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re: heat recovery
15 jan 2003
greg haas wrote:
>>...a sufficiently long horizontal common pvc pipe inside a larger pipe full
>>of greywater will make a dandy heat exchanger, approaching 100% efficiency
>>as the owner's wallet grows thinner.
>something that struck me when first reading of your idea was that a
>greywater drain that size might not "fill up" during a normal shower.
that's an advantage, if it is always full, with a drain at the top.
if the cold and hot vessels have a large volume compared to a shower,
they can exchange heat all day, vs just during the shower. the cold
water can also gain heat from basement air long after the greywater
cools, if the air is warmer than the cold water.
>in your "dandy" heat exchanger is there enough contact surface area
>between hot water & internal (cold water) pipe to get the heat
>exchange you talked about?
sure. i assume the inner pipe is completely submerged in greywater before,
during and after the shower. an 8'x6" pipe contains about 12.5 gallons of
water, enough for a 10 min shower at 1.25 gpm. its 12.56 ft^2 of surface
with about 4 btu/h-f-ft^2 and a 28 gph constant flow make ntu = 12.56x4/28
= 1.8, so an equal rate counterflow heat exchanger would have e = ntu/(ntu+1)
= 0.64, ie 64% efficiency. or somehwat lower, given a finite volume and pulsed
vs constant flow, and thermally insulating crud, ie fouling.
this would work better with a $150 42 g 20" diam x 35" long galvanized tank
with 20 ft^2 of u30 surface. ntu = 20x30/28 = 21 makes e = 21/22 = 0.956.
we might put one in a 2'x2'x4' epdm-lined plywood box near the ceiling...
>were you talking about sizing the internal (cold water) pipe so that the
>greywater would be "under enough pressure" to slide up the wall of the
>external pipe and completely encompass the internal pipe?
more like a bathtub, with the cold water tank completely submerged and
unpressurized grey water that drains out through a bulkhead fitting above
the top of the cold tank, from a dip tube near the bottom. it might be
useful to have another bulkhead fitting hear the bottom to periodically
drain or backflush the crud out.
nick
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