SLB Type Long Helical Static Mixer

Product Overview

The SLB type long helical static mixer is the low-pressure-drop specialist of the ywmixing product line. Inside a cylindrical housing, a single helical element — typically with a pitch of 1.5-3.0 pipe diameters per turn and a total length of 8-15 pipe diameters — imparts a gentle, continuous swirl to the process flow. The element is fabricated from sheet metal (typically 1.0-2.0 mm thick for SS316L, 2.0-4.0 mm for carbon steel) twisted to the specified helix angle, with edge profiles designed to minimize flow separation. The result is a mixer that delivers 3-8 μm dispersion precision with σX ≤ 3% while losing only 0.03-0.08 bar at 1.5 m/s water service in a DN100 housing — about 5-10× lower pressure drop than the equivalent SV corrugated plate mixer.

This combination of fine dispersion and very low ΔP makes the SLB the mixer of choice for processes where the pumping-energy penalty is a constraint: gravity-flow lines, long transfer pipelines, in-pipe heat-exchanger and reactor cooling circuits, and any service where the system pressure budget is already allocated to other equipment. It is also the preferred model for viscous service up to 100,000 cP, where the long-helix geometry provides smooth flow transition and avoids the flow separation that limits the more aggressive SK mixer above 1,000,000 cP.

Across the ywmixing family (SV, SK, SX, SH, SL, plus the new SD, SDB, SLB), the SLB occupies a unique position: it is the only model that approaches the SV's 1-2 μm dispersion precision without paying the SV's pressure-drop penalty, while still handling the medium-viscosity range that the SV cannot reach. For a process engineer sizing a low-ΔP continuous blending line, the SLB is typically the first mixer considered before falling back on the SD or SDB plate mixers or the SK helical-twist mixer.

Working Principle

The mixing action of the SLB is generated by the continuous swirl imparted to the flow as it traverses the long helical element. Unlike the short-helix SK mixer — which forces the flow through 180°/270° turns every 1-1.5 pipe diameters and produces intense but localized radial mixing — the SLB's gentle, progressive twist distributes the mixing over the full element length. At any cross-section, the flow has both an axial component (carrying it down the pipe) and a tangential component (imparted by the helix), and the radial concentration profile is continuously reworked by the secondary swirl as the fluid advances.

Quantitatively, the tangential velocity at the housing wall is v_θ = v_axial · tan(α), where α is the helix angle. For the standard SLB geometry (1.5-3.0 pipe diameters per turn), α is in the range 12-25°, giving v_θ/v_axial = 0.21-0.47. This ratio is the dimensionless swirl number S = v_θ/v_axial of the geometry, and it controls the radial mixing rate. Higher S (tighter helix) produces faster mixing at higher ΔP; lower S (looser helix) produces gentler mixing at lower ΔP. The SLB is biased toward lower S (1.5-2.0 D per turn is most common) to keep ΔP minimal while still meeting the σX ≤ 3% target over an 8-15D housing length.

Pressure drop across the SLB is governed by Δp = ½·ρ·v²·K·(L/D), where the loss coefficient K per unit L/D is in the range 0.4-0.8 for the standard helix geometry. This compares to K = 1.5-2.5 per plate for the SD and K = 1.2-2.0 per SK element. For a 10D housing, the total Δp at 1.5 m/s water service in a DN100 unit is approximately 0.04-0.08 bar — low enough that the SLB can be installed in gravity-flow lines and long transfer pipelines without requiring pump upgrade. In the turbulent regime (Re > 4,000), the SLB achieves 3-8 μm dispersion precision with σX ≤ 3%. In the laminar regime (Re < 2,300), the mixing is driven by the secondary flow pattern (Dean vortices in the curved flow) and the dispersion coarsens to 8-15 μm with σX in the 5-8% range.

Two additional effects make the SLB particularly useful beyond simple blending. First, the swirl component augments radial heat transport; the inside-to-outside heat-transfer coefficient is 1.5-2.5× higher than an empty pipe at the same flow rate. This makes the SLB an effective in-pipe heat-transfer enhancement device for reactor cooling and viscous-fluid heating. Second, the swirl persists for 3-8 pipe diameters downstream of the housing, so the mixing continues to develop in the downstream pipe — a useful effect when the immediate downstream fitting (a valve, an instrument, or a reactor inlet) would otherwise see an unmixed stream.