What should actually be considered when purchasing a screening machine?, Remscheid
Dipl.-Ing. Sigurd Schütz, RHEWUM GmbH
The purchase of a screening machine looks trivial at first glance. If you want to get it right, a multitude of influences have to be taken into account. Often, for reasons of simplification or for commercial reasons, the pure acquisition costs come to the fore. The following article is intended as a practical guide to help and guide you in selecting a screening machine.
First considerations in advance
Before you contact potential suppliers, you should be clear about the general conditions for the new screening machine. Keep in mind that every increase in quality requirements leads to an increase in costs, as the required screen areas have to be increased. In production engineering, therefore, the basic principle is: as exact as necessary and as imprecise as possible.
An often underestimated question is: Should the machine be planned into an existing building or is the building still to be erected? While directly driven screening machines hardly emit any vibrations and are therefore statically negligible, these must be taken into account with vibrating machines. The statics of the building must be adapted to the machine type and may become a cost driver. Experience has shown that vibrating screening machines at great building heights can cause lasting damage to weak structures. Furthermore, the height situation must often be taken into account in the construction: The steeper a screening surface is, the greater the height requirement. A flat screening machine requires less overall height, but the screening process is significantly slower than with a steeply inclined screening machine. In screening tests, which most manufacturers carry out at no extra cost when purchasing a screening machine, the perfect screening angle can easily be determined in addition to other parameters.
In addition to the question of available overall height, it must be clarified whether an existing screening machine should be replaced 1:1 or whether the original screening parameters should be changed in the course of a modernisation, e.g. an increase in capacity, purity or output or even additional separations. Once these points have been determined, the connections have to be clarified in case of a rebuild. How does the fresh product reach the screening machine and how do the screened fractions leave the plant section?
What criteria are used to select the screening machine?
In addition to the spatial conditions, several other criteria must be taken into account. At the very beginning are the product requirements. What should the end product look like? Is it a matter of removing dust, do agglomerates (lumps) have to be separated or is it a correct classification? What quantity of product do I want to process and what is the particle size distribution before the screening machine? Unnecessary, too high requirements mean a large screening surface. This, in turn, has a decisive influence on pricing. Similar to production, the question is not how high can I define the product requirements, but what product quality I really need. Do I want to realize several separations in one or more screening machines? If a high product quality is required, it is important that the screening parameters amplitude and frequency can be adapted to the respective classification.
The actual product criteria also play a role. Is the product dry and granular or sticky? Is it hygroscopic? Or even electrostatic? Does the product change in the course of time, as is usually the case with natural products, e.g. sand? Or is it always the same, as is often the case with chemically manufactured products? Does it have to be sieved dry or wet? Is it to be dehydrated again afterwards? How sensitive is the product? Is it brittle and does it break during handling? Is it highly abrasive or relatively harmless? All these factors have a decisive influence on the choice of the right screening machine. If these points are disregarded, satisfaction with the screening results and service life cannot be expected. Every product and every task requires a screening solution that is precisely tailored to it.
In addition to the purely process-related requirements, the selected screening machine must also fit into the building. In addition to the pure installation area, it is more important to note that there is also sufficient space around the screening machine for inevitable maintenance work. The overall height required for a screening machine depends mainly on the inclination of the screening machine. To simplify matters, it should be noted that flat vertical screening machines have less height, but due to their design they offer lower specific screening capacities than inclined machines.
How fail-safe must the screening machine/screening plant be? Is it possible to shut down the screening machine for several hours to change wearing parts or must production be continuous, so that a machine may have to be on standby? If this is impossible, fail-safe screening machine types should be selected without exception. Especially in flat-standing screening machines with one drive, a malfunction inevitably leads to screening and thus plant standstill. In the case of inclined screens, the angle is often chosen according to the angle of repose of the product, so that the product trickles gravimetrically over the screen fabric without further assistance. Such a solution in combination with a variety of drives is currently the safest solution if availability is one of your important issues.
How to choose the right sieve cloth?
In principle, the screening medium used is largely responsible for the product qualities produced. Without high-quality screening media, even the best screening machine cannot achieve what it could with the appropriate equipment. Do you have special requirements for the sieve cloth to be used? Can you shut down the screening machine at any time to replace the screen cloth or does it impair production? How quickly and easily can a screen cloth be changed? Fast enough so as not to influence subsequent processes? Do the screen cloths have to be made of a certain material in order not to contaminate the screenings with the inevitable abrasion? Or is the screenings extremely abrasive, which requires a special solution? A distinction is usually made between metal and plastic solutions.
In the case of metal solutions, the question then arises as to whether normal spring steel can be used, or a more wear-resistant stainless steel - and this in what quality? Does it have to be magnetic so that it can be separated in food production using a magnet? Or, in the case of stainless steel, is the question of corrosion resistance? Is a sieve cloth useful or do you directly use whole sieve trays? If the feed material has a tendency to stick and the screen separation is large, bar grates and vibrating finger grates are also popular.
If a synthetic fabric is used, the material properties must be clarified. If the screenings are used as food, polyurethane screen fabrics should be avoided, as most polyurethanes are wear-resistant but do not have FDA conformity, which is essential in the food industry.
|What accuracy do I need for my mesh size? Due to the system, the manufacturing accuracy of steel fabrics is greater than that of plastic fabrics, especially if they have a carrier fabric and are cast or even punched. Due to their plasticity, die-cut screen fabrics tend to deform during the die-cutting process, with the result that the mesh width changes from the entry to the exit of the die-cutting tool, as well as when the carrier fabric is cut through. As an alternative to casting and punching, screen fabrics can also be inserted into the carrier material by means of a laser, without deformation, with free form of the screen meshes (Polymesh).|
What should be taken into account when selecting a supplier?
What experience does the company have with an identical or comparable task? Does the supplier know the local (safety) regulations (FDA or ATEX)? Is the contact person competent and does he already know the task from a previous order? What qualifications and experience does he have? Who will handle the project? Who is then the contact person? Where is the production? Is the quality assured? What vertical range of manufacture is ensured? Are the right materials used (corrosion/abrasion)? Are environmental and ethical aspects taken into account during production? Is a Code of Compliance lived? What service life do I expect for the machine? Am I sure that the supplier will be able to ensure the service of the machine and the supply of spare parts over the period? Is there upward compatibility in the supply of spare parts? As a manufacturer, do I have to stock spare parts? If so, which ones are absolutely necessary? Are the expected costs for service and spare parts reasonable?
It is easy to see that choosing the right screening machine is not easy and requires careful consideration and planning. Do not let this put you off. Our screening machine experts will be happy to advise you with their knowledge and many years of experience. We at RHEWUM are sure that there is a specific screening machine for every application. Where others are seven, we pay attention to the special needs of our customers. Please contact us now.