Fan and Duct Sizing for Solar Air Heating Collectors



- Process over view (flow rate, pressure drops in collector and ducts, fan sizing to drop, noise, temperature)

- Collector pressure drop

- Duct pressure drop (mention corrugated ducting)

- Matching fan to system -- fan curves, fan noise, temperature capability

- Some potential fans



A more precise way to size the fan.

As has been mentioned, 2.5 to 3 cfm per sqft of collector gives good efficiency,
and also tends to give about the right temperature rise for an efficient air
heating collector. So, about 160 to 200 cfm for this 64 sf collector.

When picking the fan, what you want is a fan that will deliver the 160 cfm with
the pressure drop through the collector and ducting. Some fans that are rated
at 200 cfm with no pressure drop will deliver very little air when working
against the pressure drop for an air collector and ducting. Basically, the zero
pressure drop (free air) rating of the fan means very little.

So, try to figure out the pressure drop for the collector and ducting and pick a
fan that will deliver the 160 cfm through that pressure drop.

If you build the screen collector, the pressure drop through the collector is
likely to be in the 0.1 inches of water area with 3 cfm/sf flow. For the
testing Scott and I did last year, I measured the pressure drops for the ones we
built, and some of the pressure drops are listed here:

The pressure drop for the ducting can be estimated with a calculator like this
There are several of these calculators out there. You should pick a ducting
diameter large enough to keep the duct pressure drop down. I'd say less than
0.1 inches of water.

Adding the collector loss and the ducting loss, you have about 0.2 inches of
water pressure drop at around 3 cfm per sqft flow rate.
So, you want a fan that delivers the about 160 cfm with a 0.2 inch pressure

Some manufacturers provide fan curves for their fans that show the flow rates
against pressure drop -- for example, the Fantech FG fans:
The FG 4 inch diameter is a bit short on flow rate for you at 110cfm with a 0.2
inch pressure drop. The 5 inch diameter FG looks pretty good with 130 cfm on
the regular model and 190 on the XL model.

A lot of other companies make this kind of fan (Elucient etc), and I think that
most of them will have similar fan curves. This type of fan does pretty well on
not falling off too rapidly as it sees some flow resistance. Other types of
fan, for example regular axial flow fans, may fall off very rapidly.

This may be more work than people want to go to, but it is a way to not be
disappointed when you install the fan and it delivers half the flow you want.

It would be nice if the group could start accumulating a list of fans that work
well with typical air collectors with an indication of what the fans actually
deliver in flow rate when hooked up to a typical screen absorber air collector.



Potential fans


The 10 inch Dayton fan DAYTON Axial Fan, 115VAC - Axial Fans - 4WT44-4WT44 - Grainger Industrial Supply

axial flow fan for solar heating systems  (fan curve for the Dayton fan)