You have a full To Do list for the day and show up at the plant ready to rock it. You don’t 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 won’t 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 can’t 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 you’ve getting uneven final product. Your customers are starting to complain, and now you had a fire. What’s 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 they’ve been that way and the last person to inspect just ignored them. Good thing you started going back to the gym cause it’s 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 shouldn’t 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, it’s 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. There’s a groove that looks like it’s going around the whole drum at this point. And it looks like there are more places about to break through. You’ll 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.
If you have a rotary biomass dryer, chances are you’ve dealt with rocks, fires and explosions, and non-uniform moisture content of final product.
Rocks got you down?
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 don’t 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.
Controlling moisture content
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.
Ounce of Prevention
Dealing with Rocks
How can you get product to go one way and rocks to go the other? Using basic physics, it’s 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 you’ll 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.
Dealing with Air Leaks
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.
Drum Shell Breaches: Do an internal inspection and make sure there are no breaches through the drum shell.
Pressure Relief Systems: Occasionally pressure relief systems can be the source of air leaks.
Manways & cleanouts: check manways and cleanouts to ensure they are sealed properly and not allowing in air.
Airlocks: The blades on airlocks need to be replaced or refurbished every 2 years, more often in harsh conditions. Worn airlock blades can be a significant source of transient air.
Dealing with Uneven Moisture
Repair or replace current flights: That should be easy enough.
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.
Flighting Design Changes: Drum flighting’s purpose is to re-entrain the product into the hot drying gases after falling out of the gas stream. Signs that your flights are overloaded:
- Uneven outfeed product flow — surging
- VOC creation, high emissions
- Uneven final product moisture content
- Excessively High Inlet Temp
- Low outlet temperature
- Buildup in ductwork
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 aren’t dry enough, you put them back in.
Whether you seem to have problems that originate from rocks, air leakage, or flighting design, there’s 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 aren’t 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? It’s 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 don’t have to ruin that new shirt you just got for Father’s Day
By Becky Long – Thompson Dryers
Published in Process Heating Magazine February 2017