A large appliance has been recovered down to fifteen inches of mercury vacuum and a brazed joint now has to be heated and taken apart. Why do technicians raise the system back to atmospheric pressure with dry nitrogen first?
Correct Answer
B) Because a system under vacuum draws air and contaminants in through the opening
A system sitting below atmospheric pressure pulls in whatever is around it the moment the joint opens, including humid air, torch fumes and shop dust. Bringing the system up to atmospheric with dry nitrogen removes that pressure difference, and a slow nitrogen flow through the tubing while the joint is heated also keeps the inside of the copper from oxidizing into the scale that later plugs driers and metering devices.
Why This Is the Correct Answer
Below atmospheric pressure, the system is a sink for everything outside it, so the instant the joint parts it inhales moist air and combustion products. Raising the pressure to atmospheric with dry nitrogen removes the pressure difference so nothing is drawn in, and keeping a small nitrogen flow through the tubing during brazing prevents the copper oxide scale that forms on hot bare copper.
Why the Other Options Are Wrong
Option A: Because refrigerant left under vacuum will ignite once the torch is applied
The reason for breaking the vacuum is contamination, not ignition. Refrigerant does not burn, though it does break down into irritating products on hot surfaces.
Option C: Because the evacuation level must be released before the appliance may be opened
There is no requirement to release a vacuum before opening an appliance. The level is a minimum to be reached, and the appliance may be opened once it is met.
Option D: Because nitrogen cools the joint enough to keep the copper from overheating
A nitrogen flow does very little cooling. Its purpose is to displace air from inside the tubing so the copper does not oxidize while it is hot.
Memory Technique
A vacuum is a mouth. Do not open it near a dusty roof and a lit torch.
Reference Hint
Break a vacuum to atmospheric with dry nitrogen before opening or brazing, and keep a light nitrogen flow while heating.
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?
A fleet garage technician holds only the vehicle air conditioning certification issued under Section 609 and is asked to recover the charge from the shop's own rooftop air conditioner. What does that credential allow?
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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A shop's office manager, who holds no technician certification, is sent to a supply house to pick up a new recovery machine for the commercial crew. What does the sales restriction require at that counter?
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