A chiller charged with R-123 has been shut down long enough to warm to a room temperature of about seventy degrees F. What does a gauge on the machine show?
Correct Answer
C) A slight vacuum, because the refrigerant's saturation pressure at that temperature is below atmospheric
R-123 boils in the low eighties Fahrenheit at atmospheric pressure. When the machine sits at a room temperature below that boiling point, the saturation pressure of the refrigerant inside is less than atmospheric, so the machine holds a slight vacuum even sitting idle. That is the defining behavior of a low-pressure appliance and it is why these machines take in air over time. Warm the room past the refrigerant's boiling point and the machine would go slightly positive instead.
Why This Is the Correct Answer
Saturation pressure tracks temperature. With R-123 at seventy degrees F, the corresponding pressure is below fourteen and seven tenths psia, so a gauge referenced to atmosphere reads on the vacuum side. The machine is not equalized with the room because it is a closed vessel; it holds whatever pressure its refrigerant temperature dictates.
Why the Other Options Are Wrong
Option A: Roughly seventy psig, since gauge pressure tracks the room temperature in degrees on a warm day
Gauge pressure does not track room temperature in degrees, and the two numbers matching is a coincidence being read as a rule. Seventy psig would be an idle reading for a high-pressure refrigerant, not for this family.
Option B: Roughly thirty psig, the same reading an idle R-22 system would give sitting in that same room
Thirty psig is a believable idle reading for a high-pressure system, which is exactly why it is offered here. A low-pressure refrigerant cannot build that much pressure at room temperature no matter how long it sits.
Option D: Exactly atmospheric pressure, because a machine standing idle this long has equalized with the room around it
The machine is sealed, so it never equalizes with the room however long it stands. Its pressure is set by the saturation temperature of the refrigerant inside, which at this temperature is below atmospheric.
Memory Technique
Room cooler than the boiling point means the gauge goes negative. Boiling point is the tipping line.
Reference Hint
R-123 boils in the low eighties F at atmospheric pressure, so below that temperature the machine sits in a vacuum.
More epa-608-type-3 Questions
Some jobs on a chiller count as a major repair under the refrigerant rules, which changes what a technician is allowed to do with a badly leaking machine. Which job falls into that category?
An owner asks how many pounds of refrigerant the rules allow to be left inside a chiller once the recovery has been signed off. What is the correct answer to give him?
The motor of a hermetic centrifugal chiller sits inside the pressure boundary rather than outside it on a shaft. What follows from that construction?
An oxygen monitor is used to decide whether a chiller machine room is safe to enter after a refrigerant release. Below what oxygen reading is the atmosphere treated as oxygen deficient?
A low-pressure chiller's rupture disc has a discharge line running from it. Where does that line have to terminate?
A refrigerant vapor detector in a chiller machine room reaches its alarm level. Besides sounding, what is that alarm expected to do?
Which piece of equipment requires a technician to hold Type III certification before it may be serviced?
What property of the refrigerant is what actually places an appliance into the low-pressure category?
During normal operation, what condition exists inside the evaporator of a running low-pressure centrifugal chiller?
In what unit is the required final recovery vacuum for a low-pressure appliance normally expressed?
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