FREE Certified Energy Manager: Steam, Industrial and Thermal Storage Questions and Answers

Question 1

What does enthalpy equal at 400 degrees Fahrenheit for saturated steam?

Accepted Answer

1201 Btu/lb

Answer

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:

Accepted Answer

Improved

Answer

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.

Accepted Answer

False

Answer

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.

Accepted Answer

True

Answer

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?

Accepted Answer

7,500 MMBtu/year

Answer

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.

CEM - Certified Energy Manager Practice Test

CEM - Certified Energy Manager Practice Test

FREE Certified Energy Manager: Steam, Industrial and Thermal Storage Questions and Answers