These asphalt plant aggregate dryer flights are different.  They even look different.  They have a big hole in them!  They remind me of today’s kids expensive jeans that come new from the store with holes already in them.  When I was a kid, I wore holes in my jeans but they didn’t come that way from the store!  The hole in these flights is not about making some sort of fashion statement.  It serves a very important purpose to make aggregate dryers work better than ever before.  Of course, the particular shape of the hole is important.  The specific shape was determined by extensive prototype testing to ensure an even aggregate veil.  It also provides for good showering on the uplift side of the drum when lightly loaded and limits showering in that same area when heavily loaded.  I know this post is a little long, but it spells out the how and why of the unique design and performance of FlexFlightTM.  Read on if you’re interested or just curious.

Why do your Dryers need FlexFlightTM showering flights?  FlexFlightTM flights are the most effective showering flights available today for rotary aggregate dryers and especially for hot mix asphalt plants.  The name FlexFlightTM indicates the unprecedented flexibility of applications.  It performs exceptionally well to produce all kinds of mixes, including high RAP up to 70% and 100% virgin without changing flights. The following paragraphs explain why all this is true and further describe capabilities.

Counter flow rotary aggregate dryers are the most effective and efficient means of drying and heating stone aggregate and sand for hot mix asphalt production that the industry has ever employed.  However, they are not perfect, and, in fact, most do not perform nearly as well as they could.  When this type of dryer does not perform well, the cause of the poor performance is almost always the flight system.  All dryers contain a quantity of steel fabrications, called flights, that are attached in circumferential rows to the inside wall of the rotary drum.  As the drum rotates, the flights scoop aggregate from the bottom of the drum and drop it through the stream of hot burner gases.  This is called showering.  Showering produces a complete, or, more often, incomplete, veil of aggregate across the inside of the drum.  The purpose of showering is to expose the maximum aggregate surface area to the hot burner gases.

Most of the many differently designed flights in use today fail to create a complete veil of aggregate across the full width of the dryer drum.  When the feed rate of aggregate is less than that narrow range, the flights do not fill completely, and showering doesn’t take place on the uplift side of the drum’s rotation but is delayed until the flights get near or pass over the top point of the rotation.  This leaves an open “path of least resistance” for the burner gases to escape without touching aggregate.  When you need the heat the most is when conventional flights fail to deliver.  Some of these conditions are:  high RAP percentage, high moisture content, high internal moisture, SMA mixes, and open graded friction coarse (OGFC) mixes.  A similar thing happens on the high production rate side of the range.  Flights have too little volume capacity in each flight and/or are too few to handle the volume.  Therefore, some aggregate just gets rolled around and is never lifted and showered.

These showering deficiencies cause a number of problems.  Here are a few:

  1. Fuel efficiency is reduced.
  2. Drum exhaust gas velocity is higher than normal.
    1. Fines carry-out is increased in volume and particle size.  This increases the difficulties of fines management with associated out-of-spec. voids and erratic AC content.
    1. Baghouse and other exhaust system components reach their limits well below rated production.
    1. Mix production rate is reduced.
  3. Aggregate is not completely dried, especially with respect to internal moisture.
    1. Internal moisture will tend to steam out later and cause mix to cool quickly in the trucks or in silos.
    1. Steam may continue to come out even in the mat, which causes the mix to tear under the screed and shove in front of the roller.
  4. Some aggregate is not heated to the intended mix temperature.  This causes poor coating and cold spots in the mat with the result of uneven compaction.

The reason you need FlexFlightsTM is that, correctly applied, they solve every one of these problems.  They also expand the efficient production range of the plant.

FlexFlightsTM shower aggregate completely across the Drum diameter regardless of how full the flights are and make a good even veil.  Even the best competitive flights in the industry tend to provide a very thin veil on the uplift side of the drum when the flights are lightly loaded.  That may sound like something no one really should care about but that is exactly when you need the best veil.  Flights are lightly loaded when making high RAP mixes.  That’s when you need the best possible performance from your dryer.  When making high RAP mixes, you will be limited by high baghouse temperature, if you are not getting great performance from your dryer.

On the other hand, when fully loaded, the best competitive flights have a problem of allowing aggregate to flood out on the uplift side.  The veil in that part of the drum is so heavy that hot burner gases can’t penetrate it and are forced to the other side of the drum where they bypass much of the wet aggregate.  FlexFlightTM solves that problem too.  The fact that it has a hole rather than a notch, restricts the amount of material that can discharge early.

Much of the “magic” of FlexFlightTM showering flights is connected to their ability to work extremely well with variable speed drum drives.  Flights that produce good even veils at all production rates respond well to speed changes by allowing the operator to effectively adjust veil density to get the optimum thermal efficiency and exhaust gas temperature.  This capability separates mix temperature control from exhaust gases temperature control.  The operator can control mix temperature with the burner firing rate while independently controlling Baghouse inlet temperature with drum speed.

With FlexFlightTM showering flights and a variable frequency drum drive, your plant will make better mix at lower cost.  It will make high RAP mixes and open graded mixes as easily as it makes virgin mixes. It will generally eliminate quality and efficiency problems related to aggregate internal moisture.  The mix will store better and retain temperature better in transport to the job.  The bottom line is that the bottom line for your HMA business will be stronger.