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What does the mixing system of the fermenter include?

    The fermentation tank is composed of many important components, one of the most important components is the stirring system, which includes: motor and gearbox, stirring paddle, stirring shaft, shaft seal (end face shaft seal) baffle. The following is a detailed introduction by Bioreactek company as follows:

     1.Motor and gearbox

    The motor and gearbox are placed outside the tank. For small glass fermentation tanks, single-phase electric motors can be used, while large glass fermentation tanks generally use three-phase motors. For large glass fermentation tanks, since the speed of the motor is generally much higher than the stirring speed, the speed must be reduced through the gearbox. The laboratory small glass fermenter can be continuously variable and does not require a gearbox. In batch culture, each stage of cell growth has different requirements for shear force and oxygen transfer. In order to reduce power consumption, it is best to use an adjustable speed motor.

    2. Stirring paddles
    Can be divided into radial flow stirring paddles and axial flow stirring paddles according to the initial direction of the fluid motion generated by stirring. The radial flow stirring paddle pushes the fluid outward, meets the inner wall and baffle of the glass fermenter, and then turns back up and down both sides to generate secondary flow. Axial flow paddles start the flow in the axial direction. Generally speaking, a glass fermenter with an axial flow impeller has a lower power factor, and the energy consumption required to achieve the same mixing effect is much lower than that of a radial flow impeller. The shear force caused by the radial flow impeller is greater than that of the axial flow impeller, which is beneficial to break up the air bubbles, thereby increasing the overall oxygen transfer rate constant, but can cause damage to some cells. Therefore, radial flow impellers are mostly used for the cultivation of aerobic bacteria and yeast that are insensitive to shear force, while axial flow impellers are mostly used in shear-sensitive biological reaction systems. For large fermenters, the design of the mixing configuration of these two types of stirring paddles can be used to fully utilize the advantages of each.

    3. Stirring shaft
    The stirring shaft can extend into the tank from the top or from the bottom. The former is called upper stirring and the latter is called lower stirring. Generally speaking, the manufacturing and installation cost of the upper agitator is slightly higher than that of the lower agitator. However, when using lower stirring, the solid particles in the medium or the crystals formed by the soluble components after the water volatilizes will damage the shaft seal and increase the maintenance cost. Different sizes of vented glass fermenters have different layers of stirring paddles. Small vented glass fermenters generally have only one layer of stirring paddles, while large vented glass fermenters generally have 2 to 4 layers of stirring paddles to improve mixing and mass transfer.

    4. Shaft
    Seal The main function of the shaft seal is to prevent microorganisms in the environment from invading the glass fermenter and the leakage of the culture medium. Mechanical transmission parts are often one of the main causes of bacterial contamination, so the key to shaft seal design is to avoid bacterial contamination and leakage, and aseptic sealing materials should be used as much as possible.

    In order to prevent large vortexes on the liquid surface during stirring, and to promote the mixing of fluids in the tank in all directions, the baffle is also installed on the tank body corresponding to the stirring paddle. The baffle is designed to satisfy the “full baffle condition”. The so-called full baffle condition means that when a baffle or other accessories are added to the glass fermenter, the stirring power is no longer increased. The number of baffles is usually 4 to 6, and the width is 0.1 to 0.12D. The full baffle condition is the minimum condition to achieve the elimination of liquid surface vortex.