CRST Piping, Tubing & Brazing Techniques 2 — Questions and Answers
Question 1: In refrigeration copper tubing work, what does the term 'swaging' refer to?
- Cutting tubing at an angle using a hacksaw
- Expanding the end of a tube so another tube of the same diameter can be inserted for a brazed connection (Correct answer)
- Bending tubing to a precise angle without a bender
- Pressure-testing a completed joint with refrigerant
Correct answer: Expanding the end of a tube so another tube of the same diameter can be inserted for a brazed connection
Swaging mechanically expands the end of a copper tube to accept another tube of the same nominal size, allowing a direct brazed joint without a separate coupling fitting.
Question 2: In what direction should horizontal suction lines be sloped to promote oil return?
- Level with zero slope to minimize pressure drop
- 1/4 inch per foot sloping toward the compressor (Correct answer)
- 1/4 inch per foot sloping toward the evaporator
- 1 inch per foot sloping toward the condenser
Correct answer: 1/4 inch per foot sloping toward the compressor
Horizontal suction lines should slope 1/4 inch per foot toward the compressor so that gravity assists oil in draining back toward the suction inlet.
Question 3: Which gas is the recommended medium for pressure-testing refrigerant piping after brazing is complete?
- Oxygen from a welding cylinder
- The system refrigerant charge
- Dry nitrogen (Correct answer)
- Compressed air from a shop compressor
Correct answer: Dry nitrogen
Dry nitrogen is inert, oil-free, moisture-free, and non-reactive, making it the safe and industry-accepted gas for pressure-testing refrigerant piping without contaminating the system.
Question 4: Why must oxygen NEVER be used to pressure-test or purge a refrigeration system?
- It causes the copper tubing to turn green from oxidation
- Oxygen combined with compressor oil under pressure creates an explosive mixture capable of detonation (Correct answer)
- It is too costly compared to alternative gases
- It reacts with the refrigerant to form a non-condensable gas
Correct answer: Oxygen combined with compressor oil under pressure creates an explosive mixture capable of detonation
Oxygen in contact with petroleum-based compressor oil under pressure can form a highly explosive mixture—similar to a diesel engine—posing a severe fire and detonation hazard.
Question 5: What is the recommended minimum bend radius for ACR copper tubing to prevent kinking, flattening, or cracking during bending?
- Equal to the tube outside diameter
- 1.5 times the tube outside diameter
- 3 times the tube outside diameter (Correct answer)
- 8 times the tube outside diameter
Correct answer: 3 times the tube outside diameter
Bending copper tubing to a radius of at least 3 times the outside diameter distributes stress gradually enough to prevent the tube wall from kinking, collapsing, or cracking.
Question 6: In a horizontal suction line where pipe size must be reduced, which type of reducer fitting is preferred and why?
- Concentric reducer, because it centers both pipes on the same axis
- Eccentric reducer with the flat side down, so oil can drain continuously along the pipe bottom (Correct answer)
- Concentric reducer, because it creates less turbulence
- Eccentric reducer with the flat side up, to keep vapor at the bottom
Correct answer: Eccentric reducer with the flat side down, so oil can drain continuously along the pipe bottom
An eccentric reducer installed flat-side-down keeps the bottom of the pipe at a consistent elevation, preventing a low-point pocket where oil could accumulate and block flow.
Question 7: What is the primary purpose of vibration eliminators (flexible connectors) installed in refrigeration suction and discharge lines near the compressor?
- To act as check valves preventing refrigerant backflow
- To absorb compressor vibration and prevent metal fatigue or cracking from transmitting through the piping (Correct answer)
- To increase refrigerant flow velocity in the lines
- To allow for seasonal thermal expansion of the piping
Correct answer: To absorb compressor vibration and prevent metal fatigue or cracking from transmitting through the piping
Vibration eliminators isolate compressor mechanical vibration from the connected piping, preventing cyclic stress that causes metal fatigue, cracking at brazed joints, and noise transmission.
CRST Piping, Tubing & Brazing Techniques 2
In refrigeration copper tubing work, what does the term 'swaging' refer to?