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?
- ARoughly 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.
- BRoughly 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.
- A slight vacuum, because the refrigerant's saturation pressure at that temperature is below atmospheric
- DExactly 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.
Why C is correct
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.
What this question is testing
This checks that you can apply the pressure temperature relationship to a low-pressure refrigerant and predict what a gauge will show. It also tests whether you understand why these machines take in air even while sitting idle. Applying saturation data to an idle machine is a step beyond reciting the definition, so the item measures working command of the pressure temperature relationship.
On the job
This is a daily judgment call. You walk up to an idle machine, see the gauge sitting in a vacuum, and have to decide whether the machine is normal or has lost its charge. On a low-pressure chiller a slight vacuum at room temperature is exactly right. The same reading on an R-134a machine would mean the charge is gone. Knowing which is which keeps you from condemning a healthy machine or from ignoring a real loss of charge.
Memory technique
Room cooler than the boiling point means the gauge goes negative. Boiling point is the tipping line.
Exam tip
Compare the refrigerant's boiling point to the temperature in the question. Below the boiling point means vacuum, above it means slight positive pressure.
Where to look it up
R-123 boils in the low eighties F at atmospheric pressure, so below that temperature the machine sits in a vacuum.