A system charged with a zeotropic blend has lost most of its charge through a large leak, and the leak has now been repaired. What should be done with the small amount of refrigerant still in the system?
- ALeave it in place and add fresh blend until the nameplate weight is reached
Topping off works with a single-component refrigerant. On a blend that has lost most of its charge as vapor, it produces a mixture that matches no published data.
- Recover it, then evacuate and weigh in a full charge of fresh blend
- CLeave it in place and add only the heavier component of the blend
Blend components are not sold separately and no technician may compound one in the field. Trying to correct composition by addition is not a service practice.
- DRecover it and return it to this same system once the repair is verified
Returning the remainder to the same appliance is permitted in general, but this particular remainder is no longer the labelled product, so it would carry the problem back in.
Why B is correct
A large vapor leak fractionates a zeotropic blend, so the refrigerant left in the system is enriched in the heavier components. Adding fresh blend on top of it produces a mixture of unknown composition with the wrong pressure and temperature behavior. Recovering the remainder and weighing in a complete fresh charge is the only way to get the system back to a known refrigerant at a known weight.
What this question is testing
This tests the practical consequence of blend behavior, which the exam builds several questions around. The candidate has to connect what happens physically during a vapor leak with what the charge on the ticket has to be at the end of the job.
On the job
This is a daily decision in commercial refrigeration now that blends have replaced most single-component refrigerants. The tell is the size of the loss: a system that is a couple of pounds low can be topped up, while one that emptied itself through a split tube has almost certainly fractionated. The recovered remainder is not waste either. It goes to a reclaimer, who sorts and reprocesses it to specification.
Memory technique
A blend that has bled off is a recipe with ingredients missing. You cannot fix a recipe by pouring in more of the finished dish.
Exam tip
Small loss, top it up. Big loss on a blend, take it all out and start clean.
Where to look it up
Zeotropic blends fractionate during a vapor leak, so a large loss calls for full recovery and a fresh weighed charge.