A plating shop replaced a PTFE immersion heater in an uninsulated steel tank with a heater sized using the same calculation as before. The new heater delivered the correct wattage, but the tank took longer to reach setpoint and the heater ran continuously during production. The tank had been insulated two years earlier, but the insulation had been removed during a maintenance project and never reinstalled. The heat loss had increased, and the heater was now undersized for the actual load.
Tank insulation has a direct effect on the required PTFE immersion heater wattage. An uninsulated tank loses heat through its walls at a rate that can be 5–10 times higher than an insulated tank. The heater must replace that lost heat to maintain setpoint.
How Insulation Reduces Heat Loss
Heat loss through a tank wall depends on the temperature difference between the bath and the ambient air, the surface area of the wall, and the thermal resistance of the wall and any insulation. Insulation adds thermal resistance, which reduces the heat loss for a given temperature difference.
For a steel tank wall at 60°C in a 20°C room, the heat loss without insulation is approximately 200–300 W/m². With 50 mm of insulation, the loss drops to 20–40 W/m². With 100 mm, it drops to 10–20 W/m².
| Insulation Thickness | Wall Heat Loss at 60°C | Relative Loss | Annual Energy Cost (100 m²) |
|---|---|---|---|
| None | 200–300 W/m² | 1.0× | $12,000–18,000 |
| 25 mm | 50–80 W/m² | 0.25× | $3,000–4,800 |
| 50 mm | 20–40 W/m² | 0.12× | $1,200–2,400 |
| 75 mm | 12–25 W/m² | 0.08× | $720–1,500 |
| 100 mm | 8–15 W/m² | 0.05× | $480–900 |
Wall heat loss and energy cost by insulation thickness for a 60°C bath
How Insulation Changes the Required Wattage
The heater must deliver enough power to replace the heat lost through the walls and surface and to heat the incoming workpieces. If the tank is uninsulated, the wall losses are higher, and the heater must be larger.
For a 2,000-liter tank at 60°C with 100 m² of wetted surface, the wall loss without insulation is 20–30 kW. With 50 mm of insulation, the loss drops to 2–4 kW. The difference is 18–26 kW of heating capacity that must be supplied by the heater.
| Tank Volume | Wetted Surface | Uninsulated Wall Loss | Insulated Wall Loss (50 mm) | Additional Heater Capacity Needed |
|---|---|---|---|---|
| 1,000 L | 50 m² | 10–15 kW | 1–2 kW | 9–13 kW |
| 1,500 L | 75 m² | 15–23 kW | 1.5–3 kW | 13–20 kW |
| 2,000 L | 100 m² | 20–30 kW | 2–4 kW | 18–26 kW |
| 3,000 L | 150 m² | 30–45 kW | 3–6 kW | 27–39 kW |
Wall heat loss and additional heater capacity required for uninsulated tanks
The Effect on Surface Losses
Surface losses occur through evaporation and radiation. An open tank surface at 60°C loses 300–500 W/m². A floating ball blanket or a tank cover reduces that loss by 60–70%.
For a 2,000-liter tank with a 1.5 m² surface, the open surface loss is 450–750 W. With a cover, the loss drops to 135–225 W. The difference is 315–525 W of heating capacity.
Insulation and Heater Sizing
The heater should be sized for the actual heat loss of the tank. If the tank is insulated, the heater can be smaller. If the tank is uninsulated, the heater must be larger.
A practical approach is to calculate the heat loss for the actual insulation condition and size the heater for that load plus a margin for production load. A heater that is sized for an insulated tank but installed in an uninsulated tank will be undersized and will run continuously.
When Insulation Changes the Watt Density Requirement
Adding insulation reduces the heat loss, which reduces the required wattage. But the insulation also affects the heater surface temperature. A smaller heater operating at a higher watt density may be needed to deliver the reduced wattage within the available space. The higher watt density raises the surface temperature and accelerates degradation.
The optimum is to insulate the tank and specify a heater with a large surface area and low watt density. The insulation reduces the wattage requirement, and the large surface area keeps the watt density low.
When Standard Heaters Cannot Match the Insulation Condition
Tanks with damaged or missing insulation often require larger heaters than the original specification. A custom PTFE immersion heater configuration with a large surface area, a low watt density, and a flow-optimized position addresses the root cause of undersizing. The specification process requires the tank dimensions, insulation condition, operating temperature, and required wattage. For facilities with varying insulation conditions, this application-specific approach ensures the heater matches the actual heat loss.
