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.
A mathematical model of heat and mass transfer in rotary dryers is established.Moisture content of flexible filamentous particles and gas in dryers can be studied.Relation of temperature and humidity of tobacco and hot gas can be analyzed.
Rotary dryers are of significance to a wide range of industries to process flexible filamentous particles, such as chemicals, food and tobacco. However, heat and mass transfer of flexible filamentous particles in the rotary dryer have not been studied deeply. In this paper, a mathematical model of heat and mass transfer in a rotary dryer is established. The influence of moisture content and humidity on flexible filamentous particles and hot gas in the rotary dryer is studied, as well as the differences of temperature and humidity of particles and hot gas after entering the dryer. In a counter-current cascading rotary dryer, results show that dehydration of flexible filamentous particles in the lower end of the dryer is significantly higher than that of the upper end under the same drum wall temperature. Heat of flexible filamentous particles and gas flow are likely to increase. Most heat of gas flow come from the latent heat of vaporization in particles. Results on temperature and humidity of particles under different operating conditions can be obtained, and then suggested an optimum condition.
Heat and mass transfer of cut-tobacco particles occurred in the rotary dryer. Shown are the hot gas and particles in a counter-current cascading rotary dryer, and the hot gas at the top of dryer, while cut-tobacco particles in the bottom. Heat transfer from wall to particles and wall to hot gas when the temperature of wall was higher than the hot gas were researched in this paper. Mass transfer between hot gas and cut-tobacco particles in the rotary dryer was also studied.Download : Download high-res image (79KB)Download : Download full-size image
Heat and mass transfer between air and soybean seeds in a novel dryer was investigated.A two-phase model, with an appropriate set of constitutive equations, was used.The simulated results were in a good agreement with the experimental data with low deviations.The seeds quality was measured and related to the operating variables.
This study analyzed the simultaneous heat and mass transfer between soybean seeds and the air in a non-conventional rotary dryer developed by our research group, named as roto-aerated dryer. A two-phase model, with an appropriate set of constitutive equations, was used to describe the drying process. The system of equations was numerically solved using the normal collocation technique. The simulated results shown good agreement with the experimental data. The average difference between experimental and simulated results was of 1.5% for seed moisture content and of 6.3% for soybean temperature.
Hot air or gas flows through the cylinder and then transfers to the product, warming it and evaporating the water, therefore temperature of heating media inside the dryer is the mean parameter which control the process.
Lifting flights are of different shape which assures the wet product cascading from the top of the cylinder is exposed to the flow of hot gas. Flights shape is studied to optimize the thermal transfer from hot gases to wet product to reduce the size of the cylinder.
The interior of the cylinder is fitted with spiral flights at the feed end to quickly move the feed product into the active section, where longitudinal parallel lifting flights pick up the product and cascade it in thin even sheets, so that it will dry most advantageously.
Combustion chamber is refractory lined and combustion air cool outside shell of the chamber and at the end of the chamber conditioning air is added to hot gases to keep operating temperature requested.