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re: solar hot water heater for home: give me a push...
26 oct 2002
daestrom wrote:
>> shower drain -----------
>> | out --> |
>> | | | ----- |
>> | | | | | |
>> -- ------------------------------ --
>> warm | |
>> water <--|-/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\-|-< cold water in
>> out | |
>> ---bf-------------------------------------
>>
>> | 45" |
>>
>> the drain would enter and leave through ts at the ends of the 4" pipe,
>> and the 1/2" soft copper tubing would enter and leave through bulkhead
>> fittings in glued- or screwed-on endcaps...
after a visit to the plumbing supply store, gluons look better than screwons,
with 4"-2" pvc saddle ts vs 4" ts.
>> with a 3.5" diameter coil and 1' straight ends, 50' of copper would make
>> 48/(3.5pi/12) = 52 turns, 45"/52 = 0.86" on center.
>>
>> at 1.25 gpm, with 30 btu/h-f-ft^2 of film conductance, ntu = au/cmin
>> = 48x0.5pi/12x30/(1.25x8) = 18.8, so e = 18.8/19.8 = 0.95. with no crud,
>> it might be 95% efficient, with 100 f drain water heating 55 f cold water
>> to 55+e(100-55) = 97.7 f.
>i like your idea, but i think you're being somewhat optimistic. from the
>waste water standpoint, you have a 3 inch pipe that is 45 inches long.
but the wastewater fills the 4" pipe, contacting every bit of the 1/2"
copper coil surface.
>...the 30 btu/h-f-ft^2 isn't going to happen because of the film on
>the waste side.
it might be close, if it's backwashed often. howdy reichmuth measured
10 btu/h-f-ft^2 with slow moving water on one side of a pvc pipe and
a 12% solids still manure slurry on the other.
>consider that the supply water travels through 48 feet of tubing with a
>volume flow rate of 1.25/7.48 = 0.167 ft^3/min. with a velocity in the tube
>of 0.167 /(.5^2xpi/4/144) / 60 = 2.04 ft/second (not ideal, but
>respectable). the waste water is traveling through 45 inches of 3-inch pipe
>with a 'washboard' surface formed by the coil. the waste water traveling
>velocity would be... 0.167/(3^2xpi/4/144)/60 = .06 ft/second. this very low
>'tube velocity' would mean the thermal layer on the waste side would be
>prohibitive.
i measured that 30 btu/h-f-ft^2 with still water on both sides
of a paper cup in a pot...
>also, the gaps between the tube coils would probably be completely stagnant.
like the water on both sides of the cup.
>the only tubing cross-sectional surface that would see any heat transfer
>would be a narrow arc on the inside surface of the coil.
i disagree, altho i'm thinking it might be nice to have a 2" pipe full of
air in the middle to shorten the warmup time and force the water to flow
nearer the coil vs down the middle.
>make the coils a little tighter so some of the waste water flows around
>the outside as well as through the middle and you might get twice that
>(60 degree arc on tube wall facing inward and another 60 degree arc on
>tube wall facing outward).
good idea. maybe wind it loosely around the 2" pipe, sweat it onto the
bulkhead fittings, then twist it tighter while gluing the end caps on.
>by the way, if the film conductance is 30 btu/h-f-ft^2, then you need a
>factor of 60 to get from gpm to gph. if you also left it out of the
>calculation for 19.8, it cancels and doesn't really change the answers.
>but i've been sitting here trying to figure out how you came up with 19.8.
>unfortunately, all my thermo books are at work, so i'm doing this from
>memory.
i'll try again, slower...
the copper coil has a = 48x0.5pi/12 = 6.28 ft^2 of surface.
u = 30 btu/h-f-ft^2. at 1.25 gpm, cmin = 1.25x8x60 = 600 btu/h-f.
ntu = au/cmin 6.28x30/600 = 0.314, so e = 0.314/1.314 = 0.239. urk.
bummer. looks like the 60 does matter.
nick
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