You have a full To Do list for the day and show up at the plant ready to rock it. You dont make it 10 feet inside the gate before being told there was another fire in your rotary dryer last night. Luckily the literal fires have been put out, but now you get to deal with the metaphorical ones. So much for the To Do list. The Board wont be very happy to hear you had to put off your reports again. Since the plant is still down, you decide to open the dryer up for an internal inspection. You cant remember when the last time that happened probably the last time it caught fire. You try to stay out of the dryer operations altogether to work on more important things. Today you are going in. This is getting ridiculous. Your capacity has been down for several months and youve getting uneven final product. Your customers are starting to complain, and nowyou had a fire. Whats going on?
Your guys open the dryer up for you. You crawl in very carefully, wow that hatch is small and take a look around. You notice some bent flights and wonder if that is new or theyve been that way and the last person to inspect just ignored them. Good thing you started going back to the gym cause its pretty tight in here. A glint catches your eye, but whatever it was disappears when you shine light on it. On a whim, you turn your flashlight off. Something is wrong. You shouldnt be able to see anything. But you can. There are a half dozen pin pricks of light that you can see. But with all the flighting, its likely there are more and they are just being blocked. Turning the flashlight back on, you carefully make your way back in the drum, trying not to rip your new shirt on the flights. Then the board and the wife would be upset with you.
You are able to identify one of the pin pricks through the shell and get up close to inspect. That is a definite wear spot. Theres a groove that looks like its going around the whole drum at this point. And it looks like there are more places about to break through. Youll have to get a welder in here to patch those before starting back up. The last thing you need is the drum breaking in half and falling on the floor. Looks like there will be some Googling in your near future.
Rocks and/or sand cause problems in biomass rotary dryer systems and any downstream equipment. Some of the common problems are wear through the shell, bent or broken drum flighting, damaged conveyors and excessive wear on crushing equipment.
What do you do about these problems? You can weld the shell back together. You can repair or replace flighting or conveyors. You can replace stuff on the crushing equipment. Replace replace replace. But what if you can significantly reduce the amount of rocks that go through the dryer in the first place? How wonderful would that be?
Why dont you want daylight showing through the shell? Among other reasons, you want as few air leaks as possible into your dryer. Chances are your dryer is a direct-fired system. This means that the nothing separates the burner and flame from the gases circulating in the dryer. Most wood dryers use solid fuel burners, while some will use natural gas or propane heaters. Direct solid-fuel burner dryers will typically have hot embers going through the system at all times, just waiting to ignite something. Dry product is kindling. What does a fire need? 3 things: fuel, ignition (or heat), and oxygen. Direct fired dryers have fuel and ignition per definition. Oxygen must be limited. The burner uses oxygen, so oxygen content is already lower than atmosphere. By employing a recycle loop, oxygen levels can be driven down even lower. However, if there are breaches in the dryer such as poor drum seals, holes in the drum, holes in the ductwork, poor airlocks, etc. air can be sucked into the system. If these breaches are large enough or if there are enough small ones to admit enough oxygen, you can get a fire or explosion in your dryer system.
It is generally accepted that rotary drum dryers are the best option for high capacity drying of bulk solids with uniform shapes and sizes as well as with a range of shapes and sizes. Flighting should be designed for a specific product, product sizes and moisture variations, gas velocity, and capacity. If your rotary drum dryer is struggling to achieve uniform moisture content, and is still being operated inside design criteria, it may be time to consider a new flighting design. However, if your dryer used to process the same material at your current rates just fine, you may have flight damage either from fire, rocks, or a slug of heavy material. Bent and damaged flighting causes changes like overloaded flights, underloaded flights, wrong product drop points, etc. Any of these will cause the material to no longer shower evenly, disallowing the product the correct amount of contact with drying gases, leaving you with a very inconsistent end product.
How can you get product to go one way and rocks to go the other? Using basic physics, its easy to understand that drying biomass will be carried by the gas stream in the dryer drum from inlet to outlet in a concurrent system. But rocks are heavy enough to fall through the gas stream. Unless your flights are designed to push material toward the exit, rocks can easily get stuck in an endless loop wreaking havoc on the interior of your drum. Wherever there is a space big enough for a rock, pebble, or grains of sand to go around in circles at the inlet, between rounds of flights, at drum seams for inboard track drums, at the outlet the rocks will start cutting a groove in the metal. Most biomass dryers are carbon steel, which is a relatively soft metal, so the rocks can cut a path like water in a canyon. Eventually youll be able to see daylight through the shell during inspections.
Rock Return System: Instead of the rocks getting stuck in a rut literally rotary drum technology has been developed to direct rocks back toward the inlet of the dryer. Many dryer companies have developed special designs to encourage forward movement of rocks and other heavy objects. Using this technology throughout the whole drum will ensure the most rocks traveling toward the front of the dryer, preventing rocks from continuing to circle inside the drum or exit through the back and wreak havoc downstream. The rocks will travel toward the inlet end of the drum and be bounced into the gas inlet. These rocks can be removed during routine maintenance.
Rock removal conveyor: It is likely you will not know whether you need a rock removal screw or not until after you have installed and monitored the effectiveness of your rock return system. In many drums with this system, rocks will pile up at the inlet of the dryer. If scheduled routine maintenance is not keeping up with the rocks, it is time for a rock removal screw. The rock removal screw will be at the inlet of the drum, placed to receive rocks being deflected back toward the inlet. It will need a heavy-duty airlock to minimize the chance of air leaking into the system.
Stop bars and wear plates: If you are not concerned about removing rocks, but would like to stop them from wearing a groove, you can add stop bars at places where rocks tend to get stuck and travel around in circles indefinitely. A stop bar is placed parallel to the drum axis and perpendicular to the travel of circling rocks to interrupt the path of the rock and bounce it back into the gas stream where they will be able to continue through the drum. You can also add wear plates to spots susceptible to wear by rocks. Wear plates can be welded or bolted into your drum.
Seals: The biggest source of transient air in a rotary dryer system is typically the seals. We find that high-temperature rubber belting works best. The surfaces that the seal is in direct contact with also plays an important role in the effectiveness of the seals. The rounder and smoother these surfaces, the better seal you can achieve. Good seals enhance personnel safety and dryer efficiency and should be maintained accordingly. (Long. more about seals in 2016 PH article) Long, B. (2016, April). Diagnosing Rotary Drum Malfunctions and Maintenance Tips to Avoid Them. Process Heating.
Rock Return System: Adding a rock return system can potentially extend drying time of larger pieces of biomass. Depending on the design of the rock return system, the heavier pieces can bounce back toward the inlet allowing them more time to dry and giving you a more uniform final product.
If flights are overloaded, the material will shower prematurely. This causes most of the material to fall on one side of the drum, which can cause drying gases to flow through the drum unevenly. When the gas flow is no longer even, the drying rate is no longer even, which may cause some of the particles to overdry while others underdry.
Dryers are more complex than most people would think. Since almost everyone has a clothes dryer at home, we tend to think of all dryers as simple, mundane objects. However, an industrial size rotary dryer has many design considerations that your clothes dryer will not. At home, if you hear something banging around in your dryer, you go remove it. Problem solved. If your clothes arent dry enough, you put them back in.
Whether you seem to have problems that originate from rocks, air leakage, or flighting design, theres always room for improvement. Inspections are the #1 defense against unplanned shutdowns. You must know what is going on inside and outside your dryer system. Is there a significant air leak somewhere? Is it through the drum seals? Shell breaches? Ductwork, etc.? Is there a problem with the flighting? Do you have missing or broken flights that need to be replaced or repaired? Does your flighting look fine, but you still arent getting desired capacity and product quality? It may be time for a redesign. Are rocks carving up your dryer? Adding a rock return system, stop bars and/or wearplates can go a long way. Do you need to add a rock removal screw? Its like trying to decide whether you want the leather, sunroof, and heated seats or not.
Fast forward several months. Once again you have a long To Do list for the day. You walk into the plant and instead of being met with emergency, you are met with a report that your dryer production is up, the final product has uniform moisture content, and there have been no unplanned shutdowns for weeks. Now you can get to that To Do list and keep your board happy. And you dont have to ruin that new shirt you just got for Fathers Day
Rotary dryers are mainly used in the chemical and mineral industry. In the area of food, their most common applications are for dehydrating waste materials (citrus peels, vegetable trimmings) and animal feedstuffs (alfalfa). Rotary dryers consist of a metal cylinder with internal flights or louvers (Fig. 22.21). The cylinder is slightly inclined. The material is fed at the high end and discharged at the low end. Hot air is blown in cocurrent or countercurrent direction. As the cylinder rotates, the material climbs in the direction of rotation. When it reaches a position where its angle of repose has been exceeded, the material falls back to the bottom of the cylinder (Fig. 22.21). Most of the drying takes place while the material falls through the air blast. Using very hot air or combustion gases, rotary dryers can also function as roasters for nuts, sesame seeds, and cocoa beans. A detailed method for the design of rotary dryers, based on a heat exchange approach has been described by Nonhebel (1971).
Rotary dryers are often used for particulate material. Particles and hot air are continually fed to the drum. These large rotating drums have lifting flights which carry the particles upward as the drum rotates. The particles leave the lifting flight near the top of the drum and fall through the air stream. Heat is transferred to the particles both from the air and from contact with the dryer. The drums may have concentric sections so that the particles and air traverse the length of the drum up to three times. Residence time is on the order of minutes. Friable material, such as wafers or flakes, may be dried on trays or belts instead of in drums. Very fine material, such as fiber board furnish, might be dried in a tube dryer in which the air carries the fiber through the tube in seconds.
For particulate solids, a rotary dryer may help promote uniform and more rapid drying (Fig. 14.14). In the rotary cascade dryer, the material is placed in a rotating cylinder through which a hot air stream is passed. Flights on the cylinder wall lift and cascade the product through the air. In a variant, louvers are used instead of flights so that the product is mixed and rolled instead of dropped. The dryer is typically sloped, so that the product enters and gradually falls toward the discharge end. In direct rotary dryers, the air is passed through burners, and directly comingles with the product. Rotary dryers have been used to dry seeds, corn gluten, distillers grains, and some fruit.
A rice combine harvester usually performs with less loss of paddy; however, the potential shortcoming is that the paddy must be harvested at high moisture content, that is, ranging from 20% to 28%. The high moisture content of harvested paddy is conducive to rapid deterioration in quality such as discoloration, yellowing, germinating, and damage to milling quality.
The only practical means of preventing grain quality deterioration is immediate drying of high moisture paddy, because sun drying, the conventional method, is inadequate to guarantee the quality and quantity of the produce. Thus there is a high demand for mechanical drying facilities.
Most mechanical dryers available are suitable for rice millers and farm cooperatives that handle thousands of tons of paddy. Small-scale dryers were developed for farm use, such as a fixed bed dryer and solar rice dryer (Exell and Kornsakoo, 1977); however, those were not widely accepted because of the potential inconvenience in loading/unloading of paddy and unequal drying.
Jindal and Obaldo (1986) and Puechkamutr (1988) worked on accelerated drying of high moisture paddy using conduction heating for a rotary dryer. Their studies demonstrated the potential of high temperature for quick drying of paddy without significant damage to the grain. This technique is promising from an energy consumption point of view.
Puechkamutr (1985) developed a rotary dryer for paddy based on conduction and natural convection heating. Paddy was effectively dried from moisture content of 23% to 16% (w.b.) using a pipe heat exchanger at surface temperatures of 170C200C with a residence time of 3070s. Rapid drying and good milling quality of the paddy could be achieved with such a dryer.
A combination conductionconvection heating type rotary dryer was developed for on-farm drying as a first stage. It consisted of double cylinders: the external cylinder with 500mm diameter, attached to an inside surface with straight flight; and an inner cylinder, hexagonal in shape with an outer tray and firing device installed inside as a part of the inlet cylinder. The grain cascaded inside the external cylinder with a concurrent flow of air. Experimental results showed that about 3% of moisture content could be removed with single pass with a small reduction in milling quality (Likitrattanaporn, 1996).
Another study of a combined conductionconvection type rotary drum dryer was made by Regalado and Madamba (1997) on thermal efficiency. The fresh ambient air forced inside the drum in a counter flow direction of grain brought evaporative cooling of the hot grain as shown by the increase in moisture reduction whenever air velocity was increased.
A further improved prototype of a combined conductionconvection type rotary drum dryer used ambient air that was forced inside the drum in counter flow to the direction of the cascading grains. The grain was heated by conduction heating as drying proceeded and followed by convection heating as cooling occurred of the heated grain. The results showed that its partial drying capacity was approximately double that of the predryer developed by the International Rice Research Institute requiring only a single pass operation. Neither drum surface temperature nor ambient air velocity and their interaction influenced total milling recovery and head rice recovery.
Likitrattanaporn et al. (2003) designed and developed a combined conduction and convection heating rotary dryer for 0.5t/h capacity using liquefied petroleum gas (LPG) as the heat source, to dry high moisture paddy under farm conditions. The main aim was to find an affordable way of drying field paddy on the day of harvesting to facilitate handling and for higher returns of produce for the farmer. Emphasis was placed on operating conditions in which up to 3% moisture could be removed in a short time while grain quality should be closed to fresh paddy. Performance of the rotary dryer in terms of moisture removal, residence time, energy consumption, and milling quality were evaluated.
An experimental rotary dryer designed with concurrent flow system comprising two primary parts, a double cylinder and a discharge cover, is shown in Fig. 12.1. Forward movement of paddy takes place by inclination angle and rotary motion of the cylinder, while air is blown through the cylinder by the suction fan located on top of the discharge cover. A 1-hp motor with 1:60 reduction gear was used for driving the rotary dryer. The LPG lamp on the entry end heats up the air and heated air moves to other end by suction fan. During forward motion, paddy first contacts the outer surface of the inner cylinder where conduction heating takes place followed by a cascading action along the inside of the external cylinder resulting in convection heating. After this the paddy falls into the discharge cover and out of the dryer, while the suction fan sucks the moist air.
Relatively less moisture was removed during the last (third) pass at temperatures of 100C and 110C, that is, 1.5% and 1.7%, respectively. At 120C temperature, moisture content of 2.1% could be removed. Clearly, this is because there was less free water available at the third pass of drying.
The conduction and convection zones are shown in Fig. 12.2, along with the inlet and outlet temperatures of grain and the hot air. It can be seen that high temperature in the conduction zone can remove a higher amount of water than in the convection zone, which is, in turn, sucked out by hot moist air. It can also be observed that outlet grain temperature was dropped to the safe range (max. 52C) within a very short time (23min).
To demonstrate the dryers heat exchange efficiency, comparison of the effects of conduction heating and convection heating on moisture removal showed that the major moisture content of paddy was removed by the conduction heating for all temperatures, whereas the convection heating could remove moisture less than 0.4%.
Being designed as a mobile unit for drying paddy in the field, energy consumption is one of the most important aspects of consideration. The difference in weight before and after running a pass was recorded. A statistically insignificant difference was found in weight of LPG consumed at all temperatures. The average power consumption was, however, 0.6kWh and power of 0.46kg/h LPG. It was estimated that the operating cost of removing up to 1% of the moisture content of 1t of paddy was $0.23 in the first pass. The cost would increase up to $0.33 in the second pass and subsequently increase in the third pass depending on the availability of free moisture.
Likitrattanaporn et al. (2003) designed and developed a combined conduction and convection heating rotary dryer for 0.5ton hr1 capacity using liquefied petroleum gas (LPG) as the heat source, in order to dry high moisture paddy under farm conditions. The main aim was to find an affordable way of drying field paddy on the day of harvesting to facilitate handling and for higher returns of produce for the farmer. Emphasis was placed on operating conditions in which up to 3% moisture could be removed in a short time while grain quality should be closed to fresh paddy. Performance of the rotary dryer in terms of moisture removal, residence time, energy consumption, and milling quality were evaluated.
An experimental rotary dryer designed with concurrent flow system comprising two primary parts; a double cylinder and a discharge cover is shown in Figure 10.1. Forward movement of paddy takes place by inclination angle and rotary motion of the cylinder, while air is blown through the cylinder by the suction fan located on top of the discharge cover. A one horse power motor with 1:60 reduction gear was used for driving the rotary dryer. The LPG lamp on the entry end heats up the air and heated air moves to other end by suction fan. During forward motion, paddy first contacts the outer surface of the inner cylinder where conduction heating takes place followed by a cascading action along the inside of the external cylinder resulting in convection heating. After this the paddy falls into the discharge cover and out of the dryer, while the suction fan sucks the moist air.
Relatively less moisture was removed during the last (third) pass at temperatures of 100C and 110C, i.e. 1.5% and 1.7%, respectively. At 120C temperature, moisture content of 2.1% could be removed. Clearly, this is because there was less free water available at the third pass of drying.
The conduction and convection zones are shown in Figure 10.2, along with the inlet and outlet temperatures of grain and the hot air. It can be seen that high temperature in the conduction zone can remove a higher amount of water than in the convection zone which is, in turn, sucked out by hot moist air. It can also be observed that outlet grain temperature was dropped to the safe range (max. 52C) within a very short time (23min).
To demonstrate the dryers heat exchange efficiency, comparison of the effects of conduction heating and convection heating on moisture removal showed that the major moisture content of paddy was removed by the conduction heating for all temperatures, whereas the convection heating could remove moisture less than 0.4%.
Being designed as a mobile unit for drying paddy in the field, energy consumption is one of the most important aspects of consideration. The difference in weight before and after running a pass was recorded. A statistically insignificant difference was found in weight of LPG consumed at all temperatures. The average power consumption was, however, 0.6KWh and power of 0.46kg/hr LPG. It was estimated that the operating cost of removing up to 1% of the moisture content of 1 tonne of paddy was 0.23$ in the first pass. The cost would increase up to 0.33$ in the second pass, and subsequently increase in the third pass depending on the availability of free moisture.
Dried citrus peel is one of the most common feeds. It is manufactured by pressing peel through a rotary dryer and adding citrus molasses to help the drying process and help prevent the peel from burning. The moisture content of dried peel must be below 10%. Many experiments published in the 1970s have shown that dried orange pulp, partially or completely replacing cereals in concentrate mixtures, are particularly useful in reducing feeding costs in dairy cows, have no influence on production, and have a good palatability. Dried pulp has also been used in swine, which have been shown to utilize it at a ratio of up to 2025%. Besides its use as a substitute for maize, up to 20% in diet has no influence on the growth and production of laying hens. The dried pulp can be pelletized and is consumed more easily by ruminants with advantages of storage, shipping, and microbial spoilage. Pellets made from dried pulp have different dimensions, and several factors affect their characteristics, such as the energy used in pelletizing and the proportions of citrus molasses (about 515% of the total weight gives excellent results) used as binding agents.
Thermal desorption is a technology of physical separation based on heating the contaminated soil to volatilize water and organic contaminants. Soils are heated in a thermal desorption system, the rotary dryer being the most commonly used equipment. Thesystems require the treatment of the off-gas to remove particlesand contaminants. Its effectiveness depends on the contaminant. Decontaminated soil usually returns to the original site. Based on the operating temperature, these processes can be categorized into two groups: high-temperature thermal desorption ranging from 320 to 560C and low-temperature thermal desorption ranging from 90 to 320C. Thermal desorption can be used in a place where some other cleanup methods cannot be used, such as at sites that have a high soil contamination, and can be a soil remediation method that is faster than others.
Thermal methods may also be applied as an in situ technique. In this case, heat is applied to soil to volatilize semivolatile organic compounds (SVOCs), which can be extracted via collection wells and treated. It is a particular case of SVE. Heat can be introduced into the subsurface by electrical resistance heating, radio frequency heating, or injection of hot air or steam. Thermal methods can be particularly useful for dense nonaqueous phase liquids (DNAPLs) or light nonaqueous phase liquids (LNAPLs).
Weve built a reputation on building the best rotary dryers in the industry. All of our dryers are custom designed to suit the unique processing needs of your material. Whether you require low or high inlet temperatures, short or long residence times, counter current or co-current flow, FEECOs design team can design a rotary drum dryer for your application.
Rotary dryers are a highly efficient industrial drying option for bulk solids. They are often chosen for their robust processing capabilities and their ability to produce uniform results despite variance in feedstock.
The drum is positioned at a slight horizontal slope to allow gravity to assist in moving material through the drum. As the drum rotates, lifting flights pick up the material and drop it through the air stream in order to maximize heat transfer efficiency. When working with agglomerates, the tumbling action imparted by the dryer offers the added benefit of further rounding and polishing the granules.
All FEECO equipment and process systems can be outfitted with the latest in automation controls from Rockwell Automation. The unique combination of proprietary Rockwell Automation controls and software, combined with our extensive experience in process design and enhancements with hundreds of materials provides an unparalleled experience for customers seeking innovative process solutions and equipment.
Rotary dryers are known as the workhorse of industrial dryers. They are able to process a wide variety of materials, and can lend a hand in nearly any industry requiring industrial drying solutions. Some of the most common industries and materials in which rotary dryers are employed include:
Unlike direct dryers, indirect dryers do not rely on direct contact between the material and process gas to dry the material. Instead, the rotating drum is enclosed in a furnace, which is externally heated. Contact with the heated drum shell is what dries the material.
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Our rotary dryers are built to the highest quality standards, with longevity in mind. The best part about buying a FEECO rotary dryer, is that you get the security of knowing your equipment is backed by over 60 years of experience, material and process knowledge, and a proven track record.
Metso has installations all over the world that serve almost any industry's cooling and drying needs. What sets Metso apart from the competition is the specially designed lifters that allow the material to be showered into the hot or cold gas stream. A low velocity is required, which means a low particulate carryover occurs, which improves the product yield.
This system can be installed as standalone units or Complete systems deliveries including PLC, Dust Collection, Product Conveying (Feed and Discharge) Exhaust Gas Handling, Heat Exchanging, and Fuel Delivery Systems
The Rotary Drying and Cooling equipment utilizes a hot gas stream or cold gas stream that mixes with the product. The product is mixed with the air stream by utilizing lifters that shower the product in the air stream. The mechanical lifting aides in breaking clumps, which aides in the drying process. The equipment is set on an incline and conveys material down the cylindrical shaft.
Rotary Drying and Cooling is used in a wide variety of applications and industries where there is a need for a large throughput of material to be processed. The Rotary equipment is suited for different sized particles, large and small, and different moistures as well. Product temperatures can be easily controlled in the equipment, and the process know how of Metso allows for a consistent process product.
The rotary dryer also known as tumbling dryer is an equipment employed to minimize the moisture content of feed materials by bringing it in direct contact with a heated gas. It consists of an inclined long drum or cylindrical shell often fitted with internal flights or lifters; rotated slowly upon bearings through which the material to be dried flow with a tumbling/cascading action in concurrent (for heat-sensitive materials) or counter-current flow with the heating air or gases.
The movement of the material is due to the combined effect of inclination of the shell to the horizontal and the internal tumbling action or mechanical turn over thus the name tumbling dryer. The nature of the feed determines the directions of gas flow through the cylinder and it is relative to the solid. This drying equipment can also perform batch or continuous processing of the wet feed.
A rotary dryer is said to be of the direct type if, by virtue of its design, heat is added to or removed from the solids by direct exchange between the gas and solids. The direct heat dryers are the simplest and the most economical class. They are used when direct contact with the hot gas or air is not detrimental to the fed.
When high temperature is required for the drying process in a direct-heated rotary dryer, a combustion chamber is used and when low temperature is required on the other hand, for thermolabile materials, steam coil is used.
Although there is an infinite variation of rotary dryers, which present characteristics suitable for drying, chemical reactions, mixing, solvent recovery, thermal decompositions, sintering and agglomeration of solids, the main types of rotary dryers include;
1. Excessive entrainment losses in the exist gas stream is possible especially if the material contains extremely fine particles due to the large gas volumes and high gas velocities that are usually required.Get in Touch with Mechanic