GeorgeGitt: The following is courtesy of the current rendition of NFPA-90A:
6.4* Smoke Detection for Automatic Control.
6.4.1 Testing. All automatic shutdown devices shall be tested at least annually.
22.214.171.124 Smoke detectors listed for use in air distribution systems shall be located as follows:
(1) Downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 944 L/sec (2000 ft3/min)
(2) At each story prior to the connection to a common return and prior to any recirculation or fresh air inlet connection in air return systems having a capacity greater than 7080 L/sec (15,000 ft3/min) and serving more than one story
126.96.36.199 Return system smoke detectors shall not be required where the entire space served by the air distribution system is protected by a system of area smoke detectors.
188.8.131.52 Smoke detectors shall not be required for fan units whose sole function is to remove air from the inside of the building to the outside of the building.
184.108.40.206 Smoke detectors provided as required by 6.4.2 shall automatically stop their respective fan(s) on detecting the presence of smoke.
220.127.116.11 Where the return air fan is functioning as part of an engineered smoke-control system and a different mode is required, the smoke detectors shall not be required to automatically stop their respective fans.
Reading 18.104.22.168, I would interpret that the duct smoke is not required in your application.