On a low-pressure centrifugal chiller running at load, how do the evaporator and condenser sit relative to atmospheric pressure?
Correct Answer
D) The evaporator is under a vacuum while the condenser sits slightly above atmospheric
Pressure follows saturation temperature. The evaporator holds refrigerant near the chilled water temperature, well below the refrigerant's atmospheric boiling point, so it sits in a vacuum. The condenser holds refrigerant near the condensing water temperature, which is above that boiling point, so it carries a small positive pressure. That split is what makes the machine both leak air in at the low side and require a rupture disc rated only a little above atmospheric on the high side.
Why This Is the Correct Answer
The evaporator runs cold enough that the refrigerant's saturation pressure falls below atmospheric, producing the vacuum that defines the class. The condenser runs warm enough that the saturation pressure rises slightly above atmospheric. Understanding this split explains both why air enters at the low side and why the relief device is set so low.
Why the Other Options Are Wrong
Option A: The whole machine sits in a deep vacuum from the evaporator through the condenser
The condenser must reject heat at a temperature above the cooling water, and at that temperature the low-pressure refrigerant carries a small positive pressure rather than a vacuum.
Option B: Both vessels operate at several hundred psig, as in any other chiller
Several hundred psig describes a high-pressure refrigerant. Low-pressure machines operate within a few psi of atmospheric across the whole circuit.
Option C: The evaporator is slightly above atmospheric while the condenser is under a vacuum
This reverses the machine. The cold vessel is the one in a vacuum, and the warm vessel is the one that goes slightly positive.
Memory Technique
Cold side sucks, warm side pushes, and neither one is far from the room.
Reference Hint
Low-pressure appliance evaporators run below atmospheric pressure; the condenser runs slightly above it because the condensing temperature exceeds the refrigerant's atmospheric boiling point.
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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