Water-for-Injection Cooler Underperformance Before Sterile Filtration
A 5 m³/h water-for-injection stream must be cooled from 80°C to 25°C before a 0.22 µm filter. The existing plate heat exchanger has 8.0 m² of area; the vendor clean overall coefficient is 1200 W/m²K. Chilled water enters at 7°C and leaves at 12°C at design. After sanitization, with the same flows, the WFI outlet is 34°C and the pressure drop rises from 0.6 to 0.9 bar. The filter is rated for a maximum 30°C. Production proposes reducing WFI flow to 4 m³/h to meet the limit. The exchanger was cleaned-in-place but not opened for inspection. There is a local temperature gauge but no chart recorder. The batch record requires a continuous temperature record.
1. Calculate the design and actual heat duties, LMTDs, and overall coefficients. Is the problem fouling or undersizing?
Read the worked answer
At design, mass flow = 5 m³/h × 1000 kg/m³ / 3600 = 1.389 kg/s. Q = 1.389 × 4.18 × (80 − 25) = 319 kW. Cold-water flow = 319 / (4.18 × 5) = 15.3 kg/s, or about 55 m³/h. Countercurrent LMTD = ((80 − 12) − (25 − 7)) / ln(68/18) = 37.6 K. Required UA = 319 / 37.6 = 8.49 kW/K, so U = 8.49/8.0 = 1061 W/m²K. The vendor 1200 W/m²K gives about 12% margin, so the exchanger is not obviously undersized at design. At the observed 34°C outlet, Q = 1.389 × 4.18 × 46 = 267 kW. Cold outlet = 7 + 267 / (15.3 × 4.18) = 11.2°C. LMTD = ((80 − 11.2) − (34 − 7)) / ln(68.8/27) = 44.7 K. UA = 267 / 44.7 = 5.97 kW/K, giving U = 746 W/m²K. That is about 30% below design and 38% below the clean vendor value. The increased pressure drop supports fouling, scale, or a flow-path problem, not simple undersizing.
2. Evaluate the proposal to reduce flow to 4 m³/h. Will it reliably meet the 30°C filter limit?
Read the worked answer
At 4 m³/h, hot-side capacity = 4.18 × (4000/3600) = 4.644 kW/K. Using the actual UA of 5.97 kW/K and the same chilled-water flow, solve Q = 5.97 × LMTD together with Q = 4.644 × (80 − T). The result is an outlet near 28°C, so it meets the 30°C limit with about 2 K margin. If chilled-water flow is reduced proportionally, the outlet is about 28.5°C, still marginal. This is not a robust fix: the exchanger is already fouled, the margin is thin, and the local gauge is not a continuous record. Any further fouling, air binding, or flow imbalance could push the filter over temperature. Reducing flow also lowers filter flux and may change filtration performance. The proposal can be used only as a temporary, validated operating limit with continuous temperature monitoring and an alarm, not as the permanent correction.
3. Recommend corrective and preventive actions, including monitoring and documentation.
Read the worked answer
Open and inspect the exchanger; look for scale, gasket intrusion, plate deformation, and blocked channels. Clean and passivate as appropriate, replace damaged gaskets, and re-test thermal performance. Verify chilled-water flow and temperature, vent air, and confirm the pressure drop returns to baseline. Install a calibrated continuous temperature recorder with a high alarm at 30°C and an interlock or diversion if the limit is exceeded. Re-qualify the cooler at the intended flow, including worst-case cooling-water temperature. Add a preventive maintenance frequency based on fouling trend, not a fixed calendar guess. Update the batch record and SOPs to require the continuous record and alarm response. If the exchanger cannot maintain margin, add area or redundancy rather than relying on reduced flow.