Problem 2.8: Using the 1 evaporation rule of thumb a tower

From Chapter 2 Problem Set. Level 3: System Integration.

Question

Using the 1% evaporation rule of thumb, a tower recirculates 1,500 GPM with a 10°F temperature drop. Estimate the evaporation rate in GPM. Then verify your answer using an idealized latent-only energy balance and 1,000 BTU/lb. (Q = m × cₚ × ΔT)

Answer

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Rule of thumb: 1% of 1,500 GPM = 15 GPM evaporation for a 10°F drop. Verification: Q = 1,500 GPM × 8.34 lb/gal × 60 min/hr × 1.0 × 10°F = 7,506,000 BTU/hr. Evap = 7,506,000 ÷ 1,000 = 7,506 lb/hr ÷ 8.34 ÷ 60 = 15 GPM evaporation. The rule of thumb holds.