A metal finishing plant needed a replacement PTFE immersion heater for a 700 mm deep sulfuric acid anodizing tank. The tank was 1,000 mm wide and 1,600 mm long, with a working volume of approximately 1,050 liters. The existing heater had been a standard straight unit that protruded into the workpiece path and had failed after 13 months. The maintenance team needed a heater that would fit the deeper tank, maintain full immersion, and deliver the required wattage at a safe watt density.
A 700 mm deep tank provides more usable heater length than a shallow tank, but the heater must still be sized to fit the available space and to maintain safe immersion at the minimum liquid level.
The Immersion Depth Constraint
A 700 mm deep tank does not provide 700 mm of usable heater length. The liquid level must be high enough to cover the workpieces, and the heater must remain fully submerged at the minimum liquid level. In practice, the usable heated length in a 700 mm deep tank is 450–550 mm, depending on the freeboard and the level fluctuation.
A straight heater with a 500 mm heated length provides 500 mm of heated length. For a 5 kW duty at 1.0 W/cm², the required surface area is 5,000 cm². At a 32 mm sheath diameter, the surface area is approximately 100 cm² per 100 mm of length. A 500 mm heated length provides only 500 cm²-far short of the requirement. A larger sheath diameter or a multi-pass configuration is needed.
| Tank Depth | Usable Heated Length | Surface Area at 32 mm Sheath | Surface Area at 50 mm Sheath | Heated Length Needed for 5,000 cm² |
|---|---|---|---|---|
| 500 mm | 300–350 mm | 300–350 cm² | 471–550 cm² | 1,429–1,667 mm |
| 600 mm | 350–450 mm | 350–450 cm² | 550–707 cm² | 1,111–1,429 mm |
| 700 mm | 450–550 mm | 450–550 cm² | 707–864 cm² | 909–1,111 mm |
| 800 mm | 550–650 mm | 550–650 cm² | 864–1,021 cm² | 769–909 mm |
Heated length and surface area requirements for a 5 kW duty at 1.0 W/cm²
The L-Shaped or U-Shaped Heater Solution
An L-shaped or U-shaped PTFE immersion heater bends the heated element to fit more surface area into a deep tank. A typical L-shaped heater with a 500 mm vertical section and a 600 mm horizontal section provides 1,100 mm of heated length-more than twice the length of a straight heater in the same tank depth.
At a 50 mm sheath diameter, 1,100 mm of heated length provides approximately 1,728 cm² of surface area. That is still short of the 5,000 cm² required for a 5 kW duty at 1.0 W/cm². A multi-pass configuration is needed.
| Heater Configuration | Heated Length | Surface Area at 32 mm | Surface Area at 50 mm | Fit in 700 mm Tank |
|---|---|---|---|---|
| Straight, 500 mm | 500 mm | 500 cm² | 785 cm² | Yes, but insufficient area |
| L-shaped, 500 + 600 mm | 1,100 mm | 1,100 cm² | 1,728 cm² | Yes |
| U-shaped, 500 + 400 + 500 mm | 1,400 mm | 1,400 cm² | 2,199 cm² | Yes |
| Multi-pass, 4 × 500 mm | 2,000 mm | 2,000 cm² | 3,142 cm² | Yes |
| Multi-pass, 7 × 500 mm | 3,500 mm | 3,500 cm² | 5,498 cm² | Tight fit |
Heater configurations for a 700 mm deep tank
Fitting the Heater Into the Tank
The heater must fit into the available space without obstructing the workpieces or the flow. In a 1,000 × 1,600 mm tank, the heater can be positioned along one wall, leaving the central volume for the workpiece rack. A multi-pass heater with a 500 mm vertical section and a 600 mm horizontal section fits along the 1,600 mm wall.
The heater must be positioned so that the heated section is fully submerged at the minimum liquid level. A 700 mm deep tank with a 600 mm liquid level and a 100 mm freeboard provides 500 mm of usable depth. The heater's heated section should be positioned in the lower 450 mm of the tank, with the top of the heated section at least 50 mm below the minimum liquid level.
Flow and Temperature Uniformity
A multi-pass heater in a deep tank can create flow shadowing if the passes are too close together. The fluid between the passes may become stagnant, reducing heat transfer and creating hot spots. The spacing between passes should be at least 30–40 mm to allow flow between them.
If the tank has a circulation pump, the heater should be positioned so that the flow passes through the gaps between the passes. If the tank relies on natural convection, the heater should be positioned so that the rising thermal plume can escape without being trapped.
| Pass Spacing | Flow Between Passes | Hot Spot Risk | Recommended Application |
|---|---|---|---|
| <20 mm | Poor | High | Not recommended |
| 20–30 mm | Moderate | Moderate | Low watt density only |
| 30–40 mm | Good | Low | Most applications |
| >40 mm | Excellent | Very low | High watt density or viscous fluids |
Pass spacing and its effect on flow and hot spot risk
When a Standard Heater Cannot Fit the Tank
A 700 mm deep tank with a 5 kW duty at 1.0 W/cm² requires a multi-pass heater that standard product lines may not offer. A custom PTFE immersion heater configuration with a multi-pass design, a 50 mm sheath diameter, and 30–40 mm pass spacing addresses the root cause of space constraint. The specification process requires the tank dimensions, liquid level range, required wattage, bath chemistry, and flow conditions. For facilities with deep tanks, this application-specific approach provides the required surface area within the available space.
