The Rectangular Tank Depth Question
The PTFE immersion heater's vertical position in a rectangular tank affects the natural convection pattern and the temperature uniformity. The heater too shallow heats only the upper zone, leaving the cold floor. The heater too deep concentrates the heat at the bottom, promoting stratification.
For rectangular tanks, the characteristic dimension is the tank width, not the diameter. The optimal depth-to-width ratio determines whether the buoyant plume can circulate through the full tank volume.
The Natural Convection Pattern in Rectangular Tanks
The heater creates a buoyant plume that rises from the heating surface. The plume reaches the surface, spreads, cools, and descends along the tank walls. The circulation cell extends from the heater to the surface and back.
The circulation effectiveness depends on the heater depth. The shallow heater creates a small circulation cell near the surface. The tank bottom stays stagnant. The deep heater creates a long plume that entrains the full tank depth.
| Depth-to-Width Ratio | Circulation Pattern | Temperature Uniformity | Bottom Zone Condition |
|---|---|---|---|
| 0.1 (very shallow) | Surface-only cell | ±5°C | Stagnant, cold |
| 0.3 | Partial depth cell | ±3°C | Slow circulation |
| 0.5 | Full depth cell | ±1.5°C | Adequate circulation |
| 0.7 | Deep cell, strong plume | ±1°C | Good circulation |
| 0.9 (near floor) | Plume from the floor | ±2°C | Strong circulation, sludge pickup |
The Recommended Ratio
For rectangular tanks with natural convection, the recommended immersion depth-to-width ratio is 0.5–0.7. The heater positioned with the bottom at 50–70% of the tank width below the surface creates the plume that entrains the full tank depth. The temperature uniformity of ±1–1.5°C is achievable.
The ratio applies to the vertical heaters or the heater bundles with significant vertical extent. The horizontal floor-mounted heaters follow different placement criteria-the floor coverage area is the primary factor.
The Bottom Clearance
The heater bottom should remain 100–150mm above the tank floor. The clearance prevents the sludge pickup into the plume and allows the floor cleaning access. The clearance is independent of the depth ratio.
For the tanks with heavy sludge, the larger clearance of 200–300mm is recommended.
Summary
The optimal immersion depth-to-tank-width ratio for uniform heating with a PTFE immersion heater in a rectangular tank is 0.5–0.7. The ratio creates the natural convection plume that circulates the full tank depth. The temperature uniformity of ±1–1.5°C is achievable. The bottom clearance of 100–150mm prevents the sludge pickup.
Engineering support for the heater placement optimization is available upon submission of the tank dimensions, the heating duty, the fluid properties, and the sludge characteristics.
