I have an extractor fan which is advertised as displacing z cubic meters of air per hour.

That fan evacuates a rectangular box of dimensions a x b x c.

That box is supplied air through three separate passive intake round holes of diameter d1, d2 and d3.

Those intake holes are in turn fed by one circular duct, diameter d4 and length l, and two passive intake holes (d5 and d6) which feed in to volumes of e x f x g and h x i x j respectively.

Is there, and what is, a formula to determine the proper diameters of each of the circular holes such that I can regulate the cubic meters of air eventually flowing through the d4 x l duct and the e x f x g and h x i x j volumes to pre-determined values?

esselte said:

I have an extractor fan which is advertised as displacing z cubic meters of air per hour.

That fan evacuates a rectangular box of dimensions a x b x c.

That box is supplied air through three separate passive intake round holes of diameter d1, d2 and d3.

Those intake holes are in turn fed by one circular duct, diameter d4 and length l, and two passive intake holes (d5 and d6) which feed in to volumes of e x f x g and h x i x j respectively.

Is there, and what is, a formula to determine the proper diameters of each of the circular holes such that I can regulate the cubic meters of air eventually flowing through the d4 x l duct and the e x f x g and h x i x j volumes to pre-determined values?

chuck away the box.

The box is actually a filter. I could chuck it away, but I’d have to replace it with a filter which feeds the main extractor fan through the same three sources.

The filtering aspect is an integral component of the whole set-up and can nt be discarded.

esselte said:

The box is actually a filter. I could chuck it away, but I’d have to replace it with a filter which feeds the main extractor fan through the same three sources.

The filtering aspect is an integral component of the whole set-up and can nt be discarded.

OK but it does lower the capacity of the fan.

roughbarked said:

esselte said:

The box is actually a filter. I could chuck it away, but I’d have to replace it with a filter which feeds the main extractor fan through the same three sources.

The filtering aspect is an integral component of the whole set-up and can nt be discarded.

OK but it does lower the capacity of the fan.

and forces the fan motor to work overtime.

roughbarked said:

esselte said:

The box is actually a filter. I could chuck it away, but I’d have to replace it with a filter which feeds the main extractor fan through the same three sources.

The filtering aspect is an integral component of the whole set-up and can nt be discarded.

OK but it does lower the capacity of the fan.

It will do, but I’ve no way of determining by how much.

I’m hoping to get a formula by which I can determine each intake size as a ratio to each other intake. I will then cut each hole out increasingly large to this ratio in a running set up, until I get to a point where the fan does not appear to be straining itself.

Actually, it’s not quite that simple because the round duct will be of a certain pre-determined diameter, and one of the volumes will need to have holes cut for a pre-determined cubic meter per time extraction. The remaining volume can have holes and cubic meter extraction of any values required to feed the fan comfortably through the filter.

esselte said:

roughbarked said:

esselte said:

The box is actually a filter. I could chuck it away, but I’d have to replace it with a filter which feeds the main extractor fan through the same three sources.

The filtering aspect is an integral component of the whole set-up and can nt be discarded.

OK but it does lower the capacity of the fan.

It will do, but I’ve no way of determining by how much.

I’m hoping to get a formula by which I can determine each intake size as a ratio to each other intake. I will then cut each hole out increasingly large to this ratio in a running set up, until I get to a point where the fan does not appear to be straining itself.

Actually, it’s not quite that simple because the round duct will be of a certain pre-determined diameter, and one of the volumes will need to have holes cut for a pre-determined cubic meter per time extraction. The remaining volume can have holes and cubic meter extraction of any values required to feed the fan comfortably through the filter.

sounds fiddly.

why does it need to be filtered? Is it extracting into a breathing space?

>where the fan does not appear to be straining itself<

I would suggest that you measure the power consumption, rather than guessing by the noise.

I went through a series of calculations to find something similar when I designed the fume extraction system for the factory I built a few years ago, esselte, but the calculations are at work (under several layers of rubble by now, no doubt). I can look for them on Tuesday. In the meantime, have a look here: http://www.engineeringtoolbox.com/hvac-systems-t_23.html

You may need to search a bit for what you need, though.

The fan will have a performance curve, in the same way that a pump has a performance curve.

The system will have a ‘system’ curve, depending on the size of the holes and other factors.

The operating point will be at the intersection of the two curves.

You should be aiming for that point to be at the optimum fan efficiency, which is also generally the most stable operating point as the curves tend to be orthogonal at that point.

morrie said:

The fan will have a performance curve, in the same way that a pump has a performance curve.

The system will have a ‘system’ curve, depending on the size of the holes and other factors.

The operating point will be at the intersection of the two curves.

You should be aiming for that point to be at the optimum fan efficiency, which is also generally the most stable operating point as the curves tend to be orthogonal at that point.

Yes. If the fan was designed to work with the filter box.. then the only strain will be when the filters are clogged.

What I’m trying to design is a self contained box for growing vegetables by deep water culture.

One space will contain the plants. I have a certain rate that I want to replace the air in this space (once every three to five minutes).

The ducted air hole will incorporate a cool tube HID light, the diameter of which will depend on the flange sizes of the cool tube and the extraction rate of which will hopefully be optimally energy efficient for removing the heat of the light without stressing the fan.

The second space will contain the HID ballast and the air pumps for the deep water culture, and any other electrics. This is the space where I don’t care what the airflow is, as long as its enough to stop overheating of the electrics.

The third space, the evacuation box with the fan which the duct and other spaces feed in to will incorporate the filter which I need to remove the humidity from the extracted air, as it will be vented into an area which I do not wish to over-humidify.

esselte said:

What I’m trying to design is a self contained box for growing vegetables by deep water culture.

One space will contain the plants. I have a certain rate that I want to replace the air in this space (once every three to five minutes).

The ducted air hole will incorporate a cool tube HID light, the diameter of which will depend on the flange sizes of the cool tube and the extraction rate of which will hopefully be optimally energy efficient for removing the heat of the light without stressing the fan.

The second space will contain the HID ballast and the air pumps for the deep water culture, and any other electrics. This is the space where I don’t care what the airflow is, as long as its enough to stop overheating of the electrics.

The third space, the evacuation box with the fan which the duct and other spaces feed in to will incorporate the filter which I need to remove the humidity from the extracted air, as it will be vented into an area which I do not wish to over-humidify.

It is far better not to restrict the fan.

A friend of mine grew his stuff in a sealed box trailer. The type with the peaked roof with top sides that can be lifted open. He used four unrestricted computer fans

Wonky,

Thanks for the link. I’ll have a read of it tomorrow.

http://www.engineeringtoolbox.com/air-ducts-sizing-d_1736.html

roughbarked said:

morrie said:

The fan will have a performance curve, in the same way that a pump has a performance curve.

The system will have a ‘system’ curve, depending on the size of the holes and other factors.

The operating point will be at the intersection of the two curves.

You should be aiming for that point to be at the optimum fan efficiency, which is also generally the most stable operating point as the curves tend to be orthogonal at that point.

Yes. If the fan was designed to work with the filter box.. then the only strain will be when the filters are clogged.

I compulsively collect electronic stuff. I have a fan in my stash which moves 100 cubic meters per hour. This is much more than I imagine any of the individual components will need, so my current design is based around using a fan I already have to exhaust multiple things, rather than buying multiple new fans.

I do also have a few PC fans lying around, but i don’t know how much air they move.

I had considered fitting them to the otherwise passive intakes. Would this be a good idea to reduce the strain on the exhaust fan?

>I do also have a few PC fans lying around, but i don’t know how much air they move. <

I have one installed in my bedroom window to get rid of hot air. I did track down the capacity of it once, but I can’t recall what it was. It is available somewhere on the net.

esselte said:

roughbarked said:

morrie said:

The fan will have a performance curve, in the same way that a pump has a performance curve.

The system will have a ‘system’ curve, depending on the size of the holes and other factors.

The operating point will be at the intersection of the two curves.

You should be aiming for that point to be at the optimum fan efficiency, which is also generally the most stable operating point as the curves tend to be orthogonal at that point.

Yes. If the fan was designed to work with the filter box.. then the only strain will be when the filters are clogged.

I compulsively collect electronic stuff. I have a fan in my stash which moves 100 cubic meters per hour. This is much more than I imagine any of the individual components will need, so my current design is based around using a fan I already have to exhaust multiple things, rather than buying multiple new fans.

I do also have a few PC fans lying around, but i don’t know how much air they move.

I had considered fitting them to the otherwise passive intakes. Would this be a good idea to reduce the strain on the exhaust fan?

Remember that computer fans are used to keep computers from overheating. They are used in confined spaces. Computers potentially can easily get very hot but still operate efficiently at high temperatures.. so too do plants.

Your high capacity fan should not be required unless the space to evacuate is large.

esselte said:

roughbarked said:

morrie said:

The fan will have a performance curve, in the same way that a pump has a performance curve.

The system will have a ‘system’ curve, depending on the size of the holes and other factors.

The operating point will be at the intersection of the two curves.

You should be aiming for that point to be at the optimum fan efficiency, which is also generally the most stable operating point as the curves tend to be orthogonal at that point.

Yes. If the fan was designed to work with the filter box.. then the only strain will be when the filters are clogged.

I compulsively collect electronic stuff. I have a fan in my stash which moves 100 cubic meters per hour. This is much more than I imagine any of the individual components will need, so my current design is based around using a fan I already have to exhaust multiple things, rather than buying multiple new fans.

I do also have a few PC fans lying around, but i don’t know how much air they move.

I had considered fitting them to the otherwise passive intakes. Would this be a good idea to reduce the strain on the exhaust fan?

From what you have said, there should be no strain on the exhaust fan.

morrie said:

esselte said:

roughbarked said:

Yes. If the fan was designed to work with the filter box.. then the only strain will be when the filters are clogged.

I compulsively collect electronic stuff. I have a fan in my stash which moves 100 cubic meters per hour. This is much more than I imagine any of the individual components will need, so my current design is based around using a fan I already have to exhaust multiple things, rather than buying multiple new fans.

I do also have a few PC fans lying around, but i don’t know how much air they move.

I had considered fitting them to the otherwise passive intakes. Would this be a good idea to reduce the strain on the exhaust fan?

From what you have said, there should be no strain on the exhaust fan.

true.

morrie said:

>I do also have a few PC fans lying around, but i don’t know how much air they move. <

I have one installed in my bedroom window to get rid of hot air. I did track down the capacity of it once, but I can’t recall what it was. It is available somewhere on the net.

40 to 80 CFM

roughbarked said:

morrie said:

esselte said:

I compulsively collect electronic stuff. I have a fan in my stash which moves 100 cubic meters per hour. This is much more than I imagine any of the individual components will need, so my current design is based around using a fan I already have to exhaust multiple things, rather than buying multiple new fans.

I do also have a few PC fans lying around, but i don’t know how much air they move.

I had considered fitting them to the otherwise passive intakes. Would this be a good idea to reduce the strain on the exhaust fan?

From what you have said, there should be no strain on the exhaust fan.

true.

I was getting the impression from rough barked that the filter was going to stress the fan.

I guess that depends on the nature of the filter, however.

On the exhaust fan, could I safely connect it up through a light dimmer component to control its speed, or is there a more fan-specific potentiometer type thingy I should buy, or should I not do this with any fan that snt specifically designed to run at varying speeds?

esselte said:

roughbarked said:

morrie said:

From what you have said, there should be no strain on the exhaust fan.

true.

I was getting the impression from rough barked that the filter was going to stress the fan.

I guess that depends on the nature of the filter, however.

On the exhaust fan, could I safely connect it up through a light dimmer component to control its speed, or is there a more fan-specific potentiometer type thingy I should buy, or should I not do this with any fan that snt specifically designed to run at varying speeds?

There are fan speed controllers that are used for ceiling fans. I imagine you could use one of those.

esselte said:

roughbarked said:

morrie said:

From what you have said, there should be no strain on the exhaust fan.

true.

I was getting the impression from rough barked that the filter was going to stress the fan.

I guess that depends on the nature of the filter, however.

On the exhaust fan, could I safely connect it up through a light dimmer component to control its speed, or is there a more fan-specific potentiometer type thingy I should buy, or should I not do this with any fan that snt specifically designed to run at varying speeds?

As far as it goes More than one indoor grower has come home to find the fire brigade had been called who in turn called police.

stress a fan and you have a fire

A common one is in fan forced heaters where the fan vents get fouled by household fluff.