Secondary Cooling Nozzles: Continuous Casting Design and Selection

In a continuous caster, the strand leaves the mold at roughly 1,200–1,300 °C with a solidified shell only a few centimetres thick. Everything that happens to the strand from the mold exit to the cutting torch is secondary cooling: hundreds of secondary cooling nozzles spraying water or air-mist onto the surface to extract heat at a controlled rate. Get that cooling even, and the strand is sound. Get it uneven, and you get bulging, surface cracks, internal cracks and a cast that has to be scrapped.

This guide covers how secondary cooling is zoned, why air-mist beats water-only for the modern caster, how specific water flow is set, and the nozzle layout rules that make cooling uniform.

Why the Strand Needs Zoned Cooling

The strand is hottest and the shell thinnest right below the mold. Cooling must be aggressive there but gradually gentler as the shell thickens. The surface temperature should fall from about 1,200 °C at the mold exit to roughly 700–900 °C before the straightener, with the cooling rate matched to the steel grade.

That requirement is met with 8 to 15 cooling zones down the strand, each with its own water flow setpoint:

Zone	Position	Typical cooling duty
Foot zone	Just below the mold	Intense, fast quench of the shell
Upper spray zones	First 2–4 m	High water flow, coarse drops
Middle zones	Mid-strand	Moderate flow, finer control
Lower zones	Near straightener	Low flow, gentle, uniform

The change in specific water flow from foot zone to lower zones is typically 2:1 to 5:1. The foot zone may run at 2–3 L/kg while the lower zones run at 0.3–0.6 L/kg.

Water-Only versus Air-Mist

Two nozzle families serve secondary cooling:

Water-only sprays. Flat fan and full cone nozzles at 2–10 bar, mounted in staggered rows. Simple, high flow, proven. The limitation is turndown: a hydraulic nozzle’s flow scales with the square root of pressure, so cutting flow by half requires cutting pressure to a quarter, which changes the spray pattern. Water-only systems handle the upper zones well but struggle to deliver very low, very even flow in the lower zones.

Air-mist sprays. Water is atomized by compressed air at the nozzle, giving fine drops (100–400 µm) at low water flow (0.5–20 L/min) with pattern width controlled independently of flow. Air-mist provides the wide turndown that water-only cannot. Flow can be varied over a large range while keeping the spray pattern constant. That is why air-mist dominates the lower and middle zones of modern slab and bloom casters.

The trade-off is operating cost: air-mist consumes compressed air continuously. The standard answer is air-mist in the zones where control matters and water-only where flow is high and simple.

Specific Water Flow: The Number That Sets Everything

Specific water flow is the litres of water per kilogram of steel cast: the overall water budget of the secondary cooling system. Industry practice for continuous casting runs roughly 0.5–1.0 L/kg, with the higher end for high-carbon and peritectic grades that need aggressive cooling and the lower end for crack-sensitive grades.

The value is set by the steel grade, section size and casting speed:

Low-carbon and medium-carbon steels: 0.5–0.8 L/kg
High-carbon steels: up to ~1.0 L/kg, with high-carbon wire grades at the top end
Peritectic grades: cooling must avoid the peritectic temperature range, so the budget is tuned zone by zone rather than maximized

Spray heat transfer scales with water flow: heat transfer coefficient rises roughly with water flux to the power 1.2–1.3 (HTC ∝ Q^1.23 is a commonly quoted relationship). That non-linearity is why small local flow variations produce disproportionate temperature variations, and why nozzle uniformity is the dominant design criterion.

Nozzle Layout Rules for Uniform Cooling

Uniform cooling is the whole game. A 10 °C surface temperature variation across the strand width is enough to produce internal cracks in sensitive grades, and corner-spray errors of a few percent can be detected in the rolled product as edge defects. The rules that keep cooling even:

Overlap flat fans by 40–50%. Adjacent flat fan nozzles across the strand width must overlap so the coverage is even; the standard is roughly 40–50% overlap at the strand surface, with the spray angle and standoff chosen to give a wide, even distribution.

Stagger the rows. Rows of nozzles are staggered so the edge of one spray covers the centre of the next, avoiding a repeating hot-cold pattern down the strand.

Control the corners separately. Corners cool faster than faces because heat can leave through two surfaces. Corner nozzles are typically switched off or throttled. Corner spray cutback can add 50 °C to the corner surface temperature and reduce corner cracking.

Keep the angle stable. Pattern width at the strand surface depends on spray angle and standoff: width ≈ 2 × standoff × tan(angle/2). The mounting frame must hold nozzles at fixed standoff; any nozzle that drifts changes local cooling.

Match nozzle type to zone. Water-only flat fans in the high-flow zones, air-mist in the low-flow control zones, and dedicated corner nozzles with independent setpoints.

How Spray Heat Transfer Works on the Strand

The water that hits a 1,000 °C strand surface does not cool by simply getting warm. It goes through a boiling curve with three regimes, and the nozzle determines which regime dominates:

Film boiling. At the hottest surface temperatures, water evaporates immediately on contact and forms a continuous vapour film that insulates the surface. Heat transfer is poor. The water skates on a steam cushion. This is the regime at the foot zone, and the spray must be vigorous enough to break through it.

Transition boiling. As the surface cools, the vapour film becomes unstable and liquid intermittently touches the surface. Heat transfer climbs steeply.

Nucleate boiling. Liquid contacts the surface, boils in nucleating bubbles and carries heat away efficiently. This is the regime where heat extraction is highest, and where the spray should operate for most of the strand length.

The practical consequence: at the top of the strand the water mostly evaporates in film boiling and does little cooling per litre, which is why the foot zone needs heavy flow; further down, the same litre of water in nucleate boiling extracts far more heat. Spray design has to be matched to the local surface temperature, not applied uniformly.

Air Consumption: The Price of Control

Air-mist nozzles consume compressed air continuously, and that cost is often underestimated. A typical air-mist nozzle at 1–6 bar air runs roughly 5–30 Nm³/h depending on size and setting. A caster with 200 air-mist nozzles running 8–12 hours a day is consuming a very significant share of the plant compressed-air budget.

Two rules keep the bill sane:

Use air-mist only where control pays. The middle and lower zones, where flow must be low and even, justify the air. The foot and upper zones, running at high water flow, do not need it: water-only flat fans there.
Size the air supply per zone, not per nozzle maximum. Most nozzles never run at maximum air; a header sized for the average with a margin for the peak is cheaper to run than one sized for worst case everywhere.

Nozzle Selection Data for Secondary Cooling

Real secondary cooling layouts are built from a small set of published nozzle classes, not from catalogue ranges. The table below is the working window for strand cooling as quoted in caster engineering literature:

Parameter	Water-only zones	Air-mist zones
Nozzle family	Flat fan, full cone	Flat fan air-atomized
Water pressure	2–10 bar	1–8 bar
Air pressure	-	1–6 bar (commonly 2–3 bar)
Water flow per nozzle	25–300 L/min	0.5–20 L/min
Droplet size	Coarse, 500–2000 µm	Fine, 100–400 µm
Spray height (standoff)	200–400 mm	150–250 mm
Spray angle	40–120° (65–110° typical)	80–120° major, 12–16° thickness
Typical coverage	0.3–1.0 m per nozzle	0.3–0.8 m per nozzle
Air-to-water ratio	-	~10:1 (thin-slab practice)

Two numbers in that table decide most of the layout. First, impingement density: water sprays in secondary cooling typically deliver 1.7–33.3 kg/(m²·s) onto the strand surface, and heat transfer rises with that flux roughly as HTC ∝ Q_w^1.23, so doubling the local flux increases heat extraction by about 2.3×, which is why a single clogged nozzle creates a hot spot, not a warm spot. Second, the fine-zone air-mist window: 3–16 L/(m²·min) at 1–2.5 bar air covers the low-flow, high-uniformity range that water-only nozzles cannot hold, because a hydraulic nozzle at low flow falls off its designed spray angle.

Material selection is straightforward in this service: 316L for process water with good filtration, hardened stainless for scale-laden water or descaling adjacent zones, and ceramic or carbide inserts where the spray is aimed at the hottest, dirtiest surfaces. Brass and zinc-plated bodies have no place in a caster. The scale erodes them in weeks.

Zone-by-Zone Nozzle Duty Table

The practical starting point for any secondary cooling design is the zone duty table: surface temperature window, water intensity, and the nozzle family that serves it. A typical slab caster looks like this:

Zone	Surface temperature window	Water intensity	Nozzle family	Compressed air
Foot zone	~1200 → 1050 °C	2–3 L/kg	Full cone or high-flow flat fan, short standoff	No
Upper spray zones	1050 → 950 °C	1.5–2.5 L/kg	Flat fan, 25–300 L/min per nozzle	No
Middle zones	950 → 850 °C	0.8–1.5 L/kg	Flat fan + air-mist mix	Yes, fine zones
Lower zones	850 → 750 °C	0.3–0.6 L/kg	Air-mist flat fan	Yes, continuous

Read the table as a turndown ladder: each step down the strand needs roughly half the water of the step above, at finer control. The foot zone runs coarse and hard because it must break the vapour film on a 1200 °C surface; the lower zones run fine and gentle because the strand is one degree of over-cooling away from centre segregation. That is why a single zone-by-zone water budget, written as specific water flow, precedes every nozzle size decision on the machine.

Worked Example: Sizing a Slab Caster Secondary Cooling Zone

Take a 1.8 m wide slab caster running a 240 mm thick slab at 1.2 m/min, aiming for a specific water flow of 0.8 L/kg in the upper zones. The steel throughput: 1.8 × 0.24 m cross-section × 1.2 m/min × 7.85 t/m³ ≈ 4.07 t/min, so the upper-zone water budget is 0.8 L/kg × 4070 kg/min ≈ 3,256 L/min across the zone.

Nozzle count per row: a 90° flat fan at 400 mm standoff covers 2 × 0.4 × tan(45°) = 0.8 m. With 50% overlap, effective coverage per nozzle is 0.4 m, so a 1.8 m strand needs 1.8 ÷ 0.4 = 4.5 → 5 nozzles per row. If each nozzle runs 40 L/min at 3 bar, one row delivers 200 L/min; the 3,256 L/min budget therefore needs about 16 active rows in that zone. Because the flat fan widens as flow rises (a typical 90° fan opens to roughly 98° at the top of its range), the layout is checked at the operating flow, not the catalogue angle. A row set at minimum flow develops gaps at maximum flow.

Air-mist check for the same strand: the fine zone runs 3–16 L/(m²·min), so for 1.8 m width × 12 m zone length = 21.6 m², the water window is roughly 65–350 L/min. At a 10:1 air-to-water ratio that is 650–3,500 Nm³/h of air, which is why mills stage air-mist to the zones that need it and run water-only elsewhere.

Dynamic Control and Modern Practice

Secondary cooling is no longer a fixed table of zone flows. Modern casters close the loop: thermocouples embedded in the strand surface or pyrometers at zone boundaries feed a solidification model that adjusts each zone’s flow setpoint continuously. Published results hold strand surface temperature within ±4 °C of target across the full length. A precision that matters because the difference between 0.9 and 1.2 L/kg specific cooling decides whether carbon segregates at the billet centre.

Three control-side practices change nozzle selection:

Wide turndown per zone. When a zone must swing from 100% to 20% flow without changing spray pattern, the nozzle choice is forced to air-mist. No hydraulic nozzle holds its angle across a 5:1 turndown.
Fast valve response. Pneumatic valves with response times under 1 s allow per-segment flow changes as the strand accelerates or slows; slow valves smear the cooling curve and defeat the model.
Nozzle status monitoring. Pressure and flow transmitters per zone, plus pattern checks, catch a plugged nozzle before it writes a defect into 200 tonnes of steel. A plugged corner nozzle is invisible in the average temperature and unmistakable in the rolled product.

The same dynamic-control argument pushes water quality: a model holding ±4 °C cannot cope with scale build-up changing effective orifice area week by week. Filtration upstream of the zones is part of the control system, not a maintenance option.

Defects That Trace Back to the Nozzles

When secondary cooling is wrong, the strand tells you, usually in the rolled product. The defect families that nozzle selection and maintenance directly affect:

Longitudinal facial cracks. Caused by surface temperature cycling into the brittle range. Uneven spray, missing nozzles and blocked orifices create local cold spots that trigger cracks.

Internal cracks (midway and centre). The interior of the strand cools by conduction; when the surface temperature swings, the solidification front can pull apart. Uniform surface cooling is the first defence.

Rhomboidity and off-corner cracks on billets. Corner cooling asymmetry, typically a corner nozzle set at the wrong angle, flow or distance, distorts the billet cross-section.

Surface reheating between zones. If zones are spaced too far apart, the strand surface reheats between them, and the reheating cycle is a crack driver. Zone spacing and nozzle coverage must overlap.

Shell thinning (breakout risk). Excessive local cooling in the foot zone thins the shell; a breakout is the catastrophic failure mode of a caster and always gets traced back to cooling control.

Maintenance and Pattern Verification

Nozzles in a caster environment die quietly: scale and debris plug small orifices, spray angle drifts as mounts loosen, and abrasive scale erodes orifices so flow rises above setpoint. A maintenance plan that treats nozzles as consumables pays for itself:

Daily: visual pattern check on the empty strand; log pressure and flow per zone.
Weekly: clean and inspect foot-zone and corner nozzles (highest heat load).
Monthly: rotate nozzle bodies; check angles and standoff on the mounting frame.
Per campaign: verify flow against setpoint with a flow meter; replace any nozzle more than 10% off spec.
Water quality: filtration upstream is cheaper than unblocking hundreds of nozzles. Scale inhibitors pay for themselves in nozzle life alone.

Secondary Cooling Nozzle FAQ

What pressure do secondary cooling nozzles run at? Water-only zones run 2–10 bar; air-mist nozzles run 1–6 bar air with water at 1–8 bar. Pressure per zone is set by the flow requirement, not the other way round.

Why are flat fan nozzles standard for strand cooling? The flat fan lays down an even strip of coverage across the strand width with a defined spray angle, which makes overlap calculation and uniform coverage straightforward. Full cones are used in some foot-zone layouts where circular coverage from a tight spacing is preferred.

Can one nozzle type serve all zones? No. The foot and upper zones need high flow at coarse drop size; the middle and lower zones need low flow with fine, controllable drops. One nozzle cannot span that range. The layout uses two or three families.

How do I calculate nozzle count for a strand width? Coverage per nozzle = 2 × standoff × tan(angle/2); divide the strand width by coverage minus the overlap (40–50%) to get nozzles per row, then multiply by the number of rows. A 1.8 m wide strand with 90° fans at 400 mm standoff and 50% overlap needs 5 nozzles per row (0.8 m coverage, 0.4 m effective pitch).

What happens if I run more water than the specific flow target? Flow above the target overcools the surface, increasing thermal stress and crack risk, and wastes water and pumping energy. The specific water flow is a design budget, not a maximum to approach.

How often should air-mist nozzles be replaced? Depends on water quality and running hours; with filtered water, inspect monthly and replace on flow drift. With dirty water, plan a campaign-based replacement schedule.

What is the difference between primary and secondary cooling in casting? Primary cooling is the mold: the copper mould extracts heat from the shell as the strand forms. Secondary cooling is everything below the mold exit: hundreds of nozzles spraying water or air-mist directly onto the strand surface to grow the shell. The mold sets the initial shell thickness; the secondary cooling zone decides whether the rest of the strand solidifies soundly.

How many nozzles does a typical caster have? Several hundred. A slab caster’s secondary cooling zone commonly runs 8–15 zones with 5–10 nozzles per row across the width and multiple rows per zone. A 200–400 nozzle installation is typical, which is why maintenance planning and water quality dominate operating cost.

Selection Checklist for a Secondary Cooling Duty

Strand section (slab, bloom, billet) and casting speed
Steel grade and target cooling rate / specific water flow
Zone-by-zone water budget and turndown requirement
Water-only versus air-mist decision per zone
Spray angle and standoff to give the required coverage with 40–50% overlap
Corner cooling strategy (independent nozzles, cutback)
Water quality: filtration to protect small orifices
Material: 316L for process water; hardened steel for abrasive conditions
Pattern verification before commissioning

For the nozzle hardware behind the water-only zones, the flat fan range covers the angles and flow classes; for the pattern fundamentals see the spray patterns guide and for the wider cooling picture the cooling nozzle selection guide. Send the strand section, grade and zone layout to the BoreJet team and the nozzle sizes, angles and air-mist options come back sized, not guessed. For the run-out table side of the same plant, the strip cooling nozzle guide covers laminar and air-mist headers built on the same design logic.