Problem 13.10: Using the Perfect Storm scenario 90 condensate

From Chapter 13 Problem Set. Level 3: System Integration.

Question

Using the Perfect Storm scenario (90% condensate return dropping to 37%), estimate the fuel cost of heating the additional cold makeup water. Assume: steam production = 25,000 lb/hr, condensate returns at 180°F, makeup temperature = 55°F, boiler efficiency = 80%, gas cost = $10/MMBTU, 8,760 hours/year operation. Show your work using dimensional analysis.

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Answer

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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 × $10=\ast \ast$181,500/yr**. This exceeds the $130,000estimateinthePerfectStormbecausetheoriginalusedashorteroperatingseasonandlowerfuelcost-thepointisthatthemagnitudeisenormousrelativetothe$10,000 pump replacement.

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