Mechanical Conveying Q&A
Excellent question! Your problem is not uncommon and there are solutions to prevent this problem from reoccurring. Most bulk materials are easily conveyed using screw conveyors. However, some bulk materials have a tendency to pack under pressure while being conveyed. As a screw conveyor rotates, bulk materials are moved forward with each revolution of the screw. There is a gap between the outside diameter of the screw and the inside diameter of the trough or housing. Bulk materials fill this gap and form a layer between the screw and housing. As the screw rotates, the screw flight shears through the bulk materials in the gap. This shearing action puts pressure on the bulk material. Bulk materials that have a tendency to pack will form a hard layer in the bottom of the trough. Most of the time the layer will break loose in chunks and be conveyed downstream. A new layer will form and then break loose again.
In very rare applications a bulk material will form a permanent layer in the bottom of the trough. This layer will build up over time and cause the screw to deflect in the center. The screw is pushed up in the center during each rotation because each end of the screw is held fixed by the drive unit and end bearings. This condition puts enormous cyclical forces on the center pipe of the screw.Every revolution of the screw causes a complete reversal of the forces on the center pipe. The center pipe of the screw will fatigue and fail typically in the center. It’s very similar to taking a piece of wire and bending it back and forth until it breaks. In your application you are adding water to the flyash which is making the problem worse. The flyash/water combination is sluggish to convey and sticks to the bottom of the trough because it builds up and hardens like concrete. The outside diameter of the screw flights will become shiny as they wear against the layer of material in the trough. This is a key indicator that you have buildup. In some extremely abrasive applications the outside diameter of the flights will actually wear down, causing excessive horsepower draw and screw failure.
Screw Conveyor Fatigue Failure
There are multiple solutions to your problem. A liner can be added to the trough to reduce the gap between the outside diameter of the screw and the inside diameter of the trough. A smaller gap will cause the build up to break loose easier. We would recommend a liner made of abrasion-resistant material such as AR-235 for your application. AR-235 is an abrasion-resistant metal with an average hardness of 235-BHN. A second solution would be to add weld-on hardsurfacing to the outside diameter of the screw. The hardsurfacing will increase the outside diameter of the screw, therefore minimizing the gap between the screw and trough. The hardsurfacing is also very hard and rough and will act as a cutting edge to cut and break up the layer of material in the trough. A third solution would be to increase the stiffness of the screw by mounting the screw on larger pipe with heavier wall thickness. A 6 or even 8-in. pipe with wall thickness greater than ½-inch would create a very stiff screw assembly. The stiffer screw assembly will resist the buildup of product in the trough. Usually, a combination of any of the proposed solutions is required to permanently eliminate the screw failures.
A screw feeder is the most preferred way to meter ground limestone to a paddle mixer. Screw feeders can be designed to be totally enclosed and dust-tight to prevent any product leakage. Since this is a heavy industrial application, we would recommend using adjustable flanged gland seals on the drive and tail shafts to keep the fine, ground limestone contained in the screw feeder. The screw feeder would typically be designed with a U-trough and a bolted cover. Bolting the cover on 6-in. centers would ensure that there is no product leakage. You could also use a tubular housing instead of the U-trough to make sure there is no product leakage.
A screw feeder can accurately meter the ground limestone to the paddle mixer. Screw feeders are volumetric metering devices and when used with an AC variable frequency drive can accurately meter within 0.5-%.
If the distance between the silo and the paddle mixer is more than 20 ft you may consider using a short screw feeder to meter the product from the silo to a transfer screw conveyor. The transfer screw conveyor would then convey the product to the paddle mixer. It is difficult to design a screw feeder longer than about 20 ft without adding an internal hanger bearing. We would not recommend using an internal hanger bearing in a screw feeder for your application.
The configuration you have proposed has been attempted many times with limited success. It is very difficult to meter bulk materials from two different inlet points on a screw feeder. Each inlet will be flood-loaded, meaning the screw feeder is 100-percent full in the inlet area at two different points along the length of the screw feeder. As the screw rotates, the bulk material is moved toward the discharge. If the flight pitch remains constant between both inlet points, the screw flights will be 100-percent full as the bulk material passes the second inlet point. Since the flights are 100-percent full, no material will enter the screw feeder from the second silo. The bulk material will shear through the head load from the second silo until the first silo is empty. The horsepower and torque requirements of the screw feeder will increase due to the extra demand. If the flight pitch increases between the first and second inlet point, bulk materials will be allowed to enter the screw feeder at the second inlet. However, if the bulk material is free-flowing, some of material entering the second inlet may flood back toward the first inlet causing the screw feeder to jam.
The most conservative solution to metering bulk materials from two silos is to use a metering device on each silo. Typically, the best metering devices are screw feeders and rotary valves. Each silo would have a screw feeder or rotary valve and would meter the bulk material to a screw conveyor, which would then transfer the bulk material to your process. You would then be able to feed from each silo individually or at the same time, giving you maximum flexibility. Downtime due to material flow issues would be virtually eliminated.
You have asked two great questions, the first of which you answered yourself. A screw feeder is the perfect solution for accurately metering a dry PVC powder from a hopper to the feed throat of an extruder. The hopper of the screw feeder will store the necessary capacity to handle the demand of the extruder. The screw feeder will be designed to accurately meter and keep a constant flow of PVC powder to the feed throat of the extruder. The hopper and screw feeder must be designed together to promote mass flow so the extruder is never starved for product. Your second question involves heating the PVC powder as it is stored and metered to the extruder. A heating jacket on the hopper and screw feeder trough may be able to provide enough surface area to heat the PVC powder to the desired temperature. We would need to know a little bit more about the heating requirements in order to design the proper solution for you. This application is perfect for KWS Manufacturing because of our dual expertise in bulk material flow and heat transfer. The heating jacket on the hopper and screw feeder are typically coded pressure vessels. KWS is the only manufacturer in the U.S. that can satisfy both requirements. Our strength is providing engineered solutions to our customers. We would love to have an opportunity to help you solve this problem.
Great question! The two different flighting types that you are referring to are Helicoid and Sectional.
Helicoid Flighting is manufactured as one continuous helix from carbon or stainless steel. Special Helicoid flight rolling machines are required to create the flighting. Raw materials in the form of strip stock are fed into the Helicoid rolling machine and cold formed as they go through a set of cone-shaped dies. The dies form the raw material into a continuous helix of a specific outside diameter and pitch. The material actually gets compressed at the outer edges of the flight during the rolling process making it thinner than the inner edges of the flight. The surface of the flights actually hardens as it is cold rolled in the dies making it more abrasion resistant.
Sectional Flights are manufactured from sheet or plate. Metal donuts of a specific outside and inside diameter are cut on a plasma, water-jet or laser burn table. The metal donuts are split so they can be formed into a helix in a special press. Each helix or flight is one revolution. The flights are joined (welded) end-to-end to make a continuous helix.
Design Considerations: Screws manufactured from helicoid flighting are more cost-effective when compared to sectional screws. The helicoid flight rolling process maximizes material usage with very little scrap. Helicoid flighting takes less time to produce because it is formed as a continuous helix and cut to the exact screw length. It also takes less labor to weld the helicoid flighting to the center pipe. However, helicoid flighting is limited to standard CEMA sizes for diameter, flight thickness and pitch because of the limitations of the helicoid rolling machines.
Screws manufactured from sectional flights allow for greater variation in material type, material thickness and overall screw design. Since the sectional flights are made from sheet or plate, you can use a wide variety of material types depending on your application. For example, if you are handling a very corrosive chemical and need a high nickel alloy to prevent corrosion, then a sectional screw can easily be manufactured for your application. Or if you need 1/2 inch thick flights with cutting teeth to chop up and convey a lumpy bulk material, then again sectional flighting would be your choice.
Applications: We offer both light duty and heavy duty helicoid flights and screws for your application. Light duty helicoid is generally used for conveying non-abrasive, free-flowing bulk materials such as grains, ice or polyethylene pellets. Heavy duty helicoid is great for moderately abrasive bulk materials such as limestone, cement or fertilizer. Sectional flights and screws can be used in any application but are mainly used in heavy duty and extremely abrasive applications such as alumina, flyash or glass cullet. There are numerous applications where heavy helicoid flighting can be used in place of sectional flighting with no loss in performance and at a cost savings to you.
KWS stocks a full line of both helicoid and sectional flights and complete screw assemblies. Due to many years of experience, we can help you choose the right type of flighting and screw conveyor for your application. Our goal is to give you a very cost-effective solution that fits your budget and provides many years of maintenance free operation.
The length and size of the heat transfer processor is determined based on your heat load requirements. We will need a few more pieces of information in order to determine the heat load. Based on the information you provided, we are cooling a bio-based product from 1,000 to 150°. We now need to know the flow rate of the product you are cooling, typically in lbs. per hour. Also, if you have a better description of the product being cooled or if you know the specific heat value of the product, we will use this information to calculate the heat load.
Once we calculate the heat load, we will calculate the surface area required for the heat transfer processor and design the cooling medium flow to match the heat load requirements of the application. Typically, for a cooling application like yours, we use plant process water at 90° F for the cooling medium. If another cooling medium is available, then please let us know. The length and size of the heat transfer processor is determined after we calculate the required surface area.
Cooling a hot bio-based product from 1,000 to 150° F is very feasible using a cooling screw conveyor or heat transfer processor. Once we know your flow rate and determine the specific heat value of the bio-based product, we can easily size the unit and solve the problem for you.
First of all, it is very important to understand the application better in order to provide a good, working solution.
Since the fiberglass strands are fairly short, a screw feeder would be your best bet for metering the chopped product from the bin to the weigh hopper. The shape of the bulk bin will determine how well the product flows to the screw feeder. Chopped fiberglass will mat together and bridge over small discharge openings causing problems. The bulk bin discharge and screw feeder inlet will need to be designed properly to promote uninterrupted flow. We also need to know the amount of time required to move the product from the bulk bin to the weigh hopper. You may want to move 2 lbs of chopped fiberglass in 1 min. or even 30 secs. Screw feeders are sized based on volumetric flow rate in cubic feet per minute. We would just convert the rate in lbs per minute to cubic feet per minute. Since you are metering chopped fiberglass to a weigh hopper, it is very important to know the accuracy required for the weigh hopper. We want to make sure we do not over feed the weigh hopper by running the screw feeder too fast. The screw feeder must be controlled by a variable frequency drive (VFD). The VFD will allow the screw feeder speed to be reduced greatly so a small amount of product can be metered to the weigh hopper to “top off” the 2 lb batch of chopped fiberglass. The whole process can be controlled with control logic and feedback from load cells on the weigh hopper to the VFD.
Once all of the critical design parameters have been established, a viable screw feeder system can be proposed to satisfy your application.
The best way to prevent product leakage from a screw conveyor is to use a flanged cover that is bolted on 12-inch centers with a good, compressible gasket between the cover and U-trough. A flanged cover has both sides turned down to provide rigidity. Typical cover thickness is 12 to 14 gauge. A thicker cover may be required for very dusty applications because it provides additional rigidity. Bolting the cover on 12-inch centers will make the conveyor dust-tight. We also recommend using closed-cell foam gasket material that is easily compressible. The gasket material is typically a nitrile rubber blend that is available in black and food-grade white color. The compressible gasket will provide a positive seal even if the sealing surfaces are irregular.
The preferred clamping method is to bolt the covers using standard industrial grade fasteners. Bolted covers on screw conveyors eliminate many safety hazards and prevent injuries. Most of the time, there is no need to get inside of a screw conveyor unless you need to maintain a hanger bearing. If you have wash-down or clean out requirements, then clamping the covers becomes the preferred method. De-Sta-Co makes an excellent toggle clamp that provides adjustable clamping force and allows easy access to the inside of the screw conveyor. Another great clamp is a pivoting C-clamp that is made by Witte. The Witte clamp has a hand knob so you can apply a large amount of force to seal the covers. We use both the De-Sta-Co and Witte clamps in food and chemical applications because easy access to the inside of the screw conveyor is required for clean out.
Determining the appropriate combination of cover thickness, gasket material, and clamping method is the key to solving leakage problems in any industrial application.
Your questions are very valid. Leakage at the tail shaft of a bucket elevator is very common when handling very fine bulk materials at high temperatures. As you described, the take up has a sliding seal arrangement to allow for thermal expansion and chain stretch. Locking down the take up will put undue stresses and forces on the chain and shafts of the elevator, causing premature failure and therefore, is not recommended. Modifying the take-up or the seal would be a much better solution.
Since you have an internal gravity take up in the elevator, the take up can be modified or redesigned with sleeve-type bearings that are internal to the boot section of the bucket elevator. These sleeve bearings are very common and are typically a cast material that is very hard and tough. In this case, the tail shaft needs to be hardened in the bearing area to match the bearing. This design is used in thousands of industrial applications including the cement and minerals processing industries. The boot section will be totally enclosed with no tail shaft projections through the side walls. The leakage will be totally eliminated with this design.
A second option is to use a take up on the head section of the bucket elevator. The head shaft can be designed to adjust vertically to take up thermal expansion and chain slack. The tail shaft would be fixed and have packing gland seals to prevent product leakage. This modification would be more costly and time consuming than modifying the internal gravity take up.
The most cost effective option for minimizing product leakage at the boot section is to use seal materials that are better suited for the application. There are new seal designs on the market, but most do not function well at high temperatures. However, high temperature seal materials are available that can withstand temperatures well over 450-degrees F. Most of these materials are ceramic fibers and can be woven for rope packing for packing gland seals. In your application, a packing gland seal utilizing high temperature rope packing would be the most economical replacement for the graphite/felt seals. The packing gland seal has been around for a hundred years, but is still probably the best option for your application.
Yes! A cooling screw conveyor or heat transfer processor can be used to cool almost any bulk material. Heat indirectly transfers from the product by introducing a heat transfer medium such as cool water through a special trough jacket and/ or through the pipe and hollow flights of the screw processor. Achieving the specified exit temperature of the product is accomplished by calculating the surface area of the screw processor and designing the system flow to match the heat load requirements of the application.
In other words, the size of the heat transfer screw processor required for your application is based on the volumetric flow rate and the amount of heat needing to be removed from the hot product. We need to know the inlet and desired outlet temperatures of the product being cooled and the temperature and flow rate of the cooling medium, which is typically water that is available at the plant. We use this information to determine the Heat Load, or the amount of heat needing to be removed from the product. Then, we size the heat transfer processor to handle the heat load with a conservative factor of safety.
Once we determine the heat transfer requirements for your application, we can size the heat transfer processor that best meets your needs. Typically, we can cool your product from 1,400 to less than 150-degrees F and extend the life of your downstream equipment indefinitely.