SL Type Single-X Static Mixer

Product Overview

The SL type single-X static mixer is the model in the ywmixing range that we reach for when the process problem is fundamentally a heat-transfer problem with a mixing requirement attached, rather than the other way around. The element is a single set of crossed bars — an X-shape mounted in a short cylindrical housing, with a typical free cross-section of 75% of the pipe bore. The element's job is to interrupt the thermal boundary layer that forms at the inside wall of any pipe carrying a hot or cold fluid, and to push the bulk of the fluid into repeated contact with the wall. The result is a 3-5× increase in the inside-film heat-transfer coefficient compared to an empty pipe at the same flow rate.

The SL is also a competent mixer in its own right, but its dispersion precision is medium — coarser than the SV, SX, and SH. That is a deliberate trade-off. The X-bar geometry is a single interruption per element, not a multi-stage split-and-recombine geometry, so the SL achieves only modest reduction of concentration non-uniformity. Where the SL excels is in duties that combine blending with thermal management: a heated reactor feed where the reagents must reach the reaction temperature at the same time they reach the reactor, a viscous product that must be cooled from 150°C to 40°C before it can be packaged, or a process stream that must be conditioned to within ±1°C of a setpoint.

The viscosity envelope is the SL's most distinctive feature. It is rated for fluids up to 10 cP (water-like) or for polymer solutions at any viscosity — the "polymer media" exception in our specification reflects the fact that high-molecular-weight polymer solutions are shear-thinning and behave like low-viscosity fluids once they are moving. This dual envelope means the SL can be specified for both the heating side (low-viscosity heat-transfer fluid in the jacket) and the process side (viscous polymer in the pipe) of a single process.

Working Principle

The SL element consists of two rectangular bars welded or mechanically fastened at the pipe centerline to form an X. The bar width is typically 0.10-0.15 times the pipe diameter, and the bar thickness is 3-5 mm for SS316L or 5-8 mm for carbon steel. The element length is 1.0-1.5D. In a typical installation, two or three elements are stacked inside a housing of overall length 4-6D, with no rotation between elements (the X orientation is the same for every element, so the flow sees the same X-pattern repeatedly).

From a heat-transfer perspective, the X-bars work by two mechanisms. First, the bars physically obstruct the flow near the wall, forcing fluid from the cooler (or warmer) wall region into the bulk. This disrupts the thermal boundary layer and replaces it with a thinner one each time the fluid passes an element. Second, the bars shed vortices into the downstream flow, which mix the wall-adjacent fluid with the core fluid through turbulent (or pseudo-turbulent in laminar flow) eddy transport. The net effect is that the temperature profile across the pipe becomes much flatter than in an empty pipe, where the wall-adjacent layer can be 5-10°C different from the core even in fully developed flow.

From a mixing perspective, the X-bars split the flow into four quadrants, similar to the SX but without the rotation between elements. Each quadrant is forced to merge with its neighbor at the trailing edge of the X, producing a 4-to-1 stream reduction at each element. Over 3 elements, the fluid has been split 64 times — which is enough to bring σX down to about 5%, but not enough to reach the 1-5% band that the SV, SK, SX, and SH can deliver. For most heat-transfer-dominated processes, 5% is more than sufficient, because the downstream thermal relaxation takes care of the remaining non-uniformity.

The SL is one of the few static mixers in the industry that is explicitly designed to operate in the laminar regime. In laminar flow (Re < 2,300), an empty pipe is a poor mixer because the parabolic velocity profile means fluid in the center moves much faster than fluid at the wall. The X-bars force the slow wall fluid to re-merge with the fast core fluid, eliminating the parabolic profile over a length of about 3-5 elements.

Technical Specifications

Parameter	Value
Internal Structure	Crossed bars forming single X-shaped units; 2-3 elements per housing (no inter-element rotation)
Dispersion Precision	Medium — σX ≤ 5% with 3 elements (not the precision leader)
Viscosity Range	≤ 10 cP for Newtonian fluids; or any viscosity for shear-thinning polymer media
Pressure Drop	Moderate; 1-2 velocity heads per element (lowest of the five models)
Non-Uniformity Coefficient	σX ≤ 5% with 2-3 elements (in turbulent service); σX ≤ 3% with 3 elements (in laminar service with thermal relaxation)
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 DN600
Flow Rate Range	0.05 to 1,200 m³/h (model and pipe-size dependent)
Heat-Transfer Enhancement	3-5× inside-film coefficient vs empty pipe at the same flow rate

Material Options

SS316L is the most common material for SL elements in chemical and pharmaceutical service. The crossed-bar geometry produces a moderate residual stress pattern in the bars, but unlike the SK twisted plate, the SL bar stresses are below the threshold that requires L-grade specification. Standard 316 is sometimes used when the welded joint can be located away from the high-stress region. SS304 is the default for water, glycol, and other heat-transfer fluids in HVAC and process-cooling service. The lower alloy content translates to a 15-20% cost saving, which matters for the large-diameter SL mixers (DN300-DN600) that are used in district heating and cooling distribution systems.

Carbon steel (A36 or A516 plate) is the workhorse for oil-industry SL installations — crude oil preheating, heavy-product cooling, and asphalt temperature conditioning. Carbon steel SL mixers in DN200-DN600 are among our highest-volume products, used in refineries and petrochemical plants worldwide. For mildly corrosive service, the carbon steel is internally coated with a phenolic or epoxy coating at 150-250 μm thickness; this coating is rated for continuous service up to 120°C and pH 4-10.

PVC and PP SL mixers are produced in sizes DN15-DN200 for low-temperature chemical and water-treatment service. The plastic crossed bars are machined from solid stock and solvent-welded to a plastic divider ring, which is then inserted into the plastic housing. Hastelloy C-276 is specified for the most aggressive chemistries — typically DN25-DN100 SL mixers in sulfuric acid coolers, hydrochloric acid absorbers, and chlor-alkali cell cooling loops. The cost premium is 5-7× over SS316L, justified by the 10+ year service life in media where stainless would fail within 12 months.

Installation Methods

The four standard connection types apply to the SL, with flanged connections dominating the larger sizes and threaded connections dominating the smaller sizes. Flanged connections (ANSI B16.5 Class 150 / 300, DIN PN16 / PN40, JIS 10K / 20K) are standard for DN50 and above. The SL housing is supplied with integral flanges; the connecting pipe flanges must be aligned to within 1 mm to avoid inducing bending stress on the housing.

Threaded connections (NPT or BSP) are used for DN15-DN50 mixers in skid packages and OEM equipment. The threaded end is typically NPT for North American installations and BSP for European and Asian installations. Threaded SL mixers are usually supplied with a hex flats on the housing for installation in confined spaces. Welded connections are preferred for permanent installations in high-pressure service (above 16 bar) and in high-temperature service (above 200°C). The housing is supplied with weld-prep ends matching the pipe schedule.

Tri-clamp connections are used for the food, dairy, beverage, and pharmaceutical SL installations — typically the smaller sizes (DN25-DN100) for in-line product heating and cooling. The sanitary design specifies Ra ≤ 0.8 μm surface finish and full CIP/SIP capability. Tri-clamp connections also make the SL mixer easy to remove for inspection or for changeover to a different product in multi-product facilities.

For all installation types, the SL requires a minimum straight-pipe run of 2D upstream and 2D downstream. The 2D figure is the same as the SX because the bar geometry is similarly tolerant of inlet velocity-profile distortion. For heat-transfer service specifically, the SL is usually installed inside a jacketed pipe section (or inside a shell-and-tube heat exchanger shell) so that the heat-transfer surface surrounds the mixer.

Typical Applications

Heat exchange enhancement. The SL's primary use is as a passive heat-transfer enhancer inside jacketed pipes and shell-and-tube heat exchangers. For a typical service — cooling a 100 cP process fluid from 80°C to 40°C in a DN100 jacketed pipe — an SL mixer reduces the required jacket length by 60% compared to an empty pipe at the same flow rate. The 3-5× inside-film coefficient improvement is documented in our technical library and has been independently verified in dozens of refinery and chemical-plant installations.

Viscous product heating/cooling. The food oils industry cools hydrogenated oils, shortenings, and margarine from 80°C to 40°C in jacketed scraped-surface heat exchangers, but for less demanding temperature ranges, a jacketed pipe with an SL mixer provides adequate heat transfer at much lower capital cost. The SL's tolerance of high-viscosity polymer media (via the shear-thinning exception) makes it the default choice for these duties. Typical applications include chocolate tempering, butter cooling, and shortening crystallization pre-cooling.

Oil industry mixing + heat transfer. Refineries use SL mixers in crude unit preheat trains, where the crude is heated from ambient to 250°C in a series of heat exchangers that recover heat from downstream process streams. The SL's combined mixing and heat-transfer enhancement is particularly valuable at the phase-change points — where the crude transitions from liquid to two-phase liquid-vapor, and where uniform temperature distribution is critical to prevent hot spots that would coke the heat-exchanger tubes.

Low-viscosity chemical blending with thermal management. Many chemical reactions are exothermic or endothermic, and the feed streams must be preheated or precooled to the reaction temperature before they enter the reactor. The SL mixer, installed in the feed line just upstream of the reactor, simultaneously heats (or cools) the feed to the reaction temperature and blends the reagents to within σX ≤ 5%. This eliminates the separate pre-mix tank and pre-heater that older plant designs required, saving both capital and floor space.

District heating and cooling distribution. Large district heating and cooling systems use SL mixers in the building substations to condition the circulating water to the building's required temperature setpoint. A DN300 SL mixer in a 5,000 m³/h hot-water district heating loop provides the required mixing and heat transfer in a single 1.5-meter housing, replacing the larger mixing tank and circulation pump that older substations required. The carbon steel construction with internal epoxy coating is the standard material for this service.

Model Comparison

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

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