What is the history behind the Duo Drum "CF"?
For the past several years the market has pushed toward total separation of the drying and mixing areas in drum mix plants. In 1990 ALmix introduced the parallel flow Duo Drum to the market. The results were flawlessly clean stacks and the end of fighting light ends in baghouses. The trend away from these parallel flow Duo Drums toward the counterflow Duo Drum "CF" came about for three reasons. First, the relatively low stack temperature associated with the counterflow process allows a higher proportion of recycle material to be processed without endangering the baghouse. Second, the proximity of the mixing drum inlet to the burner in the counterflow process facilitates the burning off of any hydrocarbons created in the mixing process. Third, the ALmix innovation of variable speed drying and mixing drums allows the counterflow dryer to be operated without the baghouse mudding often associated with counterflow drying.
Why the Duo Drum design?
There are a host of reasons why we have chosen to use two drums instead of one drum with different zones. The foremost reason is simplicity. Simplicity is the main reason drum mixers have captured 98% of the plant market over the last twenty years. Simplicity means less expensive to buy, move, set up and maintain. With independent drums all component parts are easily accessible. There are no drums within drums or burners within drums to access and maintain. Each unit becomes an easily transportable load without overweight restrictions.
Are there any operational benefits to having separate drums?
Both the drying process and the mixing process require time...And in drum mix plants time means length. Single drum units reduce drying time and mixing time in an effort to stay within the confines of mobility. The last stages of the batch plant era saw dryers grow from 20' to 30' and even 36' in length. This period of dryer elongation came about for two reasons (1) increased fuel efficiency, and (2) the realization that wet materials require time as well as heat to properly dry. Our MODEL 100 DUO DRUM "CF" features a 32' long drying drum and a 16' long mixing drum. These lengths allow sufficient drying time for the wettest of materials and allow time in the mixing drum to transfer heat from virgin to RAP prior to AC injection.
What can I do with separate drums that I can't do with a single drum?
What would you say if you were told that you must run your pugmill at the same speed you run your dryer? What would you say if you were told that regardless of what mix you were making you must keep the mix in the pugmill for exactly the same amount of time? Senseless right? That's why ALmix Duo Drums are equipped with variable speed drives on each drum. When lower production is needed the drying drum speed may be reduced to lengthen residence time and increase the material veil. When lower mix temperatures are needed the drying drum speed may be increased to shorten residence time and decrease material veil to increase stack temperature. On the mixing drum side, when high percentage RAP mixes are being produced, mixing drum speed may be decreased to allow additional time for heat transfer prior to AC injection.
What is the advantage of a low air flow mixing drum?
Only a very small negative air flow is maintained in the mixing drum to remove steam and prevent fugitive dust. This extremely low air flow allows the addition of dry type additives without the immediate evacuation of a high percentage of the additive to the bag collector. Single drum units cannot prevent this problem. The result is an additional load on the baghouse and large amounts of dust in transit.
How does the Duo Drum design effect production?
The one forgotten or at least untold fact about all drum plants which separate the drying and mixing process is the effect such separation has upon production rates. Whether you use one drum or two, when you isolate the mixing process from the hot gas stream the consequence is that you lose the dust trapping ability of the mixed asphalt. The drying section of such drum mixers becomes identical in process to a batch plant dryer. That means that drum gas velocities traditional to dryers (in the range of 850 fpm) as opposed to drum mixers (in the range of 1,000 fpm) should prevail. It has been demonstrated over many years that attempts to circumvent these ideal drum velocities result in premature wear, the need for primary dust collectors and the problem of immense amounts of return dust. It is for these reasons that we at ALmix insist upon conservatively rating our Duo Drums.