A chemical plant installed four coated heating plates in a 2,000-liter pickling tank to provide 16 kW of heating capacity. The plates were mounted on two opposite walls, with two plates on each wall. The plates on each wall were spaced 60 mm apart. Within six months, the inner plates-the ones facing each other across the gap-showed blistered coatings along the edges, while the outer plates remained in good condition. The gap between the plates was too narrow for adequate flow.
Panel spacing is a critical design parameter when multiple heating plates are installed in the same tank. The spacing determines whether fluid can flow freely between the plates or whether the gap becomes a stagnant zone where heat accumulates and the plates overheat.
Why Panel Spacing Matters
Each heating plate generates a thermal plume that rises along its surface. When two plates are mounted close together, their plumes interact. If the gap is wide enough, the plumes rise independently, drawing cool fluid from below and maintaining flow along both surfaces. If the gap is too narrow, the plumes merge and the flow between the plates becomes restricted.
In a narrow gap, the fluid is heated from both sides. It rises faster than the fluid on the outer surfaces. The gap becomes a chimney with a strong updraft, but the flow at the plate surfaces may be uneven. The lower portion of the gap may be starved of flow, while the upper portion has high velocity. The uneven flow creates hot spots on the lower portions of the facing surfaces.
| Panel Spacing | Flow Between Plates | Chimney Effect | Hot Spot Risk | Recommended Application |
|---|---|---|---|---|
| <30 mm | Poor | Strong | Very high | Not recommended |
| 30–50 mm | Moderate | Moderate | High | Low watt density only |
| 50–80 mm | Good | Moderate | Low | Most applications |
| 80–120 mm | Excellent | Weak | Very low | High watt density |
| >120 mm | Excellent | None | Very low | Large tanks |
Panel spacing and its effect on flow and hot spot risk
The Chimney Effect
When two heated plates face each other, the gap between them acts as a chimney. The fluid in the gap is heated from both sides, becomes less dense, and rises. Cooler fluid is drawn in from the bottom. The chimney effect can be beneficial if the gap is wide enough to allow smooth flow, but it can be detrimental if the gap is too narrow.
In a narrow gap, the chimney effect is strong but the flow is uneven. The fluid accelerates as it rises, and the pressure drop at the bottom of the gap restricts the inflow of cool fluid. The lower portion of the gap receives less flow, and the plates there run hotter.
Spacing Guidelines for Different Plate Configurations
The recommended spacing depends on the plate orientation, the watt density, and the fluid properties.
For vertical plates facing each other, a minimum spacing of 50 mm is recommended for low watt density (0.8–1.0 W/cm²) and 80 mm for higher watt density (1.2–1.5 W/cm²). The spacing should be measured between the plate surfaces, not between the mounting brackets.
For plates mounted on the same wall, side by side, the spacing can be smaller because the plates do not face each other. A 30–40 mm gap is sufficient to prevent thermal interference between adjacent plates on the same wall.
For horizontal plates stacked vertically, the spacing should be at least 100 mm to allow the thermal plume from the lower plate to escape without being trapped by the upper plate.
| Plate Configuration | Recommended Spacing | Minimum Spacing | Notes |
|---|---|---|---|
| Vertical, facing each other, low WD | 50–80 mm | 40 mm | Measure between surfaces |
| Vertical, facing each other, high WD | 80–120 mm | 60 mm | Higher WD needs more space |
| Vertical, side by side, same wall | 30–50 mm | 25 mm | Plates do not face each other |
| Horizontal, stacked | 100–150 mm | 80 mm | Plume must escape |
| Vertical, perpendicular | 40–60 mm | 30 mm | Plumes interact less |
Spacing guidelines for multiple heating plate configurations
The Effect of Spacing on Temperature Uniformity
Panel spacing affects not only the plate surface temperature but also the temperature uniformity in the tank. Plates that are spaced too closely create a localized hot zone between them. The fluid in that zone is hotter than the bulk, and the workpieces passing through that zone may be over-heated.
Plates that are spaced widely distribute heat more evenly. The plumes from each plate rise independently and mix with the bulk fluid. The temperature across the tank is more uniform.
| Spacing | Local Hot Zone | Bulk Temperature Uniformity | Workpiece Quality Impact |
|---|---|---|---|
| <30 mm | Strong | Poor (±3–5°C) | Visible variation |
| 30–50 mm | Moderate | Moderate (±2–3°C) | Minor variation |
| 50–80 mm | Weak | Good (±1–2°C) | Negligible |
| >80 mm | None | Excellent (±0.5–1°C) | None |
Panel spacing and its effect on temperature uniformity and workpiece quality
When Spacing Cannot Be Increased
If the tank cannot accommodate wider spacing between plates, the options are to reduce the number of plates, reduce the watt density, or use a different plate configuration. A single large plate with a large surface area may be better than multiple closely spaced plates.
A custom heating plate configuration with a spacing optimized for the tank and the process addresses the root cause of flow restriction between plates. The specification process requires the tank dimensions, plate count, plate orientation, watt density, and required wattage. For facilities with multiple plates in one tank, this application-specific approach improves flow between plates and extends plate service life.
