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re: not needed--pyjamas
18 dec 2001
lpogoda wrote:
>>...now that winter is here again, i'm thinking again about making
>>a greywater heat exchanger in the basement with several plastic 55
>>gallon drums and a sump pump to move shower and kitchen sink water
>>back up into the waste line after it cools from about 100 to 80 f.
>there's really no need to keep wastewater around or to jury-rig solutions.
wintertime heat recovery is useful, altho it may not be economical.
>...taking the relatively high-grade heat of shower water and
>turning it into low-grade space heating heat is kind of wasteful,
>given that alternatives exist.
this hot water and heat both come from off-peak electricity, so the
"grade of heat" seems unimportant.
>see http://www.oikos.com/esb/49/gfx.html (i'm not affiliated in any way).
assuming this is the gravity falling film heat exchanger site, not
everyone has 5' of fall between the first floor and waste pipe exit,
or $300, or the cost of installation, and this gfx product is only
rated at 60% efficiency. one big problem with greywater heat exchange
is fouling which reduces efficiency. i have never seen any gfx
performance test data taken after some crud accumulation.
i might cut the tops off a few free 3' tall 55 gallon plastic drums and
put them under a basement workbench to collect warm graywater, connecting
them together near the bottoms with some heater hoses pushed into holes
drilled 1/8" smaller than their diameter. a submersible pump with a level
switch (eg northern's $47 item# 108981-c151) might empty one of the drums.
with 120 gallons of 110 f water entering every 24 hours, warming 70 f air,
this might be an ashrae hof heat exchanger with minimum heat capacity rate
40 btu/h-f and 0 capacity rate ratio, so ntu = 25ft^2n(1.5btu/h-f-ft^2)/40
= 0.94n, and effectiveness e = 1-exp(-0.94n), ie 61% for 1 drum, 85% for 2,
94% for 3, and 98% for 4...
nick
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