Suction system

Principle: the Giffard effect
This system, closed to the GIFFARD created in 1848 by the engineer GIFFARD in order to supply with water the locomotives steam boilers is also called:
"suction", "injection" or "aspiration" system.
It uses the formation of a partial vacuum, created in a kind of pump, by the compressed air. The generated aspiration is used to involve in the air flow the abrasives grains stored in a capacity and project them with a nozzle. The efficiency of the system rests on the fortunate balance of the respective diameters and the relative positions of the air injector and the exit nozzle.
In order the abrasive transport takes place, the feeder piping must be connected with a storage hopper and must receive a fluid mixture of particles in the air.
In order to avoid the pipe obstruction by the abrasive which, by its mass, goes against the grains transportation, it is necessary to facilitate its flow by fluidifying it.
Therefore, one uses at the level of its conveyance, a nozzle allowing the regulation of the induced air outflow and the one of the abrasives grains. The control of the shot blasting spray gun is ensured by a compressed air induction valve.

Different processes

Basic system with Ti
A suction or injection projection system is thus necessarily fed from an abrasive tank, by an air/ abrasive mixer.
The simplest is a Ti connection whose main branch is horizontal and which is connected by the center to the hopper. One of the horizontal entries is connected to the aspiration of the suction nozzle, the other one to the atmosphere.
However, this simple system may jam when stopped, particularly when fine products are used. This explains the development of the system admitting the air, whatever the circumstances are. There are generally based on the fact that a freely flowing out abrasive forms a heap whose slopes are constant and always occur according the natural rest angle of each product.

System with a plunging rod
More recently, a system allowed resolving very simply and efficiently this old problem of obstruction, even for the very fine products.
It is a plunging rod introduced into the abrasive and made of two concentric tubes, making it possible to the air to infiltrate between both walls, until the core of the abrasive mass.
For the finest abrasives, a special feeder fixed connector with a real layer fluidifyed by air suppression through a porous disc, made it possible to ensure the flowing of powders formerly used with water only.
This system is very much used on the portative device for sandblasting, micro shot-blasting or glass blasting operations.

Gravity system
It is a derivative of the previous system, in which the abrasive supplying towards the spray gun body is not made by aspiration but directly by gravity from a storage hopper located on top.
An induced air system and an abrasive measure ensure a good regularity of the flow which does not depend anymore on suction.
The proportion air/ abrasive remains very precise and gives a permanent feature of performances. That is why this projection system is used above all in the shot peening treatments.
Nevertheless, this system is little or not spread in Europe.

Uses of system

The projection material is simple, not expensive to purchase and maintain. It is often used in low dimensions sleeves stations, either controlled manually, or set up on fixed or mobile adjustable supports. The output of this system is relatively low considering the low outflow of projected abrasive, but the treatment quality makes it use for cleaning, decarbonizing, or satin finishing operations on parts of all kinds, on tools, etc..
A fine or superfine shot blasting is often required with this kind of installations.
The number of spray guns is not limited and the machines can receive all the parts handling equipments, that is to say:
- rotary table,
- crossing conveyor, etc.
The spray guns movement can be automated.
The treatment can be continuous while the stored charge lasts, or uninterrupted with the recirculation of the charge to project.