Working order principle

Basic principle
This system, also called "direct pressure" system, is totally different from the previous one (suction). It made it possible to develop the sandblasting technique.
The grains are no more sucked up under a fluidifyed form by the pump vacuum in order to be projected by a spray gun, but are propelled by compressed air from a pressurized tank, in a unique pipe, towards a "nozzle" in which they are accelerated (Picture 1).
In order to allow the grains driving, the pressure in the tank must be equal to the one of the vector in the shot blasting pipe (R1).
This equi-pressure permits a grains free flow, by simple gravity in the projection piping, through an opening situated under the tank hopper (V1).
The hopper tank being in super pressure at the time of the projection, it must be closed.
The balanced air admission is made through a pipe situated on top of the abrasive mass (R).
An opening on top of the tank allows its reloading when the internal pressure is brought back by decompression to the equal value of the atmospheric pressure (R2).
Its closing, formerly made manually with a screw-tap, is made today automatically thanks to a valve trapped in the opening (C1).

Advantages of the system
- it ensures the outflow of more important abrasive flows, with a more profitable use of the compressed air power.
- the projection piping length influences less on the output than with a very critical suction system.
- the fire distance between the nozzle and the surface is larger than for suction, and it gives the grains a more important kinetic energy.

System evolution

Sandblasters with double bodies
While the suction machine can project uninterruptedly, in so far as it is fed with air and its hopper with abrasive, the pressure tank is dependent on its own capacity.
It is possible to fill it up only when the pressure on either side of the valve (C1) is balanced in order to allow its going down.
The projection from a pressure tank is thus discontinuous.
To overcome this noticeable drawback, in particular on the automatic machines, a second tank acting as an air lock is installed on top of the main tank. The main tank is always under pressure while working, whereas the air lock tank is:
- either under pressure and communicates through the valve with the main tank (transfer period of the abrasive),
- either depressurized and communicates with an abrasive storage hopper (reloading period).
Both tanks can be built in the same enclosure which can be pressurized, with piloted working and transfer controls.

The constitutives elements

Evolutions
The evolution of the shot blasting systems under gaseous vectors, from its origin with primitive machines to the present day, is characterized by the abundance of the researches tending to improve the efficiency.
The suction machines were subjected to little evolution, from the projection point of view, except the search for more lasting constitutive materials for the realization of the injectors, the nozzles and the mixers. On the contrary, the pressure machines knew many improvements for each part of the machine.

The transfer valve
Numerous systems, with cones, domes, balls, brought back with springs or counterweights, were used to reload the tank with abrasive and put it under pressure.
Today, these systems are replaced by:
- a free valve fixed on a hollow rod with a lot of play and put on a vertical tube in the center of the tank which ensures the compressed air feeding. Under the action of pressure, it flattens against its seat and remains firmly maintained to it.
- a valve controlled independently by a hydraulic jack which comes and pushes the valve that became too heavy in the low pressure works. This kind of assisted valve is used for the continuous cycle projection.

The mixer/ measure
Many mixers with registers, needles, injectors, diaphragms, membranes, valves, rotary discs or mantles were and are still used nowadays. (Picture 4)
The mixer should allow an optimal mixture of air/ abrasive, with a fluid outflow towards the nozzle, while limiting as much as possible the pressure drops.
An advanced model allows not only realizing a harmonious mixture, with measuring out variations depending on external adjustments, but also offers the possibility of an instantaneous stopping of the projection, with or without purging the projection pipe.

The projection nozzle
This is the major piece on which depends a lot the process efficiency and which is too often uncared-for. The nozzle which is subjected to an intense and continuous abrasion should be lasting.
It will be constituted of materials with a high resistance and adapted to the specific use of an abrasive.
- alloys with a high chromium and manganese content,
- tungsten, boron or silicon carbides,
- ceramics.
All the aspects relative to the nozzles efficiency and design formed the subject of many researches. Besides the classical straight cylindrical/ conical type and its by-products, in Té, with veered shot or with annular shot, the only evolution is the application of the convergent/ divergent, type "venturi". (Pict 5)
This technique based on the maximal acceleration of the particles, and the improvement in the outflow, made it possible to realize "supersonic" nozzles, calculated like a reactor nozzle whose speed when coming out is higher than the speed of sound.
Nota:
The nozzles diameters varied between 3 and 15 mm, generating compressed air consumptions from 0.5 to 15 Nm3/mn under 6 bars.
As an example, in the chart below, you will find some outflows for the more usual cylindrical nozzles. (Picture 5 opposite: Venturi nozzle)