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re: question about a solar chimney to move air through a greenhouse (techy
12 may 2004
mark & shauna   wrote re:

>...what kind of air flow one could expect to achieve using a 
>solar chimney of sorts in place of large fans in a greenhouse?

cfm = 16.6asqrt(hdt), with vent area a ft^2 at top and bottom and height
difference h in feet and temp diff dt (f). for instance, on a 70 f day,
a 10' tall 80 f greenhouse with a 100 ft^2 slot at the bottom and a
100 ft^2 slot at the peak would have 16.6(100)sqrt(10x(80-70)) = 16.6k
cfm of airflow. 
 
>the situation is, we are off grid and have a couple decent size 
>greenhouses which to date have been using simple vents for cooling and 
>air flow. however, we are not getting enough flow which is causing some 
>problems. we have actuated vents in the gables at the peak which open at 
>a set temperature now but even when full open the air in the greenhouse 
>is not moving much on a calm day.

newer plastic film greenhouses and older glass greenhouses have doors
or roll up sides near the ground and a closable slot along the ridge.
 
>there is no way we can build another power system to drive large enough 
>fans so i thought of the solar chimney idea. perhaps an 18"x18" i.d. 
>chimney of a given height, black on the outside, etc. i am wondering
>what kind of flow we could achieve on an average sunny day via this
>method. any guesses?

...18"x18" sounds small. a 20'x100' greenhouse might collect 0.8x20x100x250
= 400k btu/h in full sun (or 40% of that under 60% shadecloth.) ignoring
latent heat (and the overwatering that requires), keeping the greenhouse only
10 f warmer than outdoor air requires 400k = 16.6asqrt(10)10^1.5, ie a
= 241 ft^2 vents, eg 2 100'x2' roll up sides and a 3' slot along the ridge. 

nick

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