Boiler Principles, Combustion, and Thermodynamics
What this domain tests
This domain tests whether you can reason in energy terms: how heat moves into water, how much of the fuel's heat you actually capture, and what the burner needs to release that heat cleanly. Expect calculation items on Btu input and output, evaporation rate, and efficiency, plus conceptual items on heat transfer and combustion.
Heat and steam fundamentals
- Sensible heat raises water temperature with no phase change; latent heat converts saturated water to saturated steam at constant temperature (about 970 Btu/lb at atmospheric pressure).
- Enthalpy (total heat) of saturated steam is sensible plus latent heat; read it from steam tables at the absolute pressure you are actually at.
- 1 boiler horsepower = 33,475 Btu/hr, roughly 34.5 lb of steam per hour evaporated at 212°F.
- Heat reaches water by radiation (furnace), convection (tube bank, economizer, air preheater), and conduction through the metal.
- Gauge pressure plus about 14.7 equals absolute pressure. Steam tables are written in absolute pressure.
Combustion essentials
- Theoretical air burns all the fuel exactly; excess air is supplied to drive the reaction to completion, typically 10–20% for gas and more for oil.
- The three T's are temperature, turbulence, and time. Lose any one and you get CO, smoke, or unburned fuel.
- Oxygen in the flue gas indicates excess air; CO indicates incomplete combustion. Stack temperature is usually the largest controllable loss.
- Fuel values to remember: No. 2 oil about 140,000 Btu/gal, natural gas about 1,000 Btu/ft³.
Worked example
A boiler burns 100 gal/hr of No. 2 oil and produces 10,000 lb/hr of steam at 150 psig from 200°F feedwater. Steam enthalpy at 150 psig saturated ≈ 1,195 Btu/lb; feedwater at 200°F ≈ 168 Btu/lb. Output = 10,000 × (1,195 − 168) = 10,270,000 Btu/hr. Input = 100 gal/hr × 140,000 Btu/gal = 14,000,000 Btu/hr. Efficiency = 10,270,000 ÷ 14,000,000 = 73.4%. Reasoning shown: subtract feedwater enthalpy, keep both sides on the same one-hour basis, and sanity-check against 75–85% for a well-tuned packaged boiler.
Traps that cost marks
- Reading steam tables with gauge pressure instead of absolute pressure.
- Believing more excess air improves efficiency; it raises stack loss and can increase NOx.
- Confusing boiler horsepower with motor horsepower on the burner nameplate.
- Looking for superheated steam temperatures in a saturation table.