HVAC System Design and Operation Flashcards

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

Answer: Reducing fan speed with a VFD to maintain duct static pressure

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

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

Answer: 12.5%

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

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

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

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

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

Answer: When outdoor air temperature goes above return air temperature

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

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

Answer: Q = 1.08 × CFM × ΔT

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

In the latent heat equation Q = 0.68 × CFM × ΔW, what units does ΔW use?

Answer: Grains of moisture per pound of dry air

The 0.68 constant assumes the humidity ratio difference is given in grains per pound of dry air.

What is a common result of 'low delta-T syndrome' in a chilled water plant?

Answer: More pumping and extra chillers staged on at partly loaded conditions

A low coil ΔT means more flow is needed for the same load, which pushes up pump energy and forces extra chillers online.