Cooling Tower Water Balance and Rising Condenser Approach at a Pharmaceutical Plant
A pharmaceutical plant in Borg El Arab rejects 1,500 kW through an open induced-draft cooling tower. Circulating water is 260 m³/h with a 5 K range and a 3 K approach; the tower serves the chiller condenser and a process water loop. Water treatment runs 4 cycles of concentration with sulphuric acid and dispersant, and make-up comes from a well at 350 ppm hardness as CaCO₃. Last month the plant logged 2,900 m³ of make-up, and the condenser approach has risen from 1.0 K to 2.8 K over three months. The tower stands 40 m from the boundary fence and 25 m from the air intake of the packaging hall. Blowdown goes to the municipal sewer without treatment and the environmental inspector has asked for an analysis. Ambient design wet bulb is 24 °C and the plant runs 24 h/day.
1. Build the water balance for the tower and reconcile it with the 2,900 m³/month of logged make-up. What does the discrepancy tell you?
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Evaporation. E = Q/h_fg = 1,500 kW ÷ 2,450 kJ/kg = 0.612 kg/s = 2,204 kg/h ≈ 52.9 m³/day.
Blowdown at 4 cycles. B = E/(COC − 1) = 52.9/3 = 17.6 m³/day.
Drift at 0.02% of 260 m³/h = 0.052 m³/h ≈ 1.25 m³/day.
Make-up = 52.9 + 17.6 + 1.25 ≈ 71.8 m³/day ≈ 2,154 m³ in a 30-day month.
Reconciliation. Logged make-up is 2,900/30 = 96.7 m³/day, about 35% above the calculated figure. Holding evaporation and drift constant, blowdown must be 96.7 − 52.9 − 1.25 = 42.5 m³/day, which implies an actual COC of E/B + 1 = 52.9/42.5 + 1 ≈ 2.2, not the 4 the treatment contractor believes he is running. The plant is therefore bleeding about 25 m³/day of conditioned water it need not lose - cost in water, acid, dispersant and biocide, and extra hydraulic load on the tower.
Prove it rather than assume it: measure make-up and blowdown with meters (not estimates), take conductivity of make-up and circulating water simultaneously, and confirm running hours. Check for overflow at the basin, leaking float valves, pump gland service water and process-side leaks. Add a conductivity-controlled blowdown valve rather than a fixed bleed, and re-baseline the treatment programme at the corrected cycles.
2. The condenser approach has moved from 1.0 K to 2.8 K. Quantify the effect on plant energy and set out your diagnosis and remedy, given that the measured cycles of concentration are low.
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Energy consequence. The approach rise of 1.8 K lifts condensing temperature by roughly 1.8 K, and at about 2.5% efficiency loss per K that is over 4% at the same duty. On a condenser duty of roughly 1,200 kW this is about 50 kW, or around 300 MWh a year at 6,000 equivalent hours - a substantial and avoidable cost, on top of capacity derate during the hottest weeks.
Diagnosis must be evidence-led. Scale is the reflex answer, but with cycles actually around 2.2 rather than 4 the scaling risk is low and the acid dose may be wasting money. The more likely culprits are: biofilm and algae on the tube surfaces and in the basin; silt and suspended solids from the well depositing in low-velocity tubes; air ingress on the pump suction or at the water box, raising the effective approach; or a genuine tube-side flow reduction from a partly closed valve or fouled strainer.
Actions: isolate and inspect the condenser heads and a tube sample; measure tube-side flow and pressure drop against the design; check the circulating pump suction for air entrainment and vortexing; brush or chemically clean as the inspection dictates; add sidestream filtration and a basin cleaning regime; fix the cycles at the true measured value and control blowdown on conductivity; install a biocide programme with dip-slide monitoring. Then trend the approach weekly as the performance KPI, with an alarm at 1.5 K above clean baseline.
3. The environmental inspector asks about the untreated blowdown discharge and the plant's HSE manager raises Legionella risk, pointing at the packaging hall air intake 25 m away. Advise the client.
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Discharge. Cooling-tower blowdown carries elevated TDS, acid, dispersant, biocide and possibly heavy metals from corrosion; it cannot be sent to the municipal sewer without checking the discharge limits in the Egyptian environmental law and its executive regulations, and without the discharger permit. Require: a holding/sampling tank, pH correction if acid dosing is used, periodic analysis against the permitted limits, records retained, and a written procedure for batch release. Where limits cannot be met, the blowdown needs pre-treatment or reuse, which also fits the site's water-scarcity economics.
Legionella and cross-contamination. A 25 m separation to the packaging hall intake is not defensible for an open tower with a visible plume - the risk is aerosol drift being drawn into a clean area. Require either relocation of the tower or the intake, high-efficiency drift eliminators (drift at or below 0.005% of flow), and a written water safety plan aligned with ASHRAE 188/WHO guidance: named responsible person, risk assessment, biocide regime with alternating oxidising and non-oxidising agents, monthly dip slides, periodic Legionella sampling of the basin and the system, and a shutdown/cleaning procedure.
Also flag: well abstraction likely needs its own licence and metering, and the treatment contractor's chemical storage needs secondary containment and an SDS on site. Put all of this in a dated action plan to the client - an inspector's visit is not the time to discover the gaps.