Mechanical Conveying Q&A
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.
A chain conveyor is a very efficient method for conveying fine potash dust from a dust filter and up a 20-degree incline. The chain conveyor will lose some efficiency as some of the fine dust falls back down the incline. The chain speed may need to be increased to overcome the inefficiency of conveyor. The chain conveyor will be more efficient as you reduce the angle of incline below 20 degrees.
A screw classifier is the solution for separating the small plastic particles from the liquid. A screw classifier is basically an inclined screw conveyor with a large reservoir or tank for holding the combined liquids and solids. The solids are allowed to fall to the bottom of the tank by gravity. The inclined screw rotates at a very slow speed to prevent turbulence and conveys the solids out of the liquid to a discharge point. The reservoir or tank has a wier on the backside to allow the liquids to reach a specific level before discharging. The liquids can then be recycled back into your process.
KWS designs and manufactures screw classifiers for many different industries including plastics recycling and water treatment. The screw classifier design for your application is based on total flowrate of solids and liquids. The reservoir or tank must be large enough to allow the solids to settle out without turbulence from the incoming flow. The inclined screw is sized based on the solids flowrate. Please submit the flowrate of both the solids and liquids and we would love to help you with your application.
All of the bulk material tables published by screw conveyor manufacturers are based on the Material Characteristic tables in the CEMA 350 book. The CEMA 350 book is the “bible” for screw conveyor design and was first published in 1971. CEMA is the Conveyor Equipment Manufacturers Association and is an industry group dedicated to the advancement of the conveyor industry. CEMA created the 350 book as a guideline for designing and manufacturing screw conveyors. The book is updated almost every year to add new materials to the Material Characteristics tables. I am the Chairman of the Screw Conveyor Engineering Committee for CEMA and we have made several updates and improvements to the CEMA 350 book in recent years. You can order an electronic copy of the CEMA 350 book from the CEMA Web site at www.cemanet.org. The guidelines in the CEMA 350 book apply for most applications and are conservative. Bulk materials are classified by density, size, flowability, abrasiveness, and other factors. Each bulk material is given a code which corresponds to basic design and materials of construction guidelines. For example, a bulk material such as Portland cement is dense at 94 lbs cu/ft, is very fine, has good flowability, is moderately abrasive and can aerate when conveyed. The CEMA material characteristic code is 94A10026M from the CEMA 350 book. The corresponding component series code is 2D, or heavy-duty. Minimum materials of construction thickness are given for each conveyor diameter.
WAM Inc. is a European-based manufacturer with offices located in the U.S. WAM is also a member of CEMA and follows our industry guidelines. They also have their own designs and standards for screw conveyors that make them unique to the industry. You probably just need to specify that you want your equipment designed and built to CEMA standards if you are more comfortable with the CEMA standards.
All of the major screw conveyor manufacturers are active members of CEMA, The Conveyor Equipment Manufacturers Association. The CEMA website is www.cemanet.org. Each screw conveyor manufacturer that is a CEMA member will be listed with their contact information. You can also call the CEMA office and speak with one of the staff. The CEMA staff is very knowledgeable and helpful. They will be able to direct you to a screw conveyor manufacturer in your area. Almost every screw conveyor manufacturer has an office or sales representation in the Midwest. Also, the CEMA staff will help you locate belt conveyor manufacturers as well as companies that make accessories for the bulk material handling industry. Since your company is a manufacturer of conveying systems and components then you may qualify to be a CEMA member. Being a CEMA member has many advantages. The main goal or mission of CEMA is to advance the conveying industry by creating technical and safety standards. You will also gain knowledge on industry trends and any legal issues that may affect your business.