Pull thru vs blow over

[bcampbell]

I am curious. Having worked a lot with exhaust fans, I believe that blowing air over an evaporator would result in more air flow than pulling air through the evap. Do you all agree?

[NoNickName]

No, the more air is pulled or pushed if the fan is standing away from the coil, so to have a lower speed through the fins.
A lower speed not just have a lower pressure drop, but also a better exchange because of longer crossing times.

It doesn't matter whether you pull or push. Just put the fan inches away.

[US Iceman]

Do you all agree?


No. Fans react to differences in static pressure. The only way to increase their capacity without changing the speed of rotation is to reduce the static pressure the fans operate at.

If the fan is in a draw-thru or blow-thru configuration the volume flow should be the same.

On coils, the fans are normally draw-thru because the air flow velocity is more uniform over the entire coil face. This is not the fact with blow-thru coils.

[Abby Normal]

A housed blow through fan will have a 'non-ducted loss' as the air leaves the blower outlet.

In cooling however, in particular when the load is extremely sensible in nature, the blow through fan will result in less total airflow and total cooling needed because the coil deals with the fan and motor heat directly.

A draw through fan under these situations has to move a higher volume of air, and has to over cool this volume of air to compensate for the downstream fan/motor reheat.

[smpsmp45]

Though this has no direct link to the msg there is one more difference. in Blow through system the Fan motor is exposed to higher temp compared to Suck Through for the obvious reasons. That does matter a lot in an textile air washer system design. But ofcourse as US ICeman has pointed out the Such thru shall have more uniform flow across the coil.