heat transfer in indirect rotary dryer

direct heat or indirect heat for rotary dryers | bulk-blog

direct heat or indirect heat for rotary dryers | bulk-blog

One of the most important factors in determining thetype of rotary dryer that is most appropriate for a certain process is the usage of direct or indirect heat. Rotary dryers are capable of utilizing either direct convection style heat transfer or indirect heat transfer, in which hot surfaces within the dryer provide heat by conduction and radiation. The determination of direct or indirect drying is contingent on the properties of the material to be dried, the process conditions and the desired end product.

Rotary dryers are versatile machines, and can be configured to meet a wide range of needs and applications. In terms of direct drying, hot air enters the rotating cylinder at one end, passes through it and exits at the other end. Flights welded to the internal walls of the cylinder lift and shower the wet material, bringing it into direct contact with the hot air. Configuration of the flights depends on the characteristics of the material being processed. This configuration is customized to efficiently handle the product as it flows through the dryer. The length and diameter of the cylinder are also customized according to the capacity of the dryer and the characteristics of the product.

Typically, a rotary dryer is filled to 10-15 percent of cylinder volume. The length of the cylinder is usually at least four times the diameter in order to achieve efficient heat transfer and temperature equilibrium between the product and the air stream.

The airflow in direct heat rotary dryers can be either counter or parallel to the conveyance of the material inside the dryer. Parallel flow dryers expose the wet feed material to the hottest air temperature. They are very efficient for handling high moisture content sludge and filter cakes. Because of the rapid heat exchange, there is significant evaporative cooling of the product. This cooling effect allows for drying a wide range of materials with varying heat sensitivities without thermally degrading the materials.

Counter flow dryers expose the driest material to the highest air temperature. This configuration is usually best for processes that require a specific elevated discharge product temperature or very low moisture content.

Indirect heat rotary dryers are most frequently used for products with small particle sizes that could result in excessive dust generation. They are also used for processes that require extreme control of the dryer exhaust. Indirect heat dryers depend on conduction and radiation to transfer the heat from the cylinder walls to the wet feed. A stationary furnace surrounds the rotary cylinder and heats the cylinder walls. Little or no airflow is required to pass through the dryer. Indirect dryers can provide only a limited amount of heat transfer area. They cannot efficiently handle materials that tend to stick to the heated walls of the cylinder.

Rotary dryers designed by Heyl & Patterson arebuilt to eachcustomers requirements. Because everydryer is custom engineered and the original drawings are maintained in-house, any unit can be servicedwith upgrades or repairs according to its original specifications for the life of the equipment. Heyl & Patterson also provides superior customer support before, during and after the purchase oneach product supplied by the company.

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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.

indirect drying vs. direct drying

indirect drying vs. direct drying

When it comes to drying bulk solids, rotary dryers are often the industrial drying equipment of choice. Their robust build, reliability, and aptitude for handling variation in feedstock makes them an ideal choice for efficiently drying bulk solids of nearly all types.

When referring to rotary dryers, its usually safe to assume that a direct-fired rotary dryer is the topic of discussion. Direct dryers are used more frequently than their indirect counterparts, because of the efficiency they offer. And while direct fired rotary dryers are most often the selected drying method, indirect rotary dryers are a valuable alternative for specific processing needs.

Direct-fired dryers rely on direct contact between the material and process gas to efficiently dry materials. Efficiency is further increased by the addition of lifting flights affixed to the internal walls of a direct rotary dryer. These flights lift the material, carrying it over and cascading it through the stream of process gas, in order to maximize heat transfer between the material and drying medium.

Indirect dryers rely on the heat emanating from the drums shell to dry the material via conduction and radiation. Subsequently, indirect dryers would not benefit from the addition of lifting flights, since there is no process gas moving through the drum. Furthermore, because the furnace exhaust is kept separate from the process gas, indirect dryers have the added benefit of requiring less exit gas treatment.

Unlike their direct-fired counterparts, which are typically constructed of carbon steel, indirect-fired rotary dryers are constructed using a high temperature alloy, in order to maintain the integrity of the drum, despite the constant exposure to high temperatures that the shell must endure.

Direct fired rotary dryers account for the lions share of bulk solids drying applications, and are used throughout a multitude of industries, including fertilizer and agriculture, mining and minerals, specialty chemicals, and more. They offer heavy-duty processing and consistent reliability.

Indirect-fired rotary dryers are better suited for niche applications. Indirect-fired dryers are ideal for processing dusty, fine materials, or materials that cannot be exposed to oxygen or combustion gases during processing. This is commonly seen with food grade materials, highly combustible materials, organics such as grains and pulps, and fine materials such as pigments.

FEECO has been a leader in the field of thermal processing for over 60 years, providing both direct and indirect rotary dryers to meet the varying demands of a wealth of industries. For more information on direct or indirect rotary dryers, contact us today!

principles of heat transfer as applied to rotary dryers, rotary kilns, and rotary coolers

principles of heat transfer as applied to rotary dryers, rotary kilns, and rotary coolers

Whether you are using a rotary dryer, rotary kiln, or rotary cooler, there is one thing that all these instruments have in common: they all use methods of heat transfer to carry out their jobs. Heat transfer is how heat moves from one source to another. Understanding the types of heat transfer, and how they differ, is an important part in understanding how a rotary dryer, cooler, or kiln works. Knowing how heat transfer works can help in sizing a machine, and can also help to anticipate where heat will be lost in a system.

Even though we might not notice it, we think about heat transfer all the time From cooking, to global warming heat transfer is constantly at work around us. There are a couple basic rules to remember when thinking about heat transfer. First, heat ALWAYS transfers from a hotter object to a cooler one; the opposite of this is just not possible. Second, some materials can transfer heat better than others.

Materials are categorized into two basic groups, depending on their ability to transfer heat. Conductors are materials that transfer heat very well. Some of the best conductors are the non-ferous metals, such as copper and aluminum. Ferrous metals such as steel and stainless steel are also good conductors. The opposite of a conductor, is an insulator. Wood, rubber, ceramic, and brick, are all examples of insulators, or items that do not transfer heat well.

There are three types of heat transfer: conduction, convection, and radiation. Conduction is the transfer of heat between two materials through direct contact. Think of touching your hand to the stove this is a prime example of how conduction works. The heat of the burner, a physical object, is transferring heat to your hand, through means of touch. Other examples of conduction include curling irons, and cooking an egg on the sidewalk.

With convection, which is the primary mode of heat transfer for gases and liquids, heat transfers from a heat source, be it a liquid or gas, to a physical object. For example, a pizza cooks in the oven by means of convection: the heat from the oven is transferring from the heat source to the colder object, ie. the pizza. Convection works by the moving of the liquid or gas atoms in response to the heat. Once moving, the atoms can move around, in a sense, to surround an object, therefore heating it. Other examples of convection include hot air balloons, and boiling water.

The third type of heat is radiation. Radiation is the transfer of heat through a direct path.This is different from conduction and convection, because it does not require physical contact (conduction), and because it is a direct path, it cannot surround an object (convection). Radiation works similar to light. An easy way to understand radiation is by thinking of getting a tan. If you are sitting on a beach, the sun, in direct contact youre your skin, will tan your skin. But if you put an umbrella over yourself, you are blocking the sun rays. The sun rays dont go around the umbrella, like convection, they are just simply blocked. Other examples of radiation include the sun heating the planets, and a fireplace heating a house. However, because heat rises in a fireplace, most of the heat from conduction is lost through the chimney, leaving the primary mode of heat transfer to your house as radiation.

All of these principles apply to the operation of rotary dryers, rotary coolers, and rotary kilns. Depending on the type of system in use, heat transfers to or from a material differently. In the case of a rotary dryer, the material is being heated directly by the gases, ie. convection. However, the material is also getting heat from the shell of the rotary dryer through means of conduction. In an indirect fired rotary kiln, conduction is the only source of heat transfer. The refractory is heated and transmits the heat from the refractory to the material by direct contact. Radiation is also present in all of these scenarios, because heat is being transferred through the atmosphere.

Heat transfer is an important component to consider in the operation of rotary dryers, rotary kilns, or rotary coolers. Knowing how heat transfer works can help in sizing a rotary drum, and being able to anticipate how heat will be lost throughout the system.

how do indirect heat rotary dryers work?

how do indirect heat rotary dryers work?

Because the heat source is separated from the processed material, air velocities within the rotating drum are kept to a minimum or are non-existent. Heat transfer is accomplished through conduction of the heat from the shell to the product and radiation of heat off the shell. The internal flighting inside the rotating drum is specially designed to gently roll the material being processed, constantly exposing new material to the heated shell to maximize the heat transfer process. Flights are designed to avoiding showering the product as that actually decreases heat transfer due to the minimum airflow inside the shell.

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