouse has 550 bags 6 in. around by 10 ft long. Each of these bags has 15.71 sq ft of cloth. A pleated bag will have nearly 10 times as much cloth per bag. This means our baghouse would only need 55 bags to clean the same amount of air. As you can imagine, this will allow the size of our baghouses to shrink dramatically.
I think fugitive smoke issues are going to become more important, as they have at the paving show. We will need to control the blue smoke we get under our loadout silos. This will probably be done with either tertiary fans, which will move the smoke back to the burner, or with smoke chokes under the silos. I see condensers on our AC tank ventilation systems.
Another seldom considered issue is odor control at our plants. To us, hot asphalt smells good. But to the general public it stinks. As more and more neighborhoods complain about the proximity of an asphalt plant, I think we are going to have to find ways of masking or eliminating the smells produced by our plant operations.
What about the next five years? Zoning issues are going to become critical. Unfortunately, in the past our industry wasn’t very good at public relations. All too often we were not sensitive enough to our neighbors around our AC plants. People complained of truck traffic, noise and noxious odors. As long as we had our permits, we ignored the concerns of our neighbors and soldiered on, oblivious to the damage we were doing to our image. These days we site our plants in poorer neighborhoods because they don’t have the money to hire attorneys to deny us a permit. As time passes, it’s going to get tougher and tougher to site an asphalt plant in an urban setting. It’s possible that no new plants will be allowed in or close to our larger cities. This, in my opinion, will place a high premium on smaller, highly portable asphalt plants. Larger companies might have two or three 200 to 300 ton per hour plants in the same market area where they would have had a single 400 or 500 ton per hour stationary plant in earlier years. Cities will probably issue temporary use permits for work in their jurisdictions, allowing these smaller plants to move in, set up, do the job then move out again.
I think the near future also will see the death of the wet-scrubber at our asphalt plants. I do not think they will be outlawed, I think they will be buried in rules and requirements, like weekly water testing and certification, making their daily operation cost prohibitive. While all these issues will drive the cost of road repairs skyward, it will be perceived as necessary to preserve our way of life.
What about the next 10 years? I recently read an article in Scientific American where someone was discussing the use of sound waves to clean smoggy air. His theory was that ultra-high frequency sound waves caused the smog-laden dust particles to cling to each other and, by weight, rain out of the air to be recovered and processed by some as yet unrevealed magical process. Returned to their former benevolent state, the dust particles were then returned to Mother Earth to begin the process all over again. This article begs the question: Can ultra-sound be used to clean air in an asphalt plant?
As fuel costs increase, I think we are going to be looking for ways to reduce its use at our AC plants. Two promising technologies exist that could be used to help reduce our use of fossil fuel: infrared and microwave energy.
I’ve often thought of building an infrared preheating unit to warm cold feed aggregates prior to their introduction to the drum. Imagine the fuel savings if you could raise the aggregate temperature to 150°. But the real benefits would come when this technology is applied to the RAP. I don’t think it is a difficult problem, but I haven’t found anyone willing to expend the funds needed to experiment.
Where are we going to be in 30 years? Given the finite amount of land available and the trend toward land conservation, I can’t believe there will be much new road construction in the continental U.S. I think what we will see will be mostly on-site regeneration and repavement of the existing roads similar to our road train paving shows today.
In emergent economies such as South America, Asia and Russia I think they will be where we are now or perhaps were 10 years ago. I’ve traveled extensively in these areas and can tell you from first hand experience: There are huge areas without roads, and those that do exist are vastly inferior to American roads.
