While this article is directed toward Boeing drum plants, these basic steps work on ALL parallel flow drum mix plants.

  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. 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. Most operators, with a little patience and a willingness to read the service manual, could fiddle with the ratio control or the belt scales and get them to work after they failed. A testament to the quality of these plants is the number of them still working all around this country. I've encountered early Boeings from Alaska to Florida, in the deserts of the southwest and in the mountains of Maine.
  As good as they are, first generation Boeing plants have what, in today's operating environment, I call a serious flaw. In an effort to reduce particulate carry-through in the dryer and produce a superior AC coating on the aggregate, they introduce the oil very early in the dryer/mixer in what they called the 'boiling zone'. Plants with this configuration are easy to identify because the oil injection tube enters the dryer/mixer on the burner end. To understand the 'flaw' we must first understand the process these plants utilize.
  In this generation Boeing plant the burner is set back from the face of the drum and a Boeing 'Pyro-cone' is used to keep the open flame away from the oil/aggregate. This cone is a stainless steel unit about 5' long, about 2.5' in diameter with holes around its perimeter to draw in a layer of cooler air around the flame, thus insulating the oil from direct contact with the fire. At start-up, the aggregates are introduced into the dryer at ambient temperature, 60 to 70 degrees in the lower states -a bit cooler in Alaska. They immediately encounter a series of J-flights designed to produce a heavy veil in the nose of the dryer, thereby soaking up a large quantity of the burner's radiant energy. The aggregate's temperature rapidly reaches the boiling point and the moisture it contains is turned to steam. AC oil is injected at this point, between a quarter and a third of the way down the drum. As the steam vacates the aggregate it foams the AC, which helps it coat the rock. At the same time the steam escaping from the pores of the rock leaves behind a vacuum that draws in the oil. The theory was that this process yielded a superior product with less particulate emissions. It actually worked very well with the oils of the day.

    THE PROBLEM:
  In today's operations we are using a different breed of oil. Some are polymer modified, others use rubber in varying amounts and still others contain who-knows-what as DOT engineers strive to come up with a more durable pavement. What all of these oils have in common is that they don't like exposure to open flame. When it happens, they build dense clouds of nice blue smoke that are a magnet to environmentalists and DEQ folks.
  As these early Boeing plants have aged they've gone through numerous repairs and rebuilds, not all of which were faithful to the original 'boiling zone' concept of oil injection in the dryer/mixer. Some modifications actually exacerbate the problem of blue smoke. In economically frugal times, the first thing to go was usually the 'Pyro-cone' since it is made of expensive stainless steel. A variety of things conspire to destroy this unit, but by far the most common is excess heat. A poorly adjusted burner/exhaust system can quickly lead to overheating the unit. Once the thing glows in the yellows it distorts and can easily become an impediment to air flow. When this happened, many companies simply removed the thing and moved the burner up next to the drum in the conventional location. The result was that the oil was now exposed to the direct flame, with predictable results.
  Inside the drum, J-flights, being expensive, were replaced with standard L flights as they wore out and their number was reduced in a misguided effort to save money. Consequently, the heavy veil, so critical to Boeing's concept, was lost and the oil was subsequently exposed to the ravages of contacting the flame stream. The internal flight modifications, combined with the elimination of the 'Pyro-cone' produced a plant that smoked noticeably under all but the best conditions.
  Another problem surfaces when an older Boeing plant with early oil injection is not well maintained. Part of the start-up sequence of a drum plant is the asphalt 'on/off delay' timers. These units start or stop the oil flow according to the specifications set by the factory or the operator. In the case of the 'on' timer when the plant is started up and the aggregate turned on the belt scales send a signal to the ratio (AC/aggregate) computer which in turn fires the timer. The timer, set to between 45 & 150 seconds, counts down to '0', then turns on the AC. The purpose of this delay is to allow the aggregate to enter the dryer/mixer and travel down it past the spot where the oil injects. When the oil injects prior to the arrival of the aggregate it is directly exposed to the flame of the burner and produces an ugly yellowish smoke which billows out the exhaust stack like a huge sign saying, "Please, Mr. DEQ man, give me a big ticket and jerk my operating permit." The 'off' timer's job is to shut down oil flow just prior to the last of the aggregate arriving at the oil injection spot. If this timer fails or is set to shut off too late the oil is again exposed to the open flame with the same results as before.

    THE CURE:
  In recent years these parallel flow drum plants have gained a reputation as smokers. It is well deserved when they are modified as described above. If you own one of these plants don't despair, there is a cure. In the past, I've experienced a large degree of success by modifying the oil injection system.

FLIGHTS:  The process is quite simple. First we need to look at the mixing flights. Early drum plants used LIFTING flights in the mixing zone. It doesn't take an asphalt plant GURU to see that you do NOT want to spill oil coated material through the super-heated airstream. To stop this I take all of these flights and cut them off at the upper angle so that they simply rise 90 degrees to the shell. Then I cut 45 degree saw-teeth into the upper edge. This causes the hot mix to be simply stirred on the bottom of the shell rather than being carried up and spilled through the airstream. This alone usually cures the plant from smoking, but further work will help with the non-visible hydrocarbons in your stack that could cause you to fail a stack test.

OIL INJECTION LINE:  Now we need to make sure the oil line is in the correct location. Some early plants had the injection line at the burner end of the drum. Not a great idea. On these plants we need to put a new oil injection pipe in the rear of the drum, installed so that it ends in the middle of the second to the last row of flights on the discharge end. This pipe should be installed so that when the drum mixer is set-up with the proper slope the injection pipe is lower at the discharge end than it is at the inlet end. This is very important, it prevents slugs from plugging the pipe. Once the pipe is sloped correctly and tack welded in place, heat and bend the last 6" so that the pipe runs almost parallel to the shell of the drum, about 18" above the flights. Install weld-on 4 bolt flanges on each end, they'll prevent oil leaks and ease any future changes. Next, run an angle iron from each side of the pipe up to the rear of the knock-out box as high and far apart as practical. Turn them up so that the open part of the V is toward the sky. The V will fill up with mix and provide a self replenishing, wear-forever surface. It's a good idea to build a trough out of angle iron on top of the injection pipe so that it, too, will fill with mix and last forever.
  Now, move the oil lines and divert valve to the rear of the dryer and mount them on the frame near the knock-out box. Be sure to mount the divert valve higher than the drum line. This will eliminate all the low spots between the drum line and the divert valve. Oil left standing in an unheated low spot could congeal as it cools, causing a blockage that will require an outside heat source to clear.
  The next step is to build an adjustable spray bar to go on the end of your injection tube. The purpose of this device is to allow you to move the oil injection point fore and aft in the drier to control the amount of fines in your finished product. I take a 3" piece of black iron pipe 4' in length and weld 4, 3" couplers equally spaced along it. After installing it on the injection tube I plug three of the openings with pipe plugs. In the fourth, install a nozzle (pipe nipple) that reaches to within 6" of the mixing flights. The installation of a weld-on 4 bolt flange finishes the unit. It is a good idea to have several 1' sections of threaded pipe, complete with threaded flanges, ready so that you can adjust the injection point farther toward the inlet, should the need arise.
  Additionally, you will need to move your AC tank and pump pallet (if so equipped) to the rear of the drum. This will involve lengthening the power leads and all control wiring. A word of caution: The control wiring is best left to a qualified electrician with control experience.
  If your plant still has the 'Pyro-cone' on it you can either eliminate it or keep it as you see fit. I personally eliminate them since they are an unnecessary piece of equipment requiring attention from the operator during on-going operations. If you remove it, some burners will require that you install a poured combustion chamber at the inlet end of the drum.
  Once you re-start your plant after the above modifications you should take a mix sample and have an extraction/gradation run on it. Pull a sample of the cold feeds as near to the drum inlet as possible. Compare the mix sample to the cold feed sample gradation. Obviously, you are interested in the amount of fines removed as a result of the previous modifications.
If you operate in a state where the aggregate is accepted on the basis of cold feed analysis, the issue to address is the amount of fines being drawn into your dust collection system. In this case you would want the oil to inject as early as possible, yet not generate smoke. Start with the nozzle in the last hole. If your stack is pristine, add a foot long section of pipe.
  If you operate in a state that accepts the aggregate on the basis of results obtained after the material emerges from the dryer/mixer the issue becomes controlling the amount of 40s thru 200s in the finished product. If you want to remove more 200s, move the oil injection point toward the discharge point. To retain more, simply move the injection point toward the burner. Experiment. A surprising amount of change can be realized through moving the oil injection point a small amount.

    NOTE:
  Some plants will show a bit of 'salt and pepper' in the mix emerging from the drum when utilizing only the last two rows of flights as the mixing zone. In most cases this is not a problem. Keep in mind that what really counts is what the mix looks like when it lands in the truck. As the hotmix travels up the drag slat it is being mixed for another 20 seconds or so.
  With the above modifications and a keen eye on temperatures your Boeing drummer shouldn't smoke. With luck and a good maintenance regimen it should soldier on well into the next century.

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