Posted by nicksanspam on August 8, 2006, 2:35 pm
> Sorry to have wasted your time folks...
Maybe not.
> Consider the heating situation. I wanted to use the same air over and
>over. Take the heat out and pump it into the house. Then the same air then
>goes back into the pipe and is cooled by the ground, ideally to about 55 F.
Suppose the heat pump cools it to 35 F and the ground warms it to 55.
>It then goes back through the heat pump again. The house needs, say,
>10000Btu/hr.
The ASHRAE HOF says damp (20% mc) sand has about 18 Btu-in/h-F-ft^2, or
maybe more, for upward heatflow with evaporation below and condensation
above, so you might get 10K Btu/h with a 10 F dT from 1000/18 = 55 ft^2
of pipe, eg 53 ft of black corrugated 4" pipe with holes, so we can add
water to the soil if needed. This pipe costs about $30/100', and it has
lots of surface compared to smaller water-filled pipes with the amount
of surface exposed to drier soil as a limiting factor.
>... Specific heat of air is about 0.23 Btu/lb/deg F. Air density is about
>0.075 lb/cubic ft Do the sums and we find that we need nearly 1000 cubic
>feet of air per minute
With a 20 F temp change and no change in moisture content, we need to move
10K/20/0.24 = 2083 lb/h of air, ie 2083/60/0.0771 = 450 cfm. But if the air
that goes to the heat pump has 100% RH at 55 F (w = 0.009233) and the air
that leaves it has 100% at 35 (w = 0.004277), each pound of air can supply
another 1000dw = 5 Btu, ie a total of 0.24x20+5 = 9.76, so we only need
10K/9.76 = 1025 lb/h, ie 222 cfm.
How much fan power is needed to pull that air through 4 4"x100' pipes?
The friction loss might be about 0.2 "H20. It looks like Grainger's 23W
3VU71 $64 fan might work up to 176 F. It can move 665 cfm at 0" SP and
550 at 0.1" and 350 at 0.2".
Nick
Posted by Alan C37 on August 9, 2006, 9:55 am
Nick,
Many thanks. Perhaps I should not give the idea up just yet.
With such a short pipe I need to be sure that the air would actually be in
the pipe for long enough to warm (or cool) sufficiently in the time it is in
the pipe each circuit. Come to that will the sandy soil around the pipe
recover its temperature quickly enough? I should probably set up a simple
test to see what can be achieved since I find calculating this stuff in
detail more than a little difficult.
Thanks for the information about blower capacity. I had assumed i would
need a much bigger one than the one you mentioned.
The dealer I am considering for the purchase of the heat pump tells me
that any such non-standard installation would completely void all warranties
so I will have to think about that too. Maybe a much cheaper system and no
warranty coverage would be worth the risk. Such things can be bought in
Canada for around $1000 or so. The excellent two zone Mitsubishi model we
have in mind will cost me about $8000 (installed), which is a little high in
my opinion. Decisions, decisions!!
Yet another possible variant of the ground source heat pump might be to
bury a long length of water pipe and circulate water or anti-freeze
relatively slowly through it. Near the heat pump outside unit would be some
sort of radiator, perhaps finned pipes, to warm/cool the air which then is
used by the heat pump.
Thanks again,
Alan C
>> Sorry to have wasted your time folks...
> Maybe not.
>> Consider the heating situation. I wanted to use the same air over and
>>over. Take the heat out and pump it into the house. Then the same air
>>then
>>goes back into the pipe and is cooled by the ground, ideally to about 55
>>F.
--
Posted via a free Usenet account from http://www.teranews.com
Posted by nicksanspam on August 9, 2006, 12:58 pm
> With such a short pipe I need to be sure that the air would actually be in
>the pipe for long enough to warm (or cool) sufficiently in the time it is in
>the pipe each circuit.
I don't think we need to worry about that.
>Come to that will the sandy soil around the pipe recover its temperature
>quickly enough?
We might find the thermal resistance of a collection of coaxial half-
cylindrical shells in series, starting with a 24" diameter shell 1" thick
with 24Pi/2/12 = Pi ft^2 of surface and a thermal resistance of 1/(18Pi)
= R0.0177 and adding the resistance of a 26" half-cylinder 1" thick, and
so on. The total comes to about R0.5, out to a 240" diameter. So 4 100' pipes
might have a thermal conductance of about 1/0.005 = 200 Btu/h-F. With a 20 F
temp diff, they might supply 4K Btu/h. Then again, if pump groundwater into
the pipes... 6K/20 = 300 lb/h, ie 0.6 gpm. Maybe we need fewer holes.
Nick
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