CEM Boiler and Steam Systems 3 — Questions and Answers
Question 1: A flash steam recovery system on a high-pressure condensate line generates flash steam for use at lower pressure. What thermodynamic principle governs this?
- Isentropic expansion of liquid water reducing enthalpy
- Throttling of saturated liquid causes partial vaporization as pressure drops, releasing flash steam (Correct answer)
- Sensible heat in condensate converts entirely to latent heat
- Conduction of heat from hot condensate to cooler feedwater
Correct answer: Throttling of saturated liquid causes partial vaporization as pressure drops, releasing flash steam
When high-pressure condensate is throttled to lower pressure, some liquid flashes to steam because enthalpy is conserved and saturation temperature drops.
Question 2: Which blowdown strategy is most energy-efficient for controlling boiler water Total Dissolved Solids (TDS)?
- Periodic manual blowdown at maximum volume intervals
- Continuous blowdown with heat recovery using a blowdown heat exchanger (Correct answer)
- Increasing makeup water flow to dilute dissolved solids
- Intermittent surface blowdown without heat recovery
Correct answer: Continuous blowdown with heat recovery using a blowdown heat exchanger
Continuous blowdown with a heat exchanger recovers thermal energy from the blowdown stream, minimizing both water and heat losses.
Question 3: What does the term 'boiler horsepower' (BHP) represent?
- The mechanical shaft power output of a steam turbine
- The evaporation of 34.5 lb/hr of water at 212°F, equivalent to approximately 33,475 BTU/hr (Correct answer)
- The electrical generation capacity of a cogeneration plant
- The rated pressure in pounds per square inch at the boiler outlet
Correct answer: The evaporation of 34.5 lb/hr of water at 212°F, equivalent to approximately 33,475 BTU/hr
One boiler horsepower equals the evaporation of 34.5 lb/hr of feedwater at 212°F, a traditional unit equal to about 33,475 BTU/hr.
Question 4: In a steam system, what is the primary function of a pressure-reducing valve (PRV)?
- To increase steam pressure for high-temperature process loads
- To step down steam pressure to match lower-pressure distribution or end-use requirements (Correct answer)
- To automatically vent excess steam during pressure surges
- To measure and record steam flow for billing purposes
Correct answer: To step down steam pressure to match lower-pressure distribution or end-use requirements
A PRV reduces high-pressure steam to a lower set-point pressure suitable for specific process or building heating loads.
Question 5: Which factor most directly determines the 'M' factor (fuel-to-steam efficiency) used in boiler performance assessment?
- Rated nameplate pressure of the boiler
- Ratio of useful heat absorbed by steam to the heat input from fuel combustion (Correct answer)
- Age and material of the boiler tubes
- Steam distribution pipe insulation R-value
Correct answer: Ratio of useful heat absorbed by steam to the heat input from fuel combustion
Fuel-to-steam efficiency measures how much of the fuel's heat content is transferred into the steam, accounting for stack losses, radiation, and blowdown.
Question 6: Foaming in a boiler steam drum is most likely caused by:
- Insufficient combustion air leading to CO formation
- High concentrations of dissolved solids or surfactants in the boiler water (Correct answer)
- Excessive superheating of steam above design temperature
- Low feedwater temperature entering the economizer
Correct answer: High concentrations of dissolved solids or surfactants in the boiler water
High TDS or oil contamination causes surface tension reduction in boiler water, leading to foam and carryover of water droplets into the steam.
Question 7: What is the recommended approach when a facility wants to lower steam system operating pressure to save energy?
- Immediately reduce boiler setpoint pressure without evaluating end uses
- Audit all end-use equipment to verify they can operate at lower pressure before reducing system pressure (Correct answer)
- Replace all steam traps with higher-capacity models first
- Install additional makeup water treatment equipment
Correct answer: Audit all end-use equipment to verify they can operate at lower pressure before reducing system pressure
Lowering steam pressure reduces losses but requires confirming all process equipment, controls, and steam traps will function correctly at the new pressure.
CEM Boiler and Steam Systems 3
A flash steam recovery system on a high-pressure condensate line generates flash steam for use at lower pressure.
What thermodynamic principle governs this?