CEM Energy Management and Control Systems 2

Question 1

A variable frequency drive (VFD) on a pump or fan saves energy because fan/pump power varies with:

Accepted Answer

The cube of speed

Answer

By the Affinity Laws, fan and pump power varies with the cube of rotational speed, so a 20% speed reduction yields nearly 49% power reduction.

Question 2

Economizer control on an air-handling unit saves cooling energy by:

Accepted Answer

Using cool outdoor air for free cooling when outdoor conditions are favorable

Answer

When outdoor air is cool and dry enough, economizers open dampers to use outdoor air for cooling, reducing or eliminating mechanical refrigeration energy consumption.

Question 3

Supply air temperature reset saves energy in an air-handling unit by:

Accepted Answer

Raising supply air temperature during mild conditions, reducing chiller and reheat energy

Answer

Supply air temperature reset raises the cooling coil leaving temperature during mild loads, allowing the chiller to operate more efficiently and reducing simultaneous heating for reheat.

Question 4

A Direct Digital Control (DDC) system differs from pneumatic controls primarily because it:

Accepted Answer

Uses digital microprocessors and electronic sensors for precise, programmable control

Answer

DDC uses digital controllers with electronic sensors and software-based control logic, providing far greater precision, data logging, remote access, and programming flexibility than pneumatic systems.

Question 5

Chiller sequencing control in a plant with multiple chillers saves energy by:

Accepted Answer

Staging chillers on/off to operate fewer units at higher, more efficient part-load ratios

Answer

Chillers generally operate most efficiently at 70-90% load; sequencing ensures fewer chillers run at higher efficiency rather than many running at very low, inefficient part-load conditions.

CEM - Certified Energy Manager Practice Test

CEM - Certified Energy Manager Practice Test

CEM Energy Management and Control Systems 2