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re: heat recovery
14 jan 2003
iain blackie wrote:
>>...i've been thinking of putting an 6"x8' horizontal pressurized cold
>>water thick-walled pvc pipe inside an 8"x10' unpressurized thin-walled
>>greywater pipe that drains to the sewer through a large bulkhead fitting.
>>mcmaster-carr sells 10' of 6" pipe for $56.61 and 6" caps for $9.75 each.
>>the cold water might enter the 6" pipe via a reinforced garden hose, then
>>flow into the water heater via the drain fitting. who sells 8" dwv pipe?
>i had in mind a tank, rather than a pipe-in-pipe. as the volume of
>outgoing water is the same as the incoming, at best you can only get 50%
>of the heat back - the 40c drops to 22.5c and the 5c rises to 22.5c.
not true, with a counterflow heat exchanger.
>the tank would take in the warm water at the top and drain it out at the
>bottom. the incoming water would go through a coiled pipe starting at
>the bottom, up to the warmest water at the top.
a coiled pipe has to be very large for 80% efficiency at, say, 2 gpm.
many basements have sewer outlets near the ceiling, so a greywater output
near the floor probably needs a pump. a heat exchanger up in the rafters
uses no floorspace and contributes more space heat upstairs, vs wasting
heat to the basement. a tank (vs coil) within a tank can smooth out the
daily hot water flow and do heat exchange and long-term tempering even with
no flow, preheating a significant volume of cold water with basement air.
>in the uk, i'm told the average family of 4 uses 4000 kwh of hot water,
>the scheme above should get 80% of 50% back - 40% overall.
an average 1557 btu/h, enough to heat 28 pounds of water per hour from
55 to 110 f. c = 28 btu/h-f and a = pi6/12x8 = 12.6 ft^2 and u = 4 make
ntu = 12.6x4/28 = 1.8 and e = ntu/(ntu+1) = 0.64, ie 64% efficiency. an
8" inner pipe ($85+2 $25 endcaps) would raise ntu to 2.4 and e to 71%.
a 19' 4" pipe inside a 20' 6" pipe would make ntu = 2.8 and e = 0.74.
how would you improve this?
nick
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