CEM Certified Energy Manager: Steam, Industrial and Thermal Storage 5

Question 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)?

Accepted Answer

625,000 BTU/hr

Answer

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

Question 2

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

Accepted Answer

Eutectic salt storage system

Answer

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.

Question 3

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

Accepted Answer

Live steam blowing through to condensate return lines, wasting energy

Answer

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

Question 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:

Accepted Answer

2.75

Answer

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

Question 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)

Accepted Answer

96,000 gallons

Answer

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.

CEM - Certified Energy Manager Practice Test

CEM - Certified Energy Manager Practice Test

CEM Certified Energy Manager: Steam, Industrial and Thermal Storage 5