How Does the 316 Stainless Steel Sheath Wall Thickness Affect the Rate of External Fouling from Polymerizing Media in Electric Immersion Heaters for Resin and Adhesive Tanks?

External fouling, the build-up of polymerised material on the sheath surface, is a persistent and costly problem for chemical process engineers running electric immersion heaters in resin manufacture, adhesive production and polymer processing. Fouling layers function as thermal insulators and reduce heat transfer efficiency. The heater will have to run at increased internal wire temperatures. Fouling at a critical thickness might cause the heater to overheat and fail, which may need tank shutdown and cleaning. The fouling rate of the 316 stainless steel sheath is influenced by wall thickness through two mechanisms: the surface temperature at a particular watt density and the thermal reaction during cleaning cycles. Thicker walls run hotter for the same power output, which can expedite polymerisation fouling, but also allow higher tolerance for frequent mechanical cleaning. This paper evaluates the relationship between 316 sheath wall thickness, surface temperature and fouling rate in polymerising media and provides suggestions for resin and adhesive tank heating selection.

Polymerisation fouling on 316 surfaces: Effect of temperature
Many resin and adhesive systems undergo polymerisation by free-radical or condensation processes that are exponentially dependent on temperature. The Arrhenius relation indicates that the rate of polymerisation on a heated surface typically doubles for each 10–15°C rise in temperature. The rate of polymerisation fouling on a sheath surface at 100 °C in an unsaturated polyester resin at a bulk temperature of 80 °C is about 0.1 mm/week for a 316 sheath. On the surface at 120oC the pace climbs to 0.4mm each week. At 140°C it is 1.5 mm/week - fast enough to foul the heater completely in one month. The crucial surface temperature threshold for most polymerising media is roughly 15-25°C above the bulk resin temperature. Below this level fouling is gradual and controlled. Above it, the fouling proceeds with great rapidity. A thicker 316 sheath creates a greater surface temperature for a given watt density. Thus the thick-walled heaters at the same bulk temperature and power output are more prone to quick fouling than the thin-walled heaters.

How Wall Thickness Affects the Fouling Cleaning Cycle
Sheath wall thickness influences both ease of cleaning and risk of sheath damage if periodic cleaning is undertaken (usually every 2–4 weeks). The cleaning of thin-walled sheaths below 1.0 mm is easier as the fouling layer is thinner at the time of cleaning as the lower surface temperature reduces the fouling rate. However, mechanical cleaning procedures like scraping, brushing or high pressure water jets can damage thin walls by denting or puncturing them. Sheaths thicker than 1.8 mm have thick walls and are not damaged by severe mechanical cleaning, but they foul faster and form thicker, more tenacious deposits that are harder to remove. The ideal wall thickness for polymerising media is thus a compromise between being thick enough to resist cleaning and thin enough to keep the surface temperature below the threshold for fast fouling. This optimum is in the range of 1.2 mm to 1.6 mm for most resin applications.

Suggested Wall Thickness for Polymerising Media Service
The following table lists the recommended 316 sheath wall thickness for electric immersion heaters for glue and adhesive tanks, based on bulk media temperature, predicted fouling rate and cleaning technique.

Bulk Media Temperature Polymerization Rate at Bulk Temperature Recommended Watt Density Recommended 316 Wall Thickness Sheath Surface Temperature at Recommended Thickness Expected Fouling Rate Recommended Cleaning Method 
20 – 40 °C (cold start, slow cure)Very low 8 – 12 W/cm2 1.0 – 1.4 mm 40 – 60°C < 0.05 mm/week Chemical flush only 40 – 60°C (warm, mild cure)Low 6 – 10 W/cm2 1.2 – 1.6 mm 60 – 80°C 0.05 – 0.15 mm/week Chemical or soft brush 60 – 80°C (active cure) Moderate 4 – 8 W/cm2 1.4 – 1.8 mm 80 – 100°C 0.15 – 0.40 mm/week Chemical and mechanical 80 – 100°C (rapid cure) High 3 – 6 W/cm2 1.6 – 2.0 mm 100 – 120°C 0.40 – 1.00 mm/week Frequent chemical cleaning 100 – 120°C (very rapid cure)Very high 2 – 4 W/cm² 1.8 – 2.5 mm 120 – 140°C 1.00 – 2.50 mm/week Requires special non-stick coating
>120 °CVery high Below 2 W/cm2 Not recommended Over 140oC Rapid failure Change to steam or external heating
At bulk media temperatures above 100°C, active polymerisation is not suggested for 316 stainless steel sheaths regardless of wall thickness. The surface temperature necessary for obtaining practical watt densities is inevitably going to be above the threshold for rapid fouling. In these situations, engineers might want to look at external tank heating (either jackets or coils) or changing to a non-stick coated sheath. PTFE or fluoropolymer coatings on 316 sheaths can provide a low-energy surface that polymerised materials do not stick to firmly and can lower fouling rates by 70–90%. The coatings, however, add expense and may limit the maximum operating temperature to 200–260 °C, depending upon the coating type.

Design Changes to Minimise Fouling Without Increasing Wall Thickness
Three design improvements are shown to decrease the fouling rate without increasing the wall thickness in cases when a thin-walled 316 sheath below 1.2 mm is desirable for thermal response in polymerising fluids. The first is to improve the circulation over the heater surface. By increasing the fluid velocity from stationary to 0.5 m/s, the surface temperature can be reduced by 10–15°C for the same watt density, due to the improved fluid-side heat transfer coefficient. Lower surface temperature directly affects polymerisation rate. The second change is to cycle the heater at a greater watt density rather than running it continuously at a lower watt density. A 12 W/cm² heater lasting 5 min on and 10 min off could have the same average power as a heater running at 8 W/cm² continuously, but the peak surface temperature is higher. The high temperature operation may in fact enhance fouling, so that intermittent operation is only advantageous if the off-time allows the sheath to cool below the polymerisation threshold so as to prevent deposit build-up. The final change is to polish the sheath surface to a mirror shine of less than 0.2 microns Ra. Smoother surfaces are easier to clean , because polymers have fewer nucleation sites to adhere to . Electropolishing of 316 sheaths can lower fouling rates by 20-40% compared to as-swaged finishes . In applications where fouling is unacceptable for critical applications, engineers should specify a dual-sheath design with a removable outer sleeve. The outer sleeve, constructed from thin-wall 316 or a non-stick coated material, gathers the fouling and can be replaced frequently without abandoning the entire heater. This technique has upfront costs but reduces long-term operational expense in severe fouling service. Always ask the heater manufacturer for the projected surface temperature for the requested watt density and wall thickness when specifying heaters for polymerising media. Compare this temperature to the established polymerisation threshold for the particular resin or glue. A 10°C difference can be the difference between six months of trouble-free operation and weekly cleaning shutdowns.