RE: Constructing a greenhouse

Wim:
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)?
-BN


> 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 pictures
> showing all (semi-)desert I might venture that a less water consuming system
> should be considered. After all, in an 18 cubic meters' enclosure at 100%
> humidity you'd only need 1.7 g of water evaporating per degree temperature
> 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 33m2.
> 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 materials.
> For single glass, plastic sheets, corrugated plastic and the like it's about
> 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 construction.
> Double glass/plastic should be able to half the amount of cooling needed.
> 
> 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 evaporation
> 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 idea.
> If the air temerature is hotter outside than inside, you will need misters
> at the ridgeline (on a different valve), as then, the airflow will reverse;
> i.e. air will come in through the roof vents, be cooled by the ridge misters,
> 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. Make
> them
> > all the way down the ridge on both sides, and easily openable (hand-crank
> 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 the
> > 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 vents.
> > 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
> >
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