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re: passive hrv
13 apr 2003
david delaney  wrote:

>has anyone ever seen a design for an efficient  passive heat recovery
>ventilator (no fans) for a house. the hrv to provide, say, 100 cfm of
>well moderated fresh intake air when it was 32f to -5f outside and 68f
>inside (0c to -25c outside, 18c inside)?

...100 cfm seems like a lot, compared to 15 cfm/occupant, altho most
houses would naturally leak at least this much. condensing moisture
from outgoing air seems doable. how to humidify incoming air? maybe
just keep the house air dry in wintertime, and wear those little
star trek alien smoking harmonica thingies under our noses.
 
>a passive hrv might be built with a counter flow parallel plate exchanger
>designed to fit into a tall insulated closet sized compartment on the
>interior of an exterior wall.  cold exterior input air would enter at
>the top and fall into the house, heating on the way. warm interior input
>air would enter at the bottom and rise out of the house, cooling on the way.

that's a nice thought, but in most houses, with no fans, air would flow in
through both paths and out through upstairs cracks and holes in wintertime,
if the closet were downstairs. seems like it needs at least one fan.

>the warm input air might be taken from some distance away. ("input" and
>"output" are relative to the hrv, of course.)

maybe put the closet upstairs, with some ducts downstairs? 

>the flow pattern in the exchanger might become cross-flow at the top
>and bottom to facilitate input and ouput. the plates might be steel or
>aluminum roofing sheet...

how about polyethylene film? e = 0.9 = ntu/(ntu+1) makes ntu = 9 = au/cmin 
= a/3/100, which makes a = 2700 ft^2, eg 42 8'x8' layers. e = 0.9 makes
tho = 68-0.9(68-32) = 35.6 and tco = 32+0.9(68-32) = 64.4, which makes
dt = 3.6 f, and 100 cfm = 16.6vsqrt(8'(3.6f)) makes the total duct area
v = 1.12 ft^2, eg an 8'x0.14' (1.68") closet with 42 layers in 1.68".
hmmm. how about 1/4" per layer in an 8'x1' closet?  

nick




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