Properly Diagnosing the Need for a System Evacuation
The NEWS’ Trainer Panel looks at the situations surrounding an HVAC system evacuation
Knowing how, when, where, and why to perform an HVAC system evacuation is an essential component of any technician’s repertoire. No two situations are exactly alike, and properly addressing the needs of each individual system is critical.
The NEWS’ Trainers Panel has decades of experience with these circumstances, and a few members recently took the time to address the best methods, tips, and tricks for handling a system evacuation or finding ways to avoid it entirely.
WHAT IS A SYSTEM EVACUATION?
According to Dennis Silvestri, lead instructor, MRS Educational Training, New Haven, Connecticut, the function of a system evacuation is to remove air and moisture that has entered into the system. This occurs when installing the suction and liquid lines where the ends of the tubing are “open” or when a system is opened up for repair or when a system loses its refrigerant.
Martin Easley, lead HVAC instructor, Valley College, Martinsburg, West Virginia, said the actual purpose of a system evacuation is to ensure the system is completely dehydrated of moisture and that non-condensables are completely removed in order to optimize life cycle and system performance. The only two acceptable materials in a refrigeration system are oil and refrigerant.
Where there is some discretion amongst the panel is deciding how long a system evacuation should actually take.
Nicholas Griewahn, associate professor of HVACR, Northern Michigan University, Marquette, Michigan, believes the length of time required for evacuation is really irrelevant.
“The evacuation should continue for as long as necessary to meet the goal,” he said. “Most of the time, the goal is around 500 microns of mercury (Hg). Other technicians will measure vacuum based on inches of Hg, but that isn’t as accurate as using microns. If there are no leaks or large amounts of contamination present in the system, the length of time it will take to reach an acceptable vacuum level will depend on the size of the system and the length of any line sets as well as the size of the vacuum pump used.”
There are several factors that need to be considered, and Easley said considering those factors is much more important than time requirements.
“System size, coil and piping integrity, and the cfm rating of the vacuum pump are the most relevant factors in determining the length of time to complete an evacuation,” he said. “Care must be taken by pressurizing the system to ensure there are no leaks in the system before a system evacuation can even begin.”
WHEN IS IT DONE?
While the actual amount of time spent on a system evacuation may not be critically important, knowing when the job is complete certainly is.
David Richardson, curriculum developer and trainer, National Comfort Institute, agreed with Griewahn’s assertion that a micron gauge is vital because, otherwise, you’re guessing.
“Those who are successfully pulling the required levels of vacuum on a system are using some inventive techniques to speed up the process,” he said. “Some contractors use special rigs to speed up their evacuation times. These contractors remove the valve cores from the service valves with a core remover and use large diameter hoses through custom fittings to properly evacuate the system. This method also allows a technician to isolate the refrigeration side of the system from the evacuation hoses to watch for a rise in vacuum levels to assure the evacuation is complete.”
The magic question for Silvestri has always been how long to evacuate.
“You will find many different opinions offered by service people on this,” he said. “The correct and true answer lies with the micron gauge. Our industry tells us to evacuate properly to 500 microns. You reach this level and then the system is dry and tight inside. Now, how long does it take to reach this value? That depends on factors, like what are the lengths and diameters of the suction and liquid lines, how large are the condenser and evaporator coils, how hot and humid is the outside air, and has any actual liquid water entered the system?”
Easley said the best practice for achieving a proper vacuum is to use the triple evacuation method. “[This method] is purging with an inert gas, such as dry nitrogen, after the initial vacuum of 1,000-2,000 microns and then evacuating the system again, repeating the purging process, and finally, on the third evacuation, pulling the system down to 250 microns and holding for 30 minutes,” he said.
Griewahn offered what he called a “reality check” for technicians.
“Some systems cannot always be evacuated to the level you would like,” he said. “Technicians who work on supermarket systems know this well. Most large systems that have extensive refrigeration lines will always have some tiny leaks present, no matter how good of a job is done with leak detection and repair. This is especially true with parallel rack systems that are older. Disassembling a branch with 500 feet or more of piping, components, and joints to find every last microleak can be impractical. Because of a tiny leak, reaching your evacuation goal after the system has been opened for repairs may be impossible. Reach the lowest vacuum achievable and use purging and nitrogen to eliminate as much contamination as possible before opening the valves or recharging the system. Don’t take this as an excuse to be sloppy though. Many branches of a multiplex system are perfectly leak-free, and any small microleak you can locate and repair is a testament to your workmanship and determination. If you’ve got leaks, always make a strong attempt to find and repair them.”
AVOID THE SHORTCUTS
Panel members said that trying to take shortcuts during a system evacuation can be costly.
“There are numerous hidden reactions that can take place inside the refrigeration side of an HVAC system that simply can’t be quantified,” said Richardson. “The presence of moisture and air combined with heat from a compressor can lead to various forms of internal damage. It may take years for this harm to occur to a point that it results in system failure. Corrosion, sludge, breakdown of the compressor oil, and eventual compressor failure can be caused by these two contaminants. When care is taken to properly remove moisture and air, the life of a compressor can be extended dramatically. When this care is not taken, the moisture and air can quickly attack the refrigerant and oil, causing various harmful reactions inside the refrigeration side of the HVAC system.”
Easley said attempting shortcuts on evacuation can result in a system that is not as efficient as it should be and not operating up to full capacity due to non-condensables.
“Premature system failure is another consequence that can be a result of moisture in the system causing the compressor to experience an electrical burn-out,” he said.
THE NEWS’ TRAINER PANEL
- Joseph Adeszko, program coordinator, HAC department, Moraine Valley Community College, Palos Hills, Illinois;
- Dennis Silvestri, lead instructor, MRS Educational Training, New Haven, Connecticut;
- Nicholas Griewahn, associate professor of HVACR, Northern Michigan University, Marquette, Michigan;
- Martin Easley, lead HVAC instructor, Valley College, Martinsburg, West Virginia;
- Bryan Lee, HVAC instructor, and program manager, Fresno City College in Fresno, California;
- Joe Marchese, author, instructor, and former HVACR contractor; and
- David Richardson, curriculum developer and trainer, National Comfort Institute (NCI).
Editor’s note: The NEWS’ Trainers Panel consists of some of the country’s best HVAC educators, instructors, and trainers. Their insights are used to answer technical questions from the field and suggest solutions to everyday HVACR problems. They detail proper maintenance techniques, solve troubleshooting issues, and find solutions to difficulties that are seen from coast to coast. If you’re interested in becoming a member of the panel, or have a question you’d like answered, contact The NEWS’ products and education editor, Nick Kostora, at firstname.lastname@example.org.
Publication date: 8/21/2017