Sunday, July 14, 2013

DIY air-air heat exchangers

"yreysa" <gary@...> mentions Shurcliff's example:

It has about 36x65"x19"/12^2 = 309 ft^2 of heat exchange surface with an approximate U = 0.75 Btu/h-F-ft^2 2-sided slow airfilm conductance, so AU = 309x0.75 = 232 Btu/h-F, with NTU = 232/100 = 2.32 and effectiveness E = NTU/(NTU+1) = 70% at 100 cfm and 82% at 50 cfm.

Room air at 70 F and 50% RH has 0.00787 pounds of water per pound of dry air and weighs 0.075 lb/ft^3. Freezing all the water out of a 30 cfm airstream would make 24hx60m/hx30cfmx0.075lb/ft^3x0.00787lb/ft^3 = 25.5 pounds of ice per day. Melting ice takes 144 Btu/lb. If the exhaust fan runs without the intake fan for T hours per day to melt ice and T(70-32)30 = (24-T)60x30x0.075x0.00787x144, T =  2.84 hours per day, with a 12% duty cycle. Condensation and ice could form in the outgoing airstream between Coroplast flat plates, with incoming cold air inside the plate corrugations.

But (IMO) this is so rarely needed in the US that it makes more sense to just run an exhaust fan with a humidistat if the RH reaches say, 60% in an "airtight house." Swedish houses have 0.025 ACH. A SIP house might have 0.1 or 0.2. An average US house has about 1 ACH. On an average day, a 4,000 ft^2 house can provide ASHRAE's standard 15 cfm for a full-time occupant with a natural air leakage of only 15x60/(4Kx8') = 0.028 ACH, ie about 0.56 ACH on a 50 Pa blower door test.

Bill Shurcliff also suggested a "lung" with an external bellows that could turn all the cracks and crevices in a house envelope into very efficient bidirectional latent and sensible air-air heat exchangers. ORNL's preliminary experiments used a bellows that was too small to exhaust all the air from a 2x6 stud cavity.

To avoid that restriction, a 90 watt 2470 cfm Lasko 2155a reversible window fan could be in an interior wall that divides the house in two partitions with a humidistat in series with a repeat cycle timer like Grainger's $83.90 2A179 (with its $4.26 5X582 socket) which can reverse the fan airflow with adjustable off and cycle times from 1.2 seconds to 300 hours.

If every 10'x10' envelope section contains a 4x10'x6"x1/32" crack and 30 cfm flows through 4,096 ft^2 of envelope and each section has a "duct" with 40/32/12 = 0.104 ft^2 of cross-sectional area, the entire envelope has 4096/100x0.104 = 4.27 ft^2, and the average air velocity is 2x30/4.27 = 14 fpm. Each wall section has 40x0.5x2 = 40 ft^2 of heat exchange surface, so Cmin = 30 and NTU = AU/Cmin = 4096/100x40x1.5/30 = 82 and E = 1-exp(-82), ie 100%. Not bad :-)

The cycle time should be long enough that stale house air clears the path to the outdoors before the fan reverses. If it isn't, we'll know, because the humidity and CO2 concentration in the house won't decrease much as the fan runs.

This could also dehumidify a house... says Energy Star dehumifidifiers typically remove 4 pounds of water per kWh (and add heat to a house if not part of an air-conditioner.) Philadelphia has an average 76.7 F temp and a 67.2 daily min and 0.0133 humidity ratio in July. ASHRAE says 80 F with a 0.0120 humidity ratio is comfortable.

A Lasko fan with a smart ventilation controller could remove 60x2470x0.075(0.0120-0.0100) = 22.2 pints per hour of water vapor from house air with a 0.0120 humidity ratio when the outdoor humidity ratio drops to 0.0100 using 90Wh, at 22.3/0.090 = 247 pints/kWh, using 988 times less energy than an Energy Star dehumidifier.

And this could happen at night in summertime, when outdoor air is cooler, since house materials can store dryness as well as coolth. Concrete weighs about 150 lb/ft^3 and stores about 25 Btu/F-ft^3 and 1% of its weight by volume as the RH of the surrounding air rises from 40 to 60%.   


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