Bryan Elliott wrote: I've got access to a long level straight run so I thought I'd take advantage of it.
If anybody knows of any pitfalls to watch out for I'd like to know.
I don't have any hands on experience with earth-tubes, but I think that 'level' is not what you're after. My understanding is that the standard is sloping, usually away from the house, enough that there are no low spots and any liquid that ends up in the pipe flows away from the house. This isn't just leaks at joints, or entry at the end; there is condensation to consider.
A dust settling box at the intake seems like a good idea. There is a diagram of one, and some other info, here:
http://www.homeintheearth.com/tech_notes/earth-tubes/earth-tube-design-for-earth-sheltered-homes/
Other misc. pitfalls I have read of:
Mold. See above re: slope and condensation!
I've heard of some folks installing a cable at time of system install, so that a rag or something can be pulled through for cleaning, or even to detect if there is a mold problem.
Radon. If it's an issue in your area, earthtubes can be an entry point.
As far as size goes, 1 foot of 8" pipe has a volume of about 0.6 cubic feet. So, 400' of 8" pipe is 240 cubic feet of air; I would expect a reasonable fan to take several minutes to move that much air through a pipe of that length.
Seeking out examples of successful/unsuccessful systems might be less work than doing all the math! First you'd need to determine what cooling wattage you need, followed by the amount of air you will need to move to get that from a given delta-T, and how long air will need to stay in the tubes to arrive suitably chilled... from there you could figure out how many pipes/fans it would take to provide that air... Whichever route you take, I would be inclined to overbuild, since so much of the expense/hassle is the installation rather than the cost of adding more pipe.
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