A low-pressure chiller that has been shut down for months keeps a heater energized in its compressor oil sump. What is that heater for?
Correct Answer
C) It drives refrigerant back out of the oil, so the oil does not foam badly at start up
Refrigerant dissolves readily in oil, and the colder the oil the more it holds. When a machine starts and the sump pressure drops, that dissolved refrigerant boils out all at once and the oil foams, which can starve the bearings and trip the machine on low oil pressure. A sump heater keeps the oil warm enough that the refrigerant stays out of it. The same behavior is what makes the last stage of a recovery so slow.
Why This Is the Correct Answer
Solubility of refrigerant in oil rises as the oil gets colder, so a cold sump on an idle machine can hold a great deal of refrigerant. Heating the oil drives it back out into the machine before startup, which prevents the sudden foaming that follows a pressure drop. The same physics explains why an oil charge keeps releasing refrigerant during recovery.
Why the Other Options Are Wrong
Option A: It keeps the oil thin enough for the pump to move it at the moment the machine starts
Viscosity is a side benefit, but chiller oils are chosen to pump at machine room temperatures without help.
Option B: It keeps the sump above the dew point so that moisture cannot condense onto the oil
The sump is inside a sealed refrigerant circuit, so there is no room air available to condense moisture onto the oil.
Option D: It keeps the bearings warm so that clearances stay within tolerance during a cold start
Bearing clearances on these machines are not managed with a sump heater, and the bearings are lubricated rather than heated.
Memory Technique
Cold oil is a sponge. Keep it warm and it stays a lubricant.
Reference Hint
Refrigerant dissolves in compressor oil and comes out when the pressure drops; sump heaters keep the oil warm to prevent foaming at startup.
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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