CEM Boiler and Steam Systems 2 — Questions and Answers
Question 1: What does boiler 'turndown ratio' describe?
- The ratio of maximum to minimum firing rate while maintaining stable operation (Correct answer)
- The rate at which steam pressure drops during shutdown
- The percentage of heat lost through the stack at low load
- The angle of the burner nozzle relative to the combustion chamber
Correct answer: The ratio of maximum to minimum firing rate while maintaining stable operation
Turndown ratio is the range from maximum to minimum firing rate at which a burner can operate stably, affecting part-load efficiency.
Question 2: Which condition best describes 'wet steam' in a steam distribution system?
- Steam at 100% quality (dryness fraction)
- Steam containing entrained liquid water droplets below 100% quality (Correct answer)
- Superheated steam that has lost sensible heat
- Steam condensed completely to liquid in the pipe
Correct answer: Steam containing entrained liquid water droplets below 100% quality
Wet steam has a dryness fraction below 1.0, meaning it carries entrained liquid droplets that reduce heat transfer efficiency.
Question 3: A steam trap that fails open causes which primary energy problem?
- Back pressure that stalls condensate return
- Live steam blow-through resulting in direct energy loss (Correct answer)
- Waterlogging of heat exchangers
- Increased boiler feed water temperature
Correct answer: Live steam blow-through resulting in direct energy loss
A failed-open steam trap allows live steam to pass through continuously, wasting the latent heat contained in that steam.
Question 4: What is the primary purpose of a deaerator in a boiler feedwater system?
- To increase feedwater pressure before entering the boiler
- To remove dissolved oxygen and carbon dioxide from feedwater (Correct answer)
- To filter suspended solids from makeup water
- To preheat steam before it enters the turbine
Correct answer: To remove dissolved oxygen and carbon dioxide from feedwater
Deaerators use steam stripping and mechanical action to remove corrosive dissolved gases, particularly O₂ and CO₂, protecting boiler internals.
Question 5: Which heat recovery device captures energy from boiler flue gas to preheat combustion air?
- Economizer
- Air preheater (recuperator) (Correct answer)
- Condensate return tank
- Flash steam vessel
Correct answer: Air preheater (recuperator)
An air preheater (recuperator) transfers heat from hot flue gases to incoming combustion air, improving combustion efficiency.
Question 6: Increasing excess air beyond the optimal level in a boiler burner primarily causes:
- Higher flame temperature and improved combustion
- Increased stack losses due to sensible heat carried away by excess air (Correct answer)
- Reduced NOx emissions without efficiency penalty
- Elimination of CO formation in the flue gas
Correct answer: Increased stack losses due to sensible heat carried away by excess air
Excess air above the optimal level carries additional sensible heat out the stack, reducing thermal efficiency even though combustion is complete.
Question 7: What is the significance of the 'critical pressure' of steam for energy managers?
- The minimum pressure at which a safety valve must open
- The pressure above which liquid and vapor phases become indistinguishable (374°C, 22.1 MPa for water) (Correct answer)
- The pressure differential required to move steam through distribution mains
- The design pressure stamped on the ASME boiler nameplate
Correct answer: The pressure above which liquid and vapor phases become indistinguishable (374°C, 22.1 MPa for water)
At the critical point (22.1 MPa, 374°C for water), the distinction between liquid and vapor disappears, which is relevant for supercritical power plant design.
CEM Boiler and Steam Systems 2
What does boiler 'turndown ratio' describe?