ASHRAE HVAC System Design and Operation 2

Question 1

In a VAV system, what primarily reduces supply fan energy as zone loads drop?

Accepted Answer

Reducing fan speed with a VFD to maintain duct static pressure

Answer

As VAV boxes close, a VFD slows the fan to hold the static pressure setpoint, cutting fan power sharply.

Question 2

According to the fan affinity laws, if fan speed drops to 50%, fan power drops to roughly what fraction?

Accepted Answer

12.5%

Answer

Fan power varies with the cube of speed, so 0.5³ = 0.125, or about 12.5%.

Question 3

What is the main goal of a trim-and-respond duct static pressure reset strategy?

Accepted Answer

Lower the static pressure setpoint until the most-open VAV damper is nearly fully open

Answer

Resetting static pressure down to just meet the most demanding zone saves fan energy while still satisfying all zones.

Question 4

With a differential dry-bulb high-limit, when does an airside economizer stop using extra outdoor air?

Accepted Answer

When outdoor air temperature goes above return air temperature

Answer

Once outdoor air is warmer than return air, bringing in more of it would add to the cooling load.

Question 5

Which equation gives sensible heat in Btu/h for standard air?

Accepted Answer

Q = 1.08 × CFM × ΔT

Answer

For standard air, 1.08 combines density, specific heat, and 60 min/hr.

ASHRAE - American Society of Heating, Refrigeration and Air Conditioning Engineers Certification Practice Test