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Earth Pipe - Page 2

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Posted by nicksanspam on May 5, 2005, 12:03 pm

Good idea, until crippled with actual numbers :-)

NREL says an average July day is 76.7 F in Phila, with w = 0.0133 and
a 67.2 daily min and a 64.9 F dew point and a 54.3 F deep ground temp.
A 5K Btu/h "earth air conditioner" with 80 F indoor air might look like
this, viewed in fixed font:

          1/G  1/800
54.3 F ---www---www--- 80 F
        <---------- I

G is a ground-to-pipe conductance and 800 is the approximate air-water
conductance of a $50 MagicAire SHW2347 2'x2' heat exchanger or a $5 used
1984 Dodge Omni automobile radiator. I = 5K Btu/h = (80-54.3)/(1/G+1/800)
makes G = 257 Btu/h-F. Table 7 on page 22.33 of the 1993 ASHRAE HOF suggests
k = 0.65 Btu/h-ft-F as a min conductance for clay soil. A foot of 1/2" PE
pipe (0.722" OD) with 0.189 ft^2 of surface and a 10 Btu/h-F-ft^2 combined
wall and inner water film conductance has 1/(0.189x10) = 0.529 Btu/h-F of
thermal resistance. A foot of clay soil adds ln(1/(0.722/12)/(2Pi0.65)
= R0.688, ie G = 1/(0.688+0.529) = 0.822 per foot of pipe, so we need
257/G = 313 feet of pipe.

But the soil warms up. If a house in Phila needs 2weeksx10h/dayx5KBtu/h
= 700K Btu/year (not much), and the soil has 313xPix1^2x30 = 29.5K Btu/F
of thermal capacitance, it might warm 700K/29.5K = 24 F over 2 weeks. So
that won't work.

We might improve it by putting the pipe under stone in an L' trench with an
EPDM liner, with another pipe or a night sprinkling system in the trench
to keep the lower part filled with water. With lots of evaporative surface
compared to the heatflow and the rock's shading and low airflow resistance
and poor point contact thermal resistance to downward heatflow, the trench
water temp might approach the dew point vs the wet bulb temp.

          1/G  1/800
64.9 F ---www---www--- 80 F
        <---------- I

I = 5K Btu/h = (80-64.9)/(1/G+1/800) and G = 565 Btu/h-F make G = 565/0.539
= 1048' of pipe, eg 3 $0 400' rolls in a 400' trench. If the dew point is
higher than 64.9 for 2 weeks and the trench water warms 10 F, 700KBtu/10F
= 70K/(1x400Wx62Btu/F-ft^2) would make the trench 1' deep X W = 2.8' wide.

A small window AC with a clogged air filter might provide dehumidification
to w = 0.012, within the ASHRAE 55-2004 comfort zone. A slow ceiling fan
might provide equivalent comfort with w = 0.0133 and no dehumidification.


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