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Mixing
and dosing
Water Vortex Mixing technology offers new opportunities
By Bruno Segers, Mervers and Neil Cathie, Chemineer
Static mixers, which are now capable of handling widely varying flow
conditions for additive injection and chemical dosing applications in
the water and wastewater industry, are making a significant contribution
towards reducing capital costs.
According to Chemineer, a company with more than 30 yerats’ experience
in water industry mixing applications, new Water Vortex Mixers (WVM)
present an opportunity to optimize a mixer’s length against mixture
quality parameter, so mixers can be specifically tailored to individual
mixing problems. With test data independently validated, using both
computational fluid mixing (CFM) and laser induced fluorescence (LIF)
mixture quality measurement techniques by the world-renowned BHR Group,
a WVM offers greatly improved mixing efficencies at low flow rates
(0.1-0.3m/s).
Less maintenance requirements
Designed with low head loss and an open, non-blocking structure, a
WVM also features a simple, non-intrusive wall additive injection point
and a pre-distribution tab. Compared to more complex multi-hole sparger
injector designs, this combination improves quantifiable mixture quality
by at least 10% and greatly reduces maintenance requirements. The WVM is
available in a range of Drinking Water inpectorate (DWI) regulation 25
approved coatings, as well as 316l stainless steel to meed OGCP
requirements. Many of the first WwTWs applications used a Kenics helical
element KMS mixer. This is still widely used today for pipes of 150mm
diameter or less, for gas-liquid dispersion applications and where
mixing is required at low flow conditions.
The early 1990s saw the advent of aan innovative static mixer that
generated multiple vortexes to create highly engery-efficient (low head
loss) radial mixing effects. This mixer, which has become known as the
high efficiency vortex (HEV) set the standard for ultra-low head loss
mixing.
Over the las 12 years, more than 1,000 full-scale HEV mixers, with
diameters ranging from 50-2,200mm and for open channels up to 6m wide,
have been installed worldwide in water and WmTWs. Market research has
shown the Kenics HEV mixer is being increasingly accepted by the
industry for two main reasons – its low head loss and its open,
non-blocking structure.
Conversely, and HEV requires relatively long and expensive injectors and
is not that will-suited to low flow rate conditions. Building on the key
advantages of the HEV, the new range of WVM allows the main poject
design parameters of length and pressure drop to achieve a certain
coefficient of variation, CoV (2), or/x is the standard deviation of
additive concentrations in any samples divided by the mean mixed
concentration of the additive, a commonly used and quantifiable measure
of mixture quality, within a certain time to be optimesed, for a WVM,
the precise number of banks of mixing elements depends on the mixture
quality required for the process. The dimensions, angle of attack and
spacing of the trapezoidal mixing elements varies, while the
pre-distribution tab and the injection point design are the same for all
WVM models.
CFM Software
The pre-distribution tab was visually modeled and optimized using
CFM software. Additives injected all the wall of the pipe are spread
into a horseshoe shape by the pre-distribution tab – the use of which
allows the simplest of injectors to be utilized – either a flanged
nozzle or a threaded boss, to which the chemical injection line can be
connected. A proprietary injection fitting with corporation cock
shut-off device can also be fitted to the wall-mounted boss if required.
The pre-distribution tab improves quantifiable mixture quality (CoV) by
at least 10% over a wide range of low rates compared to the use of a
multi-hole spranger injector. The improvement in mixing at low Reynolds
number (velocity 0,1 m/s) conditions has proved to be the greatest. The
bank of WVM mixing elements can be arranged in line with eacht other but
off-centre with respect to the pre-distribution tab. The horseshoe
shaped pre-distributed additive is then cought by the tips of the first
bank of WVM elements and drawn into the axially oriented spinning
vortices behind them.
Each element creates two vortices and each bank creates eight. The
successive banks maintain and intensify vortices throughout the length
of the mixer and for a considerable length downstream.
Research and test model
The pressure loss across the WVM models can be predicted by
correlations developed from research carried ou by the BHR Group to
determine friction factors for each WVM model and for a range of
differing Reynoleds numbers and banks of mixing elements.
Quantifiable mixture quality CoV has been determined during the same
test programme, over the same ranges of Reynolds numbers and numbers of
banks of elements, and using LIF techniquesto measure radial homogeneity.
A WVM enables individually project optimized mixers of a simpler design,
having improved performance at lower Reynolds numbers (and at lower cost
than was the case with an HEV mixer) and the following key features:
§ simple, non-instrusive wall injection point
§ optimization of length versus pressure drop parameters to suit the
application is now possible
§ improved performance at low flow rates (0,1-0,3m/s)
§ lower capex and opex
§ test date validated by independent organization, the BHR Group
§ design approved for us by MW for United Utilities projects <<
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