How to Choose the Correct Insertion Length and Diameter for an Over-the-Side PTFE Heater in a Circular Tank?

An over-the-side PTFE heater appears straightforward-just hang it from the wall. However, the heat remains confined close to the wall if the horizontal reach is too short; if it is too long, it collides with the opposing side. It's a tank geometry exercise to get the measurements correct. Heating efficiency, fluid circulation, and equipment longevity are all directly impacted by the insertion length diameter over the side PTFE heater choice for a circular tank. Hot areas, inadequate convection, or mechanical damage result from improper size.

Important Dimensions Described: Horizontal Leg and Vertical Leg
An L-shaped arrangement is common for over-the-side heaters. It is necessary to specify two crucial dimensions:

Horizontal leg (insertion length): The radially inward distance measured from the tank wall to the heated section's furthest point.

The length of the heated section that descends into the liquid is known as the vertical leg (immersion depth).

Bending radius and structural rigidity are influenced by a third parameter, the heater diameter, which is typically between 10 and 30 mm for PTFE encased elements. However, whether the heater fits inside the tank securely while encouraging natural convection depends on the diameter and insertion length taken combined.

Getting to the Central Zone with the Horizontal Leg
In order to prevent heat trapping close to the boundary layer, the horizontal leg of the heater must extend the heated part far enough from the tank wall. The rising hot liquid sticks to the wall surface if it is placed too close to it, reducing heat transfer and speeding up localised scaling. On the other hand, if the horizontal leg is excessively long, the heater could hit the circular tank's opposite wall, resulting in mechanical stress or damage to the PTFE sheath.

As a general rule, the heated area should be located in the tank's lower middle volume. This means that for a circular tank, the horizontal leg should be between 40 and 50 percent of the tank's radius, allowing at least 50 to 100 millimetres between it and the wall on the other side. This central location encourages symmetrical convection currents, which reduce temperature stratification by causing hot fluid to rise in the center, cool at the surface, and flow downward along the walls.

The Vertical Leg: Reaching the Correct Immersion Depth
The vertical leg must position the heating zone deep enough so that the entire heated length stays completely submerged despite typical liquid level fluctuations (such as evaporation and topping-off). When a PTFE heater is partially exposed to air while it is powered on, it overheats because air is far less efficient at removing heat than liquid. Above 120°C in air, the PTFE material may weaken or deteriorate.

Generally speaking, the heated length should cover between 70 and 80 percent of the typical liquid depth. To prevent sludge buildup, sediment, or materials that could insulate the surface, the heater's lowest position should be a few centimetres (usually 50–100 mm) above the tank bottom. To ensure submergence during regular level cycling, the heated section's highest point should be at least 50–100 mm below the minimum operational liquid level.

The heater requires space to suck cooler liquid from below and emit warmed liquid upward, much like a thermal smoke stack. The majority of the tank remains cold due to a tight convection cell created by a very short heated length (e.g., only 30% of liquid depth). Extremely long heated lengths (e.g., 90% of depth) run the danger of stirring up settled solids or scraping the bottom.

The Heater Diameter's Function
Two practical limitations are influenced by the heater diameter (the outside diameter of the PTFE-coated tube):

Minimum bend radius: PTFE-sheathed heaters must have a minimum bend radius of five to eight times the tube diameter at the L-shaped corner. A bigger tank might be needed because a larger diameter requires a softer bend, increasing the overall footprint.

Structural rigidity: The heater may be able to sag under its own weight and move its tip in the direction of the tank wall if it has a longer horizontal leg and a smaller diameter. Although a bigger diameter increases rigidity, it also raises the cost of materials and limits the number of heaters that can be installed across a single tank rim.

The heater's weight and the bending moment caused by the horizontal leg must be supported by the support bracket. It is advised to use a heavier-wall PTFE tube or an intermediate bracket for horizontal extensions longer than 300 mm.

A Useful Circular Tank Sizing Example
Imagine a circular tank with a normal liquid depth of 1200 mm and an interior diameter of 1000 mm.

Aim toward the central area of the horizontal leg. 500 mm is the radius. The heater axis is positioned 250 mm from the wall by a horizontal leg that is 250 mm (50% of the radius), giving the opposing wall 250 mm of space. This is acceptable.

Vertical heated length: 840–960 mm, or 70–80% of 1200 mm. For simplicity, go with 900 mm.

Bottom clearance: Position the heater so that its lowest point is 100 mm above the bottom of the tank. Because 100 + 900 = 1000 mm, the vertical leg thus extends from 100 mm above bottom to 1000 mm above bottom.

Top submergence: The minimum liquid level should never drop below 1100 mm, leaving 100 mm of the heater visible. If the level falls below that threshold, designate a low-level interlock to turn off the heater.

Steer clear of typical pitfalls
The heated area should not be positioned in the upper quadrant of the tank since it may be exposed by splashing, wave action, or level dips.

Avoid letting the heater come into contact with the bottom because settled sludge insulates the PTFE surface and can lead to localised overheating.

The moment capacity of the support bracket should not be disregarded. A long horizontal leg, such as 500 mm or more, puts a lot of torque on the tank rim bracket. It could be necessary to use stainless steel reinforcing.

In conclusion
The precise limb lengths and heater diameter are determined by the tank's diameter and depth, making the specification of an over-the-side PTFE heater a bespoke tailoring task. It is necessary to select the insertion length diameter over the side PTFE heater such that the heated section spans 70–80% of the liquid depth in the lower central volume, clearing the bottom and staying submerged at all times. For caustic baths, an appropriately sized heater provides effective, scale-resistant heating by working with the natural fluid currents rather than against them. When in doubt, a basic design of the suggested heater geometry combined with a dimensional drawing of the tank helps avoid expensive field adjustments.