Watch out for these blast gates

[BobL]

I have been meaning to test the constriction imposed by the black plastic blast gates that are commonly sold for 4" ducting.

The ones that look like this.


Apart from jamming fairly quickly and using a steep taper on the sections that connect to the ducting not everyone realizes that they only have a diameter of 91 mm


Using these gates drops the flow through 4" PVC duct from ~ 400 to ~340 CFM.
The will be caused by the smaller diameter and the added turbulence caused by the way it protrudes into the ducting on both sides.

About the only only situation these are good for is if a 4" - 2" reducer is used to connect to 2" ducting line because the 2" ducting will restrict the flow to ~120 CFM.

To maximise your flow it it worth using gates that use female connectors to the ducting. I think Chris Parks sells these?

[John Samuel]

Not only blast gates, Bob.

I picked up some black plastic fittings today. Heaven spare us from "nominal" sizing. Found the same issue, which is why my new floor vacuum unit uses a 90 mm pipe heat moulded over the black floor sweep rather than a 100 mm pipe. The same issue was found with the 4" to 2" reducer ... the 4" end literally falls into a 100 mm pipe with several mm clearance all round.

[BobL]

Not only blast gates, Bob.
I picked up some black plastic fittings today. Heaven spare us from "nominal" sizing. Found the same issue, which is why my new floor vacuum unit uses a 90 mm pipe heat moulded over the black floor sweep rather than a 100 mm pipe. The same issue was found with the 4" to 2" reducer ... the 4" end literally falls into a 100 mm pipe with several mm clearance all round.

Yeah I now avoid all the black plastic and stick with the Stormwater/Sewage stuff. I have one black blastgate on a 2" line that sucks from my power tools and that's it.