A walk-in refrigerator with a remote refrigeration system includes an evaporator coil that requires a drain. The typical installation is an indirect drain (terminating above a funnel drain or floor sink to provide an air gap).
Location: USA and Canada
I have recently seen installations and plans calling for an S-trap in the indirect drain line BEFORE the funnel drain/air gap.
[Evap Coil] ===/ /=== [S-Trap][Air Gap][Funnel Drain]====>
Image Source: HeatCraftrpd.com
My understanding was that traps provide a barrier of water that blocks gasses from entering the room through the drain.
What purpose does a trap serve immediately before an air gap?