Cooling Water Hardness Across ASEAN Regions: Predicting Pressure Drop Rise from Condenser Tube Fouling in Screw Chillers

Cooling Water Hardness Across ASEAN Regions: Predicting Pressure Drop Rise from Condenser Tube Fouling in Screw Chillers

— An engineering selection guide based on shell-and-tube heat exchanger parameters and operating boundary conditions

Water Hardness Is Not an Operating Variable; It Is a Design Boundary

In ASEAN regions (Thailand, Vietnam, Indonesia, Philippines) and South Asia (India, Bangladesh), make-up water for cooling towers is typically drawn from surface water or shallow groundwater. Total hardness (as CaCO₃) frequently ranges between 200–400 mg/L, with seasonal dry/wet cycles causing significant water quality fluctuations.

For water-cooled screw chillers, the condenser-side water loop does not operate under "standard conditions" but rather under "variable water quality conditions." The PDF clearly specifies that the SHWE series condenser design is based on a fouling factor of 0.00025 ft²·°F/Btu (equivalent to 0.0440 m²·°C/kW). This value represents the preset tolerance boundary for heat transfer degradation during the selection phase. When actual on-site water hardness causes the fouling thermal resistance to exceed this preset value, the direct physical consequence is rising condensing temperature and pressure, forcing the compressor to increase discharge pressure differential to maintain chiller capacity output.

Technical Consequences of Fouling: From Heat Transfer Attenuation to Pressure Drop Drift

Tube bundle fouling negatively impacts chiller performance in two distinct dimensions, which selection engineers and O&M teams should address separately:

Dimension 1: Increased thermal resistance (efficiency decay). Scale deposits (primarily calcium carbonate and silicate mixtures) accumulate on the inner tube wall. The thermal conductivity of scale is less than 1/50 of copper (approx. 401 W/m·K), directly elevating the heat transfer resistance between the tube wall and the water flow. This manifests as widening condenser approach temperature—i.e., the difference between refrigerant condensing saturation temperature and cooling water outlet temperature exceeds the design value.

Dimension 2: Unplanned pressure drop rise (flow safety risk). Fouling reduces the effective flow cross-section within the tubes. At the same water flow rate, velocity increases, and frictional resistance rises accordingly. Refer to the condenser water-side pressure drop data for each model in the PDF on page 10—for example, the SHWE 210H model shows 43.2 kPa under standard conditions, while the SHWE 300H shows 41.2 kPa. These pressure drop values correspond to clean tube bundle test results. When scale layer thickness reaches 0.2–0.3 mm, the measured pressure drop may drift upward by more than 30–50 kPa above the clean baseline (no percentage given; this is a qualitative projection to stress the need for adequate pump head margin during selection).

Prevention Strategies: From Material Selection to Flow Channel Geometry

Intervention against fouling risk should be addressed at the selection stage through the following three physical-level approaches:

① Tube material and surface treatment. The PDF on page 8 explicitly describes that this series of condensers employs double-sided reinforced condenser tubes. The double-sided reinforcement enhances internal turbulence to reduce laminar boundary layer thickness and delay inorganic salt deposition, while externally it improves refrigerant-side condensing heat transfer coefficients. For hard-water regions, specifiers may further consult the manufacturer regarding inner-wall coatings (e.g., Cupronickel or anti-corrosion layers). However, this option changes the overall heat transfer coefficient and requires re-calculation of the required heat exchange surface area.

② Water-side flow velocity design reference. Based on the water flow rates and connection sizes (DN100 to DN200) provided on PDF page 10, the design flow velocity within tubes generally falls within 1.5–2.5 m/s. This velocity range maintains self-cleaning effects (preventing particle sedimentation) while avoiding excessive wear or pumping losses. For high-hardness make-up water, it is advisable to maintain flow velocity above 2.0 m/s and use regulating valves or VFDs on chilled water pumps to prevent overly low velocities under partial loads, which encourage sediment accumulation.

③ Removable end covers provide physical access for mechanical cleaning. "Flooded evaporator" section, explicitly states: "Water boxes at both ends can be disassembled to facilitate maintenance." Although this description directly targets the evaporator, the condenser shell-and-tube configuration supports the same approach. During selection, sufficient tube extraction space should be preserved at both ends of the condenser. This clearance directly determines whether high-pressure water jetting or brush cleaning operations can be performed during later maintenance cycles.

Online Maintenance Strategies: Parameter Monitoring and Intervention Thresholds

For existing projects where tube replacement or coating is not feasible, the following three data-driven active maintenance mechanisms are recommended:

First, monthly monitoring of condenser approach temperature. Record the difference between refrigerant condensing saturation temperature and cooling water outlet temperature. If this approach temperature rises by more than 3°C above the baseline established during equipment acceptance (this 3°C is a general industry caution threshold; please confirm the specific baseline for each model with the manufacturer), chemical cleaning (online circulation with mild acidic cleaning agents) or physical cleaning should be initiated.

Second, online water-side pressure drop monitoring. "If the condenser outlet temperature exceeds 55°C it is recommended to contact the manufacturer for guidance." This temperature threshold directly corresponds to the condensing pressure ceiling, which is inherently linked to tube bundle fouling. Install permanent pressure sensors at both inlet and outlet points. Trigger an alarm when the measured pressure differential exceeds the clean baseline by a predetermined margin.

Third, upstream intervention in cooling tower make-up water treatment. Although the allowable cooling water inlet temperature range is broad—19°C to 50°C (PDF page 9)—water hardness is not protected by this operating envelope. Install bypass softening units (ion exchange resin) at the cooling tower basin or make-up line to reduce hardness to <100 mg/L, minimizing calcium carbonate precipitation at the source.

Conclusion

For water-cooled screw chiller deployment in hard-water regions across ASEAN and South Asia, the selection phase should not focus solely on cooling capacity (332.6–1988 kW) and COP (5.4–5.5 W/W). Equal consideration must be given to the condenser fouling factor preset at 0.0440 m²·°C/kW, the clean pressure drop baseline (41–44 kPa), and the maximum condensing temperature threshold of 55°C as critical auxiliary design inputs. During operation, integrate approach temperature drift and pressure drop drift into routine checklists, and preserve mechanical cleaning access using the removable end cover design. For mission-critical facilities such as solar manufacturing plants, hotels, and stadiums—where unplanned shutdowns are unacceptable—this strategy framework provides the physical assurance required to avoid forced de-rating operation.