A chemical plant installed four PTFE immersion heaters in a 2,000-liter pickling tank to provide 16 kW of heating capacity. The heaters were mounted on two opposite walls, with two heaters on each wall. The heaters on each wall were spaced 60 mm apart. Within six months, the inner heaters-the ones facing each other across the gap-showed blistered PTFE along the edges, while the outer heaters remained in good condition. The gap between the heaters was too narrow for adequate flow.
Element spacing is a critical design parameter when multiple PTFE immersion heaters are installed in the same tank. The spacing determines whether fluid can flow freely between the heaters or whether the gap becomes a stagnant zone where heat accumulates and the heaters overheat.
Why Element Spacing Matters
Each PTFE immersion heater generates a thermal plume that rises along its sheath. When two heaters 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 heaters 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 sheath 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.
| Heater Spacing | Flow Between Heaters | 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 |
Heater spacing and its effect on flow and hot spot risk
The Chimney Effect
When two heated sheaths 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 heaters there run hotter.
Spacing Guidelines for Different Heater Configurations
The recommended spacing depends on the heater orientation, the watt density, and the fluid properties.
For vertical heaters 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 sheath surfaces, not between the mounting brackets.
For heaters mounted on the same wall, side by side, the spacing can be smaller because the heaters do not face each other. A 30–40 mm gap is sufficient to prevent thermal interference between adjacent heaters on the same wall.
For horizontal heaters stacked vertically, the spacing should be at least 100 mm to allow the thermal plume from the lower heater to escape without being trapped by the upper heater.
| Heater Configuration | Recommended Spacing | Minimum Spacing | Notes |
|---|---|---|---|
| Vertical, facing each other, low WD | 50–80 mm | 40 mm | Measure between sheaths |
| 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 | Heaters 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 PTFE immersion heater configurations
The Effect of Spacing on Temperature Uniformity
Element spacing affects not only the sheath surface temperature but also the temperature uniformity in the tank. Heaters 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.
Heaters that are spaced widely distribute heat more evenly. The plumes from each heater 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 |
Element spacing and its effect on temperature uniformity and workpiece quality
When Spacing Cannot Be Increased
If the tank cannot accommodate wider spacing between heaters, the options are to reduce the number of heaters, reduce the watt density, or use a different heater configuration. A single large heater with a large surface area may be better than multiple closely spaced heaters.
A custom PTFE immersion heater configuration with a spacing optimized for the tank and the process addresses the root cause of flow restriction between heaters. The specification process requires the tank dimensions, heater count, heater orientation, watt density, and required wattage. For facilities with multiple heaters in one tank, this application-specific approach improves flow between heaters and extends heater service life.
