Does Blower Position in System Make any Difference? 3

[Beagle123]

Hi:

I'm building a swamp cooler for my attic, and I'm considering where to position the blower. I could position it as the first thing in the system before the heat exchanger, the pads and the ducts. Or I can position it after the heat exchanger and the pads, so it sucks air through them, then pushes the air through the ducts.

Does it make a difference if the blower is pushing air from the front, or pulling it from the back?

Thanks,
Eric

 

[ChrisConley]

If you blow through the assembly you add motor heat to the warm air, which is then cooled. If you draw through, you add motor heat after the cooling (resulting in less adiabatic cooling effect). Reheat isn't typically desired in adiabatic (swamp) cooling, so I'd recommend the blow through if you can swing it.

However, draw through results in better air distribution over the cooling pad...

 

[Beagle123]

Thank you Chris!

So if I understand you correctly, I will get exactly the same flow rate regardless of where I position the blower?

The whole reason I'm trying to move the blower to the front is because the motor may be adding about 300 watts of heat to the system AFTER the air is humidified. 300 watts = about 1000 btu.

I'm building a 2 stage cooler, so the air will travel through a heat exchanger first, so by the time the air gets to the pads the pressure should be very even.

My main concern is the flow rate. Any sacrifice in flow rate will cost more than the small energy gain from moving the blower.

Is it a scientific fact that flow will not be affected?

 

[imok2]

Fans are much more efficient at PULLING air than at PUSHING air

 

[Beagle123]

Its not a fan, its a blower from a swamp cooler. Does that make a difference?

Thanks.

 

[Beagle123]

To be more exact, then I should have the heat exchanger and the pads in front of the blower so the blower pulls air through them to achieve maximum cfm?

The heat exchanger in particular is causing a noticable pressure drop.

 

[Beagle123]

This is what the blower looks like.

http://www.wonderquest.com/swamp-coolers.jpg

 

[Zesti]

imok2 wrote

"Fans are much more efficient at PULLING air than at PUSHING air"

How would the fan know what it is doing?

What I mean is that a fan simply provides/causes a pressure difference in the airflow going through it.

To put this in numbers, let's say the fan creates a 1000Pascal pressure-difference.
Then, the fan does not know if this is from 7000Pa downstream of the fan to 8000Pa after the fan, or from 2000Pa for the fan to 3000Pa after the fan.

Atmosperic pressure is 100.000Pa so the several 100Pa's we are talking about in HVAC-fans do not matter.

On the choice of push or pull in this case:
I would say the heat added by the fan is of little consequence. I am sure it will be less than 1K. (We are talking about cooling by evaporation, right? In that case the heat from the motor might be benificial because it will help evaporation and you will get the heat from the motor into the airstream anyway, no matter where it is placed.)

I think I would place the fan in the cleanest and dryest air, so blow through.

 

[macmet]

I'm not familiar with 'swamp coolers', so I don't now what sort of temperature change you'll have in your air. But make sure that your fan is sized for the proper air temperature.

Perhaps temperature change is negligible, perhaps not. Just be sure.

 

[Compositepro]

All swamp coolers I've seen (including your picture) use blowers to suck air through the mesh. The reason is that the airflow through the mesh will be uniform. The suction of a blower draws from all directions, because the air starts with no momentum. The discharge of a blower is high velocity and directional which is what is needed to go through ducting.

 

[Beagle123]

Thanks for all your great responses.

I decided to put the blower after the heat exchanger and pads so it goes like this:

heat exchanger -> pads -> blower -> ducts

I decided that it was best to err on the side of caution, so I did it the traditional way. I included a few pics so you can see what I'm talking about.

Now I have to decide how to do the ducts. Evaporative cooling requires much more air flow than air conditioning. In fact, I think you'd always want the maximum amount you can get. I'm hoping to get 4000-5000 cfm, which my blower can handle.

I looked at home depot for the biggest ducts and vents I can find. The biggest vent I can find is a 14 X 14 box that fits between the studs and accepts a 10 inch duct. I'm thinking of using 5-6 vents. You can see the layout of my house in the attached pic.

Will 5 or 6 of these 10 inch insulated ducts allow plenty of air to flow without much friction? Should I try to use 12 or 14 inch? I don't have any feel for this because I've never done any HVAC before.

Also, will I have a problem running a duct approx 25 feet to my kitchen? I suppose there will be less flow there, but I can live with that.

ducts:

http://www.homedepot.com/h_d1/N-5yc...splay?langId=-1&storeId=10051&catalogId=10053

Am I on the right track?

Thanks in advance.

 

[willard3]

You should hire an Engineer familiar with air movement and control.

 

[RossABQ]

I'm not entirely clear on your concept here, but I'd like to point out that using a swamp cooler such as the one linked for the blower above should be used with caution in an attic; saturated air in a space with exposed fibreglass insulation can lead to mold and condensation problems. Are you in a truly arid climate?