Wednesday, June 24, 2009

The Continuing Saga of the Small Job

Following on from my previous post I spent the next two weeks locating the spray booth, the exhaust fan and constructing the connecting ducting.

I screwed a plastic gutter downpipe collecting box to the wall over the vent that I was going to use as the outlet and sealed it with acrylic flexible sealer. I used 90mm PVC pipe from the spray booth to the exhaust fan that I had mounted on an interior dividing wall in a timber box. The air exiting the fan was channelled into a plastic plant container that tapered down to about 200mm and a 90mm fitting was mounted in the base of the container. Silver foil covered spring wire ducting available for clothes driers was then sealed to the 90mm outlet of the fan and run to the vent in the brick wall. A lot of time was taken in waiting for sealant to dry sufficiently to move on to the next stage.

Before making the final ducting connection to the wall vent outlet I turned on the fan.

Well, hardly any air came out!

I removed the ducting at the fan enclosure and got a reasonable air flow.

The problem seems to be that the spiral spring ducting is not smooth inside and the ridges of the spring is sufficient to impede the air flow.

I now have to re-think the whole project.

I am going to put a new smaller spray booth as close as possible to the wall vent and cross my fingers, unfortunately I don't have a lot of room for the fan enclosure.

The spray booth and fan enclosure had previously done years of service at my old home but it was ducted with smooth bore 90mm PVC pipe.

There's a lesson there me thinks.


Sarails said...

Sounds to me your fan is spending more energy decreasing the diameter of the spiral hose than drawing air through from your spray booth.
Since the spiral hose is more commonly used in “pushing” situations, where air is forced out of the dryer to the outside, rather than what I read as a “sucking” situation where the air is being drawn out to the outside via you fan. What is happening is that the hose is being compressed at the end nearest to the fan, and this is restricting airflow down the hose from the spray booth, if you moved your fan to behind the spray booth and before the flexible hose, this will work better.
It is due to the rigidity of the PVC pipe that enabled it to work previously

Ray P said...


Maybe I didn't explain properly but the air is drawn from the spray booth through PVC pipes to the enclosed fan then blown out through the tapered plant container into the spiral ducting (which doesn't collapse) and then hardly comes out the other end about 3 metres away. I believe the spiral ridges on the inside of the ducting 'catch' the air flow and eventually stop it causing a back pressure that the fan can't cope with. The fan is about 300mm diameter and is of the type that used to be mounted in windows with louvres that would open when the fan was turned on.