Why does a technician introduce the first portion of the charge into a low-pressure machine slowly?
Correct Answer
C) Liquid flashing into the vacuum chills the tubes and can freeze the water in them
A machine under vacuum will flash liquid refrigerant to vapor the moment it enters, and flashing absorbs a great deal of heat. That heat comes out of the tube bundle and the water in it. Dumping the charge in fast can drive the evaporator below freezing and split tubes just as surely as a careless recovery. The fix is the same on both ends of the job: keep the water circulating and bring the pressure up gradually until the machine is out of the danger range.
Why This Is the Correct Answer
Flash cooling during charging pulls heat out of the tube bundle. With the machine in a vacuum, the flashing is violent and the temperature drop is fast, so tube water can reach freezing before anyone notices. Slow charging with the water pumps running keeps the temperature above the danger point.
Why the Other Options Are Wrong
Option A: The recovery cylinder will vapor lock if it is emptied too quickly at first
Cylinders do not vapor lock in the way described, and cylinder behavior is not what limits charging speed on these machines. The limit comes from what happens inside the tube bundle.
Option B: Rapid charging drives the purge unit into continuous operation for hours
The purge unit responds to non-condensable gas, not to how quickly refrigerant is added. In any case the machine is down during charging and the purge unit is not running.
Option D: A sudden pressure rise can damage the rupture disc even below its rating on the shell
A rupture disc is unaffected by pressures below its rating, wherever it is mounted. The hazard while charging a machine under vacuum is low temperature at the tubes, not high pressure.
Memory Technique
Flashing is refrigeration. Dump liquid into a vacuum and you have built an ice machine out of the tube bundle.
Reference Hint
Charging into a vacuum causes violent flashing and rapid cooling, so the charge goes in slowly with the water circulating.
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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