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mattie

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Reply with quote  #1 
Hello all
I was looking at the Missouri U fans on the build list and they specified a 3200 cfm framed ventilation fan. List here
http://aes.missouri.edu/bradford/education/solar-greenhouse/construction/materials.php

Greenhouse ventilation is important i have been looking at heat recovery ventilation units online as a time saver over building a unit or routing intake air underground.Efficiency is vital here for the greenhouse energy use to remain low.Energy recovery ventilation is a possibility.It is critical for to balance ventilation rates so that any energy inside the greenhouse is not lost to air extraction
The flow rates here seem crazy or not so bad?

Ive used a converter here hence the lists of units.

3200 cubic_feet_per_minute =>
1.51 cubic_meters_per_second
90.6 cubic_meters_per_minute
5437 cubic_meters_per_hour
1510 liters_per_second
90613 liters_per_minute
5436783 liters_per_hour
34432960 us_gallons_per_day
23940 us_gallons_per_minute
3200 cubic_feet_per_minute
28694134 imp_gallons_per_day
19935 imp_gallons_per_minute

The framed motorized shutter has a flow rate of (why flow rate needed here its a shutter? It must remain closed until fan when on pushes the shutters open?)

@ 3000 cfm conversions are.
3000 cubic_feet_per_minute =>
1.42 cubic_meters_per_second
84.9 cubic_meters_per_minute
5097 cubic_meters_per_hour
1416 liters_per_second
84950 liters_per_minute
5096984 liters_per_hour
32280900 us_gallons_per_day
22444 us_gallons_per_minute
3000 cubic_feet_per_minute
26900750 imp_gallons_per_day
18689 imp_gallons_per_minute

Also for the inflation fan which is 60 cfm
60 cubic_feet_per_minute =>
0.0283 cubic_meters_per_second
1.7 cubic_meters_per_minute
102 cubic_meters_per_hour
28.3 liters_per_second
1699 liters_per_minute
101940 liters_per_hour
645618 us_gallons_per_day
449 us_gallons_per_minute
60 cubic_feet_per_minute
538015 imp_gallons_per_day
374 imp_gallons_per_minute

Are there any cost effective means of implementing this 60 cfm inflation fan, along with the 3200 cfm and 3000 cfm ones?

Regards Mattie


mattie

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Reply with quote  #2 
Hello all
Heres a greenhouse exhaust fan and CFM calculator for anyone interested its on the same webpage as the heat load calculator Ive posted before.
http://www.littlegreenhouse.com/fan-calc.shtml

@ 24 x 12 x 12 which are the dimensions for the Misourri U it works out higher @ 4147 cfm, then again the calculator is probably based on a more common greenhouse shape.
3200/4147 x 100 =77% so a reduction of .77

Looking at 32x16x16 ft greenhouse footprint initially ,which works out at 9830 cfm here,so 9830 x.77 = 7569 CFM for a ball park figure on what may be needed for this size of greenhouse.It seems rational here that the HEX should be used on the outake and a HRV to follow for intake air,or to put another way at the same position in the greenhouse i.e at the peak.(where the exhast fan is positioned on the calculator diagram)

The HEX may pull most of the energy from the output air ,so an electrical preheat type system on HRV @ the intake air may be running a excessively.An option here as done with passivhaus and as mentioned by G_H to me is to run intake underground and use the constant year round temperature of 50 to 55 f or around 10 c in the ground to allow the intake air to gain energy ,which should drop electrical use on the air preheater in the HRV or before the HRV there is the option too to run something like this
http://www.ultimateair.com/products/accessories/warmflo-defrost-system/

Allthough i prefer to make use of the free in ground energy when ambient temperatures above ground are low, that is of course if the air can pick up enough energy over a short enough distance and I dont need large runs lots of digging high costs of materials for piping etc with large fans and watt requirements to do so.

Regards Mattie

mattie

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Reply with quote  #3 
Back again heres some info on running air underground from the website that keeps on giving,thanks again Gary.http://www.builditsolar.com/Projects/Cooling/EarthtubeNotes.htm


Garage_Hermit

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Reply with quote  #4 
Hi, Mattie,

I will respond to both of the above in one go !

first the Greenhouse CFM calculator !

Thanks for the link, very nice, I tried it for my CRAWLSPACE (!) (which has somewhat less GLASS... [rolleyes]) and it came up with 1200 CFM, which is really interesting !

I entered the dimensions: 40 x 10 x 2.5 ft, so was wondering where the difference arises... since this on the ground I have 1000 cubic feet only.

So guess I am about OK for my whole house fan (WHF), which is somewhere between 1600 and 2000 CFM (= 12-inch automobile radiator fan)

============

Regarding your question about the GH fans from Uv of Mo, these do not strike me as being strange or over the top: I guess the basic question is how to manage one air-change, for which the simple dimensions of the GH will be the starting point:...

I took a link at the UoM GH... here's the link again, for comfort...
http://aes.missouri.edu/bradford/education/solar-greenhouse/solar-greenhouse.php
quote:
Fans are rated on their CFM (or cubic feet per minute of air flow) and as a generally rule you need one CFM for each ft3 of greenhouse space: -ft2 x peak height or 24 ft x 12 ft x 12 ft=3456 cfm

I don't know if there's a typo in there, and I can't be fagged (!) to do the math, but according to the quote, this is not the way to calculate the volume of a pitched roof !  They need to calculate the volume of the lower cuboid, then that of the pitched section, and sum the 2,  and the volume of a triangular pitch is l x w x HALF the height !  Maybe they DID calculate the proper way, and that's how they get their 3456 cubic foot...  hope so ! (never mind, its just me being Monday...)[rolleyes] (I have the same shape roof on my place...)
===============

Regarding the sizing of the fan, 3200 CFM does not strike me as being exceptional - it is roughly one air-change in an hour.   I guess in one hour, you have enough time to extract the useful heat from that air, and give the plants time to extract their dose of C02...  If more air-changes were necessary, you use the vGH volume as a starting point again: 3400 x 2 = 6800 CFM etc.  So for 2 airchanges, add a second fan of the same size etc.

(FWIW, ages ago I posted a list of air-changes for various types of building, from butcher shops to hospitals, in the General Design section).  Off-hand, I don't know if GH were included...  I imagine in an industrial GH, with personnel working, it could be a criterion, as folx would be breathing humid air and mebbe suffering from heat at the same time, so I could imagine this having H&S repercussions...) (another point for U2 investigate mebbe !)
=========================

Regarding your point about the shutter, to me it is obvious that the shutter has a flow rate, as when it is open, air will flow through it ? So How Much Air ?  Answer: 3000 CFM - the shutter would therefore vent almost all of the fan's output, in near real time... sounds like normal to me...

("Shutter" is OK, but it only describes one part of what it does namely, shuts off the exhaust air flow. The other part is it OPENS it UP ! that's why I'd prefer EXHAUST  SHUTTER but that's just me...).

Like, not to  hijack the thread, I will be pulling house air into my Crawlspace, using a whole house fan, but I will also need an exhaust shutter in the CS, to direct the said air outside to waste, otherwise no air would flow in the first place etc...) (Exhaust as in "exhausted" after work has been done...) = discharge = unload = offload = DUMP
==========

I guess the INFLATION part of the question is your project and not UoMo with their rigid GH...

I assume you are referring to a fan to inflate your pneumatic tubes & keep your PE glazed sheeting taut.

Have you got a proposal to post by any chance, so we can have an eye-ball on it

Otherwise, 60 CFM (to me...) sounds suspiciously like a PC case fan !
Mebbe I am wrong - I hope so - because I would not consider this as being a good candidate for pushing compressed air to inflate tubes - they are designed to move FREE air, for *ventilation*...

I would imagine something more like a car-tire inflation pump, or a RHIB (rigid-hull inflatable boat) pump, and I would not be thinking in terms of CFM at all, but in terms of PSI, but that is just me again...

Hope this answers a few of your points, good luck with your design as always !

Cheers,
G_H

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(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...
mattie

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Reply with quote  #5 
Hello Garage Hermit
Thanks for all the info and insight.
The inflation part comes from the Missouri U design.
"The plastic covering is a double layer of 6 mil plastic. A 60 cfm squirrel cage fan pushes in outside air into the double layer. This extra insulation created by the 4 inch air gap adds about 10 degrees to the inside air temperature on a cold day"
I have a comparison U value calculation done on plastic with air vs polycarbonate sheeting which i will post later.

Regards Mattie

kcl1s

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Reply with quote  #6 
Hi All,
The high air exchange rates are needed to keep the gh temps at a reasonable level. Our one acre gh has 24 one hp 48" fans. They will all run on a 45 deg sunny day and the inside temps will be about 80 deg.

I could not open your spreadsheet on this computer but most sources (including the littlegreenhouse site) figure twinwall and double poly to have the same U value.

Keith
Garage_Hermit

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Reply with quote  #7 
I managed to open the XLX file without problems (and I don't even have an excel newer than 97...)
============

http://forums2.gardenweb.com/forums/load/strucs/msg081627251090.html

I checked several references (incl. the above) for comparative GH glazings, and they all seem to churn out more-or less the same numbers for PE sheet vs polycarbonate.

Some do mention the *fact of* having an airspace between two layers of PE - one even mentions that it is *warm* air that is blown in between; another says that certain PE sheeting has better IR characteristics, than others, and therefore holds the heat in, better...

Yet none of the sites I checked actually mention the insulant properties of the enclosed AIR GAP, so I think that Mattie's analysis is very interesting...

as supported by the following articule... http://sustainabilityworkshop.autodesk.com/buildings/total-r-values-and-thermal-bridging

salient points:

  • using PE film to create an.d AIR GAP also introduces air FILMS, which are insulant in their own right;
  • beyond one inch thick, the utility of an air space declines, whence the advantage (for THICK air spaces...) of dividing, by increasing the number of PE sheets, to create additional barriers to heat;
  • use extra insulation at frames, to prevent thermal bridging ("anti-frame effect")
r has a high resistance to heat conduction, but it has almost no resistance to heat radiation, and little resistance to heat convection outside of the thin air film touching surfaces. When conduction, convection, and radiation all occur at the same time, the overall thermal resistance of air spaces becomes virtually independent of gap width when it is greater than around 1" (2.5 cm).  - See more at: http://sustainabilityworkshop.autodesk.com/buildings/total-r-values-and-thermal-bridging#sthash.7rE1JWii.dpuf
Air has a high resistance to heat conduction, but it has almost no resistance to heat radiation, and little resistance to heat convection outside of the thin air film touching surfaces. When conduction, convection, and radiation all occur at the same time, the overall thermal resistance of air spaces becomes virtually independent of gap width when it is greater than around 1" (2.5 cm).  - See more at: http://sustainabilityworkshop.autodesk.com/buildings/total-r-values-and-thermal-bridging#sthash.7rE1JWii.dpuf

G_H

__________________
(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...
Garage_Hermit

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Reply with quote  #8 
New thread sounds like a good idea, I'll wait for your lead !

G_H

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(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...
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