The benefits:
no sediment formation
no scum formation
up to twice the yield of digester gas
less residual sludge, faster drying, lower disposal costs
a higher sludge throughput rate, less throughput time
short amortisation time for draft tube mixers
For mixing, sludge in the digester tank is fed through the draft tube pipe, which is nearly as high as the tank. If the rate of circulation is high, the draft tube mixer can recirculate the entire content of the tank within a very short time. This forms a homogeneous sludge mixture for a swift digestion process with the best yield of digester gas. By comparison, this forced circulations requires a little energy input. Therefore, this process is seen as highly economic if drive effort and digester gas yield are compared.
A system with draft tube mixer from Träxler consists of three components:
1. Mixer aggregate properly dimensioned
2. Mixer seat in form of three different installation solutions
3. Draft tube pipe
Mixer with gas hood seat
Draft tube pipe
The design of the system
The vertical mixer is equipped with an impeller (1) specially designed for pumping sludge and with a rotating deflecting disc (2). The drive shaft (3) accommodates both parts and it is mounted in the lower thrust bearing and upper guide bearing (4, 6) in the seating ring (5). It is driven directly by an electric motor (7), which is fastened onto the seating ring through a motor lantern (8). The draft tube pipe (10), in which drive shaft and impeller work, is fastened in the digester tank through the draft tube support (9) and the guying’s (11). The individual pipes of the draft tube are flanged.
Träxler offers adequate solutions for three installation versions of the draft tube mixer:
a) Installation in a gas hood. This solution offers many benefits for the ongoing service of the mixer and the various installed parts, necessary for the operation of the digester tank: A perfectly fitting, well centred mounting of mixer and device as well as swift installation/uninstallation.
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b) Installation on a support plate. In addition, this solution with a plate on the concrete of the digester tank allows for a well-centred mounting of the mixer and an easy installation/uninstallation.
c) Installation directly on a concrete foundation. Of course, we also can solve this installation situation. Ask us.
Träxler TMA mixer systems are designed for two operating modes:
A) pumping from the bottom up
B) pumping from the top down
Operating mode A is the primary operating mode. Depending on the situation in the digester tank, the mode can be switched to operating mode B.
Operating mode A:
pumping from the bottom up
If the sludge is pumped from the bottom up, it is pumped up from the bottom part of the digester tank through the draft tube pipe and then, it is generously distributed on the sludge surface through the deflecting disk. This removes sediments from the bottom of the tank and prevents the formation of a floating sludge layer and scum.
Operating mode B:
pumping from the top down
If sludge is pumped from the top down, foam and floating scum from the sludge surface are removed and they are pumped down to the bottom part of the digester tank through the draft tube pipe. This operating mode should only be used in exceptional cases (foam formation) or for control purposes, e.g. once a month.
The goal of the design is an optimum of efficiency of the digester gas yield compared with the energy input. In close cooperation during the planning stage, we develop mixer for your specific case of use. The size of the mixer is determined by the volume the digester tank can hold, the form of the digester tank, the number of sludge circulations and the runtime each day. We produce mixers with flow rates of over 6,500 m³/h for digestion towers up to a content of over 12,000 m³.
Träxler develops and produces in-house individually adapted draft tube mixers with all of its components. The in-house engineering department with 3D-CAD software is the foundation along with our own mechanical production even for large components and our own on-site installation. The project organisation and the coordination of crane and diving work are part of the turnkey package. Träxler offers complete solutions from one single source.
TMA draft tube mixers from Träxler are ATEX-approved: Model Ex II 1G c/II 2G c T3, Certification No. TÜV 05 ATEX 7157 X. They are conforming to Directive 2014/34/EU for mechanical Ex-protection and they are type examined up to ATEX zone 0. This zone contains continuously or frequently an explosive atmosphere and requires increased safety. All measures for production, repair and installations are audited and certified annually by the TÜV.
Video "Splash pattern of a draft tube mixer"