The Agitator Obstacle
Cylindrical process tanks with center-mounted agitators present a classic heat exchanger design challenge. The ideal location for an immersion heater-uniformly distributed across the tank cross-section-is occupied by the agitator shaft. The heat exchanger must fit into the annular space between the shaft and the tank wall while providing adequate clearance for shaft rotation, impeller access, and tank cleaning.
A standard rectangular PTFE heat exchanger grid cannot fit into this space without either leaving large unheated zones or interfering with the agitator. A custom header geometry-circular or segmented-conforms to the available annular volume and maximizes the installed heat transfer surface.
The Annular Header Design
The heat exchanger is configured as a circular grid with a central opening for the agitator shaft. Two header arrangements serve this geometry: a continuous circular header with radial tube connections, or a segmented header composed of arc sections interconnected by flexible tube loops.
The continuous circular header is a PTFE ring positioned at the tank perimeter, either at the tank floor (for a floor-mounted grid) or suspended from the tank rim (for a vertically oriented grid). Tubes connect to the header around its circumference and extend radially inward toward the central opening. The tube length varies with the radial position on the header, creating a pattern that fills the annular space.
For large tanks where a single continuous header would be difficult to fabricate and install, a segmented design divides the annulus into sectors-typically 4 or 6. Each sector has its own header arc, tube bundle, and steam/condensate connections. The sectors can be installed and removed independently, simplifying maintenance access through the tank manway.
| Design Parameter | Continuous Circular Header | Segmented Header (4 Sectors) |
|---|---|---|
| Header shape | Full ring (360°) | 4 × 90° arc sections |
| Header diameter | 80-90% of tank ID | Same (arc section of ring) |
| Central opening diameter | Agitator shaft diameter + 200-300mm clearance | Same |
| Tube orientation | Radial (perpendicular to header ring) | Radial |
| Tube length variation | Progressive (short at inner radius, long at outer) | Progressive within each sector |
| Maximum header piece dimension | Tank diameter (requires large manway) | Arc chord length (fits through standard manway) |
| Installation | Single lift or in-tank assembly | Sector-by-sector; installable through manway |
| Maintenance access | Must drain tank for full bundle removal | Isolate and remove individual sector |
The Agitator Clearance Requirements
The central opening in the heat exchanger grid must provide adequate clearance for the agitator shaft and impeller. The clearance serves three purposes: preventing contact between the rotating shaft and the stationary tubes during normal operation, allowing for shaft deflection under load (which can be 5-15mm at the impeller for large agitators), and providing access for impeller removal during maintenance.
The minimum clearance is 150mm radially from the shaft surface for small agitators (up to 50mm shaft diameter) and 250-300mm for large agitators. The clearance is measured from the shaft surface at its maximum expected deflection, not from the static centerline.
The tube ends at the inner boundary of the annulus are supported by a PTFE guide ring that maintains their spacing and prevents individual tubes from deflecting into the shaft clearance zone.
The Thermal Performance Optimization
The annular geometry concentrates heating surface in the outer region of the tank, near the wall. This is actually beneficial for heat transfer in stirred tanks, because the impeller creates a high-velocity zone in the center and a lower-velocity zone near the wall. Placing the heat transfer surface in the lower-velocity zone maximizes the residence time of fluid in contact with the heated tubes, improving the effective heat transfer coefficient.
The tube density (tubes per unit area) can be varied with radius to optimize the heat flux distribution. Higher tube density at the outer radius compensates for the lower local velocity. Lower density near the inner radius prevents overheating in the higher-velocity impeller discharge zone.
Summary
Custom PTFE heat exchanger headers for cylindrical tanks with central agitators use circular or segmented configurations that conform to the annular space between the shaft and tank wall. The design maximizes heat transfer surface within the available volume while providing adequate clearance for shaft rotation, deflection, and impeller maintenance. Segmented headers enable installation and maintenance through standard manways. Variable tube density optimizes heat transfer to match the velocity profile of the stirred tank.
Engineering support for annular PTFE heat exchanger design is available upon submission of tank diameter and height, agitator shaft diameter and impeller dimensions, heating duty, and available manway size.
