How to Select the Correct PFA Heater Diameter for a Given Tank’s Baffle Spacing to Maintain Minimum Flow Bypass Velocity?

Baffled tanks are utilised for mixing, chemical reactions and heat transmission and the space between the baffle and the tank wall provides a flow bypass channel. To ensure heat transfer and prevent localised overheating, the flow velocity must be high enough past the heater if a PFA heater is installed in this channel. For water service, PFA heaters should have a minimum flow bypass velocity of 0.3-0.5 m/s to maintain the PFA surface temperature within 10°C of the bulk fluid at normal watt densities (3-5 W/cm2). For a certain baffle spacing (distance between baffle edge and tank wall), the heater diameter must be chosen such that the cross-sectional area of the bypass channel around the heater is large enough to sustain velocity. Rule: heater OD should be ≤ 50% of baffle gap width. Maximum OD of heater is 50 mm for a 100 mm gap between baffles. Maximum heater OD for a spacing of 50 mm is 25 mm. The flow is constricted by larger heaters, which lowers the velocity and causes overheating.

Calculation of flow bypass velocity
The velocity in the baffle gap is v = Q / A with Q the volumetric flow rate from the agitator or recirculation pump and A the cross-sectional area of the gap not occupied by the heat exchanger. W = total gap width, D = heater OD, H = tank height (assumed fully immersed). The free area is A _free = (W - D) × H . When D approaches W , A _free approaches 0 and the velocity reduces dramatically . For v ≥ 0.3 m/s, A_free must be ≥ Q / 0.3. For a typical tank with agitator delivering Q = 0.05 m3/s (3 m3/min), H = 2 m, the needed A free = 0.05 / 0.3 = 0.167 m2. Then (W - D) x 2 = 0.167. Hence W - D = 0.0835 m = 83.5 mm. If W = 150 mm, max D = 150 - 83.5 = 66.5 mm. 65 mm heater would be ok. If W = 100 mm, maximum D = 100-83.5 = 16.5 mm-a very modest heater. In this circumstance it is impossible to put the heater into the baffle gap, it has to be moved.

In the case of natural convection tanks (no pump, no agitator) the flow velocity is generated by buoyancy. The maximum velocity increase along a heater is about v = √(2 × g × β × ΔT × L), where g = 9.81 m/s2, β is thermal expansion coefficient (0.0004/°C for water), ΔT is temperature difference between heater surface and bulk (10°C) and L is heater length (1 m). v = √(2 × 9.81 × 0.0004 × 10 × 1) = √(0.0785) = 0.28 m/s-at the minimum. If you diminish this velocity in any way you run the risk of overheating. For natural convection, the heater diameter should be ≤ 30% of the baffle gap width to avoid choking.

Heater Diameter versus. Baffle Gap Selection Chart
Baffle Gap Width (mm) Flow Type Minimum Required v (m/s) Max Heater OD (mm) for v >= minRecommended Heater OD (mm) Remarks
50 Forced (pump, stirrer) 0.3 25 20–25 Tight but feasible 50 Natural convection 0.25 15 12–16 Very tiny heater
75 Forced 0.3 40 32–38 Good 75 Natural 0.25 22 20-25 Acceptable 100 Forced 0.3 58 50 Standard 100 Natural 0.25 30 25 OK 125 Forced 0.3 83 75 big heater possible 150 Forced 0.3 108 100 Typical for big tanks
200 Forced 0.3 158 150 Can fit multiple heaters
Effects of an Oversized Heater in the Baffle Space
If the diameter of the heater is too big for the baffle gap (D > 0.6 × W for forced flow, D > 0.3 × W for natural convection) it creates three problems:

Flow stagnation: Velocity dips below 0.2m/s. The heat transfer coefficient h is decreased from 500 – 1 000 W/m 2 K to 100 – 300 W/m 2 K. With constant watt density, the PFA surface temperature increases by 20-50°C.

Localised overheating: The hottest point is downstream of the heater (in the wake) The upstream and downstream temperature difference may reach as high as 20–30°C, which leads to non-uniform thermal stress.

2.2 Accelerated scaling Low velocity permits particles and bubbles to stick to the heater, resulting in scale and biofilm formation. Even more heat transfer is lost at the scale, producing a positive feedback loop.

Field inspection of an enormous heater in a baffle gap displays a definite pattern; yellowed or brown PFA on the downstream side, with white scale deposits. The upstream side may look normal. If you see this pattern, your heater is too large for the gap.

Installation Options When the Gap Is Too Small
If the needed heater diameter is larger than the maximum allowed in the baffle gap, the following solutions are suggested:

Relocate the heater: Place the heater in the center of the tank (away from baffles) or in a recirculation loop. Flow velocity in center mounting is often greater than in baffle gaps.

Use many smaller heaters Use two 25 mm heaters, rather than one 50 mm heater. Each smaller heater has reduced flow limitation, but the overall surface area (and power) might be the same.

boost the baffle gap If tank design allows, boost W by shifting baffles or by reducing the baffle edges. By increasing the distance by 25 mm, a 50 mm heater can be added where only 32 mm fit before.

Add a flow accelerator: An eductor or jet nozzle in the baffle gap increases the local velocity. A nozzle uses some pump energy, but recovers heat transmission.

Field Example:
5,000 L mixing tank with a baffle gap of 80 mm. The plant built a 63 mm PFA heater (79% gap). Measured flow velocity = 0.12 m/s. The heater was scaled-up within 3 months. PFA surface temperature was 115°C in 70°C water (45°C increase). The plant replaced two 25 mm heaters (equal total power) at each side of the baffle. The velocity around each heater was .35 m/s. Scaling was decreased and surface temperature lowered to 85°C and heater life increased from 6 months to 5 years. Two heaters cost more (2x500=500=1,000) than one 63 mm heater ($800), but the longer life and less downtime paid off in 18 months.

Conclusion: Heater OD ≤ 0.5 * Baffle Gap for Forced Flow
For a PFA heater in a baffle gap, the heater diameter should be selected such that OD ≤ 0.5 baffle gap width (W) for forced flow (pumped or agitated), and OD ≤ 0.3 W for natural convection. This gives sufficient free area to maintain minimum bypass velocity of flow of 0.3 m/s (forced) or 0.25 m/s (natural). Oversized heaters choke flow, leading to poor velocity, high surface temperature, increased scaling and early failure. Measure the gap, determine the maximum permissible OD and choose appropriately If the required OD is outside the limit, move the heater or use several tiny heaters. The baffle space is for flow, not for shoving the biggest heater in there. respect the spacing, size the heater, keep the flow. Give your heater years of service, and thank you.