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RE: Constructing a greenhouse

They make a twin wall and a triple wall polycarbonate that is great for
green house coverings and it's shatter proof as well.

Bob Carroll

-----Original Message-----
From: owner-ferns@hort.net [mailto:owner-ferns@hort.net] On Behalf Of
Bob Needham
Sent: Wednesday, September 29, 2004 4:03 PM
To: Wim.dewinter@wur.nl
Cc: ferns@hort.net
Subject: RE: [ferns] Constructing a greenhouse

Your math rocks! I've never heard of a double-glazed greenhouse,
but that also would work better in our cold winters, at the other
end of the temperature issue.
Did you mean 43.0 Deg. C (109.4 Deg. F), instead of 430 Deg. C
(806 Deg. F)?

> From: "Winter, Wim de" <Wim.dewinter@wur.nl>
> To: <ferns@hort.net>
> Bob,
> The greenhouse you're describing uses water-cooling in a way that is
> thermodynamically efficient, but not so from a point of view of water
> management. The mist in the greenhouse is carried away by convection
before it
> has evaporated and so before it has consumed its heat of evaporation.
This is
> a quick way of cooling in excess of water, but having seen Namibian
> showing all (semi-)desert I might venture that a less water consuming
> should be considered. After all, in an 18 cubic meters' enclosure at
> humidity you'd only need 1.7 g of water evaporating per degree
> decrease (18m3, 60g H2O/m3, 600 cal/g evaporation energy). The heat
flux into
> the system is determining the water consumption.
> Assuming the greenhouse is well shaded, the heat flux largely depends
on the
> materials used in the construction. Suppose the goal is to depress the
> environmental temperature of 430C by 10 degrees. How much cooling
capacity is
> needed to maintain this temperature difference?
> The greenhouse measured 3x3x2 m (lxwxh) so the total area would be
> The heatflux at 10K difference is found by
> phi = U.A.(t1-t2) = U x 33 x 10 = 330 U [Watt]
> U is the transmission coefficient which can be found for various
> For single glass, plastic sheets, corrugated plastic and the like it's
> 4.5 to 6.2 W/m2.K. Isolation glass is as low as 1.2W/m2.K
> So, taking an average of 5, the flux would be 1650 W or 1650 J/s.
> Evaporation energy of water is 2.24 kJ/g so you'll need 0.74 ml/s or
2.65 l/h
> Not too bad, only 31 l when you need it 12h/day.
> Assuming you don't open the door and no extra losses through the
> Double glass/plastic should be able to half the amount of cooling
> Rethinking this, the fridge should be constructed outside the
enclosure, e.g.
> the roof. Wouldn't that exclude the roof fromn the total surface area?
> Regards,
> Wim de Winter
> "there's always a theoretical solution"
> By the way... Is cooling realy necessary, or is stopping the
> sufficient? I'm not sure about long term stays at 430, but a short
time at
> least is harmless.
> ---------------------------------------------------------------
> -----Original Message-----
> From:	owner-ferns@hort.net on behalf of Bob Needham
> Sent:	Tue 9/28/2004 4:35 PM
> To:	cfao@iway.na
> Cc:	ferns@hort.net
> Subject:	Re: [ferns] Constructing a greenhouse
> After re-reading your post again more carefully, I have one additional
> If the air temerature is hotter outside than inside, you will need
> at the ridgeline (on a different valve), as then, the airflow will
> i.e. air will come in through the roof vents, be cooled by the ridge
> contract, and sink, flowing out through the ground-level vents.
> Sorry. I teach Science, Physics & Math, but my "paying-attention"
skills are
> low, just ask my wife! ;^>
> -BN
> > You need openable/closeable vents near the ground, and openable
vents right
> > up at the top of the ridgeline. The ridgeline vents should be LARGE.
> them
> > all the way down the ridge on both sides, and easily openable
> or
> > garage-door opener/motorized). They should reach ~25% from the ridge
to the
> > eaves. If motorized, they can be set up on a thermostat to open when
> > temperature passes a certain point, and close again when it cools
off. To
> > help even more, put misters down just inside of the ground-level
> > The evaporation will cool the air, and natural passive convection
will draw
> > the too-warm air out the ridgeline vents. No fans needed...
> >
> >                  ---/\--- <-- Ridge vents
> >                    /  \
> >                   /    \
> >                  /______\
> >                 /|      |\
> >                  |      |
> >                  |  ___ |
> >                  |  | | |
> >                  |  | | |<-- Ground-level vents
> >
> >
> > Support hort.net -- join the hort.net fund drive!
> > http://www.hort.net/funds/
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