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Help sizing blower for flow hood with altitude conversion


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#1 panacea

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Posted 08 March 2021 - 01:39 PM

Hey all, I need some help. I've purchased a HEPA filter to make my own flowhood, approximately 18 x 24 x 6". I'm not sure which fan to buy. Problem is, I live at high elevation. According to this, my conversion factor is 1.30, at 70º F.

 

Screen Shot 2021-03-08 at 11.32.07 AM.png

 

Here's the sticker on my filter:

 

IMG_5447.JPG

 

Any tips or ideas? I'm also going to use a pre-filter so I suppose that adds about .1-.3" WG? Am I correct in interpreting that the sticker is saying the filter has a static pressure of 1" WG, so let's say 1.2 with pre-filter, 1.2*1.3 (conversion factor) = 1.56" WG? Unfortunately it's hard to find fan curves online, and most of them don't go this high. Thanks for any help!



#2 TVCasualty

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Posted 08 March 2021 - 02:24 PM

What's that chart supposed to be for?



#3 panacea

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Posted 08 March 2021 - 02:33 PM

What's that chart supposed to be for?

It shows the change in static pressure with the change in air density from temperature and altitude.

 

@sandman

 

If a mod sees this can they add the word FLOWHOOD to the title please!



#4 TVCasualty

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Posted 08 March 2021 - 02:41 PM

 

What's that chart supposed to be for?

It shows the change in static pressure with the change in air density from temperature and altitude.

 

@sandman

 

If a mod sees this can they add the word FLOWHOOD to the title please!

 

 

So how often does the air temp hit 800℉ up there?

 

I don't that chart is the right one for your situation. There are a few "build a flow hood" threads around that probably have everything you need to know.



#5 TVCasualty

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Posted 08 March 2021 - 02:43 PM

 

What's that chart supposed to be for?

It shows the change in static pressure with the change in air density from temperature and altitude.

 

@sandman

 

If a mod sees this can they add the word FLOWHOOD to the title please!

 

 

 

Okay, I can do that. And for future reference I don't think tagging members notifies them that they've been tagged like it works on Twitter.



#6 panacea

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Posted 08 March 2021 - 02:54 PM

 

 

What's that chart supposed to be for?

It shows the change in static pressure with the change in air density from temperature and altitude.

 

@sandman

 

If a mod sees this can they add the word FLOWHOOD to the title please!

 

 

So how often does the air temp hit 800℉ up there?

 

I don't that chart is the right one for your situation. There are a few "build a flow hood" threads around that probably have everything you need to know.

 

 

It doesn't, but at regular temps, the SP is 1.3 times higher for any given setup. I didn't add the bold lines on that chart, if that's what you were thinking. Did you look at my (rough) math? I have researched extensively, and I can't find any blowers with published fan curves that account for resistance quite that high. I'm also unsure of the minimum CFM flow rate I want coming off the face of the filter.



#7 TVCasualty

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Posted 08 March 2021 - 03:05 PM

I'm surprised that altitude is a factor in sizing a blower for a flow hood (assuming it is).

 

Since a blower pressurizes the air an amount that's determined by the static pressure of the filter, I would think that it would act like how a turbocharger on a diesel engine works in the sense that you can drive one from sea level up to a mountain peak without adjusting anything because the blower pressurizes (and heats) the air entering the engine to meet the engine's needs regardless of the altitude.

 

The fact that the chart has temps as high as 800 degrees on it suggests that it's for a similar application and may not be applicable to your project at all. I could be wrong about all this since I've not used a flow hood at high altitude but I get the impression that altitude is not a significant factor at the volume and pressure we're dealing with when building a hood and that you'd be fine using whichever blower meets the specs of the filter.



#8 Myc

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Posted 08 March 2021 - 03:29 PM

I built a flow hood years ago based upon the specs used by most on this website.

I live at altitude.

The complete workstation has been (successfully) in-service for over 10 years - and still doing the job consistently to-date.



#9 sandman

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Posted 09 March 2021 - 10:23 AM

I'm pretty sure that chart is for aircraft since it is a pressurized vessel, and that altitude doesn't effect the static pressure of an open system like a flowhood.

 

I would contact Baker and ask them to be sure https://bakerco.com/services/support/ Just ask if there is any considerations for using their flowhoods in a lab at 7,000 ft or what have you.

 

I'm 99% sure this doesn't apply though.

 

1.0" @ 429 cfm tested S.P.  24x18 = 3 sq ft, x 100 fpm = 300cfm desired.

 

At 300 cfm volume, this filter should be approx. 0.69" S.P.

 

+0.2" for the prefilter.

 

So you need a blower that can do about 300cfm @ 0.9" 


Edited by sandman, 09 March 2021 - 12:39 PM.


#10 Jrotten

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Posted 09 March 2021 - 08:12 PM

You could do all this or just buy a fan with more than enough power, say 20% and use a fan speed controller to dial it in.  This is really the way to go no matter what since over time as a filter gets dirty it's resistance will increase.  I don't think you need to worry at all, but this advice holds for any build.  It's not a necessity, but it is the easiest way to get ideal performance.



#11 sandman

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Posted 09 March 2021 - 08:18 PM

Most squirrel cage blowers are not designed to utilize a speed controller and should not be used with one.

 

Yes, some work. But sometimes the motor wont agree with it and your speed controller will melt or the motor will burn up. The motors werent designed for speed control the way inline weed vent fans are.

 

A better way to dial a blower down is simply strips of duct tape across the input opening. 


Edited by sandman, 09 March 2021 - 08:19 PM.


#12 Jrotten

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Posted 09 March 2021 - 11:55 PM

I used an inline duct fan which is variable. Covering the intake can burn up a fan depending on operating environment because the motor is air cooled and you are reducing flow while increasing resistance. Also uses a smaller footprint. Pros and cons to everything, but I’m very happy with mine for the last 4 years and off the shelf, no loss & washable prefilters are made for direct application. I’m not that versed in squirrel cage motors beyond both of my central units being variable speed, but they are newer.

#13 sandman

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Posted 10 March 2021 - 06:25 AM

For a small filter like this one an inline type fan could work just fine.

 

Inline type fans are so different than squirrel cage blowers that a squirrel cage blower with no static pressure will burn up very quickly because they are designed to operate at load at a certain speed and without that load they overspin. Inline type fans have no such requirement. So dialing a squirrel cage blower by adding to the static pressure is no problem. Inline fans are utterly worthless in the face of static pressure.

 

Inline type fans use DC motors, squirrel cage blowers use AC induction motors. It's all a little complicated to explain but they require a certain voltage and frequency and when you adjust the voltage things get really weird on them.

 

 

 

  1. Changing Applied Voltage: This method, even though easiest, it is rarely used. The reasons are (a) for a small change in speed, there must be a large variation in voltage. (b) This large change in voltage will result in large change in flux density, thereby seriously disturbing the magnetic distribution/condition of the motor.

 

Now I used to use a speed controller for my old flowhood once upon a time. No problem at all. Everything was grand.

 

Then I built a new hood with a new slightly larger blower but still way under the rated 15 amps of the controller.

 

I was running the hood for a few hours testing it out. Went to turn it off and the controller looked funny.

 

It was half melted. Turned it off. Touched the motor of the blower. It was around 350 degrees...

 

Researched why this is happening, turns out you cant speed control most of these motors with a simple tool speed controller.

 

I assume the blower that worked fine with the controller only needed a small adjustment in speed but the bigger one needed  a little more adjustment that exasperated the problem.

 

So then I made a post apologizing for advocating speed controllers and telling bout it.

 

Anyone can google all about this. It even says specifically NOT FOR SPEED CONTROL in plain English on most of the dayton blowers from grainger.

 

SO DONT PUT A CHEAP TOOL SPEED CONTROLLER ON YOUR SQUIRREL CAGE BLOWERS! YOU COULD BURN YOUR HOUSE DOWN.

 

Please if you could refrain from telling people to add a speed controller to their flowhoods now that you also know.

 

I think I was the first one to advocate for the router speed controllers many many years ago, after noticing that they looked identical to the more expensive "fan speed controllers" and they indeed are the same things.  They just aren't good to use for squirrel cage blowers.


Edited by sandman, 10 March 2021 - 06:39 AM.

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#14 Jrotten

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Posted 10 March 2021 - 11:55 AM

That's good to know.  I know you also don't like 12" filters, but that's what I used, and I get what you're saying about the squirell cage blowers, in my limited knowledge of HVAC, they are meant to feed large duct networks and be filtered. I've seen intake size issues but I think that's more about air moved over a coil than the fan operation.

 

Ultimately it was cheapest and easiest for me to buy the 12" Alpha 2000 on ebay and the 8" duct fan on amazon.  My new 1micron no loss prefilter actually cost more than my HEPA which was needed after I had aerosolized oils to contend with increasing my HEPA  resistance.  It's a nice setup in that my fan can run for hours in 95F temperatures and nothing overheats and the prefilter does the majority of the work since it's all located in a tiny 4x8x8 room/closet meaning HEPA in large part acts to clean the whole room.  I can take the whole PC or things out of the PC, leave them in the hood and forget them overnight  and the next day without worrying about abusing my fan or filter.


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#15 Myc

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Posted 10 March 2021 - 11:58 AM

Re: Variable speed controllers and squirrel cage blowers

 

You'll need to purchase a shaded-pole blower motor. These can be controlled (within limits) with variable speed controller.

 

The only infinitely variable option is to use a VFD - Variable Frequency Drive and a motor rated for the application. This equipment is vastly expensive and probably not an option for use at home.


Edited by Myc, 10 March 2021 - 12:01 PM.

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#16 panacea

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Posted 11 March 2021 - 03:01 PM

I'm surprised that altitude is a factor in sizing a blower for a flow hood (assuming it is).

 

Since a blower pressurizes the air an amount that's determined by the static pressure of the filter, I would think that it would act like how a turbocharger on a diesel engine works in the sense that you can drive one from sea level up to a mountain peak without adjusting anything because the blower pressurizes (and heats) the air entering the engine to meet the engine's needs regardless of the altitude.

 

The fact that the chart has temps as high as 800 degrees on it suggests that it's for a similar application and may not be applicable to your project at all. I could be wrong about all this since I've not used a flow hood at high altitude but I get the impression that altitude is not a significant factor at the volume and pressure we're dealing with when building a hood and that you'd be fine using whichever blower meets the specs of the filter.

It's absolutely a factor. Airplanes taking off from "hot and high" airports face restrictions in terms of maximum takeoff weight and runway length, because as both elevation and temperature rise, air density decreases (it's called density altitude), and the turbofan engines, which are just jet-powered fans, can't produce as much thrust as they could at sea level and/or in colder temperatures. Or consider the example of a turbocharged vehicle. The turbocharger can only compress the air a given amount. If the engine performed just as well at altitude as it did at sea level, that would mean the turbocharger was creating more boost with thinner ambient air, which doesn't make any sense. This is basically what's happening with a flow hood, creating back pressure in the plenum to overcome static pressure and produce laminar flow.

 

The chart is from an industrial fan supply company, and many of those values could well be theoretical, although I can imagine applications where a couple hundred ºF come into play. Nevertheless, the reasoning stands and at my altitude, a fan will have to work harder to push a given CFM of lower-density air, effectively raising the SP of the system.

 

I built a flow hood years ago based upon the specs used by most on this website.

I live at altitude.

The complete workstation has been (successfully) in-service for over 10 years - and still doing the job consistently to-date.

Good to hear you've had success! When you say "the specs used by most on this website", what are you referring to? I'm aware of some general guidelines, but this is a sizeable investment for me so I'm wanting to calculate things well and do it right the first time. Especially since apparently variable speed controllers can't simply be wired to any old fan, although that would be nice.

 

I've heard the Dayton 1TDT8 might be a good option? But still I want to see a fan curve graph for it.

 

I'm pretty sure that chart is for aircraft since it is a pressurized vessel, and that altitude doesn't effect the static pressure of an open system like a flowhood.

 

I would contact Baker and ask them to be sure https://bakerco.com/services/support/ Just ask if there is any considerations for using their flowhoods in a lab at 7,000 ft or what have you.

 

I'm 99% sure this doesn't apply though.

 

1.0" @ 429 cfm tested S.P.  24x18 = 3 sq ft, x 100 fpm = 300cfm desired.

 

At 300 cfm volume, this filter should be approx. 0.69" S.P.

 

+0.2" for the prefilter.

 

So you need a blower that can do about 300cfm @ 0.9" 

Only the cabin of aircraft are pressurized, and that is achieved with bleed air from the engine turbines themselves, not additional fans. Also the "equivalent effective cabin altitude" is generally about 6,000 ft and doesn't vary, so this chart wouldn't apply or have any use.

 

Good idea on contacting Baker, thanks, I'll definitely do that.

Thanks for dropping by and chiming in on this thread, too.

 

I'm really just looking for recommendations of specific fans and where to buy them. I am really excited to get started, although a bit apprehensive about wiring it up.
 



#17 sandman

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Posted 11 March 2021 - 03:58 PM

electricmotorwarehouse.com, 1tdt2 looks like it should work fine assuming the altitude stuff is negligible.

https://www.electric...specs/ITDT2.pdf

 

Do let us know what baker says about the altitude. I'd like to add this info to my repertoire. Never heard the concern before.



#18 TVCasualty

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Posted 16 March 2021 - 11:27 AM

Or consider the example of a turbocharged vehicle. The turbocharger can only compress the air a given amount. If the engine performed just as well at altitude as it did at sea level, that would mean the turbocharger was creating more boost with thinner ambient air, which doesn't make any sense.



Since I've driven a turbocharged diesel from sea level to ~7000 ft in the course of a day and had no noticeable change in performance it seems to me that turbocharging works like it does because the engine acts as a HEPA filter in the sense that it creates back pressure, and so long as the atmosphere is not so thin that the blower or turbocharger can't push more air than the engine consumes or the HEPA filter allows to pass (which is where the pressure comes from) then it's performance should be just as good as if it were operating at sea level.

Airplane propellers don't push against any back pressure, so the comparison with blowers is not valid. On the other hand, jet engines are basically huge turbochargers so they work fine even at 60,000 ft. (and higher) but in thin or low-density air the limiting factor for airplane performance is the reduction in lift as air density decreases. So you can still fly commercial jets out of Denver when it's 110 degrees F, but they'll require significantly longer runways than airports at lower altitudes/higher air densities.

I'm guessing that there are probably some handy and precise technical terms for what I'm trying to describe but I don't know what they are.


Edited by TVCasualty, 16 March 2021 - 11:28 AM.


#19 panacea

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Posted 22 March 2021 - 12:08 PM

 

Or consider the example of a turbocharged vehicle. The turbocharger can only compress the air a given amount. If the engine performed just as well at altitude as it did at sea level, that would mean the turbocharger was creating more boost with thinner ambient air, which doesn't make any sense.


Since I've driven a turbocharged diesel from sea level to ~7000 ft in the course of a day and had no noticeable change in performance it seems to me that turbocharging works like it does because the engine acts as a HEPA filter in the sense that it creates back pressure, and so long as the atmosphere is not so thin that the blower or turbocharger can't push more air than the engine consumes or the HEPA filter allows to pass (which is where the pressure comes from) then it's performance should be just as good as if it were operating at sea level.

Airplane propellers don't push against any back pressure, so the comparison with blowers is not valid. On the other hand, jet engines are basically huge turbochargers so they work fine even at 60,000 ft. (and higher) but in thin or low-density air the limiting factor for airplane performance is the reduction in lift as air density decreases. So you can still fly commercial jets out of Denver when it's 110 degrees F, but they'll require significantly longer runways than airports at lower altitudes/higher air densities.

I'm guessing that there are probably some handy and precise technical terms for what I'm trying to describe but I don't know what they are.

 

Could be! Check out page 5 here: https://www.cincinna...3-internet.pdf

I'll probably call these folks and ask a couple questions soon.
The thing with the turbo analogy is that it doesn't just require the turbo to push more air than the engine consumes (which would be just a slight positive pressure), it needs to push several times more than that. If an engine is configured to run with 10 PSI of boost for full power at sea level, and that's the maximum that particular turbine can provide, then it won't be able to match that at a high elevation with lower density air, regardless of backpressure.
 
I wasn't comparing props to blowers, so not sure what you mean there. Good point about jet engines, although high altitude performance is definitely diminished (they will flame out at a high enough altitude), it just works at cruising altitude because air resistance up there is also so much lower.
 

I'm pretty sure that chart is for aircraft since it is a pressurized vessel, and that altitude doesn't effect the static pressure of an open system like a flowhood.
 
I would contact Baker and ask them to be sure https://bakerco.com/services/support/ Just ask if there is any considerations for using their flowhoods in a lab at 7,000 ft or what have you.
 
I'm 99% sure this doesn't apply though.
 
1.0" @ 429 cfm tested S.P.  24x18 = 3 sq ft, x 100 fpm = 300cfm desired.
 
At 300 cfm volume, this filter should be approx. 0.69" S.P.
 
+0.2" for the prefilter.
 
So you need a blower that can do about 300cfm @ 0.9"

I called Baker today per your suggestion, the guy is going to try to email me back with some more technical information after he asks around.
 
Just curious, how did you arrive at the .69", just divide 300/429? Is that all there is to it?
 
 
https://www.electric...specs/ITDU2.pdf
https://www.electric...1TDT8_specs.pdf
I found this place selling two promising blowers (WITH curves!), but one looks to be far too powerful and one is right on the line. [.69" SP + .2" pre filter]*1.3 elevation conversion = 1.169" SP which looks to be just beyond what the 2nd one can push on HI @ 300cfm.
 
Getting closer, thanks everyone!


Edited by panacea, 22 March 2021 - 01:08 PM.


#20 sandman

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Posted 23 March 2021 - 05:58 AM

Yea since the static pressure is relative to the face velocity, and the test pressure happened to be 1" you can just divide the desired speed by the test speed. 1/429 = 0.69/300. 






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