A technician has a vacuum pump on a system that took on moisture, and the reading falls quickly at first and then stalls at a level well above the target. Why does the last of the water take so long to come out?
Correct Answer
D) Liquid water only boils into vapor at very low pressures, and boiling chills the remaining water, which slows the process further
A vacuum pump can only remove vapor. Liquid water in a system has to boil before it can leave, and at ordinary temperatures that requires pulling the pressure down very low. As the water boils it absorbs heat from its surroundings and chills itself, which lowers its own boiling pressure and slows the process, and in a cold system the remaining moisture can even freeze. That is why a wet system holds at a stubborn plateau and why adding heat to the system, or sweeping with dry nitrogen, is the practical fix.
Why This Is the Correct Answer
Evacuation removes vapor, so every drop of liquid water must first change state. Boiling requires heat, and the only heat available comes from the tubing and the water itself, so the process is self-limiting: as it boils it cools, the boiling pressure drops, and the pull-down plateaus until heat flows back in.
Why the Other Options Are Wrong
Option A: Water vapor is heavier than air and settles into the bottom of the evaporator and the receiver, where the pump's suction cannot reach it
Water vapor is actually lighter than air, and under a deep vacuum there is no gravity separation worth speaking of. The pump reaches the whole volume; the problem is getting the water to become vapor.
Option B: The vacuum pump gradually loses capacity as the hoses and the pump body cool down over the course of a long pull
Hose and pump temperature have no meaningful effect on capacity. What sets the plateau is the state change happening inside the system, not the condition of the pump.
Option C: Liquid water has to be drawn out through the pump as a liquid, which takes far longer than moving vapor
A vacuum pump moves vapor only, and liquid water reaching it would ruin the oil. The water has to boil off inside the system first, which is exactly why the reading stalls.
Memory Technique
The pump eats vapor, not water. Water has to boil first, and boiling makes it cold enough to stall.
Reference Hint
40 CFR 82.156 sets required evacuation levels for appliances before service; moisture removal is the practical reason those levels are hard to reach on a wet system.
More epa-608-type-2 Questions
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An office building has thirty water source heat pumps on a common water loop, and each one left the factory assembled, hermetically sealed and charged with four pounds of R-410A. How is one of those units classified?
A data sheet for an unfamiliar refrigerant lists a boiling point of about fifty-five degrees Fahrenheit at atmospheric pressure. What does that tell a technician holding Core and Type II certification?
A technician holds Core, Type I and Type III certification and is sent to recover the charge from a twenty ton rooftop unit charged with R-410A. What does that combination of sections allow on this job?
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A split system's outdoor unit, indoor coil and the line set the installer ran and brazed between them are being considered. For the purposes of the refrigerant rules, what makes up the appliance here?
On a new installation the line set is brazed to a factory charged outdoor unit and an indoor coil shipped under a nitrogen holding charge, with the outdoor unit's service valves still closed. What does the recovery requirement call for before that brazing?
A rooftop unit's data plate has weathered until the charge figure cannot be read, and the technician needs the full charge to place the appliance under the evacuation and leak provisions. Where should that figure come from?
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