In baffled tanks-used for mixing, chemical reactions, and heat transfer-the space between the baffle and the tank wall creates a flow bypass channel. If a PFA heater is placed in this channel, the flow velocity past the heater must be sufficient to maintain heat transfer and prevent localized overheating. The minimum flow bypass velocity for PFA heaters in water service is 0.3–0.5 m/s to keep the PFA surface temperature within 10°C of the bulk fluid at typical watt densities (3–5 W/cm²). For a given baffle spacing (gap between baffle edge and tank wall), the heater diameter must be selected so the cross-sectional area of the bypass channel around the heater is large enough to maintain velocity. The rule: heater OD should be ≤ 50% of the baffle gap width. For a 100 mm baffle gap, maximum heater OD is 50 mm. For a 50 mm gap, maximum heater OD is 25 mm. Larger heaters choke the flow, reducing velocity and causing overheating.
Flow Bypass Velocity Calculation
The velocity in the baffle gap is v = Q / A, where Q is the volumetric flow rate from the agitator or recirculation pump, and A is the cross-sectional area of the gap not occupied by the heater. The total gap width is W, heater OD is D, and tank height is H (assumed fully submerged). The free area A_free = (W - D) × H. If D approaches W, A_free approaches zero, velocity drops dramatically. To maintain v ≥ 0.3 m/s, A_free must be ≥ Q / 0.3. For a typical tank with an agitator delivering Q = 0.05 m³/s (3 m³/min), H = 2 m, required A_free = 0.05 / 0.3 = 0.167 m². Then (W - D) × 2 = 0.167, so W - D = 0.0835 m = 83.5 mm. If W = 150 mm, maximum D = 150 - 83.5 = 66.5 mm. A 65 mm heater would work. If W = 100 mm, maximum D = 100 - 83.5 = 16.5 mm-a very small heater. In this case, the heater cannot be placed in the baffle gap; it must be relocated.
For natural convection tanks (no pump, no agitator), the flow velocity is generated by buoyancy. The maximum velocity rise along a heater is approximately 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. Any restriction that reduces this velocity risks overheating. For natural convection, the heater diameter should be ≤ 30% of baffle gap width to avoid choking.
Heater Diameter vs. Baffle Gap Selection Table
| Baffle Gap Width (mm) | Flow Type | Minimum Required v (m/s) | Maximum Heater OD (mm) for v ≥ min | Recommended Heater OD (mm) | Notes |
|---|---|---|---|---|---|
| 50 | Forced (pump, agitator) | 0.3 | 25 | 20–25 | Tight but feasible |
| 50 | Natural convection | 0.25 | 15 | 12–16 | Very small 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 | Large heater possible |
| 150 | Forced | 0.3 | 108 | 100 | Typical for large tanks |
| 200 | Forced | 0.3 | 158 | 150 | Multiple heaters may fit |
Consequences of Oversized Heater in Baffle Gap
If the heater diameter is too large for the baffle gap (D > 0.6 × W for forced flow, D > 0.3 × W for natural convection), three problems occur:
Flow stagnation: Velocity drops below 0.2 m/s. The heat transfer coefficient h drops from 500–1,000 W/m²·K to 100–300 W/m²·K. For constant watt density, the PFA surface temperature rises by 20–50°C.
Localized overheating: The hottest point is on the downstream side of the heater (in the wake). The temperature difference between upstream and downstream can reach 20–30°C, causing uneven thermal stress.
Accelerated scaling: Low velocity allows bubbles and particles to cling to the heater, forming scale and biofilms. The scale further reduces heat transfer, creating a positive feedback loop.
Field inspection of an oversized heater in a baffle gap shows a distinct pattern: the PFA is yellowed or brown on the downstream side, with white scale deposits. The upstream side may appear normal. If this pattern is observed, the heater is too large for the gap.
Installation Alternatives When Gap Is Too Small
If the required heater diameter exceeds the maximum allowable for the baffle gap, consider these alternatives:
Relocate the heater: Mount the heater in the center of the tank (away from baffles) or in a recirculation loop. Center mounting often has higher flow velocity than baffle gaps.
Use multiple smaller heaters: Instead of one 50 mm heater, use two 25 mm heaters. The total surface area (and power) can be the same, but each smaller heater has a lower flow restriction.
Increase baffle gap: If tank design permits, increase W by moving baffles or trimming baffle edges. Adding 25 mm to the gap allows a 50 mm heater where only 32 mm fit before.
Add a flow accelerator: Install a small eductor or jet nozzle in the baffle gap to increase local velocity. The nozzle consumes some pump energy but restores heat transfer.
Field Example
A 5,000 L mixing tank had a baffle gap of 80 mm. The plant installed a 63 mm PFA heater (79% of gap). Flow velocity (measured) was 0.12 m/s. The heater scaled up within 3 months, and PFA surface temperature reached 115°C in 70°C water (45°C rise). The plant replaced it with two 25 mm heaters (total power same) placed on either side of the baffle. Velocity around each heater was 0.35 m/s. Scaling reduced, surface temperature dropped to 85°C, and heater life extended from 6 months to 5 years. The cost of two heaters (2 × 500=500=1,000) was higher than one 63 mm heater ($800), but the longer life and reduced downtime paid back within 18 months.
Conclusion: Heater OD ≤ 0.5 × Baffle Gap for Forced Flow
For a PFA heater installed in a baffle gap, select the heater diameter such that OD ≤ 0.5 × baffle gap width (W) for forced flow (pumped or agitated), and OD ≤ 0.3 × W for natural convection. This ensures sufficient free area to maintain minimum flow bypass velocity of 0.3 m/s (forced) or 0.25 m/s (natural). Oversized heaters choke flow, causing low velocity, high surface temperature, accelerated scaling, and premature failure. Measure the gap, calculate the maximum allowable OD, and select accordingly. If the needed OD exceeds the limit, relocate the heater or use multiple smaller heaters. The baffle gap is for flow, not for stuffing the largest possible heater. Respect the gap, size the heater, maintain the flow. Your heater will thank you with years of service.
