Exhaust fans versus 'normal' fans? - Orchid Board

daemondamian · #1 12-31-2008, 11:10 PM

Hi folks,
must an exhaust fan be used for moving air out of a greenhouse?

Why can't a normal fan be used but facing out from the outside of the GH- wouldn't that be drawing in air from inside the GH and blowing it out?

When I say normal fan I mean this - the main round fan blades and front and back grill -which is either on a bracket or stand.

I have looked at all the calculations- trying to get the right cfm but nothing I've found comes close to what it seems I'll need for removing the air volume per minute [see below for my 'GH' dimensions] plus it's measured in litres per minute here in Australia.

I can easily can get a pretty big normal type fan and sit it in the timber frame- fill in around it. Otherwise the exhaust types fans I could get are:

EZI AIR 4 POLE INLINE FAN 300MM 240V 65W 380L/S

SKYWIDE 250MM AXIAL INLINE DUCT FAN 4 POLE 65W
Flow Rate L/Sec: 110 approx

SPECTRUM HGT-30 AXIAL INLINE FAN 300MM BLACK 65W 380L/S

SPECTRUM CUSTOMLINE C200 CENTRIFUGAL FAN 150WATT 220L/S

or

Easy Duct Thermal Transfer System
55 litres per second

Damian.

Extra info:
I'm in the process of setting up a new area for my Pleurothallidinae & Oncidinae alliance plants which is approximately 5 metres long by 2.5 metres wide [actually it's wider at one point and tapers thinner towards the other end] by 2.8 metres high.

On one end wall I'm putting in an evaporative cooler [with the back of it actually exposed outside so as to draw in new air] sitting on the bench and I have a big 3 speed tilt adjustable wall mounted oscillating fan [maybe 40cm?].

On the other end wall I am considering putting in a fan that would draw out hot air in summer and in conjuction with the evaporative cooler keep temperatures lower and humidity higher.

The setup is actually on the east side of my house [I'm in Australia]- house on one side [brick, two big windows] and solid fence on the other side- I've just put up some timber framing, polyflute, clear plastic, shadecloth etc put the area does stay much cooler than my other little shed where I've been growing everything.

Dollgoldii · #2 01-01-2009, 12:23 AM

I have a self made 4' X 8' shadehouse with a 7.5' ceiling. I have 2 X 8" regular fans placed at the ends and they worked pretty good with the four vents I have opened at the bottom part. They move air quite efficiently and the only problem I have is insulation around the fan; I just stuff newspaper into wherever I could around the fan. In a nut shell, for amateurs like myself who have more restrictions than not, I recommend 12" - 15" box fans (easier to get around as far as insultation goes. Another problem you have to solve is preventing water and wind from going into the greenhouse throung the fans. Professional grade exhaust fans usually come with covers or louvres.

Stephen

daemondamian · #3 01-01-2009, 02:30 AM

Quote:

Originally Posted by Dollgoldii

I have a self made 4' X 8' shadehouse with a 7.5' ceiling. I have 2 X 8" regular fans placed at the ends and they worked pretty good with the four vents I have opened at the bottom part. They move air quite efficiently and the only problem I have is insulation around the fan; I just stuff newspaper into wherever I could around the fan. In a nut shell, for amateurs like myself who have more restrictions than not, I recommend 12" - 15" box fans (easier to get around as far as insultation goes. Another problem you have to solve is preventing water and wind from going into the greenhouse throung the fans. Professional grade exhaust fans usually come with covers or louvres.

Stephen

Hi Stephen, thanks for your reply

and great diy professional looking GH BTW!

I'll look into the box fan option- they're available from a few places around here especially as we are [supposedly] in summer.

I'm lucky in that the end where I will have the 'exhaust' type fan is actually adjacent/just under to the covered in backporch- so it's pretty much protected. See these pics.

Regards
Damian.

P.S. just found this:

http://i47.photobucket.com/albums/f1...ruction016.jpg

from:

Ventilation fan - Greenhouses & Garden Structures Forum - GardenWeb

so maybe a good way to go is creating a square of solid material [plywood or something else] with a circle cut out in the middle the same dimensions of the fan so the fan would sit in it just so snugly and it would make it easy to put in the timber frame I've made at the porch end.

dounoharm · #4 01-01-2009, 03:35 PM

i have used a box fan as an exhaust fan in a homemade greenhouse, it worked fine for about 3 years even exposed to the weather...

dounoharm · #5 01-01-2009, 03:40 PM

what is the problem with using an exhaust fan as normally used in greenhouses? it looks like all the options you have identified cost more and require engineering to actually do the right job....i would suggest just going with the normal exhaust fan on a thermostat...much more efficient, and cheaper.

Gin · #6 01-02-2009, 12:37 PM

I used a regular box fan as an exhaust fan for a long time it was hooked to a thermostat, the louvers can be bought separate, all we did was cut out a piece of plywood . There are some fans that have a built in thermostat .
I have a friend that used an attic fan installed vertical with it's own thermostat . I ran across a regular green house fan for cheap, using it now . Gin

cb977 · #7 01-02-2009, 01:01 PM

I have two ceiling fans going at all times out in the orchid room with the doors and some windows open. Works just fine

In the cooler months, I have them set on reverse so it pushes the warm air back down towards the plants while keeping the air moving

daemondamian · #8 01-03-2009, 05:20 PM

Thanks everybody for your all replies and useful information

.

The typical box fans available locally are rounded ones, so I would have to had padded/filled in around them too.

As to buying the normal type of exhaust fan as normally used in greenhouses- this was my first preference but where I live (Australia, in a rural area) but there's not a greenhouse supplier business nearby

.

When I do get around to getting a big proper, freestanding GH

, I will probably combine getting proper exhaust fans meant for greenhouse use when I get all the other stuff I will need- that's if I don't decide to just go with vents and/or turbine ventilator/whirly things.

I have come across some more information about air stratification which got me to thinking about air flow, humidity and venting- some people have vents opening in the roof to release the hot air.

This is a discussion where some one talks about horizontal air flow removing 'good' air with hot air staying up the top- though if you had the exhaust fan up at the highest point you would get more of the hotter air being removed I think:

OrchidSafari ARCHIVES* New Program for OrchidSafari: Greenhouse Basics

This is interesting in that I read about someone who installed a the exhaust fan on the opposite end wall to an evaporative cooler or wet wall which worked well to remove heat but also removed a fair bit of humidity.

In contradiction I've read turbine ventilators do remove humidity and are installed and used to not only remove heat in summer but to remove moisture from under the roof/ceiling or in attics.

I know some one who uses whirly birds/turbine ventilators though they have said height is really important for them to be effective- my height is not that high really?

So I ended up not going with fans for exhausting air on the opposite end wall to the cooler and instead got a 30cm turbine ventilator whirly gig thing. I also found a few more exhaust fans [for bathrooms, etc] and got one [25cm AUS $49.00. that will fit under the whirly, extracting air keeping the turbine spinning.

The exhaust fan is not on a thermostat but I figure I could just turn it off and maybe cover it in winter to stop heat escaping.

I will be placing it in/on on the roof nearer towards the highest point where hot air should rise to.

I don't know how effective it will be but with misting under the benches, the evaporative cooler and trickling water down the shadecloth/matting against the fence I'd be happy if it only reduced temps just a few degrees.
Where my masies, dracs, pleuros, oncids, miltonias etc are at the moment is a mucher small potting shed, which I kept totally closed up last summer but I did set up a hose in there to release a small spray of mist [slightly dripping] and have the evaporate cooler in there [+ fan inside circulating air around].

Despite some days with temps going past 35 degrees celcius I didn't lose or have any plants visibly suffer or have masie leaf drop except for one plant that was a newbie.

I think it was keeping the humidity up around 80-90% [+ shading] that helped

.

My main concern really has been about ondontoglossums and hybrids with the cooler/cold ones like crispum, I have been told and read too that they do better if temps can be kept lower in Summer.

I will post back later on how it works out and put some

of my turbine/exhaust fan up in my Pleurothallidinae and Oncidiinae Grow House photobucket folder .

Damian.