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greywater heat exchange
31 jan 2003
daestrom wrote:
>> ...4 10 min showers per day and 20 minutes of dishwashing at 1.25 gpm
>> might involve heating 75 gallons of 55 f water to 110 with 8x75(110-55)
>> = 33k btu, about 10 kwh worth about $1. a $1,000 100% heat exchanger could
>> pay for itself in about 3 years. using hot water in say, 6 10 minute bursts
>> over 12 hours makes once-through heat exchange harder (something like 300'
>> of 3/4" copper tubing?) than storing significant volumes of greywater and
>> cold water with slow continuous heat exchange.
e = 0.9 = ntu/(ntu+1) for a counterflow heat exchanger with equal heat
capacity rates. this makes ntu = 9 = au/c = 30a/600 at 1.25 gpm with
a 60 btu/h-f-ft^2 water film on each side of the tubing (measured with
a pot in a pot in my kitchen.) this makes a = 180 ft^2, eg 917 feet of
3/4" tubing. wow.
is there an easy way to figure that a coil with more volume will be more
efficient, since we use hot water in bursts? adjust the flow rate? with no
volume, c = 600 btu/h-f at 1.25 gpm. with infinite volume, c = 75gx8/24h
= 25 btu/h-f... what's an effective flow rate formula for a coil with
volume v and water usage of n b gallon bursts per day? something like
a plug flow retention time with an upper limit of 1.25 gpm...
>> we could save more cooling the water to 5 vs 20 c, especially if
>> (oil?) house heating is cheaper than (electric?) water heating...
>
>how can using oil to heat the house help cool water from 20 c to 5?
>unless you somehow are using house heat to pre-warm the incoming 5 c
>water to 20 c??
if water heating costs more than space heating per btu, it's more economical
to heat cold water than house air with outgoing greywater. if not, it costs
less to build a water-air heat exchanger, eg a 55 gallon plastic drum or two
in series with the septic outlet.
>> greywater tends to foul heat exchangers with insulating layers of crud,
>> and we need a way to clean out or backflush holding tanks. i've thought
>> about a large galvanized tank inside a large unpressurized tank, a 4'x20'
>> horizontal pipe under the ceiling (which might come close to a counterflow
>> heat exchanger)...
>4^2*pi/4 *20 *62.2 = 15 625 lb of water! that's a lot of weight for hanging
>from any floor joists ;-)
heavy water? ...(4/2/12)^2pix20x62.2 = 109 pounds, ie 5.4 pounds per foot.
>> i could put a $130 16" diam x 38" long 30 gallon galvanized tank inside
>> a 2'x4'x3' tall epdm-rubber-lined plywood box with drains from the shower
>> and washer and a sink on top and a bilge pump with a switch near the top
>> of the box...
>2x4x3x62.2 = 1492 lb of water. be sure to brace for the weight.
the weight would be on a concrete floor. the sideways force at the bottom
would be 3x0.433 = 1.29 psi, ie 186 psf, a piece of cake.
for more of a counterflow ambiance, the box might contain a rectangular
spiral instead of a tank, with 5 turns (60') of 4" thick-walled pvc pipe
with cold water entering the coil at the bottom and warm water leaving at
the top, surrounded by stratified greywater. we could make this 42 gallon
"tank" with 20 elbows (or possibly bend the pipe at the corners?) with
water usage in 6 12.5 gallon bursts per day and the "infinite volume"
heat capacity flow rate and 63 ft^2 of u4 surface and a 0 heat capacity
flow rate ratio, ntu = 63x4/25 = 10 and e = 1-e^-ntu = 0.99996. not bad.
with equal flow rates, e = ntu/(ntu+1) = 0.91.
nick
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