Free Certified Energy Manager: Steam, Industrial and Thermal Storage Questions and Answers — Questions and Answers
Question 1: What does enthalpy equal at 400 degrees Fahrenheit for saturated steam?
- 767 Btu/lb
- 1201 Btu/lb (Correct answer)
- 825 Btu/lb
- 375 Btu/lb
Correct answer: 1201 Btu/lb
The enthalpy of saturated steam at a specific temperature is a thermodynamic property that must be determined by consulting a steam table. These tables provide precise values for various properties of water and steam, including enthalpy, at different temperatures and pressures. According to standard steam tables, the total enthalpy (h_g) for saturated steam at 400 degrees Fahrenheit is approximately 1201 Btu/lb, which accounts for both sensible and latent heat.
Question 2: In general, if cooler air enters a compressor, the compressor's efficiency is:
- Improved (Correct answer)
- Reduced
- Neither because the efficiency is not related to intake air
Correct answer: Improved
Cooler air is denser, meaning a greater mass of air can be compressed per unit of volume. When a compressor takes in denser air, it can deliver more compressed air for the same amount of volumetric displacement and energy input. This increases the compressor's mass flow rate and overall efficiency, as less energy is expended on compressing a smaller mass of less dense air.
Question 3: Heat recovery systems (also known as heat exchangers) are unable to lower temperatures below the hot gas's dew point.
- False (Correct answer)
- True
Correct answer: False
This statement is false. Heat recovery systems, or heat exchangers, are indeed capable of lowering the temperature of hot gases below their dew point. When the gas temperature drops below the dew point, water vapor within the gas will condense into liquid. This process is often intentionally utilized in condensing heat exchangers to recover additional latent heat from the condensing water vapor, further improving energy efficiency.
Question 4: Storing Thermal Energy Latent heat storage is the main method for employing ice as a storage medium.
- True (Correct answer)
- False
Correct answer: True
This statement is true. Latent heat storage is a highly effective method for storing thermal energy, and ice is a primary medium for this application. Ice stores a significant amount of energy (latent heat of fusion) when it melts from solid to liquid at a constant temperature of 0°C (32°F). This property makes it ideal for cooling applications, allowing for the storage of cooling capacity during off-peak hours for use during peak demand.
Question 5: Consider a process that consumes 10,000 MMBtu per year and is powered by a 50 percent efficient, old oil-fired boiler that burns leftover fuel oil #6. What energy savings might you expect if you upgrade to an 80% efficient natural gas-fired boiler?
- 12,500 MMBtu/year
- 20,000 MMBtu/year
- None of the Above
- 7,500 MMBtu/year (Correct answer)
Correct answer: 7,500 MMBtu/year
The process requires 10,000 MMBtu/year. With the old 50% efficient boiler, the fuel input is 10,000 MMBtu / 0.50 = 20,000 MMBtu/year. Upgrading to an 80% efficient boiler means the new fuel input will be 10,000 MMBtu / 0.80 = 12,500 MMBtu/year. Therefore, the energy savings are the difference between the old and new fuel inputs: 20,000 MMBtu/year - 12,500 MMBtu/year = 7,500 MMBtu/year.
Question 6: A manager of energy is comparing the running expenses of two boilers. One uses natural gas and has a 75% efficiency, while the other uses electricity and has a 97% efficiency. Electricity is $0.095/kWh and natural gas is $0.92/therm. Calculate the price per MMBtu for each system's water heating.
- Cannot be determined
- Electric: $28.7/MMBtu; Gas:$9.2/MMBtu
- Electric: $28.7/MMBtu; Gas:$12.27/MMBtu (Correct answer)
Correct answer: Electric: $28.7/MMBtu; Gas:$12.27/MMBtu
To calculate the cost per MMBtu, we convert MMBtu to the respective energy units and account for efficiency. For electricity, 1 MMBtu equals 293.071 kWh; thus, the cost is (293.071 kWh / 0.97 efficiency) * $0.095/kWh = $28.70/MMBtu. For natural gas, 1 MMBtu equals 10 therms; so, the cost is (10 therms / 0.75 efficiency) * $0.92/therm = $12.27/MMBtu.
Question 7: An ESCO finds chances for energy savings in the new building's design (which has not been built yet). Which of the following M&V Approaches would be the best fit to take into account the "avoided cost" (resulting from the energy savings) inside the new structure?
- IPMVP Option C
- IPMVP Option B
- None of the Above
- IPMVP Option D (Correct answer)
Correct answer: IPMVP Option D
IPMVP Option D, or Calibrated Simulation, is the best fit for new building designs or major renovations. This approach involves creating a detailed computer model of the building's energy use, calibrating it with actual data (if available from similar buildings), and then comparing the simulated energy performance of the proposed design against a baseline simulation of a standard or pre-retrofit design. This allows for the quantification of 'avoided costs' or savings from design choices before the building is constructed or fully operational.
Free Certified Energy Manager: Steam, Industrial and Thermal Storage Questions and Answers
What does enthalpy equal at 400 degrees Fahrenheit for saturated steam?