A chiller has been pressurized with dry nitrogen for a leak test and the technician returns hours later to read it. What has to be accounted for before deciding the machine has lost pressure?
Correct Answer
D) The change in temperature since the test was set, since pressure moves with temperature
A sealed volume of gas changes pressure with temperature, so a machine room that has cooled overnight will show a lower reading with no leak at all, and one that has warmed will show a higher one. Both the pressure and the temperature are recorded when the test is set and again when it is read, and the comparison is made after correcting for the temperature change. Without that correction a tight machine can be condemned or a leaking one passed.
Why This Is the Correct Answer
Gas pressure in a fixed volume rises and falls with absolute temperature. On a machine the size of a chiller, sitting in a room that swings ten or fifteen degrees between morning and afternoon, the pressure change from temperature alone is easily large enough to be mistaken for a leak. Recording temperature with pressure at both ends of the test is what makes the result meaningful.
Why the Other Options Are Wrong
Option A: The nitrogen cylinder's remaining contents, since a low cylinder cannot hold the shell up
Once the shell is up to test pressure and the supply is valved off, the cylinder plays no further part in holding the reading.
Option B: The chilled water flow rate, since moving water changes the volume available inside the shell
Water flow outside the tubes does not change the gas volume inside the shell, and the test is isolated from the water side.
Option C: The oil level in the compressor sump, since oil absorbs nitrogen and lowers the test pressure
Oil does absorb some nitrogen, but the effect is small and it is not what technicians correct for when reading a standing test.
Memory Technique
Gas does not know the difference between leaking out and cooling down. Only your thermometer does.
Reference Hint
A standing pressure or vacuum test must be corrected for the temperature change between the two readings.
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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