Evaporators and Condensers Flashcards

In a liquid overfeed (pumped recirculation) system, what does an overfeed ratio of 3:1 mean?

Answer: Three times as much liquid is pumped to the evaporator as is evaporated

Overfeed ratio is the mass of liquid supplied divided by the mass actually evaporated.

In a typical shell-and-tube water-cooled ammonia condenser, where does the refrigerant flow?

Answer: In the shell, around the tubes

Cooling water runs through the cleanable tubes while ammonia vapor condenses on the shell side.

Scale buildup inside the water tubes of a water-cooled condenser will most likely result in what?

Answer: Higher condensing pressure

Scale insulates the tubes, reducing heat transfer and forcing the condensing temperature and pressure up.

What is the main purpose of bleed-off (blowdown) on an evaporative condenser?

Answer: To limit the concentration of dissolved solids in the recirculating water

Evaporation concentrates minerals, so a portion of water is bled off to prevent scaling.

After a hot gas defrost, why are evaporator fans normally delayed before restarting?

Answer: To refreeze remaining water droplets on the coil before air movement

The fan delay lets water on the coil refreeze so it is not blown into the room or onto the product.

Where does compressor oil tend to collect in an ammonia evaporator or surge drum, and why?

Answer: At low points, because oil is heavier than and not miscible with liquid ammonia

Mineral oil is denser than liquid ammonia and does not mix with it, so it settles and must be drained from low points.

Increasing the temperature difference (TD) between room air and the evaporating refrigerant typically causes what?

Answer: More moisture removal from the room air

A colder coil relative to room air condenses more moisture, lowering room humidity.