Mix-and-Match Asphalt Plants
Seasonal maintenance offers prime time to assess sizing the puzzle pieces of optimum, efficient production
I have been to hundreds of asphalt plants and helped design and build even more, but I have never owned one. I have often thought I would like to own a brand-new counterflow drum mixer plant. I think I could make some great mix and a lot of money with that kind of plant. There is just one little problem. I seem to be a few million dollars short of the price tag. I guess everyone would rather have a brand-new plant but there is a significant capital cost advantage to piecing a plant together with good used equipment, if you know how to go about it.
If I were going the mix-and-match route to a well-functioning plant, how would I do it? It isn’t easy to get it right. There are many considerations. Some of these are the same whether the plant is to be new or composed of used equipment.
- Market size
- Site location
- Emission limits
- Plant type (batch or continuous, portable, relocatable, stationary)
- Production capacity, and so on.
These things must be addressed regardless of whether you opt for new or used. Then there are things that would have to be addressed specifically because of trying to mix and match equipment that did not all originally belong to the same plant. This approach can save a lot of money or, if not done right, it can lose a lot and result in a plant that performs poorly.
I have recently been helping a major contractor to upgrade an existing plant by modifying some components and replacing others. Some of the replacements have been new and others have been “experienced.” Some of the pre-owned equipment was not quite right for the plant. There is no shortage of pitfalls in this approach to upgrading a plant, but it can be done successfully.
Some Mistakes to Avoid
The “trust-the-nameplate” mistake. I am not going to say anyone made deficient equipment for our industry on purpose, but we have learned a lot more about how to make well-performing equipment over the years. The contractor I mentioned above brought a used pulsejet baghouse from another of their plants because the smaller existing baghouse was a bottleneck to the process. The production goal was 400 tons per hour (TPH). This rate typically calls for about 68,000 cubic feet per minute (CFM) of exhaust system capacity. The used baghouse nameplate capacity was considerably higher than that. However, the bags were 16 feet long, which was a red flag, based on some previous unpleasant experiences.
I ran a few calculations and found the air-to-cloth ratio (exhaust gas velocity through the cloth) to be fine but interstitial velocity (upward velocity between bags at the plane of the bottom of the bags) was too high. High interstitial velocity prevents the bags from being cleaned properly, which results in reduced production capacity. In this situation, the cleaning air pulse knocks the accumulated dust off of the bags, but the high interstitial velocity picks it up and puts it right back on the bags. To solve the problem and salvage the baghouse, I recommended replacing every other 16-foot-long bag with an 8-foot bag. Calculations indicated that air-to-cloth ratio, though higher, was still okay and interstitial velocity came down enough to allow proper cleaning.
The “bigger-is-better” mistake. Another common mistake is to think that a component is okay to use if it isn’t too small or doesn’t have too little capacity for the application. Oversizing. That kind of sounds right, but it’s not.
A baghouse is a good example of this one, too. An oversized baghouse may work okay but it usually comes with an exhaust fan that matches the baghouse capacity. An oversized fan can easily and unintentionally be used to pull in too much air. Too much air can cause a whole host of problems:
- Poor cleaning
- High burner excess air (which results in poor drying and excessive fuel consumption)
- Excessive dust carry-out from the dryer
- and a few others.
Also, the oversized baghouse may not be so good for the application for lack of good circulation. Areas in the baghouse may not get enough flow to stay hot and may then get wet and muddy. When bags become wet, they are effectively disabled. Wet walls and structure tend to corrode.
Consider a burner as another example of the “bigger-is-better” mistake. Suppose you have determined that your burner is the bottleneck that is preventing the plant from achieving the production rate you want. For example, you have a plant that is capable of 300 TPH except for the burner. Let’s say the existing burner is rated at 60 MMBtu/hr. For a 300 TPH capacity, you really need 75 MMBtu/hr.
You know there is a good used burner at another of your company’s plants that is not being used and it has a nameplate capacity of 100 MMBtu/hr. Should you use that burner? The answer is no. It is too big for several reasons.
First, most operating equipment has a sweet spot that is near 80% of its maximum operating rate. Because 80% of 100 is 80, the 100 MMBtu/hr burner will not be able to operate in a 300 TPH plant at a rate where it runs best. Fuel efficiency and emissions will suffer—and those emissions matter.
Failing a stack test because of an oversized burner is a real risk. Most of the burners used in the asphalt plant dryers are of the aerodynamic mixing type. They are commonly referred to as nozzle-mix burners. This type of burner, unlike premix burners, depends on violent turbulence to mix air and fuel. The turbulence that is needed is caused by high air velocity at the nose of the burner. Of course, the lower the firing rate, the lower the air velocity and turbulence. In our example, the flame diameter will probably be too big for the drum, too. Oversize flame diameter can cause damage to combustion flights and the drum shell.
Many burners are also good examples of the “trust-the-nameplate” mistake. Most plants are burning natural gas now. Many of the used burners out there were built when oil was the fuel of choice. Gas was added as an afterthought, and it wasn’t always done very well. Some of the most popular burners in the industry (avoiding naming names) will perform at about 80% of nameplate capacity when burning natural gas. There is no way you can tell which ones perform at this rate. You just have to know or know someone who does.
Consider the Whole
So far, what I have given you are a few mix-and-match issues to avoid. To upgrade a plant, you need to take a holistic approach. This simply means that all plant components must be able to support your goals for the plant.
The first step is determining the goals. This should come from market data. For example, if it is determined that a plant, which produces 300 TPH and can produce mixes with up to 45% recycled asphalt pavement (RAP), is right for the available market, that is a starting place. That plant will need a 7- or 7.5-foot-diameter counterflow dryer and a baghouse with about 52,000 to 58,000 CFM capacity. Matching components must make logical sense.
Including a 68,000-CFM baghouse in a plant with a 7-foot-diameter drum might work but it makes no logical sense and will not work as well as it could. Having a competent engineer run a few basic calculations to determine which existing components do not match your goals for the market will save time and money and go a long way toward assuring a good outcome of the project. A good approach is to select an appropriately sized, existing major component and redesign the plant as necessary around that component. Once you have selected the key component, all other components must be chosen to have a little more capacity than that one.
Having said that a 300 TPH plant needs a 7- or 7.5-foot-diameter dryer drum, be aware that the drum diameter isn’t all there is to it. The flight system is critical. Most existing flight systems out there will struggle to accommodate 45% RAP and many struggle at lower RAP percentages. A good flight system and a variable frequency drive (VFD) are necessities in most markets. There are great flight systems available today that will comfortably produce mixes with 60% RAP and with the flexibility—with the VFD—to produce 100% virgin mixes without anyone going in the drum to adjust, add, or remove flights.
With this knowledge, a mix-and-match approach to upgrading a plant can be completely successful. Assuming the condition of the equipment is correctly assessed, the greatest threats to success are hidden deficiencies—like overstated nameplate ratings mentioned above. To determine whether a nameplate rating is right will require someone with enough experience to know, or with the ability to investigate and calculate, or test.
Mix and match isn’t for everyone. Doing it this way requires the willingness and resources to do the homework. When buying a new plant, you generally are dealing with a manufacturer that knows its business. That isn’t necessarily the case when you are employing used equipment.
Whether you are putting a whole plant together, upgrading an existing one or just replacing an aged or worn component, these same principles and practices can be applied to ensure a good result. If yours is a big company with many plants, you may have good equipment that has been set aside and can be reemployed. If you have a smaller company, you are more likely to have to search and find good used equipment or introduce a new piece into the existing plant. In any case, getting a correct match of real capability is the key thing. Applying the appropriate expertise is critical to getting a good outcome. That may come from inside or outside your organization, just as long as it is the right expertise.
Malcolm Swanson is the proprietor of e5 Engineers LLC, Chickamauga, Georgia. For more information, contact him at (423) 667-6781 or firstname.lastname@example.org.