In sandblasting processes, there are two different types of blasting. In this article we will see the differences between both systems, as well as some guidelines to determine which one is more convenient depending on the application we intend to perform.
The Venturi effect refers to the phenomenon by which a fluid in motion, inside a conduit, decreases its pressure when the velocity increases as it passes through a narrow zone. The higher the velocity, the lower the pressure, and if the end of another conduit is introduced at that point of the conduit, the fluid will be sucked from it and mixed with the fluid flowing through the first conduit. In colloquial terms, it is known as projection by suction or aspiration.
By means of this system, the compressed air that feeds the blasting machine reaches the blasting gun through the distributor and sucks the abrasive, both mixed and projected through the nozzle.
Venturi gun parts
A complementary tank to the blasting machine where the abrasive is stored receives the compressed air supply. The pressure increase causes an upper buffer valve to close leaving this reservoir at the same pressure it receives. Subsequently, by means of manual or automatic operation of a pulsating device, a lower valve opens, allowing the compressed air/abrasive mixture to escape. This overpressure equipment is also known colloquially as blowers or sandblasters.
Practical differences between the two systems
On a physical level, the difference is obvious: in the case of the Venturi, a significant percentage of the energy provided by the compressor in the form of compressed air will be used to suck the abrasive from the lower tank of the machine to the gun against gravity. On the other hand, in overpressure units we do not have these head losses, so this system has a much higher performance.
On the other hand, the suction provided by the compressed air makes the Venturi effect work with abrasives (grit) that combine low particle sizes and/or low specific gravity. On the other hand, in overpressure we do not have any such limitation.
In both cases, the relationship between the internal diameter of the nozzle and the size of the abrasive particles must be considered. A small diameter nozzle is more prone to clogging when different grains are grouped together, blocking the nozzle.
The Venturi system is easy to automate as there is no container tank to be emptied, it is a continuous projection and can have several firing nozzles.
In the overpressure system, these problems can be solved with special equipment with double chambers and multiple outlet nozzles, but they make installation and maintenance considerably more expensive.
Applicability depending on the firing system
We can deduce, among others, the following questions:
The Venturi system is suitable for the treatment of small to medium sized parts, especially when cleaning, aesthetic satin finishing, micro deburring of machined parts or low roughness pickling is required.
- It may be advisable when the material is a material of low hardness: composites, alloys or of low thickness in order to avoid stresses that may warp it.
If we need higher blasting energy, either because we need a higher impact intensity, or because we are forced to use larger particle sizes (or use metallic blasting media). It is advisable (or rather essential) to perform it in direct pressure by coupling overpressure equipment.
- Likewise, all processes that require massive launches such as large structures, warehouses, maintenance of civil works or building facades must be carried out using this system.
If the structures (carpentry, metalwork, containers, truck trailers, for example) are likely to be contained within a booth or blasting room (sandblasting). We should do so because free blasting, i.e. the projection of abrasive without any collection system, is regulated by environmental and occupational safety standards, and also practically forces the use of very low-cost abrasive (sand and similar) with very poor results in terms of finish.
But beyond even these technical and health considerations, if there is no collection, filtering and selection system, processing costs increase exponentially, since most of the cost corresponds to labor, which, without the aid of these systems, spends more time on these functions than on the projection itself.
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