HVAC System Design and Operation Flashcards

A chiller runs at a COP of 6.0. What is its efficiency in kW/ton, approximately?

Answer: 0.59 kW/ton

kW/ton = 3.517 / COP = 3.517 / 6.0 ≈ 0.59.

What does a thermostatic expansion valve (TXV) control directly?

Answer: Evaporator outlet superheat

A TXV meters refrigerant to hold a set superheat at the evaporator outlet.

Where is liquid subcooling measured in a vapor-compression system?

Answer: At the condenser outlet or liquid line

Subcooling is the saturated condensing temperature minus the actual liquid line temperature.

How does an air-source heat pump typically defrost its outdoor coil?

Answer: It reverses the cycle with the reversing valve so the outdoor coil works as a condenser

Switching briefly to cooling mode sends hot gas to the outdoor coil and melts the frost.

When is a waterside economizer most useful?

Answer: When outdoor wet-bulb is low enough for tower water to cool the chilled water loop through a heat exchanger

Cold tower water passes through a plate-and-frame heat exchanger to give free cooling with the chillers off or unloaded.

How does an enthalpy (total energy) recovery wheel differ from a sensible-only wheel?

Answer: It transfers both sensible heat and moisture between airstreams

A desiccant coating lets the wheel move moisture as well as heat.

What is a key design benefit of a dedicated outdoor air system (DOAS)?

Answer: It separates ventilation and latent load handling from the zone sensible cooling equipment

DOAS conditions all ventilation air on its own, so terminal units can deal mainly with sensible loads.