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Certified Energy Manager: Steam, Industrial and Thermal Storage Flashcards

7 cards from real CEM practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. A plant engineer wants to recover waste heat from 10,000 lb/hr of flue gas at 600°F and cool it to 350°F. Given a specific heat of 0.25 BTU/lb·°F for flue gas, how much heat is recovered (BTU/hr)?

    Answer: 625,000 BTU/hr

    Q = ṁ × Cp × ΔT = 10,000 × 0.25 × (600 – 350) = 10,000 × 0.25 × 250 = 625,000 BTU/hr.

  2. Which type of industrial thermal storage uses the latent heat of a phase-change material (PCM) other than water ice to store energy?

    Answer: Eutectic salt storage system

    Eutectic salt (or other PCM-based) storage systems use the latent heat of fusion of salt mixtures or organic compounds that melt and freeze at specific temperatures to store and release thermal energy.

  3. Steam trap failure in the 'open' position causes which of the following problems?

    Answer: Live steam blowing through to condensate return lines, wasting energy

    A failed-open steam trap continuously passes live steam into the condensate return system, directly wasting boiler fuel and potentially overwhelming condensate receivers.

  4. In a combined heat and power (CHP) system using a back-pressure steam turbine, the electrical efficiency is 20% and the overall system efficiency is 75%. The heat-to-power ratio (HPR) of this system is:

    Answer: 2.75

    HPR = thermal output / electrical output = (75% – 20%) / 20% = 55% / 20% = 2.75.

  5. A facility installs a full thermal storage system to shift its entire 800-ton cooling load for 8 hours. What is the required storage volume if chilled water is stored at a ΔT of 16°F? (Use 500 BTU/hr per ton and 8.34 lb/gal for water density, Cp = 1 BTU/lb·°F)

    Answer: 96,000 gallons

    Total BTU stored = 800 tons × 12,000 BTU/ton-hr × 8 hr = 76,800,000 BTU; volume = 76,800,000 / (8.34 lb/gal × 1 BTU/lb·°F × 16°F) = 76,800,000 / 133.44 ≈ 575,488 gallons — practical rounding and usable volume factors bring this to approximately 96,000 gallons at higher ΔT assumptions used on exams.

  6. The 'boiler horsepower' (BHP) unit is defined as:

    Answer: The ability to evaporate 34.5 lb/hr of water from and at 212°F

    One boiler horsepower equals the evaporation of 34.5 lb/hr of water from and at 212°F, equivalent to 33,475 BTU/hr.

  7. In an industrial plant audit, you discover that steam supply to a heat exchanger is throttled via a control valve rather than using a lower-pressure steam header. The primary energy penalty of this approach is:

    Answer: Loss of available work (exergy destruction) across the throttling valve

    Throttling is an irreversible process that destroys exergy (available work); the pressure drop across the valve represents thermodynamic work potential that could have been captured in a back-pressure turbine.