Some high efficiency purge units send their exhaust through an activated carbon canister before the gas leaves the building. What is that canister doing?
- ADrying the gas so that the water vapor removed with the air does not condense in the vent line
The purge condenses moisture out into its own separator. The carbon bed is not there to dry the vent gas.
- Holding back the last traces of refrigerant so less of it leaves with the discharged air
- CFiltering oil mist so that the exhaust does not stain the roof around the vent termination
Purge exhaust does not carry meaningful oil mist, and a carbon bed is not an oil filter.
- DSilencing the discharge so that the purge cycle cannot be heard from the occupied floors
Noise from a purge discharge is not what a carbon canister addresses, and silencers are not made of activated carbon.
Why B is correct
Activated carbon adsorbs refrigerant vapor onto its surface, so passing the purge exhaust through a bed strips out the traces that survived the condensing stage. That directly improves the purge efficiency figure, which is measured as refrigerant lost per unit of non-condensable gas removed, and it reduces what the owner has to buy back each season.
What this question is testing
This tests knowledge of how purge efficiency is actually achieved rather than how it is defined. The exam ties the efficiency concept to hardware, because the owner's refrigerant bill is the reason the concept exists.
On the job
Owners who have replaced an old thermal purge with a high efficiency unit often see their annual refrigerant purchases fall by a large fraction, and the carbon stage is part of why. For a technician the practical items are checking the replacement date, keeping a spare canister on the shelf and recognizing that a saturated bed quietly stops working without changing anything the operator can see on the panel.
Memory technique
The carbon is a net at the door, catching the refrigerant trying to leave with the air.
Exam tip
Efficiency on a purge means refrigerant kept per pound of air removed. Carbon beds exist to improve that number.
Where to look it up
Purge efficiency is judged by refrigerant lost per unit of non-condensable gas removed; carbon adsorption reduces that loss.