THE FUTURE OF ASPHALT PLANTS

by
Cliff Mansfield

    I’m often asked what the future holds in the way of asphalt plant design. Orson Wells’ vision of the future holds the view that cars will travel through the air. We all hope that’s a few decades off because it doesn’t bode well for our industry: I don’t see a need for roads in mid-air.
For now, AC plant design is being driven by the state DOTs as they write more specialized specifications requiring yet another compromise in the functionality of our current plant designs. We’re blending polymers, latex’s and rubber in thirty year old asphalt plant designs. We’re doing open graded mixes and Super-Pave mixes in plants designed for the dense mixes of the seventies.
    Several things are influencing what happens at the manufacturers. One is the current trend toward Super-Pave mixes. Another is the ever tightening emission restrictions and, of course, there is the issue of ever increasing competition which requires plants to get more and more cost efficient. For these reasons and others, batch plants in America are nearly dead. With twice the moving parts to wear out, they are simply too expensive to operate and maintain.

    Drum mix AC plants were first introduced in the early seventies. At the time, the new technology revolutionized our industry. Plants became highly portable and substantially more cost effective to operate. It was a time of bold experimentation by a variety of entities. Among this group was Boeing Construction Equipment, part of the vast Boeing aircraft company. This group of talented individuals put together what (even today) I consider one of the best drum mix plants in existence: Their BCE-100 through 400 series portable drum plants. No, they didn't have all the gee-gaws that come on today's AC plants, but that meant you didn't have to have an electrical engineering degree to keep the thing running. They used Ramsey 10-201 belt scales, Seco DC feeder drives and simple Genco burner controls. These plants also utilized a Ramsey ratio system that featured a full manual back-up system. Most operators, with a little patience and a willingness to read the service manual, could fiddle with the ratio control, burner controls or the belt scales and get them to work after they failed. Today’s asphalt plants are using such complicated electronic control systems that the everyday plant operator is simply lost if things go awry. Most new asphalt plants have no manual back-ups what-so-ever. This situation forces the plant owner to rely on the factory for support if and when things go wrong. Unfortunately, the factory service personnel are not always readily available. For this reason, all the asphalt plants I spec out for my customers includes a full manual back-up system.
    Another trend at our asphalt plants seems to be to remove the plant operator from the mix design and compliance loop. Not long ago I was at a southern asphalt plant working on grinding a set of trunnions. This was a big, 10’ x 50’ drummer with eight feed bins making around 4000 tons per day. One day I was in the control room during a break in our grinding operations. A truck had just loaded and was approaching the elevated control house to get his ticket. I noticed that the mix in the truck seemed very fine and a bit on the wet side. I asked the operator what the mix was. He said something like “ID-2 top”. I asked him what that was. He said that it was used as a surface mix. I’d already figured that out so I asked him, “What goes in it?”
    “Bin 3 and 5 and a little sand,” was his answer.
    A little frustrated, I asked, “No, I mean what material, 3/8 - 0 or what?”
    “Whatever the loader man puts in bin 3 and bin 5,” he said.
    I asked him again and he told me that it was not his job to worry about what aggregates were used to make what mixes. That responsibility belonged to the QA/QC team that handled the testing chores at his plant. As I talked further with this 34 year old operator with ten years of experience, I realized that he knew very little about mix designs and the processes we use to keep our plants in spec. I’ve trained a lot of plant operators around this country and one of the things I try to pass on to them is a basic understanding of mix designs. With this in mind I figured that the plant operator I talked about earlier was not typical. I was wrong.
    As I traveled around last summer I started to pay attention to the level of training of the plant operators I encountered. I was surprised to notice that almost all of the younger operators were not being trained in mix design issues. These, instead, were being left to QA/QC people. Further investigation revealed that few, if any, of these QA/QC people understood how a hot mix plant actually works. I think that this is a disturbing trend in our industry. It seems to me that the plant operator needs to understand every facet of his job. Over the years, as our older operators retire, we will find ourselves scrambling to fill there positions with people who do not really understand the job. Eventually, our plant personnel will be more specialized, unable to cope with the wide variety of issues raised in daily plant operations. At that point we will be required to have 5 or 6 guys at every plant. We’ll need a plant operator, a plant oiler/ground man, a loader operator, a QA/QC man and a plant manager to hold it all together. This is the situation in most of the Southeastern asphalt plants I’ve been to. In my mind, there are simply too many people at these plants.

    Conventional drum mix asphalt plant technology has been geared to parallel-flow drying and mixing drums. In this configuration the aggregates enter the drum at the burner end and travel downhill to the discharge chute. This is called parallel-flow because the aggregates travel the same direction as the superheated air-stream. About two thirds of the way down the drum the asphalt is injected into the heated aggregate. The problem with this configuration is that the superheated air from the burner impinges on the asphalt oil and burns it, releasing blue smoke. Over the years we’ve managed to find ways to minimize the amount of smoke our drums spewed. By extending the drying zone and shortening the mixing zone we can reduce the time that the liquid oil is exposed to the superheated air stream. The mix exiting the drum might exhibit some ‘salt & pepper’ which requires that we use our slat conveyor for some of the mixing time. Usually, by the time the mix goes up the slat, through the batcher and into the truck it is sufficiently mixed that you won’t see any ‘salt & pepper’ in the load leaving the plant.
    Another thing we learned to do was to keep the hot-mix next to the shell in the mixing end of the drum. Most plants came from the factory with L shaped flights or something similar in the mixing zone. Those of us trying to combat the problem of blue smoke designed and built ‘finger-flights’ for the mixing zone. These flights do not elevate the hot mix and veil it through the air stream like the L flights. Instead, these flights mix by spilling the hot mix through a series of ‘fingers’ like those on your hand. As the asphalt spills around these fingers it is mixed, out of the air stream. Although these modifications have helped considerably, blue smoke issues in parallel flow asphalt plants are still prevalent. This problem is exacerbated when we introduce RAP since we must run our heat up to compensate for the cold RAP.

    In our environmentally sensitive times RAP has become a very important issue. Most manufacturers are now focusing on how to blend rap without the associated ‘blue smoke’ problems of the past. Counterflow dryer/mixers are the dominant theory. In this configuration the aggregates enter the drum at the opposite end from the burner. They travel down the drum in the opposite direction of the superheated air. Nearly every drum designed strictly for aggregate drying has been this configuration since it is the most cost efficient way of drying rock. While the counter-flow aggregate drying concept is not new, it is important to understand that this technology is essentially in its infancy when used on a drum mix AC plant.

    Several plant configurations have been designed to address the issue of blue smoke, all utilizing counter-flow technology. Some manufacturers are using long nose burners which extend a third of the way up into the drum. This creates an isolated air zone at the mixing end of the drum to keep the AC and the RAP from coming in contact with the superheated air stream. CMI, Cedarapids and Gencor use this approach. Astec, with their ‘DOUBLE BARREL’, has moved the mixing zone to the outside of the drum and utilizes pugmill-like tips and liners to do the mixing. In this configuration the inside of the drum looks like any standard counter-flow drying drum. The difference is that when the hot aggregates exit the drum they enter the mixing chamber. You’ve all seen twin shaft pugmills. What Astec has done is to mount a series of pugmill shanks and tips on the outside of their drum, build a mixing chamber around it and introduce the RAP and AC oil in this chamber to eliminate the smoke.

    The simplest approach, and my personal favorite, is the two drum system where one drum is used to dry the aggregate and a second, smaller drum is used to blend the AC and the RAP with the heated aggregates. ALmix pioneered this systems use in drum mix AC plants about twelve years ago. This system utilizes a counter-flow aggregate drying drum to remove the moisture and heat the rock to the desired temperature. In this drum the air velocity is up around 1,000 feet per minute. The heated aggregate is then introduced into the smaller drum where the asphalt oil and the RAP are mixed with it. Since this drum is not connected to the drying drum’s air flow system, some method must be used to remove the steam that results when 400 degree aggregate come into contact with 75 degree RAP containing 4% moisture. This is done with a small scavenge air fan. These approximately 4,000 ACFM units are generally variable speed and set to provide a minimum of suction to eliminate puffing at the drum seals. One major advantage to this system is that when we are required to blend admixes such as mineral fines we can do so in a nearly null-air environment, thus insuring that the admixes go into the mix and not into the air stream and on to the dust collection system.

    Another important manufacturing trend addresses the issue of the amount of time it takes to get a new asphalt plant up and running from arrival at the site to in spec and bug-free. In the old batch plant days this process could take three or four months. Plants were like one huge erector set with massive frames and supports for everything. Drum mix asphalt plants are much simpler designs, consisting of many fewer components, so by nature they take less time to set-up. In recent years plant manufacturers have started to make their plants more modular, with a support structure that went to ground level. We call this configuration ‘skid mounted’. Buying ‘stationary’ plants requires the construction of massive concrete foundations and the use of hard wiring and conduit. Both of these things add money and time to the project. By selling skid mounted plants equipped with plug and cord wiring we’ve reduced the set-up time to a minimum and the cost of wiring, normally $250,000 to $400,000, to less than $50,000. I recently installed a 400tph ALmix Duo-Drum plant in Miami in 8 days from first concrete pour to running in spec. This was made possible by the fact that I only needed to pour five flat pads of concrete to set the silos, drums, baghouse and feeders on. Standing the iron up took five days with eight men and myself overseeing the job. We did the wiring in one day. It was simply a matter of stretching the cords over to the control house and plugging them into the appropriate connector. We later put wire trays under the cords. This particular plant was equipped with Seltec’s Premium System which is a PLC controlled by a Pentium PC. Calibrations of the feeders, the AC oil and the anti-strip were fully automated and took most of another day. I’ve been back to this plant twice since it was first started; once to teach them how to adjust the trunnions and once to remove some flights from the drying drum to get the baghouse heat up. In this day and age I would say that this is typical for most drum plants. I read an article recently in which a plant owner stated that it took a year to get a new plant set-up, calibrated and running trouble free. Over the years I’ve set-up around 80 asphalt plants of all configurations in 10 countries. The longest I’ve ever spent on one of these projects was three and a half months. If you are thinking of a new plant, 90 days from arrival of iron to full production is more than enough time.

    What are some of the upcoming hot button issues facing our industry?
    For some reason our industry does not seem to have a glamorous image. When you ask a kid what he or she wants to be when they grow up you hear Doctor, Nurse, Fireman or Cop. Hey, some of them even want to be lawyers, god forbid! But you seldom hear a kid saying “I wanna be a road builder.” For this and other reasons in-house personnel training is becoming more important. 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 web sites for our paving companies, or maybe our asphalt paving associations. It seems clear to me that we need to do something.

    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 be 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, essentially, 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 internal modifications to the drum. Asphalt Equipment and Service Company, Renton, WA, has 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 downward of electronic prices I suspect that we will see plants leaving the factory with this option.

    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, the South, the East and the 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 from the thing, but when it was not being pulsed none was being 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 is also 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” while the high “pressure differential” is set around 6”. As the plant is running the photohelic will automatically begin pulsing the bags when the pressure reaches 6” and stop when it falls to 4”. 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 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”. 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.

    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” around by 10’ long. Each of these bags has 15.71 square feet of cloth. A pleated bag will have nearly ten 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 in 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 quite 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 and do the job then move out again.

    I think the near future will also see the death of the wet-scrubber at our asphalt plants. I do not think that 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 ten years?
    I recently read an article in Scientific American where some Poindexter lookin’ brainiac 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 that 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. They are infrared and microwave energy. I’ve often though 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 degrees. 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 a client willing to expend the funds needed to experiment. Any volunteers?

    Where are we going to be in thirty years?
    Given the finite amount of land available and the trend toward land conservation I can’t believe that there will be much new road construction in the continental US. I think what we will see will be mostly on-site regeneration and replacement of the existing roads similar to our road train paving shows today. This, of course, assumes that the space aliens haven’t shown up offering molecular transporters like on Star Trek. In emergent economies such as South America, Asia and Russia I think they will be where we are now or perhaps were ten 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.
 

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