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stmbtwle

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Reply with quote  #31 
Hmm the microwave fan might be a viable option.

However don't fall into the velocity trap. You also need VOLUME. A fan that moves a lot of air at lower speed may actually move more air/heat than a smaller, high-speed fan. What you want to measure is VOLUME, of which velocity is only a component.

Same with temperature. High temperatures mean little without volume.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay

dademac

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Reply with quote  #32 
Quote:
Originally Posted by stmbtwle
Hmm volume.


Willie,

I am soo thankful that you guys take the time to help newbies like me and discuss strategies amongst yourselves so that all can grow from the advice. My plan is the next time I open her up I want to put in a plenum on each end along with probably ZP.. Will this help with the volume? I am more than a little fuzzy on how to increase the volume. I have found it is easy to increase the speed.

Jim in Kansas
gbwillson

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Reply with quote  #33 
Jim-

Adding plenums only serve to transition airflow. A manifold should have minimal effect, if any, on the volume or amount of air passing through a collector, as it is usually empty space. Most collectors don't have or need them, but the ZP needs them to transition from a round duct to a narrow slot. 

To calculate the volume(amount of space) of air moving through a collector is easy. We normally use CFM as the unit of volume, or amount of air that moves through a collector. But to make that calculation, we use the speed of the air as it moves through the collector system along with the diameter of the ducts. 

Example: Using 6" ducts, with the air moving through the collector at 10mph(can be any unit of speed such as feet per minute, meters per second, etc.), the estimated AMOUNT or CFM moving through the system is roughly 173CFM. How did I get the CFM? I use this online calculator below, Give it a try...

http://www.calculatoredge.com/optical%20engg/air%20flow.htm


Yes, you can increase the volume or CFM of the air moving through the collector if you increase the speed of the air, BUT...only if you have actually increased the air speed and not just the fan power. Every collector system has a certain amount of resistance or static pressure that has to be overcome in order for airflow to occur. Long duct runs and certain collector designs can greatly increase this static pressure. Some fan designs move a lot more air than others in free air, but perform poorly when trying to move air with a lot of resistance. Being DIY, we often use scrounged or second hand fans, so we can't always be picky. 

As long as your fan has the capacity to move the collected heat out of the collector, your fan is fine. Lower fan speeds will increase output temps and waste BTU's, while higher fan speeds will capture the most BTU's, but can feel drafty. I like to recommend a fan that has greater capacity than the minimum needed, and use a fan speed controller to adjust the output CFM and temps as desired.  

Greg in MN

dademac

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Reply with quote  #34 
Quote:
Originally Posted by gbwillson
Jim-


Greg in MN



output today @ noon- 92 degrees, 6.2 mph, 4 " ducts, = 190 CFM 

If you figure BTUs/hour = CFM * 1.08 * Temp Rise (in F)

190 * 1.08 * 30 degrees = 6,156 BTUs/hour

Guess that's not very much, but it's a start. 

Jim - Ks

Let the Sun Shine!
dademac

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Reply with quote  #35 
OOps!!

I got caught cheating. (not really) Rookie mistake. I miscalculated my CFM so it made all of my numbers wrong. I entered the diameter of my duct instead of the radius. The new numbers look like this:

6.2 mph, 4 " ducts, = 47.61 CFM 

47.61 * 1.08 * 30 degrees = 1542 BTUs/hour

Jim - Ks

Let the Sun Shine!
stmbtwle

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Reply with quote  #36 
Dademac,

6' x 3' is 18 sq ft., so that's about 86 btu/sq ft/ hour.

AVERAGE energy input is considered to be about 317 btu/sq ft/hr, so that would be better than 25% efficiency. Considering it's winter, the actual input may be less, which would give you an even higher number. Too bad you don't have a meter.

Anyway, not "half bad" for a first try. I'M IMPRESSED!

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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