Mixing Grate Bars: Three Combinations and What Goes Wrong
Replacing a full set of grate bars is expensive and needs a shutdown, so the temptation to change only the worst rows is strong. Sometimes that is the right call. Sometimes it produces a grate that jams, burns unevenly and destroys the new bars along with the old. The difference lies in what is being mixed.
1. First mix: two different geometries on one grate
The common version is plain bars in the tail and perforated bars in the hot zone. Another is S-type bars in one area and wide or T-type bars in another. Both fail the same way, because the pitch, the section height and the air path are all tied to the bar shape.
A different pitch means the number of bars per metre of width changes, so rows of one type do not line up with rows of the other, and the end bar no longer meets the side seal. A different height means the fuel bed steps up and down from row to row, which changes bed depth and the stoking action exactly where the operator cannot see it. A different air path means the perforated rows pass most of the compartment’s air while the plain rows starve, and a starved row is a hot row. The geometry case is examined in full in will holed bars fit an existing grate, and the three biomass bar profiles are compared in S-type, wide and T-type bars.
2. Second mix: two different grades in adjacent rows
Mixing grades is more defensible than mixing geometries, and it is the basis of zone-by-zone selection: an expensive steel in the main combustion zone, an iron at the two ends. It goes wrong when the two grades are placed side by side without checking how they grow.
Thermal expansion is the first check. Take two grades differing by 1 × 10⁻⁶ per ℃ in expansion coefficient. On a 390 mm bar at 1000℃ the difference in growth is about 0.4 mm. That is the same order as a designed clearance between neighbouring bars, so one row can close the gap on the row beside it.
Hot strength is the second. A cast iron at the top of its range creeps and warps; a nickel-bearing steel at the same temperature does not. The warped row lifts its neighbour, the overlap opens, air escapes through the new gap, and the hot spot moves to the boundary between the two grades. In practice the cheap rows fail first and take the adjacent expensive rows with them, which defeats the purpose of buying them.
When mixing grades is acceptable: identical outline, pitch and height; a boundary placed at a zone transition rather than mid-zone; expansion coefficients within about 1 × 10⁻⁶ per ℃ of each other; and a written record of which grade sits where, so the next overhaul is not a guess.
3. Third mix: old bars and new bars in one set
This is the most common and the least visible. A used bar is not the same size as a new one: the face has thinned, the pitch has stretched, the pin holes have gone oval, and the engagement teeth have worn. Fit a new bar between two used ones and three things follow.
The overlap is wrong, so the new bar sits high or low and the bed depth jumps at that row. The pin holes no longer match, so the pin loads on an edge instead of over its length and eventually shears. The drive load rises, because the new bar is being pushed by worn teeth that no longer share the load evenly, and the first symptom is usually noise, then a jam.
Mixing bars from two different batches or two different foundries belongs here as well. Even within one drawing, cast tolerance and chemistry vary; two heats from two suppliers can differ enough that the bars do not interchange cleanly. When a set is replaced in stages, the spare bars should come from the same heat as the ones already in service, and the dimensional check should be done on the actual bars rather than on the drawing.
4. The three mixes at a glance
| Mix | What does not match | Symptom | Rule |
|---|---|---|---|
| Two geometries | Pitch, height, air path, seal faces | Uneven bed, one zone starved, side seal open, jams | Do not mix; choose one geometry for the whole grate or rebuild the seats |
| Two grades | Expansion, hot strength, wear rate | Boundary hot spot, warped cheap rows, early failure of the expensive ones | Allowed at zone boundaries with matched geometry and expansion |
| Old and new | Thickness, pitch, pin holes, tooth wear | New bar sits proud, pin shear, drive load and noise | Replace by row or zone, keep spares from the same heat |
5. Five conditions for a partial replacement
- The replacement bars come from the same drawing and the same heat as those staying in service.
- Thickness and pitch of the bars left in place are measured and within tolerance, not merely “still looks fine”.
- The boundary between new and old falls on a row line, never inside a row, and preferably at a zone transition.
- Pins, side seals and retaining parts are replaced together with the bars, since they wear as a set.
- The reason the old bars failed is understood. If it was burn-through at a gap, new bars in the same gap will fail the same way within weeks.
6. What we check before dispatch
We supply moving and fixed bars, side seals and matching parts for biomass reciprocating grates, dispatched from a partner factory. For a partial replacement the useful inputs are the measured thickness and pitch of the bars staying in service, the heat or batch of the previous order, and a sketch showing which rows are being changed. With those, the new bars can be matched to what is actually on the grate rather than to what the drawing says.
