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Problem 1
Explain in one sentence why dissolved solids go wherever the water goes, but suspended solids go wherever the system lets them. What physical principle governs each?
Authored answer
Dissolved solids have been dismantled at the molecular level and are governed by chemistry (equilibrium, solubility, reaction kinetics). They move wherever the water moves because they are part of the solution. Suspended solids are intact particles governed by physics (gravity, flow velocity, inertia). They settle wherever the system’s hydraulics allow them to stop moving.
Read the connected sectionProblem 2
A cooling tower has excellent water chemistry: inhibitor residuals on target, biocide fed on schedule, pH and conductivity within range. Corrosion coupons look acceptable, but during shutdown you find aggressive pitting beneath silt deposits on horizontal pipe runs. The basin has 3 inches of settled silt. Explain how these observations are consistent.
Authored answer
The bulk water chemistry is excellent, but it is only measuring the water column. The 3 inches of settled silt have created deposits on horizontal pipe surfaces. Beneath those deposits, oxygen is consumed but cannot be replenished, creating oxygen differential cells that drive under-deposit corrosion. The corrosion inhibitor, pH, and biocide measured in the bulk water are not reaching the metal surface beneath the deposits. The coupons, which are typically placed in the flowing water column on clean holders, are measuring the chemistry the pipes are not actually receiving. Excellent bulk chemistry and aggressive pitting are entirely consistent when deposits are present.
Read the connected sectionProblem 3
Describe the potential sequence of consequences through all three pillars (corrosion, biology, scale) for a windstorm that deposits a heavy layer of dust in a cooling tower basin if the solids are not removed.
Authored answer
Dust settles in low-flow zones and on pipe surfaces, creating deposits. The deposits create oxygen differential cells that drive under-deposit corrosion (Corrosion pillar). Corrosion products (iron oxide) generate additional suspended solids, accelerating the cycle. The deposits also provide shelter for biological attachment: prefabricated homes for biofilm formation that are shielded from biocides in the water column (Biology pillar). The biofilm produces EPS that traps additional particles, further stabilizing the deposit. Meanwhile, the deposit surfaces provide heterogeneous nucleation sites that lower the kinetic barrier for scale formation, particularly on heat-exchange surfaces where temperature is highest and hydration is most stressed (Scale pillar). A single windstorm, left unaddressed, cascades through all three pillars.
Read the connected sectionProblem 4
A closed chilled water loop shows brown, turbid water at 45 NTU despite having been in operation for two years. Is this a suspended solids problem or something else? What would you investigate first?
Authored answer
In a closed system that has been operating for two years, 45 NTU is a suspended-solids problem, but the source is probably corrosion rather than external dirt. Closed systems are not continuously exposed to atmospheric dust, but they can still accumulate solids from makeup water, construction debris, maintenance activities, process leaks, line breaks, and internal corrosion. In this case, the brown, turbid water most likely indicates iron oxide generated by active corrosion of carbon-steel piping. Investigate first: (a) check the corrosion inhibitor residual: is the treatment program being fed consistently? (b) filter a sample and examine the particulate: brown/red suggests iron oxide, black suggests magnetite or SRB activity, green suggests copper corrosion products. (c) check whether the system was properly flushed after construction: construction debris from two years ago can continue circulating indefinitely.
Read the connected sectionProblem 5
A cooling tower has a system volume of 3,000 gallons and a recirculation rate of 800 GPM. (a) What sidestream flow rate is needed to turn over the system volume every 4 hours? (b) What percentage of the recirculation rate is this? (c) If the tower is near an active construction site, would you recommend increasing this target?
Authored answer
(a) Required throughput = 3,000 gal ÷ 4 hr = 750 gal/hr = 12.5 GPM. (b) 12.5 GPM ÷ 800 GPM = 1.56% of the recirculation rate. (c) Yes. Near an active construction site, solids loading will be significantly higher than normal. Increasing the sidestream flow rate to turn over the system volume every 2–3 hours (18.75–25 GPM, or roughly 2.3–3.1% of the recirc rate) would be prudent. Additionally, check the filter type: construction dust may include fine particles that require a sand or multimedia filter rather than a screen or disk filter.
Read the connected sectionProblem 6
Using the concept from Stokes’ Law, explain why a hydrocyclone separator is effective for removing rust particles (density ~5,200 kg/m³, diameter ~50 µm) but ineffective for biological debris (density ~1,050 kg/m³, diameter ~20 µm). You do not need to calculate, just explain the reasoning.
Authored answer
Stokes’ Law shows that settling velocity depends on the square of the particle diameter and the density difference between the particle and water. Rust particles have a very high density difference (ρp − ρw ≈ 5,200 − 998 = 4,202 kg/m³) and a moderate diameter (50 µm). The hydrocyclone generates centrifugal force that accelerates settling. Heavy, dense particles are thrown to the outer wall and separated. Biological debris has a density barely above water (ρp − ρw ≈ 1,050 – 998 = 52 kg/m³), roughly 80× less density difference, and a smaller diameter (20 µm). The centrifugal force is not sufficient to separate particles with such a small density difference from the water. They simply recirculate with the flow.
Read the connected sectionProblem 7
Your corrosion coupon program shows carbon steel rates of 1.2 MPY, within the “good” range. But during a shutdown, you find deep pitting beneath silt deposits on horizontal pipe runs. Explain why the coupons missed this, and propose a change to the monitoring program.
Authored answer
Standard corrosion coupons are mounted in the flowing water column on clean, smooth holders. They measure uniform corrosion in the bulk water environment. Under-deposit corrosion is localized: it occurs beneath silt deposits on horizontal pipe surfaces where the metal is isolated from the treatment program. The coupon never sees this condition because it has no deposit on it. To detect under-deposit corrosion, the monitoring program should be supplemented with: (a) visual inspection of horizontal pipe runs during shutdowns, looking for deposits and localized pitting beneath them; (b) ultrasonic thickness testing on deposit-prone sections; (c) placing some coupons horizontally with intentional deposits or in spool pieces that mimic pipe conditions rather than in clean, vertical holders.
Read the connected sectionProblem 8
A 1,000-ton chiller system operates 4,000 hours per year at $0.12/kWh. The chiller draws 0.65 kW/ton. Basin turbidity averages 35 NTU and condenser approach temperature has risen 3°F above clean conditions. (a) Estimate the energy penalty of the 3°F rise (use 1.5% per °F). (b) Calculate the annual cost of that penalty. (c) A sidestream sand filter costs $8,000 installed. If it restores the condenser to clean conditions within one cooling season, what is the simple payback period?
Authored answer
(a) Energy penalty = 3°F × 1.5%/°F = 4.5%. (b) Baseline energy: 1,000 tons × 0.65 kW/ton = 650 kW. Annual energy: 650 kW × 4,000 hr = 2,600,000 kWh. Annual cost: 2,600,000 kWh × $0.12/kWh = $312,000. Energy penalty cost: $312,000 × 4.5% = $14,040/year. (c) Simple payback: $8,000 ÷ $14,040/yr = 0.57 years, or approximately 7 months. The filter pays for itself well within the first cooling season. And this calculation does not include reduced chemical cost, lower maintenance labor, or extended equipment life.
Read the connected sectionProblem 9
A water treater increases the dispersant dose in a heavily fouled cooling tower. Within 48 hours, turbidity increases from 20 to 60 NTU and condenser approach temperature actually improves by 1°F. Explain what is happening and whether this is a problem or a success.
Authored answer
The dispersant is working. The turbidity spike from 20 to 60 NTU indicates that the dispersant is lifting deposits from heat-exchange surfaces and piping into the water column. The 1°F improvement in condenser approach temperature confirms this. The insulating deposit layer on the condenser tubes has been partially removed, restoring heat transfer. The “worse” turbidity reading is actually showing material that was already in the system but hidden on surfaces, invisible to bulk water monitoring. This is a success. The correct next steps are to increase blowdown temporarily or ensure sidestream filtration is running to capture the suspended material, and to follow with a biocide treatment to kill organisms that were sheltered beneath the deposits and are now exposed.
Read the connected sectionProblem 10
You are called to a cooling tower that has not had sidestream filtration, has been running without consistent biocide treatment, and has visible silt deposits in the basin. The customer wants you to “fix the chemistry.” Write a brief explanation (4–6 sentences) of why adjusting chemical feed rates alone will not solve this problem. Reference at least two amplification pathways.
Authored answer
Adjusting chemical feed rates alone will not solve this problem because the suspended solids have created deposits that isolate surfaces from whatever chemistry is in the water. Increasing corrosion inhibitor concentration does not help when the inhibitor cannot reach the metal beneath the silt deposits. Under-deposit corrosion will continue regardless of bulk inhibitor residual. Increasing biocide dose does not help when the biocide is consumed by EPS and organic debris in the deposits before it can reach the organisms sheltered within them. The deposits must be physically removed first: the basin needs to be cleaned, sidestream filtration must be installed to prevent reaccumulation, and a dispersant program should be used to lift existing deposits off surfaces. Only then will the chemistry operate in the environment it was designed for. The customer is asking to fix a mechanical problem with a chemical solution, and the amplifier effect guarantees that approach will fail.
Read the connected section