re: revisiting greywater in a drum SOLAR.KnowledgePublications.com

re: revisiting greywater in a drum
15 mar 2005
well, gary and i are going to try this greywater heat exchanger. i just
mailed 22 pounds of plumbing to him in montana, including a drum lid with
some extra holes and attachments. he has a 400' roll of 1/2" hdpe pipe...

>its pretty flexible -- i can wind it around the 14 inch dia form i made 
>earlier, and also inside of a 5 gallon pail (11 diameter).  this was at 
>about 62f, so it might be better yet if warmed a bit.  not sure what the 
>pipe life implications are for tight winding?

the ap engineer said there might be a long-term stress problem (in 25
years) if it's wound tighter than min radius 9xod, ie 13" diameter for
nominal 1/2" pipe, but that's just a rule of thumb. not very critical.

>i've been looking at the discussions between you and daestrom, and 
>wonder if you have reached any conclusions on coil diameters and spacing 
>between coils (both horizontal and vertical). 
>
>my 2 cents (actually probably only worth about 1 cent :-) would be that 
>there should be some reasonable space between the tubes -- and also 
>between the outer tubes and the barrel.

me too. i've been thinking 1/8" horizontal space between layers and
a 1/8" vertical space every 4" (nominal 8 turns.)

>my rough thinking is that you have good heat transfer on the inside
>tube wall because the water is flowing under a pressure differential
>at fairly high velocity.  if you don't want the heat transfer on the
>outer wall to be a bottleneck, then you need to provide flow passages
>around the outside of the tube that encourage the (weaker) buoyancy
>driven flow??  on the other side of the coin, more spacing means less
>tube length.

and desirable heat transfer area and burst integration volume.

>there has to be an optimum somewhere :-)

yes...

>all in all, i'm not convinced of our ability to analytically determine 
>what the best spacing arrangement is...

we could try. i can imagine trying to balance the extra convective flow
resulting from wider spacing against the extra heat transfer resulting
from more pipe. there's also the crud factor. a 1/8" gap might more
easily clog than a 1/4" gap...

>...maybe this is a good place to do a bit of experimenting before doing
>the "final" stuffing of the barrel??  if we could work out a simple way
>to do a trial winding or two and measure the performance, i'd be willing
>to give it a go. something that only requires winding a couple hundred
>feet rather than 800 ft would be nice.
>
>what do you think??

i can't think of a good experiment at the moment. otoh, we might look
into this on paper. maybe daestrom can help. with a vertical d" gap
every 6 turns (4.33") and lots of horizontal space between layers, what
value of d maximizes heat transfer? the d" gap would provide about 22d
in^2 of bottleneck flow area. how do we convert this to a hydraulic
diameter? and 6 turns have about 35' pipe with 6.6 ft^2 of surface.
we might assume the bulk greywater is 90 f with 70 f freshwater in
the pipe.  the drum is only 34.5" tall, which limits the number of
0.722" pipe turns and d" spaces.

nick