We all want to produce as much sand as possible, so it’s common to want to push the boundaries of what your Sand Screw is capable of handling.
While it may be tempting to increase the speed of your Fine Material Washer to allow more capacity, running your unit at speeds faster than recommended isn’t such a great idea. Here’s why.
Why a slower speed is needed to retain finer sand products
Running a screw shaft too fast can be a problem for keeping finer-solids products, like mason or mortar sand, asphalt sand and manufactured sand products, from some crushers. Screw speeds that are faster than recommended can cause product-sized material to overflow the weirs with the wastewater, potentially resulting in a loss of saleable product.
Excessive speed can cause the fines to accumulate and remain in suspension until an excess concentration exists. This fine sand then builds up in the lower corners of the washer tub and, in a few hours, can build up so much that nearly the entire pool area accumulates fine solids that are typically -50 mesh (300 micron) x 200 mesh (75 micron).
You can visually observe this phenomena occurring when sand falls off the screw shaft spiral at the water line and slides back into the washer tub. Eventually, this product-sized sand may overflow. This excess material may overload and stall the screw shaft, which can lead to a halt in production until the machine is cleared of the fine material buildup.
If you notice your screw washer is filling up with fine sand, it is often recommended to reduce the screw shaft speed to correct the issue. Often, a change in sheaves and V-belts is required. The slower speed allows the material to remain on the screw flights as it travels toward the discharge and curtails fine sand from falling back into the washer tub. While a screw shaft speed reduction decreases capacity, a Sand Screw will be able to dewater fine sands retained that may have 30% to 50% passing 50 mesh or 300 micron.
Trust the manufacturer
In many cases, the speed of the screw shaft is set by the manufacturer for the application for which it was originally sold. The manufacturer or dealer should have taken your application’s data, including feed gradation, desired product, solids capacity and water volume, into account to ensure the proper size machine was selected for the job.
The screw speed is typically based on the percent passing the 50 mesh sieve for the product gradation being produced.
Reduced screw speed
In most instances, a reduction in screw speed is recommended to retain finer sand products, like mason sand, because it allows the finer-solids particles to stay on the screw shaft spiral. The slower speed allows fine material to be conveyed away to a saleable product pile. It is important to note, however, that reducing the speed of the screw shaft will also reduce the production capacity.
The chart below can help you determine the recommended screw speed for your Fine Material Washer based on the size of the machine. Fine Material Washers with larger diameter screw shafts have a greater capacity than smaller diameter machines. However, all washers will classify and dehydrate to the same effectiveness within the tonnage and gallon limits for which the machine was designed.
To determine the recommended screw speed for your Fine Material Washer, first calculate the percent of fine material product passing the 50 mesh or 300 micron sieve with silts and clays removed. Locate that number on the top row of the chart. Next, find the size of your Fine Material Washer in the first column. Follow the row over to the column containing the percent passing the 50 mesh or 300 micron sieve to discover the recommended screw speed and resulting capacity.
More speed isn’t always better
When it comes to Fine Material Washers, increasing the speed to produce more tons per hour will not work, as it can result in losing product-sized sand along the way. Stick to the manufacturer’s recommendations for screw speed to ensure optimal product retention and proper screw washer operation.
If an increased capacity is required, you may need to consider investing in a larger machine that will produce the desired tonnage without sacrificing product-sized material. Eagle Iron Works can help you determine the right size machine for your application by running a screen analysis of your material to find the percent passing 50 mesh. By doing this, they can recommend the correct screw shaft speed for your Fine Material Washer and/or provide you with the right size machine to achieve your desired production.
Water volume and the percent passing 200 mesh or 75 micron associated with a typical -4 mesh or 5mm solids feed also affects Fine Material Washer size selection. Providing your Eagle Iron Works dealer with all application data allows the machine selection to be correct for your requirements and ensures you are getting product-sized sand on the ground for your customers. Eagle Iron Works can provide full details on their wide Fine Material Washer line, with machines that have capacities ranging from 30 to 950 tons per hour.