Type II evacuation & charging: EPA 608 Practice Questions

How should R-454B be charged into a system?

R-454B is a zeotropic blend, so remove it from the cylinder as liquid to keep the composition right, and throttle it so the compressor does not receive liquid.

Why is liquid refrigerant added to the low side slowly, through a metering device or throttled valve?

Liquid entering a compressor can damage valves and bearings (slugging). Metering the liquid lets it vaporize before reaching the compressor.

What is the most accurate way to charge an evacuated split system from scratch?

Weigh in the nameplate charge, adjusted for line-set length per the instructions, then confirm with superheat or subcooling.

Which reading is normally used to check the charge on a system with a thermostatic expansion valve (TXV)?

A TXV holds superheat steady, so subcooling shows the charge. Fixed-orifice systems are charged by superheat.

A system shows high head pressure, high subcooling and low superheat. What is the most likely problem?

Too much refrigerant backs liquid up in the condenser (high subcooling, high head) and floods the evaporator (low superheat).

When a system with a water-cooled heat exchanger is charged from a deep vacuum, why is vapor added first?

Liquid entering a deep vacuum boils at a very low temperature. Raise the pressure with vapor first (above the pressure for 32°F) before adding liquid.

After brazing on a system, the technician evacuates it and the micron gauge holds at 400 microns for 10 minutes. What does this suggest?

A low reading that holds steady in a decay test shows the system has no leak and little remaining moisture, so it is ready to charge.

Why should a low flow of nitrogen pass through the tubing while brazing?

Hot copper exposed to oxygen forms scale that can clog metering devices and filters. Flowing nitrogen keeps oxygen out of the tube.

After a system has been opened for a repair, which component should usually be replaced before evacuation?

A new filter-drier captures moisture and debris that entered while the system was open.

A sight glass moisture indicator has changed color to show "wet". What should the technician do?

Moisture in the system causes acid and freeze-ups. Remove it by recovering the charge, installing a new drier and evacuating properly.

Why should a system with POE oil be left open to the air for as little time as possible?

POE is hygroscopic; moisture it absorbs is hard to remove even with a vacuum. Cap open lines and keep open time short.

What must be true of the gauges and hoses used on an R-410A or R-32 system?

R-410A and R-32 operate at much higher pressures than R-22, so gauges and hoses must be rated for them (commonly 800 psi working pressure on hoses).

What is the purpose of a triple evacuation?

Pulling a vacuum, breaking it with dry nitrogen and repeating helps carry moisture and air out, especially on wet systems. Confirm the final vacuum with a micron gauge.

A fixed-orifice system has very high superheat and low suction pressure after charging. What is the most likely cause?

Too little refrigerant starves the evaporator, so the vapor picks up extra superheat and suction pressure drops.

What is the advantage of a single-component refrigerant such as R-32 when topping off a charge?

A single-component refrigerant has no glide or fractionation. A zeotropic blend may lose more of one component in a vapor leak.