How To Choose The Right Asphalt Plant

Dec. 28, 2000
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.

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.

Sponsored Recommendations

The Science Behind Sustainable Concrete Sealing Solutions

Extend the lifespan and durability of any concrete. PoreShield is a USDA BioPreferred product and is approved for residential, commercial, and industrial use. It works great above...

Powerful Concrete Protection For ANY Application

PoreShield protects concrete surfaces from water, deicing salts, oil and grease stains, and weather extremes. It's just as effective on major interstates as it is on backyard ...

Concrete Protection That’s Easy on the Environment and Tough to Beat

PoreShield's concrete penetration capabilities go just as deep as our American roots. PoreShield is a plant-based, eco-friendly alternative to solvent-based concrete sealers.

Proven Concrete Protection That’s Safe & Sustainable

Real-life DOT field tests and university researchers have found that PoreShieldTM lasts for 10+ years and extends the life of concrete.