Spray Dryer Nozzle Type
There are many types of spray drying, which can be divided according to the process: open, closed-loop, self-inert and semi-closed loop; according to the direction of gas flow: cocurrent, countercurrent, and mixed flow; And according to the atomization method: pressure single fluid, centrifugal, and two fluid. Among them, the atomization method classification is the most used one, and the following focused on the atomization type, that is, the spray dryer nozzle type.
Spray dryer nozzle type — pressure single fluid nozzle
Pressure single fluid nozzle atomization process
The high-pressure pump is used to obtain high pressure for the liquid. When the high-pressure liquid passes through the nozzle, the pressure energy is converted into kinetic energy, and the liquid is dispersed as mist droplets when ejected at high speed. Liquid atomization is obtained when pressurized from a single fluid, which is due to the external force. Not far from the nozzle outlet, the liquid overcomes the surface tension, splits from the liquid film into thin lines. The transverse velocity and the relative speed of the surrounding air cause the liquid line to split into droplets of different sizes.
Characteristics of pressure single fluid nozzle atomization
1)Simple structure, no-noise operation.
2)The required atomization shape can be easily obtained by changing the internal structure of the nozzle.
3)Suitable for mass production.
Limitations of pressure single fluid nozzle atomization
1)When the nozzle orifice diameter is less than 1mm, it is easy to cause a blockage.
2)Not suitable for colloidal liquid with high viscosity and suspension with solid-phase decomposition.
3)The atomization nozzle is easy to wear.
Spray dryer nozzle type — centrifugal atomization nozzle
Centrifugal atomization process
The material liquid is thrown from the edge of the disk and atomized by the centrifugal force in the high-speed rotating disk (circumferential speed 90~160m/s). During the centrifugal atomization process, a high-speed rotating disk plays an important role. When the liquid is injected into the surface of the disk, the liquid is subjected to two kinds of forces. First, it is accelerated by centrifugal force and gravity to split and atomize; second, the liquid and the surrounding air contact the surface at the same time, and there is friction which promotes the formation of mist droplets.
Spray dryer nozzle type — two fluid nozzle
Two fluid nozzle atomization process
Two fluid nozzle atomization, also known as airflow atomization. The airflow small spray dryer uses compressed air to eject from the nozzle at very high speed (≥300 m/s), relying on the frictional force generated by the speed difference between the gas and liquid phases to split the liquid into mist drops. It uses the high-speed movement of steam or compressed air to make the material liquid meet at the nozzle outlet to produce liquid film splitting and atomization. Since the liquid velocity is not large and the air flow velocity is high, the relative velocity of these two makes the liquid film drawn into a wire and then split into fine droplets. The droplet size depends on the relative velocity and the viscosity of the liquid.
Limitations of two fluid nozzle atomization
It only makes sense to use steam injection when the temperature of the hot air is higher than 300℃. Generally, when the steam pressure is 0.3-0.7MPa, and the compressed air pressure is 0.2-0.5MPa, the average diameter of the droplets is 10-60μm.