Evaporators and Condensers Flashcards

What is the main purpose of maintaining superheat at the outlet of a direct-expansion (DX) evaporator?

Answer: To ensure only vapor returns to the compressor

Superheat confirms all liquid has boiled off, protecting the compressor from liquid slugging.

Why does a flooded evaporator generally transfer heat better than a DX evaporator of the same size?

Answer: Nearly all of its internal surface is wetted with liquid refrigerant

Liquid-wetted surfaces have far higher heat transfer coefficients than surfaces contacting superheated vapor.

Heavy frost accumulation on an air-cooling evaporator coil typically causes which condition?

Answer: Reduced airflow and a falling suction pressure

Frost insulates the coil and blocks airflow, so less heat is absorbed and suction pressure drops.

In a hot gas defrost system, what heat source melts the frost from the evaporator coil?

Answer: Compressor discharge vapor routed into the coil

Hot gas defrost sends high-pressure discharge vapor into the coil, where it condenses and releases heat.

An evaporative condenser rejects most of its heat by which mechanism?

Answer: Evaporation of spray water over the coil

Evaporating water absorbs large amounts of latent heat from the refrigerant inside the coil.

Non-condensable gases trapped in a condenser most directly cause which problem?

Answer: Abnormally high head pressure

Non-condensables occupy condenser surface and add their partial pressure, raising head pressure.

What is the main operating penalty of running a refrigeration system at a higher-than-necessary condensing pressure?

Answer: Increased compressor power per ton of refrigeration

A higher compression ratio requires more horsepower per ton and lowers efficiency.