Evacuation & dehydration: EPA 608 Practice Questions

What is the main purpose of evacuating a system with a vacuum pump before charging?

Evacuation (dehydration) removes air and moisture. A vacuum pump is never used to remove refrigerant; refrigerant is recovered first.

Which instrument should be used to confirm that a system has reached a deep vacuum?

A compound gauge cannot read the small pressures of a deep vacuum. A micron gauge, connected as close to the system as possible, measures it accurately.

At sea level, a perfect vacuum equals about how many inches of mercury on a compound gauge?

Standard atmospheric pressure is 29.92 in Hg (760 mm Hg or 14.7 psia), so a perfect vacuum reads about 29.92 in Hg vacuum.

What vacuum level is a common evacuation target for residential and light commercial systems, unless the manufacturer specifies otherwise?

Many manufacturers call for 500 microns or lower, followed by a decay test. Always follow the equipment instructions.

After evacuation, the pump is valved off. The micron reading rises and then levels off well above the target. What is the most likely cause?

A rise that levels off points to moisture or refrigerant still evaporating. A rise that keeps climbing toward atmospheric pressure points to a leak.

What harm does moisture left in a refrigeration system cause?

Moisture reacts with refrigerant and oil to form acids and sludge, and it can freeze at the metering device. That is why systems are evacuated and filter-driers are installed.

Why can a deep vacuum remove water from a system at normal room temperature?

As pressure drops, water boils at lower temperatures, so in a deep vacuum it turns to vapor and the pump removes it.

Why is vacuum pump oil changed often?

The oil absorbs the moisture the pump removes. Contaminated oil cannot pull a deep vacuum, so change it as the manufacturer recommends, often after each job.

Why should a hermetic compressor never be used as a vacuum pump or started while the system is in a deep vacuum?

In a deep vacuum, the insulating effect of the gas around the windings is lost, so the motor can short out and fail. Use a real vacuum pump.

Which change will shorten evacuation time the most?

The vacuum path is usually the bottleneck. Wide, short hoses and removing Schrader cores cut restriction far more than a bigger pump alone.