The Cooling Water Variable
Cooling water is not a single fluid. It varies from plant to plant-from clean once-through river water to high-cycle cooling tower water laden with dissolved solids, suspended particles, and biological activity. The shell-side fluid velocity in a PTFE shell-and-tube heat exchanger must be matched to the specific cooling water chemistry.
Velocity that is too low allows suspended solids to settle, promotes biofilm attachment, and accelerates scaling on the PTFE tube surfaces. Velocity that is too high wastes pumping energy, increases erosion risk, and may cause flow-induced vibration of the flexible PTFE tubes.
The optimal velocity is a compromise between these competing demands, and the balance point shifts depending on the cooling water chemistry. A one-size-fits-all velocity recommendation is inadequate.
The Fouling Velocity Threshold
Different fouling mechanisms have different velocity dependencies. Understanding which mechanism dominates for a given cooling water chemistry guides the velocity specification.
Crystallization fouling-deposition of calcium carbonate, calcium sulfate, or silica-is primarily controlled by temperature and concentration, not velocity. However, higher velocity reduces the wall temperature by improving heat transfer, which reduces the crystallization driving force at the tube surface.
Particulate fouling-deposition of suspended silt, corrosion products, or airborne debris-is strongly velocity-dependent. Below a critical velocity, particles settle by gravity onto horizontal tube surfaces. The critical velocity for preventing particulate settling on PTFE is approximately 0.3-0.5 m/s, lower than for metal tubes because the smooth, hydrophobic surface reduces particle adhesion.
Biofouling-attachment and growth of microorganisms-is velocity-sensitive. Biofilm formation is inhibited at velocities above 0.8-1.0 m/s, where fluid shear stress exceeds the adhesive strength of the microbial film. However, PTFE's low surface energy already reduces biofilm adhesion compared to metal surfaces.
Table 1: Recommended Shell-Side Velocity by Cooling Water Type
| Cooling Water Type | Typical Chemistry | Primary Fouling Risk | Recommended Velocity (m/s) | Basis |
|---|---|---|---|---|
| Once-through river water | Low TDS, variable suspended solids | Particulate, seasonal biofouling | 0.5-0.8 | Prevents settling; moderates biofilm |
| Cooling tower water (low cycles) | Moderate TDS, treated with biocide | Scaling, some biofouling | 0.6-1.0 | Controls scaling via wall temperature reduction |
| Cooling tower water (high cycles) | High TDS, high hardness | Severe scaling risk | 0.8-1.2 | Maximizes heat transfer to minimize wall temperature |
| Closed-loop treated water | Low TDS, inhibited | Minimal fouling | 0.4-0.6 | Energy-efficient; low fouling risk |
| Seawater (once-through) | High TDS, chloride, biological activity | Biofouling, some scaling | 1.0-1.5 | High velocity inhibits macro-fouling attachment |
| Brackish water | Variable TDS, sulfate | Scaling, biofouling | 0.8-1.2 | Compromise between scaling and fouling control |
PTFE-Specific Velocity Considerations
PTFE tubes are flexible. At shell-side velocities above 1.5 m/s, flow-induced vibration can excite tube motion. The critical velocity for vibration onset depends on the unsupported tube span. Closer support spacing allows higher velocities without vibration.
PTFE surfaces are hydrophobic. The boundary layer behavior differs from hydrophilic metal surfaces. The slip velocity at the PTFE-water interface reduces wall shear stress slightly compared to a no-slip metal surface. The practical effect is a small reduction in the velocity required to achieve a given fouling resistance.
PTFE is not subject to erosion-corrosion in the conventional sense because there is no protective film to remove. However, at very high velocities (>2.5 m/s) with water containing abrasive particulates, mechanical erosion of the PTFE surface can occur. This is rare in cooling water service but relevant for water sources with high silt loading.
Velocity Optimization with Anti-Fouling Features
The PTFE heat exchanger design can incorporate features that enhance fouling resistance at lower velocities. Helical baffles create a swirling flow that scours tube surfaces more effectively than the straight-line flow from segmental baffles. The swirling flow maintains particulate suspension and inhibits biofilm attachment at velocities 20-30% lower than those required with conventional baffling.
The lower velocity capability reduces pumping energy costs while maintaining the fouling resistance of higher-velocity designs. The energy savings over the equipment lifetime can be substantial.
Summary
Cooling water chemistry determines the optimal shell-side velocity in PTFE shell-and-tube heat exchangers. The recommended range spans 0.4 m/s for clean closed-loop water to 1.5 m/s for seawater. The velocity selection balances particulate settling prevention, biofilm inhibition, and scaling control against pumping energy and vibration limits.
PTFE's low surface energy and smooth surface provide an inherent fouling resistance that allows velocities 10-20% lower than those required for metallic exchangers in the same service. Helical baffle designs further reduce the velocity requirement while maintaining fouling control.
Engineering recommendations for PTFE shell-and-tube exchanger velocity specification are available upon submission of cooling water analysis, operating temperature range, fouling history of existing exchangers, and available pumping capacity.
