In NFPA 13 Section Figure A.23.1.5 a closed loop system with sprinkler piping entering into Heating Ventilation and Air Conditioning Equipment is shown. In what specific instances is a closed-loop system required and what additional design requirements are necessary to support this kind of system? The only applications I have seen where sprinklers enter into ductwork are as follows;
1. Semi-Conductor Scrubbers
2. Grease Duct Systems
In other words, sprinklers enter into these ductwork systems when there is a specific hazard therein that could potentially ignite within the ductwork. But for typical Heating Ventilation and Air Conditioning ductwork, which seems to be illustrated by the NFPA 13 Section Figure A.23.1.5, I stand confused when considering potential applicable scenarios.