Why should a capillary tube system be left open to the atmosphere for as short a time as possible during a repair?
Correct Answer
D) Moisture drawn in from the air can freeze inside the capillary tube and block refrigerant flow
A capillary tube is a very small bore metering device with no moving parts and no filter beyond the drier ahead of it. Humid air entering an open system leaves moisture behind, and once the unit runs, that moisture reaches the cap tube where temperatures fall below freezing and forms an ice plug that stops refrigerant flow. The unit then cools intermittently or not at all. Keeping the open time short, capping lines, and installing a fresh drier are the defenses.
Why This Is the Correct Answer
Moisture is the enemy of a capillary tube system. Water vapor drawn in while the system is open ends up at the coldest restriction in the circuit, freezes, and plugs it. The symptom is a system that cools for a while and then stops, which sends technicians chasing a problem they created during the repair.
Why the Other Options Are Wrong
Option A: Air left inside will raise head pressure until the compressor overload trips on every cycle
Non-condensables do raise head pressure, but the failure a capillary tube system is uniquely prone to is a moisture freeze-up at the metering device. That is the reason for minimizing open time.
Option B: Oil in the tubing oxidizes within minutes and turns acidic enough to attack the copper
Refrigeration oil does absorb moisture and can become acidic over time, but not within minutes of exposure. The immediate hazard is the water vapor itself.
Option C: The capillary tube loses its factory calibration whenever air passes through it
A capillary tube is a fixed length of small bore tubing with nothing to calibrate. Air passing through does not change its metering characteristics.
Memory Technique
The cap tube is the narrowest, coldest spot in the system. Whatever water gets in will find it and freeze there.
Reference Hint
EPA Section 608 small appliance service practice; recovery and evacuation procedures under 40 CFR 82.156 assume proper system dehydration before recharge.
More epa-608-type-1 Questions
An under-counter ice maker is one factory-sealed cabinet, while a modular ice machine head is joined to a remote condenser by field-run tubing. What certification does servicing the modular machine's circuit require?
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Two technicians recover from identical refrigerators with working compressors, one using a 1991 recovery machine and the other a 2015 machine. Why do their required recovery levels differ?
System-dependent recovery equipment may be used only on appliances that normally hold up to what amount of refrigerant?
What feature defines a self-contained recovery unit and sets it apart from the system-dependent kind?
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