ROTARY DRYER DESIGN 101: An In tro duc ti on to th e Con sid era ti ons in Des ign in g A Rot ary Dry er
a product of
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INTRODUCTION
The following is a collection of blog posts, originally designed as a series for the FEECO International blog. We have received such a positive response from the release of this series, that we decided to make it available as an entire article.
FEECO International has been a leading supplier of rotary dryers for over 60 years. Our rotary dryers are custom-engineered to t the specic needs of the material to be processed. Our rotary dryers are robust, and built for longevity.
What follows is a general overview of things to consider before entering the process of designing a rotary dryer. This is by no means meant to be a practical guide, but rather an introduction to the intricacies involved in the design of a rotary dryer.
For more information on rotary dryers, visit our website: http://feeco.com or contact us at
[email protected].
3913 Algoma Rd. Green Bay, WI 54311 • Phone: (920)468.1000 • Fax: (920)469.5110 • Email:
[email protected]
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DEFINING YOUR MATERIALS:
PERCENT MOISTURE So you are in need of a rotary dryer . Where do you go from there? There are a lot of factors that come into play when determining the needs of a rotary dryer, and determining your needs can easily become overwhelming. Take it a step at a time: we’ve developed this series to help you understand what goes into sizing a rotary dryer, and what might best suit your needs. At FEECO, we custom design our rotary dryers based around the material and its ideal feed rate through a rotary dryer. The rst and most important aspect in sizing a rotary dryer is dening the needs and limitations of the material to be worked with. The rst variable in this equation is the percentage of moisture the material holds in its “raw” state, or the state at which it will go into the dryer. Similarly, it is necessary to know the percentage of moisture desired in the end product, also called the “outlet moisture.” This difference in actual vs. desired percent moisture in part determines the design of the dryer. Let’s use laundry as an example. If you have a load of jeans that you just pulled out of the washer, and they are soaking wet, their percentage of moisture is going to be quite high, therefore requiring a long period of time in the dryer. However, if you forgot about
them on your laundry room oor for a day and they’ve had a chance to air dry, their percentage of moisture is going to be much lower, so you know you don’t need to put them in the dryer for as long, and the dryer does not need to work as hard to dry them.
DEFINING YOUR MATERIALS :
BULK DENSITY Bulk density is another characteristic that is important to know when sizing a rotary dryer. Bulk density is the weight of a material per a specic volume. Typically, this is calculated in pounds per cubic foot in US units or ks/m3 in metric units. Let’s say you have something that is 50 lb./ft3. This means that if you have a 1’x1’x1’ box full of said material, it will weigh 50 lbs. Let’s look at an example. Say we’ve got 1,000 lbs. of brick, and 1,000 lbs. of feathers. Which weighs more? Trick question: they both weigh the same. BUT, the bulk density of the brick is much higher. Try to t as many feathers
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as you can in a cubic foot. Now try the same with bricks. It will take a lot less bricks to ll the cubic foot. So if we go back to our 1,000 lbs., 1,000 lbs. of brick may only be 500 bricks, but 1,000 lbs. of feathers might be the size of a building. In the case of a rotary dryer, if we have to run brick through the rotary dryer, it is going to take a lot more energy per cubic foot to rotate the dryer vs. running feathers through the rotary dryer. For this reason, the rotary dryer will need much more heavy-duty bearings, gear train, trunnion wheels and motor.
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as it does for copper. Specic heat relates to how well the atoms of a substance can transfer energy among themselves. So those that can transfer very well, have a low specic heat,
DEFINING YOUR MATERIALS:
SPECIFIC HEAT Another key factor in sizing a rotary dryer is determining the specic heat of a material. Specic heat is dened as how much energy it takes to raise 1 gram of material 1 degree Celsius. In more simple terms, it’s how resistant a material is to heating. Materials can have very different values of specic heat, meaning they take more or less heat to cause a change in the temperature of the material. Water has a very high specic heat, which means that you have to add a lot of energy to raise the temperature. Metals, however, have a low specic heat, which means it takes less energy to raise their temperature. In a more specic example, let’s look at copper and water. The specic heat of water is 10 times that of copper. That means it takes 10 times the energy to raise the temperature of water,
and those that don’t transfer well have a high specic heat, because it takes more energy to transfer the heat from one atom to another. Of course, the type of fuel used to heat the material running through a rotary dryer also plays into our sizing equation.
DEFINING YOUR MATERIALS :
HEAT TRANSFER The heat transfer properties of a material are also important to be aware of before sizing up your rotary dryer. It is possible for two materials to have the same percentage of moisture, but very different heat transfer properties, which will call for different drying needs. Let’s look at an example. Let’s pretend that glass has the same percentage of moisture as clay. They should have the same drying time then, right?
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If only it were that easy. Glass holds its moisture on the outside, whereas clay holds its moisture on the inside. So in comparison, the glass will dry much faster because the moisture is all on the surface of the material. In addition to this, some materials may dry fairly quickly, until their moisture drops below a certain point, making the last remaining moisture much harder to draw out. This has to do with “driving force,” which states that the closer the remaining moisture gets to zero, the harder it is to draw out. Driving force diminishes as the percentage of moisture in a material reaches zero, therefore requiring additional time to fully dry. For example, sand dries at a fairly normal pace, but getting the last remaining moisture out of it is much more difcult than getting the initial moisture out. In this situation, a counter-current rotary dryer would be best. A counter current rotary dryer means just that:
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the stream of gas ows in opposition to the material. So the hottest area in the stream of gas is at the very end of the rotary drum, opposite the feed chute. So in the case of the sand, the sand would dry normally as it moves through the rotary dryer, but when it gets to the end of the dryer, an extra boost of heat (the beginning of the gas stream ow) will help draw out that hard to reach moisture. The opposite of a counter-current rotary dryer is a co-current rotary dryer, in which the material and gas stream ow in the same direction. It all depends on where your material needs the heat to be most in the cycle of drying…the beginning or the end.
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DEFINING YOUR MATERIALS:
MATERIAL LIMITATIONS After you have determined the characteristics of your material, it is necessary to look at the limitations set by your material, as these too will play a part in the design of the rotary dryer. For example, some materials may not be able to come into contact with oxygen. In this case, an indirect rotary dryer would be needed. A direct rotary dryer applies the heat directly to the material, via a stream of hot gas. An indirect dryer, however, transfers the heat to the material through the shell of the rotary drum. So the hot air/gases do not actually come into direct contact with the material, except for the rotary dryer itself. An indirect dryer would also be needed when a material requires absolute sterility. For example, some household items need to be safe for being around children, etc., and so it would not be acceptable to use any combustion gases to sterilize them. This would also be the case for materials that simply require sterilization through heat. Sterility can also dictate what type of material the shell of the rotary dryer will be made of. Some materials for example, cannot be allowed to cake up together. In this case, the inside of the rotary dryer would need to be completely polished smooth, with absolutely no imperfections on which the material could catch and pile up. Another limitation that can be encountered is the consistency of a material. Believe it or not, a wet, sludge-like material could lead to a re in the rotary dryer. This occurs because
the rotary dryer surface becomes like a hot frying pan. When the sludge-like, wet material hits it, it sticks. The material then continues to stick, and dry, until it becomes bone dry and potentially starts a re. In a situation like this, there may be a need to reduce the moisture to avoid sticking. This can be accomplished by diluting the moisture via the addition of dried product prior to the dryer. Some materials may also be quite fragile. Depending on how fragile the material is, we may decide to not weld any ights for the rst so many feet of the rotary dryer. This allows the material to have a chance to dry and become more durable before it is gradually introduced to the ights. The durability of the material, in combination with particle size and weight, also helps determine the maximum air velocity, or how quickly air ows through the rotary dryer. This will dene the size of fan needed for the rotary dryer. DEFINING YOUR ATMOSPHERE :
What does atmosphere have to do with rotary dryer design, you ask? Well actually, a lot. The needs of a rotary dryer can differ greatly from region to region, due to different climates. Things like humidity, temperature, and elevation can play a big part in determining the specics of your rotary dryer. Let’s take humidity for example. Imagine hanging your
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wet clothes out on the line out in the desert. Desert air, aside from being very hot, is also very dry, meaning it has the potential to hold a lot of moisture. Moisture is quickly absorbed into the air, drying everything in record time. But here in Wisconsin, hang your wet clothes on the line on one of our humid summer days, and you will be waiting a while. When the air is humid, some or much of its water holding capability is already used up, meaning it is already reaching its water holding capacity and cannot absorb much more. Here again, driving force comes into play, because the drier the air, the larger the driving force. In a
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humid atmosphere, it will require many more BTU’s to dry the material, as opposed to a dry atmosphere, where it will require signicantly less energy to dry the material. Think back to our discussion on specic heat; water is more resistant to temperature change than many other materials. So more energy is required to heat the moisture-laden air than air holding less moisture. Ambient air temperature is also an important factor to look at. When it comes to air temperature, it makes it easy to think of your dryer like a furnace. In a cold climate, you furnace is going to require more energy to heat your house. But in a warm climate,
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your furnace does not have to work very hard to heat your house. It may not even need to use much energy at all, depending on your specic situation. So if you are living in hot and Sunny Florida, your rotary dryer may not need as powerful a motor and fan as someone who’s living in Antarctica. It wouldn’t make sense to pay the price to run a super heavyduty furnace all year round in the event that you get one cold day a year. However, determining rotary dryer design can get tricky when you live in a place like the Midwest, where in the summer it’s 90º and humid, and in the winter it’s -20º and dry. In this case, it is up to you to nd the balance and gure out what makes most sense and is most efcient in your particular situation. Elevation also plays a role in rotary dryer design. At higher elevations, the air contains less oxygen. Because of this, the same volume of air that might be effective at sea level, is not as efcient in a higher elevation. Pulling the same volume of air through the rotary dryer will not be effective, because the air is less dense, so a higher volume of air is required, and therefore a bigger fan. FLIGHT DESIGN
Another key phase in rotary dryer design is determining the best ights to use inside the rotary dryer. Flights are like specially designed ns, that pick up the material as the rotary drum rotates, and then gradually drop the material through the stream of gas, like a
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clothes dryer. This falling of the material is called a “curtain.” Ideally, the curtain will span across the inside of the drum, with material falling evenly from one side to the other. An ideal curtain is created by having the optimum volume of material owing through the rotary dryer, in combination with the most efcient ight design. The ight design is in part created based on the angle of repose of the material. In more simple terms, when a ight is piled with material, the rotation of the drum will cause the material to slide out of the ight. Angle of repose of a material is at what angle (relative to a horizontal plane), the material will slide against itself off of the pile. However, when it comes to ights in a rotary dryer, it can get tricky, because you don’t want all the material to fall at once in a big clump; it needs to slide and fall evenly through the stream of air. This curtain can also be affected by the pattern in which the ights are welded into the dryer. For example, ights may be
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staggered, or they may line up ush with each other. In the section on material limitations, we discussed how if a material is too fragile to be dropped when it is rst fed into the rotary dryer, then ights may not be put in for several feet, allowing the material to dry and toughen before it is gradually introduced to lifting ights. When requested by the customer, FEECO has the ability to test ight design and pattern with a ight simulator.
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CONCLUSION
It’s easy to see how determining the design of a rotary dryer can become overwhelming. Hopefully now, you’ve got a better idea. If not, no worries…FEECO International can take care of it all, from testing your material’s char acteristics, to delivering the end product to your door. Backed by 60 years of knowledge and experience, your rotary dryer is in good hands at FEECO International.
For more information on rotary dryers, visit our website: http://feeco.com or contact us at
[email protected]. 3913 Algoma Rd. Green Bay, WI 54311 • Phone: (920)468.1000 • Fax: (920)469.5110 • Email:
[email protected]
