The Blueprint of Water

Heating Things Up: Answer Key

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⟵ 13 Heating Things Up (Problem Set) · Contents · 14 The Blueprint ⟶

Heating Things Up: Answer Key

  1. Corrosion rates double every 18°F. At 140°F (~70°F above ambient) ≈ 3.9 doublings → ~23.9 ≈ 15×. Although half the oxygen is gone, the remaining 4 mg/L is reacting 15× faster. Net corrosion potential is approximately 7.5x compared to room temperature. The paradox does not resolve until the boiling point eliminates oxygen almost entirely.

  2. 2% of 100 billion BTU = 2 billion BTU/yr. At 10 = $20,000/yr from a credit-card’s thickness of scale.

  3. Priming. The pattern (weekday mornings only) correlates with a sudden load swing: occupants arriving, thermostats activating, steam demand surging from low fire. This is mechanical surging, not chemical foaming. Chemistry is within limits, and the problem does not occur during steady-state weekend operation.

  4. (a) Softener overload / hardness breakthrough → pretreatment link. (b) Cold deaerator / elevated dissolved oxygen → feedwater/deaeration link. (c) Low cycles / increased blowdown → boiler vessel link. (d) Elevated CO₂ / low condensate pH → steam-condensate link. Also: increased makeup volume, increased fuel cost, increased chemical consumption.

  5. (a) Stoichiometric sulfite requirement: 3 ppm O₂ × 7.88 = 23.6 ppm sodium sulfite in the feedwater to neutralize the dissolved oxygen. (b) The sulfite that reacts with oxygen is consumed – it does not concentrate in the boiler. Only the excess sulfite that survives the reaction concentrates with the boiler water. To achieve a 30 ppm residual in the boiler at 10 cycles, the excess sulfite in the feedwater must be: 30 ppm ÷ 10 = 3.0 ppm excess in feedwater. Total feedwater sulfite target = stoichiometric demand + excess for residual = 23.6 + 3.0 = 26.6 ppm sodium sulfite.

  6. (a) Q = 8,000 × 1.0 × (170 − 60) = 880,000 BTU/hr. (b) Annual energy = 880,000 × 8,000 = 7.04 × 10⁹ BTU = 7,040 MMBTU. Adjusted for efficiency: 7,040 / 0.82 = 8,585 MMBTU. Cost: 8,585 × 103,024/yr**.

  1. (a) 50 µS × 40 = 2,000 µS. (b) 200 µS × 10 = 2,000 µS. (c) The conductivity is the same numerically, but the underlying chemistry is radically different. At 40 cycles on clean feedwater, TDS is composed of benign sodium salts. At 10 cycles on dirty feedwater, TDS includes calcium, magnesium, and elevated alkalinity - the species the softener should have removed. The 2,000 µS reading is within the 3,500 mg/L TDS limit for <300 psig, but the composition - not the conductivity - determines the risk. Hardness breakthrough means scaling risk is severe regardless of the conductivity reading.

  2. (a) The conductivity sensor is fouled or out of calibration, reading artificially low. The controller believes the water is clean and has closed the blowdown valve, allowing cycles to climb unchecked. (b) Actual cycles = 3,800 / 80 = 47.5. (c) At 300 psig, the ASME TDS limit is ~3,000 mg/L. If 3,800 µS corresponds to roughly 2,500–3,000 mg/L TDS (depending on ionic composition), the boiler is at or near the carryover risk threshold. Foaming, silica deposition in the steam system, and TDS-driven carryover are all concerns. Immediate action: validate the sensor, perform manual blowdown, recalibrate the controller.

  3. Do not increase chemical pumps. Diagnose the circuit. Step 1: Check deaerator temperature (explains sulfite consumption if DA is cold - elevated dissolved oxygen is overwhelming the scavenger). Step 2: Check softener output and brine tank (explains inhibitor depletion if hardness is breaking through - the scale inhibitor is being consumed by calcium instead of maintaining a residual). Step 3: Check condensate return rate (explains elevated CO₂ and low condensate pH - increased makeup alkalinity means more bicarbonate entering the boiler, more CO₂ in the steam, more carbonic acid in the condensate). A single root cause - loss of condensate return - explains all three symptoms simultaneously, exactly as in the Perfect Storm scenario. The circuit reveals the cause; the chemical tests only showed the symptoms.

  4. At 90% return: condensate = 22,500 lb/hr at 180°F, makeup = 2,500 lb/hr at 55°F. Weighted feedwater temp = (22,500 × 180 + 2,500 × 55) / 25,000 = (4,050,000 + 137,500) / 25,000 = 167.5°F. At 37% return: condensate = 9,250 lb/hr at 180°F, makeup = 15,750 lb/hr at 55°F. Weighted feedwater temp = (9,250 × 180 + 15,750 × 55) / 25,000 = (1,665,000 + 866,250) / 25,000 = 101.2°F. Additional heating required per pound: 167.5 − 101.2 = 66.3°F. Additional fuel load = 25,000 lb/hr × 66.3 BTU/lb = 1,657,500 BTU/hr. Annual = 1,657,500 × 8,760 = 14.52 × 10⁹ BTU = 14,520 MMBTU. At 80% boiler efficiency: 14,520 / 0.80 = 18,150 MMBTU of fuel input. Cost: 18,150 × 181,500/yr**. This exceeds the 10,000 pump replacement.

⟵ 13 Heating Things Up (Problem Set) · Contents · 14 The Blueprint ⟶

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