rotary dryer equations

rotary dryer design & working principle

rotary dryer design & working principle

For evaporating moisture from concentrates or other products from plant operations, Rotary Dryers are designed and constructed for high efficiency and economy in fuel consumption.Whenever possible to apply heat direct to the material to be dried, Rotary Dryers of the Direct Heating Design are used. If it is not possible to apply heat direct to the material to be dried, Rotary Dryers of the Indirect Heating Design can be furnished so that the heated gases will not come in direct contact with the material.

Rotary Dryer is a simple, inexpensive unit for reducing the moisture content of flotation concentrates, as well as chemical and industrial products. Frequently the saving of shipping weight so effected will pay for the dryer in a few months. Difficulties from freezing while in transit are also eliminated. Many industrial projects are now using Dryers for control and production purposes on many materials.

Three main types of Rotary Dryers can be supplied. The direct heat unit is used when it is permissible for the drying gases to come in direct contact with the material being dried. Partition plates increase the heating surface. Drying may be by hot air or exhaust gases from other operations. If this drying gas has a deleterious effect on the product, then an indirect type of dryer can be supplied. A further derivation is the Tedrow Steam Dryer.

Of the different types of dryers that there are the most common is the ROTARY DRUM DRYER/Kiln, This type of drier is common not only in the mining industry but you will find them in fertilizer plants, Cement plants, and peat hogs to name but a few.

The theories behind these machines are very simple, heat an air space up, and then tumble the material to be dried through this space until it is dried. All though it sounds simple there are problems that have to be solved before the required results are met. But first, so you know what we are talking about lets go through the design of a drier.

First is the KILN, this provides the heat, The BURNER is inside this portion. The fuel for the burner is usually diesel although heavy crude oil could be used in some cases. To be able to generate enough heat to dry the concentrate air must be added by way of a BLOWER. In front of the kiln is the point that the wet concentrate enters the drier. It is put into the revolving SHELL. The shell is on a slight incline. As the Concentrate is tumbled through the hot air mass of the drier it travels down this incline to the exit of the drier.

At this exit point the concentrate is either deposited straight into a storage area or taken to the storage area by a conveyor. It is also at this point that there is an EXHAUST HOOD. This provides a controlled escape passage for the fumes and water vapor that is generated by the concentrate drying. This is a very important function and the operator will have to be sure that it is open at all times. If it should become blocked the water vapor will not be able to escape. The concentrate will become wet and sticky which will result in the discharge plugging. The wet sticky concentrate will also lower efficiency level of the drier for an extended period of time. This happens because inside the drier shell are what are termed FLIGHTS these are flat pieces of metal that are bolted onto the shell.

They are there to lift the concentrate up to the top of the shells rotation and drop the concentrate through the hot air. If the water vapor isnt taken away, the concentrate becomes sticky from reabsorbing the water. This sticky concentrate will fill the spaces between the flights.

The concentrate will not be lifted and dropped through the hot air. This results in a long term condition of poor performance even after the initial problem has been cured. These flights will remain buried in concentrate. This removal of the water vapor is one of the functions of the blower. It assists the natural process of air movement as the hot air mass expands. To prevent the buildup of concentrate on the flights there are often CHAINS attached to them. As the drier revolves the chains slap the flights preventing concentrate from building up on dryers walls.

The drier shell is rotated separately from the stationary kiln section. To achieve the rotation a BULL GEAR is attached around the shell section. There are also two flat rings attached to the shell. These provide surfaces for support rollers to roll on. There is another problem that the inclined shell has, the incline causes the shell to want to slide in the direction of the incline. To prevent this additional rollers are attached to the last set of rollers.

the design of flights in rotary dryers - sciencedirect

the design of flights in rotary dryers - sciencedirect

Rotary dryers for particulate solids commonly use flights along the length of the shell to lift solids and make them rain across the dryer section. These flights are critical to dryer performance. Previous models for the prediction of the solids holdup in rotary drums have assumed that the angle of the solids level in a flight is independent of the flight geometry. They have considered only the case of extended circular flights and angular flights with two segments.

New equations were derived to predict the solids holdup in flights with three segments. Their predictions were compared to measurements performed with a pilot plant rotary dryer. Theoretical equations provided estimates of the flight holdup that were accurate enough to predict the power required to lift the solids. On the other hand, the model is not accurate enough to predict the variations in the flux of solids over the drum cross-section. This shows that the angle of the solids level in a flight can be affected by the flight geometry.

rotary dryer design 101: retention time

rotary dryer design 101: retention time

The retention time required for a given application is determined through balancing the necessary process parameters particle size distribution, air flow velocity, temperature, and more with the desired outcome; itcan then be controlled through various design parameters.

In general, the longer the required retention time, the longer the length of the drum that will be needed. However, drum length and diameter have an interdependent relationship, so length can vary as diameter changes.

Similarly, the speed of the drum (RPM), combined with the slope of the drum, can also be used to control retention time. All dryers are set on a slight decline so gravity can assist in moving material through the unit. While the maximum recommended slope for a dryer generally falls between //ft to /ft, depending on the circumstances, within that range, slope can be adjusted. Note: drum slope is a set parameter that cannot be changed on the fly once the drum is installed.

Retention time can also be controlled through the addition of dam plates. The addition of a dam plate in the dryer causes material to build up behind it (much like a dam in a river), forcing a longer retention time.

FEECO is the leading supplier of custom rotary dryers. All FEECO dryers are engineered around the specific requirements of the material and process goals to produce a system that operates efficiently and reliably. For more information on rotary dryer design, contact us today!

rotary dryer design 101: defining your materials part 1 percent moisture

rotary dryer design 101: defining your materials part 1 percent moisture

Rotary dryers are used prolifically throughout industrial processing endeavors to dry bulk solids. These industrial drying systems are often custom designed around the unique requirements of the project at hand in order to achieve the best results.

All materials will behave differently in a rotary dryer depending on the materials unique characteristics heat transfer properties, bulk density, etc.. Achieving optimal performance of a rotary dryer is the result of finding the right balance between the characteristics of the material to be processed and how the dryer works with those characteristics to promote maximum efficiency and product quality. As such, the process of engineering a custom rotary dryer begins with analyzing the material.

The first variable in this equation is the percentage of moisture that will need to be removed. This is determined through analyzing the inlet raw material and defining the required outlet moisture content of the material.

This difference between actual and desired moisture content determines what the dryer will need to accomplish, which will, in part, influence some of the key design parameters of the system. In some cases, it may reveal the need for a preconditioning step to reduce the inlet moisture in order to avoid sticking. This might include a back-mixing step, a recycle loop, or a pre-drying step to bring down the moisture of the feed.

The percentage of moisture the dryer will need to remove can impact several aspects of dryer design. Its important to remember, however, that many other factors come into play for each aspect of the dryer; the interrelation of these factors will ultimately determine the final design.

Retention time, sometimes also referred to as residence time, is the amount of time that the material will need to reside in the dryer to reach the desired results. This influences the overall sizing of the dryer.

The amount of moisture a material carries can influence the air flow configuration of the dryer. This refers to the direction of the air flow in relation to the flow of material co-current (in the same direction) or counter current (in opposing directions).

Materials that carry a significant amount of moisture may require fast initial drying. In this case, the co-current air flow configuration would be chosen, because it puts the wettest material in contact with the hottest combustion gases.

Drum diameter and length are influenced by a variety of factors that play off of each other. In particular, the amount of air flow velocity required, in combination with the necessary retention time, will be substantial factors in determining overall drum diameter and length.

There are many factors that work into the engineering process of sizing and designing a rotary dryer for optimal performance. The amount of moisture removal required sets the stage for what the drum will need to accomplish, and ultimately, how it will need to be designed to meet those specifications.

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