The Blueprint of Water

The Hero of Heat Transfer: Answer Key

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⟵ 02 The Hero of Heat Transfer (Problem Set) · Contents · 03 The Solvent ⟶

The Hero of Heat Transfer: Answer Key

  1. Adhesion is water’s attraction to other surfaces; cohesion is water’s attraction to itself. Adhesion keeps water in contact with heat exchange surfaces (enabling conduction). Cohesion holds the bulk fluid together so it moves as a connected whole (enabling convection). Without both, heat transfer stalls.

  2. When energy enters liquid water, much of it is absorbed into the Hydrogen-Bond Network as potential energy - bonds stretch and reconfigure rather than immediately increasing molecular speed. This stores energy in structure rather than temperature, dampening the rate of temperature change. That dampened response is the “shock absorber,” and it is the reason water’s specific heat is 1.0 BTU/lb·°F - far higher than most substances.

  3. Sensible heat changes temperature without changing phase - it increases molecular motion. Latent heat changes state without changing temperature. It is spent separating molecules, overcoming intermolecular attractions, and creating the vapor phase.

  4. Refrigerants are selected because their intermolecular attractions and vapor-pressure behavior allow them to boil and condense with modest pressure and temperature changes. Water’s hydrogen-bond network resists separation, requiring ~970 BTU/lb to vaporize. Refrigerants “quit” easily; water resists until an enormous energy toll is paid.

  5. 500 gal × 8.34 lb/gal = 4,170 lb. Q = 4,170 lb × 1.0 BTU/lb·°F × (180 − 60)°F = 4,170 × 120 = 500,400 BTU.

  6. (a) Sensible: 1 lb × 1.0 × (212 − 72) = 140 BTU. (b) Latent: 1 lb × 970 BTU/lb = 970 BTU. Vaporization requires 970/140 = 6.9 times more energy than heating to the boiling point.

  7. 6,000,000 BTU/hr ÷ 970 BTU/lb = 6,186 lb/hr. 6,186 lb/hr ÷ 8.34 lb/gal = 742 gal/hr ÷ 60 min/hr = 12.4 GPM.

  8. 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 ÷ 970 = 7,738 lb/hr ÷ 8.34 ÷ 60 = 15.5 GPM evaporation. The rule of thumb holds.

  9. Sensible: 1 lb × 1.0 BTU/lb°F × (212°F − 50°F) = 162 BTU. Latent: 970 BTU. Total: 162 + 970 = 1,132 BTU per pound of steam. Latent heat accounts for 86% of the total energy investment.

  10. Kinetic energy (sensible heat) allows water to absorb and release heat through temperature change - this is what chilled water loops and hot water loops rely on. Potential energy stored in the Hydrogen-Bond Network acts as a buffer, dampening temperature swings and keeping systems stable. Phase-change energy (latent heat) allows massive energy transfer in a small mass of water - this is what makes both boiler steam and cooling tower evaporation possible. Without all three, water could not serve as both the heating and cooling medium for industrial systems.

⟵ 02 The Hero of Heat Transfer (Problem Set) · Contents · 03 The Solvent ⟶

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