Comprehensive Knowledge about The Release Device of Dissolved Air Flotation

The flotation system of DAF is composed of dissolved air releases, pressure dissolved air tanks, scraper machines, air compressors and other components. This article mainly shares the relevant knowledge about dissolved gas release, and provides our readers with a reference for choosing release device.

The flotation water purification equipment was first successfully researched by Shanghai Tongji University in the late 1970s and was soon promoted nationwide.

TS, TJ and TV type of dissolved air release devices are used in China ,they are all patented products of Shanghai Tongji University.

Their similarity；

• Release almost all gases, and when the dissolved gas pressure is above 0.15MPa, about 99% of the dissolved gas can be released.

• Work under lower pressure, and when the dissolved gas pressure is above 0.2MP, they can achieve good water purification effects and save electricity consumption.

• The released bubbles are tiny, which with an average bubble diameter of 20~40μm, dense bubbles, and good adhesion performance.

Table of contents（Click to go to where you want to see）

1. Dissolved air release TS

2. Dissolved air release TJ

3. Distributed air release device of type TV
   

1. Dissolved air release TS

Physical map of TS

TS type dissolved air releases are mostly used in experimental devices. Because the outflow is relatively small, they are suitable for flotation equipment with a small amount of water.

There are 5 specifications of dissolved air release TS.Their appearance is shown in the follow picture :

P1 Appearance of dissolved air release TS

When the pressurized dissolved gas water passes through the inner hole box of the release, the energy is rapidly dissipated due to the sudden change of flow state, so that the bubbles can be fully released instantly under the condition of reduced pressure.

The flow rate and working range of the 5 specifications under different pressures are shown in the table below:

The process of forming microbubble is shown in the picture below.(P2）

Internal structure of TS

1—Water inlet；2—Hole Chamber；3—aperture；4、5—Microbubble formation chamber；6—Nozzle

The pressurized dissolved air water enters the orifice 1, and due to the sudden contraction and expansion, it reduces part of the pressure energy to enhance the kinetic energy of the air molecules and accelerate their escape from the "cage". According to the water flow, it collides in the pore chamber 2 and forms a vortex, which continues to "dissipate energy", transfer mass, and release gas due to the generation of violent turbulence, so that most of the air molecules escaping from the "cage" are gradually combined into ultra-fine bubbles under the action of molecular diffusion and turbulent diffusion.

When the water flow gradually turns into the gap 3 of the parallel disc, the flow state suddenly changes. At the extremely narrow slit outlet, the water flow cross section shrinks sharply again, the turbulence is more intense, and a local vacuum occurs, and the thinning of the liquid film and the surface renewal rate reach the peak. At this point, almost all the air molecules are released, and they diffuse rapidly along the radial direction of the disc, causing the ultra-fine bubbles to continue to grow step by step until micron-sized microbubble are formed at 4 and 5 and flow out.

At this time, the energy of the dissolved gas water has been consumed by about 95%, and the remaining energy continues to generate the turbulent flow velocity gradient G' in the nozzle 6, so that the microbubble only "grow in the same direction" under the influence of turbulent diffusion movement, and become 10 micron-level microbubble. So far, the TS-type dissolved gas release has completed the whole process of forming microbubble from pressure dissolved gas water, thereby turning the transparent dissolved gas water into the milky white gas-released water required for flotation purification. The whole process takes less than 0.3s, and the residence time in the hole box is only 10^(-3)~10^(-2)s.

The main reasons for the bubble growth during the gas release process of the release are the following three points.

(1) Turbulent flow velocity gradient G and G'

The larger the G value in the hole box, the more gas molecules jump out of the "cage", the more equal the probability of mutual collision, the closer the diameter of the superfine bubbles formed step by step, the more superfine bubbles that are "grown in the same direction", the smaller the diameter, and the more thorough the "energy dissipation" and gas release.  On the contrary, the G value in the nozzle should not be too large, because the function of the nozzle is to eliminate the residual energy in the orifice box outflow and stabilize the degassing water flow. We should try our best to reduce the unfavorable factors of the bubbles further "coexisting in the same direction" under the influence of G.

(2) Residence time t and t'

The longer the pressure dissolved air water stays in the release, the greater the probability of bubble coexistence and the larger the microbubble formed in the end.

(3) Effective collision coefficient n and n'

It is not only affected by the bubble concentration and turbulence gradient, but also related to the strength of the bubble film (and the quality of the dissolved air water). Tough and elastic bubble films are not easy to break, so the bubbles are stable and difficult to break, and the n and n' values are small. On the contrary, the film is brittle and easy to break and grow.

2. Dissolved air release TJ

principle of TS to expand the outflow and action range of a single release device and to overcome the problem that TS is easily blocked by impurities in water. It is made of cast iron and lined with stainless steel sleeve.

The appearance is as the picture below(P3)

P3 The appearance of TJ

When TJ is blocked, vacuum can be drawn from the upper interface to lift the tongue spring inside the device to remove impurities.

TJ has 5 specifications, and the flow rate and range of action under different pressures are shown in the below table.

The installation diagram of TJ is shown in the below picture.(P4)

P4 The installation diagram of TJ

The tongue reed installed in the TJ is in a compressed state under the pressure of the water pump when working normally.

When there are impurities blocking the water, open the valve behind the water ejector in Figure 4 to make the water ejector work, and rely on the vacuum created by the water ejector to pull up the tongue reed, thereby increasing the water flow channel and discharging the impurities.

3. Distributed air release device of type TV

Physical image of TV

shortcomings of the above two release devices , such as uneven water distribution and the need for a water ejector to lift the tongue. It uses a circular disc to release air in all directions in radial direction, which provides better contact conditions with floc-containing water.

The appearance of TV is shown in the below picture.(P5)

P5 The appearance of TV

When it is blocked, the compressed air can be turned on to move the lower plate downward, enlarging the water flow channel between the plates and allowing the blockage to be discharged. At the same time, in order to prevent the release device from being corroded in water, it is made of all stainless steel.

The installation diagram of TV is shown in the below picture.(P6)

TV has 3 specifications. The flow rate and range of action under different dissolved gas pressures are shown in the sheet below.