CFC System Charging & Evacuation 2 — Questions and Answers
Question 1: What is the primary reason for using a two-stage evacuation process on large refrigeration systems?
- It is required by EPA regulations for all systems
- It removes moisture and non-condensables more effectively than a single-stage pull-down (Correct answer)
- It allows the technician to charge refrigerant between stages
- It prevents the vacuum pump oil from overheating
Correct answer: It removes moisture and non-condensables more effectively than a single-stage pull-down
Two-stage evacuation breaks the process into intervals, allowing trapped moisture to vaporize and be removed more thoroughly than a single continuous pull-down.
Question 2: When charging a refrigeration system with a blended refrigerant such as R-410A, the refrigerant must be:
- Charged as a vapor only to prevent compressor damage
- Charged as a liquid to maintain proper blend composition (Correct answer)
- Mixed with nitrogen before entering the system
- Pre-heated to at least 100°F before charging
Correct answer: Charged as a liquid to maintain proper blend composition
Blended refrigerants like R-410A must be charged as liquid because vapor-phase charging causes fractionation, altering the blend ratio.
Question 3: A vacuum gauge reads 29.5 in Hg during evacuation. What does this approximately equal in microns?
- 500 microns
- 1,270 microns (Correct answer)
- 5,000 microns
- 25,400 microns
Correct answer: 1,270 microns
29.5 in Hg corresponds to approximately 1,270 microns of mercury (µmHg) on the absolute pressure scale.
Question 4: Which condition indicates that a system is not holding a vacuum after the pump is isolated?
- The micron gauge reading drops below 500 microns
- The micron gauge reading rises above the target level and stabilizes
- The micron gauge reading rises steadily and does not level off (Correct answer)
- The suction pressure equals the discharge pressure
Correct answer: The micron gauge reading rises steadily and does not level off
A continuously rising micron gauge after pump isolation indicates a leak, because moisture or non-condensables alone would cause the reading to stabilize at a higher level.
Question 5: What is the recommended recovery cylinder fill limit by weight to prevent hydrostatic overpressure?
- 100% of the cylinder's rated capacity
- 90% of the cylinder's rated capacity
- 80% of the cylinder's rated capacity (Correct answer)
- 70% of the cylinder's rated capacity
Correct answer: 80% of the cylinder's rated capacity
Recovery cylinders should never be filled beyond 80% of their rated capacity by weight to allow for refrigerant expansion and prevent hydrostatic rupture.
Question 6: After completing a nitrogen pressure test on a new installation, the technician must do what before beginning evacuation?
- Add a small charge of refrigerant to purge the nitrogen
- Recover the nitrogen using a recovery machine
- Vent the nitrogen to atmosphere through the low-side port
- Install the service valve cores and connect the vacuum pump (Correct answer)
Correct answer: Install the service valve cores and connect the vacuum pump
Nitrogen is not a regulated substance and can be vented, but service valve cores must be installed before evacuation so the vacuum pump can draw the system to the required vacuum level.
Question 7: A technician is top-charging a system with R-22. Which method is acceptable under EPA Section 608 rules?
- Adding refrigerant through the high-side port with the compressor running
- Adding refrigerant vapor through the low-side port with the compressor running (Correct answer)
- Topping off the charge without checking for leaks first
- Adding refrigerant whenever the low-side pressure drops below 0 psig
Correct answer: Adding refrigerant vapor through the low-side port with the compressor running
Adding refrigerant vapor through the low-side service port with the compressor running is standard practice and does not introduce liquid to the compressor suction.
CFC System Charging & Evacuation 2
What is the primary reason for using a two-stage evacuation process on large refrigeration systems?