C. M. Consulting
P.O. Box 407
Odell, Oregon 97044
C. M. CONSULTING
A Division of Cliff Mansfield Incorporated
THE FUTURE OF ASPHALT PLANTS
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
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
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
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
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
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
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
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
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
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.
For additional information on this subject
or help with any problems encountered
contact Cliff Mansfield,
7:30am to 9:00pm Pacific Standard Time.
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