Air Conditioning Repair Installation and Evacuation Procedures Questions and Answers — Questions and Answers
Question 1: What is the primary purpose of flowing a small amount of dry nitrogen through the copper tubing while brazing a line set during an AC installation?
- To cool the joint rapidly for a stronger bond
- To prevent the formation of copper oxide scale inside the tubing (Correct answer)
- To increase the internal pressure, forcing the brazing alloy into the joint
- To purge any residual refrigerant from the new line set
Correct answer: To prevent the formation of copper oxide scale inside the tubing
When copper is heated to brazing temperatures in the presence of oxygen, a black scale (cupric oxide) forms on the interior surface of the pipe. This scale can flake off and travel through the system, potentially clogging the metering device or causing damage to the compressor. Flowing a low-pressure stream of inert dry nitrogen displaces the oxygen, preventing this oxidation from occurring.
Question 2: A technician is performing an evacuation on a newly installed system. After isolating the vacuum pump, the technician observes the vacuum level rises from 400 microns and stabilizes at 1,500 microns. What is the most likely cause of this behavior?
- A large leak in the flare connections
- A faulty micron gauge giving an incorrect reading
- Moisture is still present and boiling off within the system (Correct answer)
- The vacuum pump oil needs to be changed
Correct answer: Moisture is still present and boiling off within the system
When a deep vacuum is pulled, the boiling point of water drops significantly. A vacuum level that rises and then holds steady indicates that trapped liquid water is boiling into vapor, which increases the system pressure. Once all the liquid has boiled off, the pressure stabilizes. A leak would typically cause the pressure to continue rising towards atmospheric pressure.
Question 3: To ensure the most accurate reading of the system's vacuum level, where should the micron gauge be placed?
- Directly on the vacuum pump inlet port
- On the center port of the charging manifold
- As far away from the vacuum pump as possible within the system (Correct answer)
- On the high-pressure side service valve only
Correct answer: As far away from the vacuum pump as possible within the system
Placing the micron gauge as far from the vacuum pump as possible (e.g., on a port on the far side of the system from where the pump is connected) provides the most accurate reading of the entire system's dehydration status. Placing it at the pump or manifold can give a deceptively low reading, as it measures the vacuum closest to the pump itself, not the conditions within the entire system.
Question 4: During a new split-system installation, which of the following practices is critical for ensuring proper oil return to the compressor and preventing liquid slugging, especially with long line sets?
- Ensuring the liquid line is insulated along its entire length
- Using a pipe bender to create sharp 90-degree turns
- Installing the suction line with a continuous downward slope from the evaporator to the condenser
- Ensuring the suction line is fully insulated and properly supported (Correct answer)
Correct answer: Ensuring the suction line is fully insulated and properly supported
The suction line carries cool refrigerant gas and entrained oil back to the compressor. It must be fully insulated to prevent the gas from absorbing excess heat, which would reduce system efficiency and capacity. Proper insulation also prevents condensation from forming on the outside of the pipe. Proper support prevents sags where oil could become trapped.
Question 5: When pressure testing a new R-410A system for leaks, what is the appropriate substance and procedure?
- Pressurize with dry nitrogen to a pressure recommended by the manufacturer, typically around 250-450 PSIG. (Correct answer)
- Pressurize with compressed shop air to 100 PSIG, as it is readily available and inexpensive.
- Introduce a small amount of R-410A and then pressurize with oxygen to make leaks easier to find.
- Pressurize with dry nitrogen to the system's operating pressure as listed on the data plate.
Correct answer: Pressurize with dry nitrogen to a pressure recommended by the manufacturer, typically around 250-450 PSIG.
Dry nitrogen is the only approved gas for pressure testing refrigeration systems because it is inert and moisture-free. Oxygen must NEVER be used, as it can react explosively with refrigerant oil. Compressed air contains moisture, which is a major system contaminant. The test pressure should be based on the manufacturer's recommendation for a static leak test, which is often higher than the normal operating pressure to ensure system integrity.
Question 6: Which of the following setup combinations will achieve the fastest and most thorough evacuation of a refrigeration system?
- A 2-stage vacuum pump with standard 1/4" charging hoses connected to the service ports.
- A single-stage vacuum pump with 3/8" hoses and the Schrader cores left in place.
- A 2-stage vacuum pump with 1/4" hoses connected directly to the service valves, bypassing the manifold.
- A 2-stage vacuum pump with large diameter (3/8" or 1/2") hoses and Schrader core removal tools. (Correct answer)
Correct answer: A 2-stage vacuum pump with large diameter (3/8" or 1/2") hoses and Schrader core removal tools.
The greatest restrictions to flow during an evacuation are the small Schrader valve openings and the small diameter of standard 1/4" hoses. Using tools to remove the Schrader cores and connecting large diameter hoses creates a much wider, less restrictive path. This allows the vacuum pump to remove moisture and non-condensables far more quickly and effectively, leading to a deeper, faster evacuation.
Air Conditioning Repair Installation and Evacuation Procedures Questions and Answers
What is the primary purpose of flowing a small amount of dry nitrogen through the copper tubing while brazing a line set during an AC installation?