Table of Contents
Steel strip gets water-cooled twice, and both times the nozzle array decides the metallurgy of the finished product. The first time is in the continuous caster, where the strand leaving the mold passes through a secondary cooling zone of pressurized sprays. The second is on the run-out table (ROT) of the hot strip mill, where the rolled strip is cooled from finishing temperature to coiling temperature by banks of laminar cooling headers. The two systems share one core problem: put water on a surface at 900 °C without creating temperature differences across the width that crack, buckle or over-transform the steel.
This guide compares the two stages directly, segment structure, water allocation, width uniformity, flat fan layout and overlap, and cooling rate control, with the published numbers that separate a good strip cooling nozzle layout from a bad one.
The Short Answer
- Strip cooling is two jobs: laminar headers cool the run-out table strip at 5 to 10 °C/s at the base rate, while caster secondary cooling grows the shell at roughly 0.5 to 1.0 L/kg specific water.
- Above about 300 °C surface temperature, water sits on a steam cushion and stops cooling, so the spray must punch through the vapour film with impact.
- Flow follows Q ∝ √P, so doubling pressure raises flow 41 percent and barely moves the angle; coverage is bought with angle and layout, not pressure.
Why Cooling Red-Hot Steel Is a Nozzle Problem
Water removes heat from hot steel three ways; the nozzle decides which dominates. Sensible heating dominates in laminar cooling: water’s specific heat is 4.18 kJ/kg·K, so each litre that warms by 40 °C carries away roughly 167 kJ. Evaporation dominates in atomized sprays: the latent heat of vaporization is about 2.26 MJ/kg, roughly 540 times the energy to heat the same kilogram by 1 °C, so evaporating about 26.5 L/min removes on the order of 1 MW, which is why air-mist nozzles pull huge heat fluxes with modest water volumes.
Impingement is what breaks the film. Above the Leidenfrost point, for water on steel, roughly 300 °C of surface temperature, a stable vapour film insulates the metal from the liquid and heat transfer collapses. A 900 °C strip sits deep in film boiling, so a nozzle that merely drips water on it is nearly useless; the water must have enough impact to punch through the vapour film. That single fact explains the architecture of both systems: laminar bars use tall, coherent columns of water, while secondary cooling sprays use high-momentum droplets from flat fan and air-mist nozzles.
Finally, flow through an orifice follows Q ∝ √P: doubling pressure raises flow by 41%, not 100%, while the spray angle stays essentially fixed. You tune cooling intensity with flow and pressure, but you buy coverage with spray angle and layout. The two are set independently, and confusing them is how uneven strip cooling happens.
Laminar Cooling on the Run-Out Table: Gravity Water in Rows
The run-out table sits between the last finishing stand and the down coiler. Strip leaves the finisher at roughly 850–950 °C (finishing delivery temperature, FDT) and must reach a coiling temperature from about 150 °C (martensitic grades) to 750 °C (soft low-carbon grades). Strip speed on the ROT reaches 20 m/s for thin gauges, so the cooling system must extract hundreds of megawatts over the roughly 100–150 m of table available.
The standard hardware is laminar cooling: water fed by gravity from headers above the strip through vertical tubes or U-shaped siphon pipes, falling as coherent columns onto the top surface, with low-pressure spray headers below the strip for the bottom surface. Mills divide the ROT into a main cooling zone and a fine cooling zone, each bank independently switchable. Published mill descriptions give the scale: one typical ROT comprises 15 water banks with 16 nozzles above the strip and 8 nozzles per bank below; another groups its main zone as 18 banks of 4 top and 4 bottom headers each, plus a fine zone of 2 banks with 8 top and 8 bottom headers: top headers flowing about 120 m³/h, bottom headers about 130 m³/h, and fine-zone headers roughly half of that.
Why laminar instead of sprayed water for the top? The tall, stable column maintains impact through the vapour film and keeps the pool of runoff water thick and uniform. The classic ISIJ study of laminar headers measured the flow regimes at the nozzle mouth: as per-nozzle flow rises, the water transitions from laminar column to rodlike flow to turbulent, and turbulent flow cools less efficiently because the column breaks up before reaching the strip. The same work reported cooling efficiency Q_w ≈ 107 × Re^−0.645 at the nozzle mouth: the more stable the laminar flow, the more heat extracted per unit of water. Laminar headers are therefore designed around nozzle diameter and flow range, not pressure.
The cooling rates are the real point. Conventional laminar cooling operates at roughly 5–10 °C/s: fine for plain carbon, too slow for modern high-strength grades. Accelerated cooling reaches 20–50 °C/s, what TMCP grades need; direct quench pushes 60–100 °C/s for martensitic steels; ultra-fast cooling (UFC) sections reach around 300 °C/s on 4 mm strip, with one study measuring a heat transfer rate of about 4.37 MW for a 4 mm carbon steel plate at that rate. Mills installing intense cooling sections at the head of the ROT report roughly 50 °C/s on 12 mm strip and up to 700 °C/s on 3 mm strip.
The metallurgical payoff: at 1 °C/s the microstructure is ferrite and pearlite; at 30 °C/s it is essentially 100% bainite; push harder and you get martensite. Faster cooling means finer ferrite and higher strength for the same alloy content: strip cooling nozzles are production equipment, not utility.
Continuous Casting Secondary Cooling: Pressurized Sprays on a Moving Strand
Secondary cooling begins where the mold ends. The strand exits the mold with a solidified shell and a liquid core, at a surface temperature of roughly 1200–1300 °C, and must be cooled so the shell grows to full thickness before straightening. Modern casters use 8–15 cooling zones, each with independently adjustable water flow, arranged between the support rolls.
Unlike the ROT, the strand is thick and slow, casting speeds of 1–6 m/min are typical, and the surface spends a long time under each spray. Cooling is done almost entirely with pressurized sprays: plain-water flat fan and full cone nozzles in the upper zones, air-mist flat fan nozzles where the surface has cooled below about 900 °C and the spray must be controllable without over-cooling. Direct spray impingement accounts for roughly 60% of the heat removed in the secondary cooling zone, so the nozzle array is the primary thermal control instrument on the machine.
Water allocation follows a strict gradient: spray intensity is highest near the mold, where the thin shell must grow fast, and decreases toward the straightening zone, where the surface must stay hot enough to avoid cracking during bending. Intensity is quoted per tonne of steel: specific cooling of about 0.5 L/kg is considered low, above 1.0 L/kg high, and studies of high-carbon billets found an optimum near 0.9 L/kg for minimizing center segregation. Heat transfer correlations show why flow dominates the response: measured heat transfer coefficients follow HTC ∝ Q_w^1.23 for plain water in the 25–300 L/min range, with impingement densities for water sprays spanning roughly 1.7–33.3 kg/(m²·s) and air-mist nozzles operating in the 3–16 L/(m²·min) range at 1–2.5 bar air.
The nozzle is also the crack-prevention device. Uneven secondary cooling produces thermal stresses that cause surface and internal cracks, and reheating between zones where the spray footprint misses is a classic cause of mid-way cracks. Modern practice actively manipulates the spray: shutting off selected nozzles in the later zones at medium and low casting speeds raises slab corner temperature by about 50 °C, keeping corners out of the low-ductility range at straightening.
The Two Systems Side by Side
| Parameter | Run-out table laminar cooling | Continuous casting secondary cooling |
|---|---|---|
| Steel condition | Thin strip, 1–25 mm, fully solid | Thick strand, liquid core, growing shell |
| Strip/strand speed | Up to 20 m/s | 1–6 m/min |
| Surface temperature range | ~900 °C down to coiling temp (150–750 °C) | ~1300 °C down to ~900 °C |
| Water delivery | Gravity laminar columns (top), low-pressure spray (bottom) | Pressurized plain water and air-mist flat fan sprays |
| Typical cooling rate | 5–10 °C/s laminar; 20–100+ °C/s accelerated/UFC | Low by design, controlled per zone |
| Typical water intensity | Headers at ~120–130 m³/h each, banks switched | 0.5–1.0+ L/kg specific water, decreasing along the caster |
| Key nozzle job | Break vapour film with coherent column; uniform top pool | Atomize fine, controllable droplets; flat footprint across width |
| Main control variable | Number of banks/headers on, water flow | Water flow and air pressure per zone |
| Failure mode | Edge-to-center temperature difference, buckling | Surface reheating between zones, corner cracks |
The table hides one asymmetry: laminar cooling is fast but coarse, secondary cooling slow but precise: the caster nudges surface temperature within a few degrees over tens of metres, while the ROT has only the distance between finisher and coiler to hit the coiling temperature.
Width Uniformity: The Real Test of a Strip Cooling Nozzle Array
Both systems fail the same way: the middle cools differently from the edges. In laminar cooling, pressure distribution inside the header creates a flow profile across its length: full-size header measurements show edge-to-centre flow differences near 78% on uncompensated designs. That is why production headers are built with a deliberately convex flow distribution (more water at the centre, less at the edges), and why modern mills add edge masking and edge-boost nozzles.
The consequences are quantified in the residual-stress literature. With an initial edge-to-middle temperature difference of 30 °C, residual edge stress measured about 107 MPa; at 120 °C it reached about 446 MPa, far beyond yield, and the edges develop edge waves. Plant data on 116 strips confirms it: shape defects concentrate at 50–55 °C edge difference; strips at 35–40 °C keep good shape. Advanced accelerated cooling attacks the same problem from the heat-extraction side: one Japanese steelmaker reported new equipment reducing temperature variance across plate width from 50 °C to 30 °C by stabilizing the boiling regime.
Top-to-bottom symmetry is the second half. On the top surface, runoff water pools and keeps cooling the strip after it leaves the header; on the bottom, gravity pulls the water away, so the bottom must be over-supplied. Published work on ultra-fast cooling found the optimal bottom-to-top water flux ratio is about 1.20 when the top headers flow 100 m³/h and about 1.15 when they flow 120 m³/h, with the ratio wrong, the heat transfer coefficient differs between top and bottom and the strip curls. The bottom headers sit between the table rollers with less room, so the nozzles must deliver the extra flow in a constrained envelope.
In secondary cooling the same problem shows up in the overlap between adjacent nozzles. Laboratory inverse-heat-transfer studies found that a homogeneous spray pattern does not guarantee a homogeneous heat flux: heat transfer coefficients can differ by tens of percent across the spray width, especially in the overlapping region between nozzles. Overlap is planned and verified by measurement, not assumed from the chart.
Flat Fan Nozzle Arrangement and Overlap for Steel Cooling
Flat fan nozzles are the workhorse of steel cooling where a controlled, even footprint is needed: secondary cooling air-mist, ROT bottom headers, edge cooling, interstand cooling. A flat fan produces a sheet whose width at a given standoff follows w = 2d·tan(θ/2), where d is the standoff and θ the spray angle. A 65° fan at 0.5 m covers about 0.64 m; a 40° fan at the 0.38 m nozzle distance used in thick-slab secondary cooling covers about 0.28 m. That last example matters: the study that found 380 mm spacing and 663.5 mm spray height optimal for thick-slab uniformity did so because they matched the flat fan coverage to the strand width without gaps.
Layout rules are the same for steel or a coating line: adjacent fans must overlap by a planned fraction of the coverage width. For tapered-edge fans the common starting point is spacing at 40–50% of the pattern width, so the thin ends land inside the thick middle of the neighbour and the row reads as one band. Even-edge fans, where the spray profile is constant across the width, are used where arrays must butt-joint with no overlap. Steel cooling headers mix the two: tapered fans for the long runs, even-edge fans at zone boundaries and strip edges.
Two nozzle-behaviour details break more steel-cooling layouts than anything else. First, spray angle is not constant across the flow range: measurements on a typical flat fan show the angle widening from about 88.8° at 1.58 L/min to 98.6° at 4.14 L/min. Design the overlap at the operating flow, not the catalogue flow. A row set up at minimum flow develops gaps at maximum flow. Second, because Q ∝ √P and angle changes with flow, the designer fixes nozzle size and pressure range first, then lays out spacing from the real coverage width at the operating point.
For air-mist secondary cooling, the flat fan dominates because its elongated footprint matches the strand width and its thin spray thickness angle (typically 12–16° for major angles of 80–120°) keeps the wet zone narrow in the casting direction, making zone boundaries sharp; spray heights in the upper casting zones run 200–250 mm. Droplet size and impact velocity control heat transfer in this regime, the heat flux correlates with droplet diameter and velocity through the Weber number, which is why air-mist nozzles are specified by air-to-water ratio (commonly around 10:1 in thin-slab practice) rather than water flow alone.
A practical flat fan reference for steel cooling duties, from the BoreJet range:
| Model | Spray angle | Flow @ 3 bar | Edge type | Typical steel-cooling use |
|---|---|---|---|---|
| FF-40 | 40° | 1.0–20 L/min | Tapered | Compact secondary cooling zones, narrow strand edges |
| FF-65 | 65° | 2.0–35 L/min | Tapered | ROT bottom headers, general strand surface cooling |
| FF-80 | 80° | 3.0–50 L/min | Tapered | Wide secondary cooling footprints |
| FF-110 | 110° | 4.0–60 L/min | Tapered | Low-standoff wide coverage, large slab faces |
| FF-145 | 145° deflector | 6.0–120 L/min | Even | Edge masking and zone-boundary butt joints |
| FF-HP | 15–40° | 2.0–20 L/min | Even | High-pressure film-breaking duties, hardened stainless |
Cooling Rate Control: Valves, Banks and Models
Both systems control cooling rate the same fundamental way: switch rows of nozzles on and off, and modulate the flow through the rows that are on. On the ROT, every header is individually switchable, typically with fast pneumatic valves: the classic ISIJ study adopted piston valves with a response time of 0.5 s and no leakage after more than 750,000 cycles, because a slow or leaking valve turns the cooling curve into a scatter plot. Control systems divide the strip into segments, track each with the rolling velocity profile (the time-velocity-distance curve), and compute which banks must be on for that segment to hit the coiling temperature pyrometer at the far end.
In secondary cooling, the control variable per zone is water flow (and air pressure for air-mist zones), and the target is a surface-temperature profile, not a single endpoint. Optimization schemes that adjust all zone flows against a solidification model hold strand temperature within ±4 °C of target. A precision that matters because the difference between 0.9 and 1.2 L/kg specific cooling decides whether carbon segregates at the billet center.
Water temperature is a control input, not a constant: the classic laminar cooling test conditions quoted 45 °C cooling water, and heat extraction per unit volume changes measurably with inlet temperature because it changes boiling behaviour. A mill that lets its cooling tower drift 10 °C between summer and winter shifts its coiling temperature curve unless the model accounts for it.
One more thing the control system must respect: the coil. The strip coils at 150–750 °C and then takes about three days to cool to ambient, so the coiling temperature, and its uniformity across the width, is what the microstructure remembers.
Worked Example: Water a 3 mm Strip Actually Needs
Take 1 m² of 3 mm strip cooling from 800 to 600 °C. Mass: 0.003 m³ × 7850 kg/m³ ≈ 23.6 kg. Heat removed at a specific heat of roughly 0.6 kJ/kg·K: 23.6 × 0.6 × 200 ≈ 2.8 MJ.
If the cooling water leaves the strip 40 °C hotter, each kilogram carries 4.18 × 40 ≈ 167 kJ, so about 17 L of water per m² must make effective contact. If the same heat were removed by evaporation, it would take only 2.8 MJ ÷ 2.26 MJ/kg ≈ 1.25 L, 13 times less water.
Now scale it: a 1.2 m wide, 3 mm strip at 12 m/s covers 14.4 m²/s, demanding on the order of 245 L/s, and real ROT systems run several times that because most water never fully heats. The published header flows add up the same way: 18 banks × 4 top headers × 120 m³/h alone is 8640 m³/h. Laminar cooling is cheap, robust and thirsty; sprays are efficient but need control.
Nozzle Wear and Maintenance in Steel Service
Steel cooling is the hardest duty a nozzle can face: radiation, scale, vibration, and water that may be recycled and gritty. Erosion is the head killer: an eroded orifice flows more at the same pressure, and a row of eroded strip cooling nozzles quietly changes the cooling curve. Because flow rises with erosion, the system compensates by closing valves, hiding the drift until a bank that should cool does not. Hardened stainless orifices (the FF-HP class) are the standard answer for high-pressure and descaling duties; for plain-water laminar service the wear is on the header orifices and siphon pipes. Inspect by measuring flow and pattern at a fixed pressure, not by calendar: every shift for descaling headers, weekly for ROT laminar bars, per-ladle for the hottest secondary cooling zones.
Which Strip Cooling Nozzle System Do You Need
Three hardware families dominate strip cooling, and the choice between them is decided by the duty, not by habit:
| System | Hardware | Typical duty | Cooling rate | Water consumption |
|---|---|---|---|---|
| Laminar flow headers | Gravity-fed columns or slit-style curtains | Run-out table top cooling of hot strip | 5–100+ °C/s (300 °C/s with UFC) | High: 120–130 m³/h per header, banks switched |
| Flat spray headers | Flat fan nozzles on custom headers | ROT bottom headers, roll cooling, interstand cooling | Moderate, uniform | Lower: sized to coverage width |
| Air-mist flat fan | Air-atomized flat fans, ~10:1 air-to-water | Secondary cooling zones below ~900 °C, edge control | Low and precise (±4 °C per zone) | Lowest: evaporation does the work |
If the job is bulk heat removal from a fast-moving strip, laminar headers win on robustness. If the job is a controlled footprint, bottom headers, roll cooling, edge masking, flat spray headers give the coverage geometry. If precision matters and over-cooling is as bad as under-cooling, air-mist flat fans keep the surface on target without flooding the strand. Most mills run all three.
The BoreJet range maps directly onto this: the flat fan range covers ROT bottom headers, roll cooling and secondary cooling; full cone nozzles handle high-intensity caster zones; and the cooling nozzle selection guide walks through all seven cooling scenarios with worked numbers.
Selection Checklist for Strip Cooling Nozzles
- Fix the temperature window first: FDT/coiling for the ROT, shell surface targets for the caster. The boiling regime decides the nozzle class.
- Choose the delivery mode: coherent laminar column for fast, robust top cooling; flat fan spray for controlled footprints; air-mist for fine, zone-accurate cooling.
- Lay out coverage from real geometry: w = 2d·tan(θ/2) at the operating flow, because spray angle moves with flow.
- Set overlap deliberately: tapered fans at 40–50% of pattern width, even-edge fans for butt joints and zone boundaries.
- Balance top and bottom: expect a bottom-to-top flux ratio around 1.15–1.20 on the ROT.
- Design the width profile: convex header flow plus edge masking to keep edge-to-center temperature difference under 40 °C.
- Size the pump for the sum, not the average: every bank can be on, and steel cooling is thirsty.
- Pick material by duty: hardened stainless for high-pressure and abrasive water, 316L for the rest.
- Plan inspection by flow and pattern measurement, not by the calendar.
FAQ
Why does laminar cooling use columns of water instead of spray nozzles on the top of the strip? A gravity column maintains a coherent, high-momentum stream that penetrates the vapour film on a 900 °C surface, and the pooled runoff keeps cooling the strip after the column passes. Sprays at the same pressure break into drops that bounce off the vapour film, so they are reserved for bottom headers and zones needing fine control. What cooling rate does a hot strip mill actually achieve? Conventional laminar cooling runs 5–10 °C/s. Accelerated and TMCP systems reach 20–50 °C/s, direct quench 60–100 °C/s, ultra-fast cooling roughly 300 °C/s on 4 mm strip. The grade decides the target: ferrite–pearlite at low rates, bainite around 30 °C/s, martensite higher.
Why does the edge of the strip cool differently from the middle? Header flow distribution, edge radiation and the vapour film all differ across the width. Uncompensated headers can show edge-to-centre flow differences near 78%, which is why production headers use a convex flow profile, edge masking and edge-boost nozzles, and why the literature says to keep the edge-to-middle difference under roughly 40 °C.
How much water does strip cooling use? A mill-scale ROT moves thousands of cubic metres per hour: one published example runs 18 banks of four 120 m³/h top headers plus a fine zone. Secondary cooling runs roughly 0.5–1.0 L/kg, about 0.9 L/kg optimal for high-carbon grades.
Does spray angle change when I change the flow? Yes: a typical flat fan widens from about 89° at 1.58 L/min to about 99° at 4.14 L/min. Set overlap at the operating flow, not the catalogue angle.
How do I size the nozzles for a new cooling header? Start from the target cooling rate and strip speed, convert to specific water flow in litres per minute per square metre, then pick nozzle flow at operating pressure and lay out spacing from the real coverage width at that flow. Work the same chain step by step in the water spray nozzle design calculation guide.
When should I use air-mist instead of plain water in secondary cooling? When the strand surface has cooled below the film-boiling range and over-cooling is the risk. Air-mist flat fans give fine droplets and a sharp footprint, air-to-water ratio commonly near 10:1, so heat flux is tuned per zone without flooding the strand.
One-Paragraph Summary
The two systems share the physics and fail on the same details: real coverage width at operating flow, deliberate overlap, edge compensation, and inspection by flow and pattern rather than calendar. If you are designing a header for either duty, start from the flat fan nozzle range, lay out the overlap by real coverage geometry, and if the duty is outside the common ranges, send the application team the standoff, width, flow and temperature window.
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Written by
Ray ChanIndustrial spray nozzle specialist. I size tank cleaning, atomizing, flat-fan and spiral nozzles against real duty conditions, flow, pressure, fluid and target, rather than catalogue numbers. Every guide here comes from actual sizing work.