CEM Certified Energy Manager: Steam, Industrial and Thermal Storage 3 — Questions and Answers
Question 1: A facility uses a steam-to-process heat exchanger with an effectiveness of 0.82. If the maximum possible heat transfer is 500,000 BTU/hr, what is the actual heat transferred?
- 340,000 BTU/hr
- 410,000 BTU/hr (Correct answer)
- 450,000 BTU/hr
- 500,000 BTU/hr
Correct answer: 410,000 BTU/hr
Actual heat transfer = effectiveness × maximum possible = 0.82 × 500,000 = 410,000 BTU/hr.
Question 2: Which factor most significantly degrades heat transfer in a steam-heated process vessel over time?
- Increasing steam pressure
- Air and non-condensable gas accumulation (Correct answer)
- Higher condensate subcooling
- Increased steam velocity
Correct answer: Air and non-condensable gas accumulation
Non-condensable gases (air, CO₂) form an insulating layer on heat transfer surfaces, drastically reducing the overall heat transfer coefficient.
Question 3: For a chilled water ice-on-coil thermal storage system, what is the typical storage capacity per ton of ice produced?
- 6 ton-hours per ton of ice
- 12 ton-hours per ton of ice
- 288 ton-hours per ton of ice
- 12,000 BTU per ton of ice (Correct answer)
Correct answer: 12,000 BTU per ton of ice
One ton of ice stores 12,000 BTU (1 ton-hour = 12,000 BTU/hr × 1 hr), which is the latent heat of fusion for one ton of ice.
Question 4: A boiler's stack gas temperature is reduced from 450°F to 350°F by installing an economizer. Assuming 1°F stack temperature reduction saves approximately 0.5% fuel, the estimated fuel savings is:
- 25%
- 50% (Correct answer)
- 10%
- 5%
Correct answer: 50%
A 100°F reduction in stack gas temperature × 0.5% per °F = 50% savings; however, practical economizer savings are typically 1–5%; the rule states approximately 1% per 40°F, so 100°F ÷ 40°F × 1% = 2.5% fuel savings, making 5% the closest realistic answer.
Question 5: In industrial process heating, 'recuperation' refers to:
- Recovering heat from exhaust gases to preheat combustion air (Correct answer)
- Storing excess heat in a thermal mass for later use
- Recycling condensate back to the boiler
- Recovering heat from cooling water in a heat pump
Correct answer: Recovering heat from exhaust gases to preheat combustion air
Recuperation is the process of using a heat exchanger to capture waste heat from exhaust gases and transfer it to incoming combustion air, improving furnace efficiency.
Question 6: What is the primary purpose of a deaerator in a steam system?
- To reduce steam pressure before distribution
- To remove dissolved oxygen and carbon dioxide from feedwater (Correct answer)
- To separate condensate from live steam
- To increase feedwater temperature above 212°F
Correct answer: To remove dissolved oxygen and carbon dioxide from feedwater
A deaerator removes dissolved non-condensable gases—primarily O₂ and CO₂—from boiler feedwater to prevent corrosion in the boiler and steam system.
Question 7: A 500-ton partial-storage chilled water TES system is sized to shift 30% of the peak cooling load. If the peak load is 1,000 tons, how many ton-hours must the tank store assuming a 6-hour discharge period?
- 300 ton-hours
- 500 ton-hours
- 1,800 ton-hours (Correct answer)
- 3,000 ton-hours
Correct answer: 1,800 ton-hours
Load to shift = 30% × 1,000 tons = 300 tons; storage required = 300 tons × 6 hours = 1,800 ton-hours.
CEM Certified Energy Manager: Steam, Industrial and Thermal Storage 3
A facility uses a steam-to-process heat exchanger with an effectiveness of 0.82.
If the maximum possible heat transfer is 500,000 BTU/hr, what is the actual heat transferred?