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Certified Energy Manager (CEM) Questions and Answers

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Viewing questions 31-40 out of questions

Questions # 31:

A cooling tower is delivering water to a chilled-water plant. The cooling tower outlet temperature has a 6°C approach temperature to the ambient air, wet-bulb temperature. The outside air conditions are 20°C dry bulb, 60% relative humidity. Using the psychrometric chart, at what temperature can the cooling tower deliver water to the chilled-water plant?

Options:

13°C

17°C

21°C

25°C

Answer

Questions # 32:

Why do electric utility companies transmit power at high voltages?

SELECT THE CORRECT ANSWER

Options:

Higher voltages are more stable

Higher voltages are safer

Transmission power losses (12R) are less

Electrical frequency is more stable

Answer

Explanation

Electric utility companies transmit power at high voltages primarily to reduce transmission power losses, which are proportional to the square of the current (I²) multiplied by the resistance (R) of the transmission lines. This relationship is expressed by the formula for power loss: P_loss = I²R.

Key Points:

Power Loss and Current Relationship:Power losses in transmission lines are directly proportional to the square of the current flowing through them. Reducing the current decreases these losses significantly.

High Voltage Transmission:By increasing the transmission voltage, the current required to deliver the same amount of power decreases. This is because power (P) is the product of voltage (V) and current (I): P = VI. For a given power level, increasing voltage allows for a corresponding decrease in current.

Reduced I²R Losses:Lower current results in reduced I²R losses, enhancing the efficiency of power transmission over long distances.

Conclusion:

Transmitting power at high voltages minimizes transmission losses, making it the most efficient method for long-distance electrical power delivery. Therefore, the correct answer is C. Transmission power losses (I²R) are less.

Questions # 33:

What term describes the difference between the control signal value and the set point?

Options:

Differential

Dead band

Error or offset

Cut off

Dead time

Answer

Questions # 34:

A motor operates with a normal and constant load. The electric current is measured to be 25% of the motor's nameplate rating. What does this mean?

Options:

The motor is operating at high efficiency

The motor is oversized and may be downsized to increase efficiency

The applied voltage is too low and should be increased to improve efficiency

The applied voltage is too high and should be reduced to improve efficiency

Answer

Questions # 35:

Using a degree-day base of 18°C, calculate the number of cooling degree days (CDD) if the outside temperature is a uniform 25°C throughout the year (365 days/year).

Options:

1,845 CDD/yr

1,955 CDD/yr

2,250 CDD/yr

2,555 CDD/yr

2,975 CDD/yr

Answer

Questions # 36:

A refrigerant-to-water heat exchanger can be added to air-conditioning equipment to heat water using the waste heat in the vapor compression refrigerant cycle. Where is this type of heat exchanger located in the refrigerant loop?

Options:

After the condenser coil, before the thermal expansion valve

After the expansion valve, before the evaporator coil

After the evaporator coil, before the compressor

After the compressor, before the condenser coil

Answer

Questions # 37:

Which electric rate tariff would best cost justify a thermal energy storage (TES) system?

Options:

High electricity rate and low peak-demand rate

Low electricity rate and low peak-demand rate

Great differential between off-peak and on-peak time-of-use electricity rates

Great differential between high season and low season electricity rates

Great differential between high season and low season demand rates

Answer

Questions # 38:

An energy-saving project saves $30,000 per year. The project life is 10 years and the company minimum annual rate of return (MARR) is 15%. How much can the project cost and still be cost effective?

Options:

$150,570

$165,903

$188,343

$198,992

$201,123

Answer

Questions # 39:

A 7 bar (gauge pressure) compressed air system has 5 leaks that are 3 mm in diameter and 3 leaks that are 6 mm in diameter. The compressed-air system is pressurized 8,760 hours per year. Electricity cost is $0.10/kWh. What is the annual cost of the leaks in the compressed air system?