Achieve better mixing and dosing

Chemineer's Neil Cathie presents a detailed analysis of the diverse range of flow conditions static mixers are now capable of handling

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 years’ experience in water industry mixing applications, new water vortex mixers (WVM) present an opportunity to optimise 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 efficiencies at low flow rates (0.1-0.3m/s).

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 Inspectorate (DWI) regulation 25 approved coatings, as well as 316L stainless steel to meet 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 an innovative static mixer that generated multiple vortexes to create highly energy-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 last 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 WwTWs. 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, an HEV requires relatively long and expensive injectors and is not that well-suited to low flow rate conditions. Building on the key advantages of the HEV, the new range of WVMs allows the main project 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 optimised. 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.

The pre-distribution tab was visually modelled and optimised using CFM software. Additives injected at 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 utilised – 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 flow rates compared to the use of a multi-hole sparger injector. The improvement in mixing at low Reynolds number (velocity 0.1m/s) conditions has proved to be the greatest. The banks of WVM mixing elements can be arranged in line with each other but off-centre with respect to the pre-distribution tab. The horseshoe shaped pre-distributed additive is then caught 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.

The pressure loss across the WVM models can be predicted by correlations developed from research carried out by the BHR Group to determine friction factors for each WVM model and for a range of differing Reynolds 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 techniques to measure radial homogeneity. A WVM enables individually project optimised mixers of a simpler design, having improved performance at lower Reynolds numbers (and at a lower cost than was the case with an HEV mixer) and the following key features:

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