CEM Certified Energy Manager: Steam, Industrial and Thermal Storage 2 — Questions and Answers
Question 1: A flash steam recovery system recovers steam from condensate at 150 psig. If condensate is flashed to 15 psig, approximately what percentage of the condensate mass becomes flash steam?
- About 5–7%
- About 12–15% (Correct answer)
- About 20–25%
- About 30–35%
Correct answer: About 12–15%
When high-pressure condensate flashes to a lower pressure, roughly 12–15% of the mass converts to flash steam due to the difference in saturation enthalpies.
Question 2: Which type of steam trap operates on the principle of density differences between steam and condensate?
- Thermostatic trap
- Inverted bucket trap (Correct answer)
- Disc trap
- Bimetallic trap
Correct answer: Inverted bucket trap
The inverted bucket trap uses the buoyancy of steam vapor inside the bucket to keep the valve closed and opens when condensate fills the bucket and sinks.
Question 3: A boiler produces 20,000 lb/hr of steam at 250°F. The feedwater enters at 212°F. If the enthalpy of steam is 1,164 BTU/lb and feedwater enthalpy is 180 BTU/lb, what is the boiler heat output in MMBTU/hr?
- 14.8 MMBTU/hr
- 19.7 MMBTU/hr
- 20.8 MMBTU/hr (Correct answer)
- 23.3 MMBTU/hr
Correct answer: 20.8 MMBTU/hr
Heat output = 20,000 lb/hr × (1,164 – 180) BTU/lb = 20,000 × 984 = 19,680,000 BTU/hr ≈ 19.7 MMBTU/hr; the closest correct answer based on common exam rounding is 19.7 MMBTU/hr.
Question 4: In a stratified chilled water thermal storage tank, the thermocline is best described as:
- The zone of maximum turbulence near the inlet diffusers
- The thin layer separating warm and cold water zones (Correct answer)
- The insulation layer on the tank exterior
- The recirculation zone at the bottom of the tank
Correct answer: The thin layer separating warm and cold water zones
The thermocline is the thin, naturally stable temperature gradient layer that separates the warm return water above from the cold supply water below in a stratified tank.
Question 5: Which industrial heat exchanger configuration provides the highest log mean temperature difference (LMTD) for a given set of inlet and outlet temperatures?
- Parallel flow
- Counter flow (Correct answer)
- Cross flow
- Mixed flow
Correct answer: Counter flow
Counter flow configuration maximizes LMTD because the hottest fluid meets the coldest fluid at the same end, maintaining a larger driving temperature difference throughout.
Question 6: Steam system insulation thickness is increased from 2 inches to 4 inches on a 4-inch pipe. The primary benefit realized is:
- Increased steam velocity
- Reduced condensate formation in distribution lines (Correct answer)
- Higher steam pressure at end-use points
- Improved steam trap discharge efficiency
Correct answer: Reduced condensate formation in distribution lines
Thicker insulation reduces heat loss to the surroundings, which directly decreases condensate formation within steam distribution piping.
Question 7: In a thermal energy storage (TES) system used for demand-side management, the 'discharge' period typically occurs:
- During off-peak electrical hours at night
- During on-peak hours when cooling loads are highest (Correct answer)
- During boiler startup in the morning
- During weekends when the facility is unoccupied
Correct answer: During on-peak hours when cooling loads are highest
TES systems discharge stored cooling energy during on-peak hours to reduce electrical demand charges and shift load away from expensive peak periods.
CEM Certified Energy Manager: Steam, Industrial and Thermal Storage 2
A flash steam recovery system recovers steam from condensate at 150 psig.
If condensate is flashed to 15 psig, approximately what percentage of the condensate mass becomes flash steam?