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re: commercial air-air exchangers ?
21 dec 1996
david j. wingert  wrote:

>we are looking to provide ventilation for a 20,000 sq. ft. building that
>is being used for machine work... the owners don't want to spend any money
>on heating/cooling all that make-up air... if you know of anything that
>might work...

how about running a few concentric polyethylene ducts from side to side near
the building ceiling, with greenhouse fans pushing outside and inside air
into opposite ends of each tube?

a $50 100' section of 36" diameter duct with 8 mph air has au ~ 5000 with
1600 cfm = cmin = cmax, roughly, and z = 1, so ntu = 5000/1600 ~ 3, and the
ashrae hof says e = 3/4 for this counterflow heat exchanger, so 30 f entering
air would be raised to about 30 + 0.75(70-30) = 60 f with 70 f exhaust air...

they might heat the building this way, if both airstreams ended up at opposite
ends of a $1/ft^2 lean-to commercial plastic film greenhouse bolted on to the
south side of the building, to collect the solar heat equivalent of about a
gallon of oil per square foot per year, with the 5 cent/ft^2 plastic film
covered with shadecloth in the summer, to make it last longer than the 4 year
guarantee period and reduce the solar heat gain of the building.


>and air change created by the comings and goings of people and pets.

that's a minor problem, unless you live in a department store, which you
can see with a few simple numbers. suppose you and your wife and two kids
and your small menagerie enter and leave the house 4 times a day and leave
the door open for 10 seconds each time, allowing 1000 cfm of 30 f air to
enter the 70 f house... airlocks on doors are not cost-effective in most
houses, unless they form part of a larger sunspace.

>> this example house needs 24 hr x (68 f - 30 f) x 125 btu/hr-f = 114,000 btu
>> to stay warm on an average january day, the approximate heat equivalent of
>> a gallon of oil.
>and so, if you are indeed off by a factor of 2 or 3 which i believe

and so? :-) where are your numbers? perhaps you believe the moon
is made of green cheese as well... harmless enough, i guess.

>try again

sometimes the world seems full of critics who just stand back and throw rocks
at things, and tell other people how to behave, vs understanding or helping.
these people remind me of animal preditors.

   cheetahs often catch hold of their prey's nose and run alongside it.
   as the victim stumbles and falls, or staggers, or tries to run, the
   cheetah holds tight, closing mouth and nostrils in one stapled hold,
   or--with larger prey--biting into the throat to cut off air. leopards
   like to leap down from trees for a quick crack of the back. lions
   improvise, each has its own specialty. some leap up from behind, like
   a terrestrial leopard; some try a daring front leap, risking hooves
   and horns to bite into neck or face.

   hyenas are far more efficient. they catch hold of flesh, not with small
   nips and throwing of weight but by smoothly and quickly transferring
   chunks of it from prey to throat. food slips instantly from toothhold to
   stomach. like human infants nursing, they seem to swallow without pausing
   for breath, as if food and air travelled in separate channels. they are
   the only predators adapted to eating bone. their dung is white with it.

                                 from personal history: hyena
                                      by joanna greenfield,
                                      in the new yorker, nov. 11, 1996

>or better yet build it in the real world with real occupants
>and a nice monitoring system and i'll be quite interested.

how nice to hear that you may become interested in this way of helping
our country stop bombing the holy land to keep the price of oil low,
on this christmas day.

physics professor paul bashus and ee prof rick perry and engineering student
carl mas and 14 year old paid helper doug lepine and i are trying to build
another real thing now, 60 times larger than last winter's outdoor structure,
with a few other volunteers, including two architects with hammers, and some
real future occupants, maybe, but we are not in the housebuilding business.
so far i'm doing most of the work and providing most of the money. perhaps you
would like to contribute a little time or money to this non-profit enterprise.

our datalogger/controller is now set up and measuring the room temp, and
talkable to by telephone with a modem and some simple help menus. we need
to wire up more temperature and wind and sun sensors and some solid-state
relay control outputs for motorized dampers, etc. the building itself needs
more work, eg insulation. the existing 10x12' cabin is now a corner of an
18'x24' structure that is mostly sunspace, with room for a solar closet, like
this (use a non-proportional font like courier to see this better on a mac):

|           |
| reflecting|    8'  10'
| pool      |hhhhhhhhhhhhhh
|           |h ----.-----.h
|           |hpc   |     |h
|36' max~2' |hpc   |cabin|h12'    pc is polycarbonate glazing
|    deep   |hpc   |     |h
|           |hpe---------|h
|           |hpe   s     |h       pe is polyethylene film
|           |hpe   s(sc) |h12'
|           |hpe   s     |h       s is an 8'x 12' strawbale wall
 ----------- h ---------- h
             h            h       h is some horse manure filling some
             h            h         drainage trenches, after a little

a slightly larger view from the ne:

                   |                    | <--we plan to put 3 55 gal drums
                    --------------------     and a dozen 10' long x 4" diam.
                  .|   .                |    $4 thinwall pvc plastic sewer
              .    |          .         |    pipes above the collar beams,
           .       | and a solar   .    |    a la norman saunders...
         .       8'| closet here     .  |
        .          | containing       . |    the sunspaces are made with
       .           | about 12 55       .|    a "foundation" consisting of
~~~|   .    8'     | gallon drums       |    pressure-treated 2x4s on edge,
--------pepepepepepe-----------------------  staked to the ground with 2'
                                             of rebar...

   the sunspace and thermal store are unusully large in this case compared
   to the original structure because we'd like to try out several different
   heating techniques, and heat pond water for showers, and grow some plants
   and perhaps put a hot tub in the sunspace. also, it's harder to make a
   small solar house work than a large one, from a physical point of view,
   as the surface (loss) to volume (storage and living space) ratio increases.

   the sunspace floors are two tons of lightweight gravel (expanded shale,
   which has been cooked at about 2000 f to make it pop like popcorn,
   lowering its heat capacity and thermal conductivity, and making it able
   to absorb 13% of its weight in water, which makes it potentially useful
   as a summertime dessicant) over 6 mil polyethylene film. at the moment
   there's too much moisture coming out of the ground, (the poly film on
   the ground needs to be better sealed at the edges) so it takes the sun
   an hour or so to melt the ice off the inside of the glazing in the morning,
   before the sunspace temp goes up to 75 f, on a 30 f day. the cabin has
   no insulation yet, and a bare cement floorslab with no insulation or
   vapor barrier underneath, and lots of air leaks in the strawbale wall.

it's 28 f this sunny morning, as i go off to work. ho ho ho.


nicholson l. pine                      system design and consulting
pine associates, ltd.                                (610) 489-0545
821 collegeville road                           fax: (610) 489-7057
collegeville, pa 19426                     email:

computer simulation and modeling. high performance, low cost, solar heating and
cogeneration system design. bsee, msee. senior member, ieee. registered us
patent agent. solar closet paper:
web site:

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