SK Type Helical Static Mixer

Product Overview

When the process fluid behaves more like cold honey than like water, the SK type helical static mixer is the model that does the work. Inside the housing, a series of flat metal blades — twisted 180° or 270° from inlet face to outlet face — sit one behind the other. Each successive element is rotated 90° to its neighbor, so the flow that has been twisted clockwise by one element is immediately counter-twisted by the next. The net effect is not droplet breakup (that is the domain of the SV and SX); it is the controlled folding and reorientation of viscous streamlines until the composition profile is uniform across the pipe cross-section.

The headline specification of the SK is its 1,000,000 cP viscosity ceiling — three to four orders of magnitude beyond the SV, SX, and SL models. That is what makes the SK the only static mixer in the ywmixing catalog that can be specified for the most punishing heavy-industrial services: Bunker C fuel oil and bitumen blending, polymer melt homogenization, animal-fat and hydrogenated-oil processing, and the activation of high-molecular-weight flocculants in sludge dewatering. None of these applications would work in a plate or bar mixer, because the fluids would simply refuse to flow through the narrow channels.

The SK also has the lowest clogging risk of any model in our line. The twisted-blade geometry has no closed channels and no small orifices; anything that will pass through the pipe bore will pass through the mixer. This makes the SK the default choice for slurries with up to 30% solids by volume, fibrous media, and other "dirty" services. The trade-off is coarser dispersion: SK delivers ≤10 μm droplets, which is fine for blending high-viscosity fluids but is not appropriate when the process needs sub-micron emulsions.

Working Principle

The internal element is a rectangular plate, typically 0.8-2.0 mm thick for stainless steel or 1.5-3.0 mm for carbon steel, twisted along its longitudinal axis. The aspect ratio (width-to-length) of the plate is chosen so that the twisted element just fits inside the housing with a clearance of 1-2 mm to the wall. Standard twist angles are 180° (used in most general-purpose applications) and 270° (used when the additional rotational shear is needed for very viscous media). The element length is usually 1.0-1.5 pipe diameters.

When fluid enters the mixer, the twisted blade forces it into a helical motion. The pitch of the helix is determined by the twist angle and the element length; for a 180° element of length 1.5D, the induced swirl is approximately one full turn over the element. As the fluid exits the first element and enters the second (rotated 90°), the swirl direction is reversed and the flow is forced to reorient. The result is a chaotic, three-dimensional flow pattern in which the original stream tubes are repeatedly stretched, folded, and re-stacked.

The dominant mixing mechanism in the SK is laminar stretching and folding — not turbulent breakup, since most SK applications operate at Re < 1,000 (often Re < 100). This is the regime in which the Kenics static mixer, the parent design of our SK model, has been most thoroughly characterized in the academic literature. The mixing length required to reach σX ≤ 5% is typically 5-8 elements for 180° twist and 4-6 elements for 270° twist. Pressure drop per element is roughly 1.5-3 velocity heads (compared to 5-10 for an SV element), which is why the SK is preferred when the process pump is marginal on head.

Technical Specifications

Parameter	Value
Internal Structure	Single helical twist blade (180° or 270°), adjacent elements rotated 90° with reversed twist direction
Dispersion Precision	≤ 10 μm (coarser than SV/SX; appropriate for viscous blending)
Viscosity Range	1 cP to 1,000,000 cP — highest in the product line
Pressure Drop	Moderate; ~1.5-3 velocity heads per element
Non-Uniformity Coefficient	σX ≤ 5% with 5-8 elements (180°) or 4-6 elements (270°)
Material Options	SS304, SS316L, Carbon Steel, PVC, PP, Hastelloy C-276
Connection Types	Flange (ANSI / DIN / JIS), Threaded (NPT / BSP), Welded, Tri-Clamp
Temperature Range	-50°C to +300°C (metallic) / -10°C to +80°C (PVC/PP)
Pipe Diameter Range	DN15 to DN500
Flow Rate Range	0.05 to 800 m³/h (model and pipe-size dependent)
Clogging Risk	Lowest of all five models — open geometry, no small channels

Material Options

Helical blades for the SK are available in the same six material grades as the rest of the line. SS316L is the most common specification for chemical and pharmaceutical service because the low-carbon formulation allows the twisted plate to be welded into the housing without subsequent sensitization. The 180° twist geometry produces a residual stress pattern in the plate, and any carbon above 0.03% in a standard 316 would risk intergranular corrosion at the weld HAZ — a failure mode that has shown up in several of our competitor's products and that we eliminate by specifying L-grade for welded assemblies.

SS304 is used for less aggressive services — water treatment polymers, food-grade thickeners, and dilute chemical streams where chloride is below 200 ppm. Carbon steel (A36 or A516) is specified for heavy-oil blending and bitumen service, where the temperature is 100-200°C and the process fluid is essentially non-corrosive to steel. Carbon steel elements are usually 2-3 mm thick to provide the mechanical strength needed for the twisting operation.

PVC and PP are used in low-temperature chemical service (below 80°C) where stainless would be over-specified. The maximum element length is restricted to about DN200 because the plastic cannot be reliably twisted in larger sizes without cracking. Hastelloy C-276 is reserved for the most aggressive chemistries — wet chlorine, hot sulfuric acid above 70%, and ferric chloride etching baths. The cost premium is 5-7× over SS316L, but the service life is measured in years rather than months for these media.

Installation Methods

SK mixers follow the same four standard connection types as the rest of the line, but the heavy-industry context of most SK applications means that flanged and welded connections dominate. Flanged connections (ANSI B16.5 Class 150 / 300, DIN PN16/PN40) are the default for DN50 and above. The mixer housing carries integral flanges; the connecting pipe flanges must be aligned to within 0.5 mm to avoid inducing bending stress on the housing. For high-temperature service above 250°C, the flange bolts are specified as ASTM A193 B7 with graphite gaskets, and the bolts are re-torqued after the first heat-up cycle.

Welded connections are preferred for permanent installations in heavy-oil and bitumen service, where the process temperature is high enough that a flanged joint would be a chronic leak source. The housing is supplied with weld-prep ends matching the pipe schedule, and the field weld is typically a full-penetration TIG weld followed by post-weld heat treatment when the wall thickness exceeds 8 mm or the service is above 200°C.

Threaded connections (NPT or BSP) are used for DN15-DN50 mixers in skid-mounted packages, such as the polymer-activation skids supplied to wastewater-treatment plants. These skids are pre-assembled, factory-tested, and shipped as a single unit; threaded connections speed up the on-site hookup. Tri-clamp connections are specified for the food and pharmaceutical SK installations — yogurt texture smoothing, margarine processing, and pharmaceutical ointment mixing — where the line must be CIP-cleaned between batches.

For all installation types, the SK requires a minimum straight-pipe run of 2D upstream and 2D downstream. The 2D figure is lower than the SV's 3D because the open helical geometry is more forgiving of inlet velocity-profile distortion.

Typical Applications

Heavy fuel oil additive blending. Marine bunker fuel and power-station heavy fuel oil (HFO, grade 380 or higher) must be dosed with combustion improvers, sludge dispersants, and pour-point depressants before being burned. The fuel itself has a viscosity of 100-500 cP at 50°C, and the additives are typically high-molecular-weight polymers that resist diffusion. An SK mixer in a DN150 line processing 200 m³/h of HFO at 80°C achieves additive distribution uniformity within σX ≤ 3% in a single pass — replacing the recirculation tank and mechanical agitator that the refinery would otherwise have to install.

Yogurt texture smoothing. Set-type yogurt and stirred yogurt both require the smooth incorporation of fruit preparations, sugar syrups, and stabilizers into a viscous cultured base (viscosity 2,000-10,000 cP at 20°C). The SK element, in a sanitary SS316L housing with Ra ≤ 0.8 μm finish, provides the gentle folding action that distributes the inclusions without breaking the fragile protein gel network. Compared to a rotor-stator homogenizer, the SK produces 60% less specific energy input and does not raise the product temperature.

Animal fat, butter, and wax blending. The food oils industry processes animal fats (lard, tallow), butter, and natural waxes at 40-80°C in the liquid state, with viscosities of 50-20,000 cP. SK mixers are used to homogenize these fats with additives such as vitamin premixes, emulsifiers (mono- and diglycerides), and antioxidants. The all-stainless construction and the absence of any moving parts in the product zone meet the hygienic-design requirements of 3-A Sanitary Standards.

High-viscosity polymer solution make-up. Water-treatment plants and paper mills activate polyacrylamide (PAM) and similar high-molecular-weight flocculants by diluting a concentrated emulsion polymer (viscosity 5,000-100,000 cP) into water at 0.05-0.2% concentration. The activation is shear-sensitive — too much shear breaks the polymer chains and destroys the flocculation performance. The SK's gentle laminar mixing provides activation in 30-60 seconds of residence time without significant chain scission, whereas a high-shear mixer would lose 30-40% of the polymer's molecular weight in the same duty.

Sludge-dewatering polymer activation. Municipal sewage sludge is dewatered on belt presses or centrifuges, with cationic polymer (typically polyacrylamide with charge density 50-80%) injected just before the dewatering device. The polymer must be activated (i.e., the emulsion inverted and the polymer chains allowed to uncoil) in 5-20 seconds of residence time. SK mixers sized for the sludge line (DN80-DN200) provide consistent activation at viscosities of 1,000-50,000 cP, allowing the dewatering plant to operate at higher solids capture (typically 96-98%) than is possible with static mixers that have more aggressive geometries.

Model Comparison

Parameter	SK	SV	SX	SH	SL
Internal Structure	Helical twist	Corrugated plates	Crossed bars	Dual-channel helical	Single-X bars
Dispersion Precision	≤ 10 μm	1-2 μm	2-5 μm	1-2 mm	Medium
Max Viscosity (cP)	1,000,000	100	10,000	1,000,000	10 (or polymer)
Pipe Diameter	DN15-DN500	DN15-DN1000	DN25-DN800	DN15-DN200	DN15-DN600
Clogging Risk	Lowest	High	Moderate	Moderate	Low
Best For	High-viscosity	Fine dispersion	Mid-range	High-viscosity clean	Heat transfer

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