Checker Bricks for a Glass Furnace Regenerator: Three Brick Types by Temperature Zone, Plus the Thickness Calculation
The regenerator checkerwork is the one part of a glass furnace that takes up volume but earns its keep by surface area. Across a single regenerator the checkerwork can span more than 600 °C top to bottom, while alkali vapour and sulphates condense in different bands. So checker brick selection is never "pick one good brick" — it is zone by zone. Below are the brick types, property targets, and the thickness and surface-area calculation for the top, middle and bottom zones, ready to check against a drawing.
1. Calculate surface area first, then talk about brick type
Specific surface area a is the first number: 40 mm hexagonal or cross-split checker brick gives roughly 38–45 m²/m³, while 30 mm gives 50–58 m²/m³. Total area A = checkerwork volume V × a. Opening a flue from 120×120 mm to 150×150 mm drops a by 15–20%.
Thin brick buys area but gives up heat storage and strength. The practical trade: when checkerwork height is constrained, as in most rebuilds of older furnaces, go to 30–35 mm to recover area; where height is available, 40 mm protects storage capacity and life. On a regenerator with 120×120 mm cells and 7 m of checkerwork, moving to 30 mm brick raises total surface area by roughly a fifth, but stored heat falls and the reversal cycle has to shorten by 5–10% to compensate. Do not shrink the flues indefinitely either — keep flue gas velocity at 2–3 m/s, and note that once blockage reaches 30%, system draught has usually already degraded by more than 30%, at which point even good heat transfer means a shutdown for cleaning.
2. Top zone, 1350–1550 °C: magnesia or magnesia-chrome, never high alumina
Direct-bonded magnesia brick: MgO ≥90%, apparent porosity ≤18%, refractoriness under load T0.6 ≥1500 °C. Where chrome is restricted, magnesia-alumina spinel brick is the substitute; where thermal shock is severe, magnesia-chrome.
Why not high alumina at the top: alkali vapour (NaOH, KOH) reacts with the SiO₂ in high-alumina brick to form nepheline-type phases with accompanying volume expansion, and the brick face spalls layer by layer. In the same service, high-alumina brick often lasts one-half to one-third as long as magnesia. Use 40 mm as standard at the top, with one or two courses of 50 mm at the very top to resist gas velocity.
3. Middle zone, 1100–1350 °C: high alumina or mullite
High-alumina brick LZ-65 (Al₂O₃ ≥65%) or mullite brick, apparent porosity ≤18%, cold crushing strength ≥50 MPa. This band is the transition where alkali condensation meets sulphate reaction, so the selection criterion is low porosity and low Fe₂O₃ (≤1.2%), not simply higher alumina content.
Put one or two transition courses between the magnesia above and the high alumina below. Do not let basic and acidic brick sit in direct contact at temperature; the interface reacts, forms low-melting phases, and the two courses weld and bulge.
4. Bottom zone, 700–1100 °C: low-porosity fireclay, focused on sulphate resistance
Low-porosity fireclay brick: apparent porosity ≤15%, Fe₂O₃ ≤1.5%, refractoriness under load T0.6 ≥1350 °C, cold crushing strength ≥40 MPa. This band sits close to the active sulphate condensation range. Sulphates soak into open pores, crystallise, expand, spall the face, close the flues, and eventually block the whole section.
Against this attack, cutting porosity works far better than raising alumina: dropping from 20% to 15% typically halves the penetration depth. The lowest one or two courses also take knocks from cleaning tools, so high-alumina brick can go back in there.
5. Laying and rejection criteria: four things you can measure
Joints: fireclay joints at 2 mm, with successive courses offset by at least half a brick. Above 3 mm the joint is attacked first by alkali vapour and becomes a penetration path.
Expansion joints: allow 10–15 mm every 1.5–2 m of height, calculated from a linear expansion of 0.5–0.8%. Neither fully tight nor fully free works; the gap between checkerwork and sidewalls also needs allowance and must not be packed solid.
Verticality: ≤3 mm per metre, ≤10 mm overall. Lean redistributes load in the upper checkerwork and is a common starting point for collapse.
Rejection signals: top settlement over 100 mm; blocked flue area over 30%; exhaust gas temperature 30 °C higher than at first start-up for the same reversal cycle. During a cold repair, inspect three to five flues with a borescope. If glass phase is hanging on the brick face or the alkali spall layer exceeds 5 mm, the brick type for that zone was wrong — it is not a laying problem.
One cost-saving mistake appears again and again: using high-alumina brick through the full height. The top deforms within six to twelve months, the checkerwork collapses, and the cold repair interval falls from 8–10 years to 4–5. The brick money saved does not come close to one cold repair outage. Stock 3–5% spare on each zone and list transition shapes separately; the classic cold-repair stall is being two courses short of magnesia at the top with the whole campaign waiting.
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