goldenchild
Well-known member
- Joined
- Aug 14, 2009
- Messages
- 1,810
IntelGold said:
What does this equate to in cfm?
Fume Hood, Will it work?
- Thread starter IntelGold
- Start date
Help Support Gold Refining Forum:
What does this equate to in cfm?
A
Anonymous
Guest
I don't make a big song and dance about it but my degree studies were in Aeronautical engineering which involved an awful lot of flow/fluid/gas dynamics.
You're trying to extract air from a complete fume cupboard here with a large volume being processed not only through pipes, but also through a severely denser medium. Whether you use solid state scrubbing or liquid scrubbing the challenge is the same i.e. you have to have (at least) the same volume of air exiting the process as entering it. Otherwise it's pointless.
The pressure differences required to take gases from one transport medium to another of higher density are exponentially increasing the denser the secondary medium is. That's actually not very easy to do cheaply when you're talking about the volumes discussed here.
You're trying to extract air from a complete fume cupboard here with a large volume being processed not only through pipes, but also through a severely denser medium. Whether you use solid state scrubbing or liquid scrubbing the challenge is the same i.e. you have to have (at least) the same volume of air exiting the process as entering it. Otherwise it's pointless.
The pressure differences required to take gases from one transport medium to another of higher density are exponentially increasing the denser the secondary medium is. That's actually not very easy to do cheaply when you're talking about the volumes discussed here.
goldenchild
Well-known member
- Joined
- Aug 14, 2009
- Messages
- 1,810
spaceships,
Would you say that it would be equally as challenging pushing the fumes through than pulling them through a system?
Would you say that it would be equally as challenging pushing the fumes through than pulling them through a system?
A
Anonymous
Guest
That's a very good question Golden, and I think it would have to be experimented with because it really does depend upon the media that will be used as filtration/scrubbing.
Taking a logical approach though. If you're trying to push it through and the power isn't there to do so then you will just create a back pressure that will cause big problems in the fume hood. All that will be achieved is blasting the contaminated around the hood and possibly the room.
Pulling it through would at least keep a negative pressure in the fume hood. The question would be how much of a negative pressure, and would this be enough to remove all the required volume ?
Taking a logical approach though. If you're trying to push it through and the power isn't there to do so then you will just create a back pressure that will cause big problems in the fume hood. All that will be achieved is blasting the contaminated around the hood and possibly the room.
Pulling it through would at least keep a negative pressure in the fume hood. The question would be how much of a negative pressure, and would this be enough to remove all the required volume ?
It would not be much difference if you tried to push or pull the air, a restriction will slow down progress or CFM through the system.
It does not matter how big the fan is, through a restriction you can only move so much volume of air, unless the fan can overcome the pressure or vacuum of the restriction in the system,(fans and blowers work with very low pressures), with fans (not compressors or vacuum pumps desighned to work with higher pressures, or under vacuum), fans or blowers will not easily work against pressure. or a vacuum, the CFM of the system can be limited by the restriction of the system, adding a larger CFM fan will not overcome this problem.
When moving air through a system the CFM through the system is not only sized by the fan, but also by the duct work taking restrictions (sizes of duct work and restrictions in the system) into consideration.
It does not matter how big the fan is, through a restriction you can only move so much volume of air, unless the fan can overcome the pressure or vacuum of the restriction in the system,(fans and blowers work with very low pressures), with fans (not compressors or vacuum pumps desighned to work with higher pressures, or under vacuum), fans or blowers will not easily work against pressure. or a vacuum, the CFM of the system can be limited by the restriction of the system, adding a larger CFM fan will not overcome this problem.
When moving air through a system the CFM through the system is not only sized by the fan, but also by the duct work taking restrictions (sizes of duct work and restrictions in the system) into consideration.
A
Anonymous
Guest
As Butcher said 100%.
My wording was not as good though.
My wording was not as good though.
Any system trying to push fumes through the scrubber better be leak free, any leak and fumes are pushed out into the room instead of through the scrubber.
If the system pulls the fumes through the scrubber then a leak is only going to pull some additional clean air through the system.
There is another thing to take into consideration too, if the pressure is positive inside the scrubber it will mean a force pushing outwards on the walls. A drum can easily handle a positive pressure. If the pressure is negative a drum could needs some sort of reinforcement as a drum easily collapses.
Göran
If the system pulls the fumes through the scrubber then a leak is only going to pull some additional clean air through the system.
There is another thing to take into consideration too, if the pressure is positive inside the scrubber it will mean a force pushing outwards on the walls. A drum can easily handle a positive pressure. If the pressure is negative a drum could needs some sort of reinforcement as a drum easily collapses.
Göran
http://www.machinemart.co.uk/images/library/product/large/06/060610035.jpg?2
I have gone ahead and bought this wood dust extractor. Its rated at 765m/3 per hr. It makes one hell of a noise. Its stove enamelled, so my next question - Will it still need bed liner? Thankyou.
I have gone ahead and bought this wood dust extractor. Its rated at 765m/3 per hr. It makes one hell of a noise. Its stove enamelled, so my next question - Will it still need bed liner? Thankyou.
goldenchild
Well-known member
- Joined
- Aug 14, 2009
- Messages
- 1,810
Wow! Until you posted that link I forgot that I had purchased one of those. I bought it and still haven't gotten around to using though. Mine is actually very quite however. Surprising since its nearly double the cfm of the one currently in use. I would say even quieter. I believe the internals are plastic as well which is a bonus. Whenever the one I'm using craps out I will be replacing it with the one in the link below. I plan on bed lining it also. Since the squirrel cage is the part that goes it should last who knows how long.
http://www.harborfreight.com/1-hp-mini-dust-collector-94029.html
http://www.harborfreight.com/1-hp-mini-dust-collector-94029.html
A
Anonymous
Guest
If I may be so bold to the OP. I think that to try and scrub both the fumes from your fume hood along with the fumes from your processing in one system is a little ambitious, and unnecessary.
You'd be better served splitting the two.
You'd be better served splitting the two.
Can someone please tell me if its better to use 2 fans inline of each other or split them into 2 separate pipes? I guess what I am really asking, is it better to have double air flow or double pressure for a fume hood. Thanks. 
Two fans inline would not work.
You can use two fans with two ducts.
Remember duct sizing (CFM) as well as fan sizing (CFM), or any openings or restrictions in the system, determine how much CFM (cubic feet per minute of air flow) through the system, not only the sizing of the duct work, but also restrictions like turns or elbows in the duct can also limit the CFM the system can handle, turns or elbows as well as the duct-works shape (like square duct-work with a sharp "L" turn or bend), can cause restrictions to the flow of air through the system.
You have to have as much air coming in as going out, the building (the fume hood is in) would need a fresh air supply from the outside, a closed room cannot exhaust air if no air is coming into the room, the fume hood opening size large or small (full open or partially closed) will determine how much air can pass through it (size CFM), the fan needs sized for the fume hood hood opening (full open door), the duct-work also needs sized large enough for all of the air to flow through it taking restrictions of the duct into consideration (size of duct, turns elbows...)
Try sucking air out of a coke bottle, no air coming in, you cannot suck air out.
Try blowing air into the coke bottle, no air leaving the bottle, you cannot blow air in.
Air, water, or other medium, trying to be forced through a small orifice builds a back pressure on the hose or duct system, as you can only push so much through that small hole. the same with pulling the medium, an orifice, or restriction would crate a vacuum as you can still only get so much through that hole over a given time {CFM or some other measurement of volume and time like GPM (gallons per minute)}, the larger the orifice the easier it is for the medium to flow through it.
An over sized duct or orifice will not cause a restriction.
An over sized fan could only push or pull as much air as the duct, openings or restrictions allow, which could set the fan motor up for problems like overheating of the motors windings...
(kind of of hard for me to explain).
You can use two fans with two ducts.
Remember duct sizing (CFM) as well as fan sizing (CFM), or any openings or restrictions in the system, determine how much CFM (cubic feet per minute of air flow) through the system, not only the sizing of the duct work, but also restrictions like turns or elbows in the duct can also limit the CFM the system can handle, turns or elbows as well as the duct-works shape (like square duct-work with a sharp "L" turn or bend), can cause restrictions to the flow of air through the system.
You have to have as much air coming in as going out, the building (the fume hood is in) would need a fresh air supply from the outside, a closed room cannot exhaust air if no air is coming into the room, the fume hood opening size large or small (full open or partially closed) will determine how much air can pass through it (size CFM), the fan needs sized for the fume hood hood opening (full open door), the duct-work also needs sized large enough for all of the air to flow through it taking restrictions of the duct into consideration (size of duct, turns elbows...)
Try sucking air out of a coke bottle, no air coming in, you cannot suck air out.
Try blowing air into the coke bottle, no air leaving the bottle, you cannot blow air in.
Air, water, or other medium, trying to be forced through a small orifice builds a back pressure on the hose or duct system, as you can only push so much through that small hole. the same with pulling the medium, an orifice, or restriction would crate a vacuum as you can still only get so much through that hole over a given time {CFM or some other measurement of volume and time like GPM (gallons per minute)}, the larger the orifice the easier it is for the medium to flow through it.
An over sized duct or orifice will not cause a restriction.
An over sized fan could only push or pull as much air as the duct, openings or restrictions allow, which could set the fan motor up for problems like overheating of the motors windings...
(kind of of hard for me to explain).
Thanks Butcher, Very nicely explained. "Try sucking air out of a coke bottle, no air coming in, you cannot suck air out.
Try blowing air into the coke bottle, no air leaving the bottle, you cannot blow air in." Really like this Example.
I have drawn up so many pics now its unreal. I have posted a pic below of the rough idea. If you ask why have I gone for 2 fans, its because they are both sucking the same amount of air x 2 because they can be adjusted so they are at exactly the same speed. The speed controller can control both at the same time. The fans also have twin speeds. If 1 fails I have the other to back it up, maybe not 100% but anything would do in a emergency. The red lines I have drawn are the shed walls. Its only a small hood, measures 40" x 24". All piping is 6" (150mm). I was going to have 5 foot of pipe with packing material in it. I now realize that instead I should use a large container which has been strengthened. The pipe would go into the water and bubble up which slightly scrubs the gases. With packing I am hoping to scrub most of the fumes before they hit the fans. I have also drawn a green square round the 2 pipes nearest the floor, it has red stripes going through it. This would be a large bucket/container filled 3/4 of the way up. The 2 pipes would then be inserted into the scrubbing media.
I really think I should do another drawing as i maybe confusing people. Sorry.
A water pump and Spray Bar would help also. Please comment. Any comments a good comment.
Thanks.
![IMG_0385[1].JPG](https://cdn.imagearchive.com/goldrefiningforum/data/attachments/17/17430-e26c6d7ea6c64b62f41564305898d8bd.jpg "IMG_0385[1].JPG")
Try blowing air into the coke bottle, no air leaving the bottle, you cannot blow air in." Really like this Example.
I have drawn up so many pics now its unreal. I have posted a pic below of the rough idea. If you ask why have I gone for 2 fans, its because they are both sucking the same amount of air x 2 because they can be adjusted so they are at exactly the same speed. The speed controller can control both at the same time. The fans also have twin speeds. If 1 fails I have the other to back it up, maybe not 100% but anything would do in a emergency. The red lines I have drawn are the shed walls. Its only a small hood, measures 40" x 24". All piping is 6" (150mm). I was going to have 5 foot of pipe with packing material in it. I now realize that instead I should use a large container which has been strengthened. The pipe would go into the water and bubble up which slightly scrubs the gases. With packing I am hoping to scrub most of the fumes before they hit the fans. I have also drawn a green square round the 2 pipes nearest the floor, it has red stripes going through it. This would be a large bucket/container filled 3/4 of the way up. The 2 pipes would then be inserted into the scrubbing media.
I really think I should do another drawing as i maybe confusing people. Sorry.
A water pump and Spray Bar would help also. Please comment. Any comments a good comment.
Thanks.
Forgot to say they have Thermally protected motors so over heating should not be an issue. Each fan would produce 530 m3/hr. I am not sure whether this would equate to 1060 m3/hr. I'm sure someone on this site will know. I like the fact that the fans can be turned up or down dependent on the Scenario. They might pull the fumes through nicely on 3/4 speed controller setting.
I also think that the first fan which is pushing air is not needed and will just cause complications in the flow of air.
Lots of thoughts going through my head at the moment. Sorry if I'm babbling.
I also think that the first fan which is pushing air is not needed and will just cause complications in the flow of air.
Lots of thoughts going through my head at the moment. Sorry if I'm babbling.
I may not understand the scrubber you are talking about, but getting that volume of air through a bucket of water would put a kink in the system.
The air should flow fairly free through the scrubber.
Again you can use a simpler small scrubber that just scrubs fumes from the reaction vessels, without scrubbing all of the fume hoods air.
If you feel you must scrub all of the air from the fume hood, the scrubber will have to be extremely large the air would not bubble through water, but flow through the scrubber easily (it may need baffles or a medium to slow it down some having to make twists and turns like flowing around some type of medium, but basically not restricted in flow, a scrubbing solution can be misted into the scrubber where the gases pick up moisture and the gases can convert to acids or are neutralized, think really tall and large scrubbers something all of your neighbors will see towering over your roof top.
Spend some more time studying scrubbers and fume hoods, it looks to me like you are working from the wrong end of the hammer, you are making all of the plans of how you will build the house, but have not yet understood how the house needs to be built.
The air should flow fairly free through the scrubber.
Again you can use a simpler small scrubber that just scrubs fumes from the reaction vessels, without scrubbing all of the fume hoods air.
If you feel you must scrub all of the air from the fume hood, the scrubber will have to be extremely large the air would not bubble through water, but flow through the scrubber easily (it may need baffles or a medium to slow it down some having to make twists and turns like flowing around some type of medium, but basically not restricted in flow, a scrubbing solution can be misted into the scrubber where the gases pick up moisture and the gases can convert to acids or are neutralized, think really tall and large scrubbers something all of your neighbors will see towering over your roof top.
Spend some more time studying scrubbers and fume hoods, it looks to me like you are working from the wrong end of the hammer, you are making all of the plans of how you will build the house, but have not yet understood how the house needs to be built.
This spreadsheet may help you understand just how big fume scrubbers have to be to handle all of the fume coming out of a hood.
The box on the left allows you to put in the diameter of the scrubber and the depth of the packing. Then it tells you how many Cubic feet per minute of air it can effectively scrub with an 8 second retention time.
On the right are opening sizes for 3 hoods. Put in the length and height of your hood opening and it will tell you the total CFM required to maintain 100 CFM face velocity. This is considered a fugitive emission.
The ideal setup is to provide an exhaust as listed to maintain the 100 CFM face velocity and this is non scrubbed air, it just makes the work environment pleasant for the refiner working in front of the hood.
The size of the scrubber determines the CFM it can handle, that size scrubber receives the fume from enclosed reactions in the hood. This is the scrubbed emission.
View attachment scrubber sizing and blower sizing.xlsx
The box on the left allows you to put in the diameter of the scrubber and the depth of the packing. Then it tells you how many Cubic feet per minute of air it can effectively scrub with an 8 second retention time.
On the right are opening sizes for 3 hoods. Put in the length and height of your hood opening and it will tell you the total CFM required to maintain 100 CFM face velocity. This is considered a fugitive emission.
The ideal setup is to provide an exhaust as listed to maintain the 100 CFM face velocity and this is non scrubbed air, it just makes the work environment pleasant for the refiner working in front of the hood.
The size of the scrubber determines the CFM it can handle, that size scrubber receives the fume from enclosed reactions in the hood. This is the scrubbed emission.
View attachment scrubber sizing and blower sizing.xlsx
A
Anonymous
Guest
butcher said:
This is what I too have politely been trying to say Butcher but it doesn't appear to be registering with the OP. He appears hell bent on doing what he set out to do regardless. Best of luck I guess.
