How To Choose The Right Asphalt Plant

Asphalt Article December 28, 2000
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So, after years of buying hot mix from the local supplier you've decided to
purchase an asphalt plant and start producing it for yourself, and perhaps sell it commercially. Important decisions will have to be made, such as AC plant type, style, size, brand and budget. Once you've made those choices, more issues crop up, such as AC tank size, storage silo size, number of cold feed
bins, type of pollution controls and more.

The batch plant:

A batch plant makes mix in batches. Aggregate is fed by the feeders to a rotary dryer where the moisture is removed. It is then transported to a set of screens where it is divided into several component sizes (3/4 in.-1/2 in., 1/2 in.-1/4 in., 1/4 in.-1/8 in. and 1/8 in. and smaller, for example) and stored in
"hot bins". These aggregates are then correctly proportioned into a weigh bin by the plant's blending computer. From there it is introduced into a pugmill and dry-mixed for a specified amount of time. The oil is introduced at the proper time and the combination is then wet-mixed for the required time.

At the end of this cycle the pug gate opens and the finished hot mix is either discharged directly into a waiting truck or it is dropped into a moveable chute, which leads to a slat conveyor and then a holding silo.

Batch plants range in size from a 250-lb baby Madsen up through 18,000-lb monsters. I've heard of a 20,000-lb behemoth but have not seen one. The plants generally are rated at one batch per minute, so a 4,000-lb plant would yield 120
tons per hour (tph), while an 18,000-lb unit would yield 540 tph.

A batch plant's strength lies in its ability to make salable hot mix out of almost any reasonable stockpile of aggregate. As one old timer put it, "You can feed 'er meteors and coprolites and I'll make spec mix, sonny."

Another strength inherent to a batch plant is its ability to switch mix
specifications mid-truck if needed. Essentially, if you supply the plant with 3/4 in. and smaller aggregate you can make any mix that uses materials contained within those parameters.

Depending on what screens you have installed in the plant, you can make 3/8 in. and smaller mix for one truck, then switch to 3/4 in. and smaller mix for the next and still be able to blend a nice 1/2 in. and smaller mix for the third.

A batch plant
utilizes numerous steps to produce hot mix. Although these steps
give the plant its versatility, it is these very steps that also
are its weakness to an operator who is making the same mix all
day long.

A batch plant spends about 30% of its time waiting
on bins to weigh up, the pug to empty and similar activities. To
an operator who does not have to make a lot of daily mix changes
and is concerned with high production, perhaps a batch plant is
not the right choice.

Drum plants

What is a drum mix
plant? Essentially, a drum mix plant is a continuous mix
facility that takes the hot-mix manufacturing process to its
basics. In the feeders it proportions the aggregates into the
correct blend to meet job requirements. This material is then
conveyed to the dryer-mixer where the first two-thirds of the
unit is dedicated to moisture removal. In the last third of the
unit the correct percentage of asphalt is injected and the
resultant material is thoroughly mixed. At this point it is
discharged into a slat conveyor for transport to a storage silo
where it is distributed to the waiting trucks.

Drum plants
range from 8-tph toys to 800-tph giants. The most common plants
are between 150 and 400 tph.

The fact that a drum plant
reduces the hot-mix process to its essentials is its big
strength. By eliminating most of the steps taken by a batch
plant to produce mix, a drum plant is able to do its job more
economically. They also will operate at a higher rate of
production for a given drum size because the mixing process is
continuous.

By the nature of their design, drum plants are
limited to producing one mix design at a time. If you introduce
properly gradated 1/2 in. and smaller aggregate into the unit
you are going to get the same thing out of it.

For a
contractor who is required to supply several different mix
designs in the same production run a drum plant may not be the
best choice. This problem can be overcome through the use of
multiple silos and a sharp operator, but multiple silos are not
as practical if you must be portable.

Styles of plants

There are two styles of asphalt plants, portable and stationary.
It's fairly easy to choose between the two, once you know the
answers to a couple of questions: Do you need to move your
plant? If so, how often will it be moved?

One
incontrovertible rule is that portable plants cost substantially
more than their stationary brethren. If you only need to move
your plant once every few years then it is possible that the
cost of moving the plant is considerably less than the cost of
portability on the original order. I recommend that companies in
this category consider buying a skid-mounted plant with flexible
wiring, like SO cord, equipped with quick disconnects.

Companies that plan to move their plants several times a year
must purchase a portable facility. When looking at these units
it is strongly recommended that a self-erect silo/drag
combination be considered. In the Northwest, a crane to set the
silo and drag conveyor can easily cost in excess of $5,000 per
move. It doesn't take a rocket scientist to see that if you move
your plant four times a year, you can pay for the difference in
cost for a self-erect in about two years.

Size of plants

How much production do you need? This is a difficult question to
answer. One of the fundamental mistakes that I see is a company
trying to size its plant to its highest production days. I
recommend a plant be sized to the average for a company's
busiest month.

New companies, without a track record for mix
production, must analyze their market and decide from there. If
you feel that you can sell 3,000 tons per day, 10% of the time
and 1,000 tons per day the rest of the production season, I
suggest that a 200-tph plant is more appropriate than a more
expensive 400-tph unit. Thinking along the same lines, it's
difficult to justify the million dollar cost of a new 350-tph
plant if you plan on making 40,000 tons a year, regardless of
how fast you can make it.

In general, a plant is most
efficient when it is running at about 80% of its rated capacity,
and plants last longer when they are not stopped and restarted
constantly. Also, a smaller AC plant with lots of storage
capacity can often turn out as much mix on a given paving day
because it might not have to stop while waiting for trucks,
while the larger plant fills its silos in a hurry and then must
wait for the silo level to go down. This dead time takes away
from productivity and adds to the cost per ton of mix.

New
vs. used plants

The advantages of a new plant are obvious:
It's new, so there is no guesswork as to its condition. It comes
with full factory support. This is very important in such issues
as DEQ air quality compliance and plant troubleshooting, should
any problems develop. Additionally, most factories will assist
you in the set-up and start-up of their plant. This is a real
plus for companies new to the world of AC plants.

Used
plants offer advantages too. Most notably, they can be
considerably less expensive. The trick is to find a used plant
that hasn't been abused to death, and one that is offered direct
from the owner or his primary broker. This is important because
a plant may be listed through numerous brokers who work together
and if you don't pick the primary listing agent you may have to
pay finder's fees to several and not even realize it.

In
general, try to find out as much information as possible about a
particular plant if it catches your eye. Don't be afraid to ask
questions. Most brokers are willing to help you. If they aren't
find another plant.

Once you decide on a particular piece of
equipment, have it appraised. Get an impartial opinion of the
plant. Several companies offer this service. It is money well
spent and quite possibly could save you from major repair bills
in the future.

Are there any substantial differences between
the brands of asphalt plant? Essentially, an asphalt plant is an
asphalt plant. Manufacturers like to extol the virtues of their
particular features, and some have valid points, but in the long
run they are all similar and when configured the same, any two
brands will do the same job: Produce hot mix.

Look closely
at the specifications sheets on each plant you are considering.
Compare them to the competition and remember: The cheapest is
not always the least expensive.

Cost of facility

New
plants run a wide range of pricing. A very popular small
manufacturer offers a 30-tph portable plant for under $150,000
complete. That same manufacturer offers a 325-tph counter-flow
double-drum with RAP capability for around $1.35 million.

Other manufacturers offer similar pricing and prices soar when
the larger sizes are considered. Five-hundred-tph plants, with
counter-flow technology and RAP capability, set-up to run in
California can exceed $4 million.

The best advice is to do
your homework, then shop for pricing. If you can't foresee a
need for a certain option, don't buy it. Keep your plant as
simple and basic as you can. This will aid you in the
troubleshooting and repairs in the future.

The used AC plant
market usually is very good. There are more plants for sale than
there are buyers. This is good for the contractor looking to buy
a used plant, but it also makes the choice of a plant more
confusing because most brokers will bury a prospective customer
in information.

The list of available plants can quickly
overwhelm the average guy and render any possibility of making
an informed decision virtually impossible without a large
investment in time and air fare traveling across the country
inspecting plant after plant. I suggest you find a broker you
trust and have him do the legwork and find you a piece of
equipment that fits your needs.

There are numerous other
factors in addition to price and size that you will need to
consider when choosing a plant. Some of these things are:

--
AC tank size. How far away from your proposed plant site is the
nearest oil supplier? How long does it take to lap a truck from
plant to the refinery and back?

These are questions that
need to be answered before you can decide on oil storage
capacity. In general, a 300-tph plant will use about 17.4 tons
of oil per hour at a mix design percentage of 5.8%. A 30,000-gal
tank holds roughly 116.7 tons of oil at 7.7 ppg (approximate
weight of PGA-58-22 @ 300 deg F).

Because you cannot pull
all of the oil out of the tank as they are set-up so that the
heaters stay immersed in oil (electric models) let's assume you
can use 114 tons of liquid. At 17.4-tph, you can run
approximately 6.5 hours on the oil you have. To stay even with
your rate of consumption you will need to get a 35-ton load of
oil every two hours. If this is not possible, you need to
increase your oil storage capacity. In general, use as much
storage as you can afford.

-- Hot-mix storage. Larger silos
generally mean less truck turn-around time in the yard, which
equates to more money on the bottom line. A large capacity silo
can also help a smaller plant perform with the big boys as far
as mix shipped per hour. This is because when the trucks are
gone the plant can continue to run longer, refilling the silo.

If you start the plant early enough in the morning to have
the silo full when the first truck loads, it will help the plant
stay ahead of the trucking for several hours. In general, buy as
much storage as you can afford. It's easy to put 50 tons in a
200-ton silo, but it's tough to put 60 tons in a 50-ton silo.

-- Cold feeds. Does your state use mixes that require the
aggregates to be broken down into more than one or two
stockpiles. If so, you will need to make sure your new plant
accommodates all the materials you will need to use. If you have
to blend sand into your mixes, you could find that you need five
feed bins.

-- Wet Wash vs. Baghouse. It's no great secret
that an asphalt plant costs considerably more when equipped with
a baghouse as opposed to a wet scrubber. But what may not be so
well known is that within a few short years the federal DEQ
regulations may obviate the use of wet scrubbers through default
by making their use so expensive that the small operator can't
afford to comply with all the requirements.

As an example,
in the Northwest, Washington state has enacted a new law that
takes effect Oct. 4 and is going to require all wet-scrubber
operators to two things. Operators will have to line all ponds
with an impermeable membrane (concrete, polyurethane or
something similar) and perform water testing on a weekly basis
(possibly more often) of any discharged liquid.

In the near
future, you may well see a wave of states following in
Washington's footsteps. With these regulations taking effect on
the state level, can the feds be far behind?

If you can
afford it, buy a plant with a baghouse, or at least price a
baghouse to fit the plant you are considering. If the feds
mandate baghouses in the near future, the cost of these units
will skyrocket. Current portable units, sized for a 200-tph
plant, sell for around $140,000 new and $70,000 or more used. It
costs additional money to retrofit a baghouse to a plant.

One last bit of advice: Start on the air quality permit process
the minute you decide to get a plant. This process, under
adverse conditions, can drag on for months and in some
situations actually kill the entire project. Know your prospects
for obtaining all the needed permits first.

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