Thermodynamics and Heat Transfer
Thermodynamics and Heat Transfer
This domain tests property relationships and heat-transfer fundamentals that explain boiler behavior: sensible and latent heat, saturation, superheat, subcooling, and conduction, convection, and radiation. Expect conceptual questions plus a simple enthalpy or heat-rate calculation.
Core rules and concepts
- Sensible heat changes temperature without a phase change; latent heat changes phase at constant saturation temperature.
- Saturation temperature rises with pressure. Steam tables use absolute pressure, so add atmospheric pressure to a gauge reading.
- Superheated steam is above saturation temperature at the same pressure; subcooled water is below saturation temperature.
- Conduction needs a temperature gradient and a medium; convection moves heat with a fluid; radiation travels by electromagnetic waves and does not need a medium.
- Fouling adds a resistance layer on tubes. It lowers heat transfer, raises stack temperature, and can cause overheating of tube metal.
Worked example
How much heat is needed to convert 1,000 lb of water at 60°F into saturated steam at atmospheric pressure? First, sensible heat: 1,000 × 1.0 Btu/lb°F × (212 − 60) = 152,000 Btu. Then latent heat: 1,000 × about 970 Btu/lb = 970,000 Btu. Total is about 1.122 MMBtu. At a higher pressure the saturation temperature is higher and the latent heat is lower, but the total enthalpy to saturated steam is not simply 970 Btu/lb multiplied by mass. Use steam-table values at the actual pressure.
Traps that cost marks
- Using gauge pressure instead of absolute pressure in steam tables.
- Assuming superheat increases the latent heat instead of adding sensible heat above saturation.
- Ignoring fouling or scale when explaining a rise in flue-gas temperature.
- Treating radiation as needing a medium.