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re: an easy way to recover heat lost down your shower drain
5 may 1997
louis kennedy wrote:
>...my main shower is over a crawl space. there was adequate (at least
>6 feet) horizontal length of drain before the down drop... i replaced the
>plastic horiziontal shower drain with a 1 and a half inch coper pipe.
>using two: 1 and a half inch x 2 inch x one half inch tees (one for each end)
>and another length of 2 inch copper pipe (to make an outside sleve over
>the 1 and a half inch copper drain)
so the internal pipe has a surface area of about 6'x1.5"/12xpi = 2.4 ft^2, and
in ashrae (1993 p 3.4) hof heat exchanger terms, the heat capacity flow rates
are equal, so z = 1 and cmax = cmin = 3gpmx8lb/galx60min/h = 1440 btu/h-f, and
if each water film has a thermal conductance of, say, 40 btu/hr-f-ft^2, the
"number of heat transfer units," ntu = 2.4x40/2/1440 = 0.033...
>the cold water supply to the shower recovers the heat from the shower waste
>water as it travels in this counter current sleve over the shower drain...
so the counterflow heat exchanger effectiveness e = ntu/(1+ntu) = 0.032...
>due to the arrangement of the pipes the cold water is reheated for the
>entire bathroom, as a result it was easy to verify the cold water is
>reheated by almost 10 degrees celcius during an average shower with this
>passive heat recovery system.
and if the shower temperature were thi = 110 f, and the cold water entered at
tci = 50 f, it would emerge at tco = tci+e(thi-tci) = 50 + 0.032(110-50)
= 51.9 f, about 1 c warmer than when it entered, or 10 times less than you
measured. i wonder what's missing here. a lower flow rate would heat up
the cold water more...
nick
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