BTU's needed to heat air

[Leigh Betsch]

I'm trying to calculate how much it would cost if I exhusted my dust collector to the outside rather than filtering and returning the air to the shop. Does anyone know how many BTU's it takes to heat one cubic foot of air 1 degree?

[Rod Sheridan]

Hi Leigh, air isn't a homogenous substance, it contains water vapour in widely varying quantities which dramatically changes the amount of energy required to change the temperature.

What would the outdoor air temp and humidity be at your loacation?

Regards, Rod.

[Leigh Betsch]

Hi Leigh, air isn't a homogenous substance, it contains water vapour in widely varying quantities which dramatically changes the amount of energy required to change the temperature.

What would the outdoor air temp and humidity be at your loacation?

Regards, Rod.

Maybe a safe estimate would be 10*F and 65% RH in the winter.

[David Keller NC]

Leigh - If I can find my psychometric chart, I can answer your question, but in the meantime, making your decision may be easier than calculating it. The first question to answer is how long your dust collector runs while you're using the shop. If the answer is that it runs semi-continuously, then it may be very expensive to exhaust it to the outside. If it runs infrequently, then it's trickier.

Another question has to do with less objective parameters - what's your tolerance for super-cold drafts? If it's low, then an external exhaust isn't good. A related question has to do with moisture - while at 10 degrees F and 65% humidity there will be little moisture drawn in, that might not be the case in summer.

Finally, how long does your heating system have to run to bring your shop up to working temperature, and how often does it run to maintain it? If the answers are "a long time" and "frequently", exhausting your DC collector to the outside may make maintaining the temp in your shop very difficult.

[Jim Kountz]

Heres the actual definition maybe you can apply it to your needs??

Definition: A British Thermal Unit (BTU) is the amount of heat energy needed to raise the temperature of one pound of water by one degree F. This is the standard measurement used to state the amount of energy that a fuel has as well as the amount of output of any heat generating device. You might be able to imagine it this way. Take one gallon (8 pounds) of water and put it on your stove. If the water it 60 degrees F. and you want to bring it to a boil (212 degrees F.) then you will need about 1,200 BTUs to do this.

[David Keller NC]

Here's what you need to calcualte this yourself:

http://en.wikipedia.org/wiki/File:PsychrometricChart-SeaLevel-SI.jpg

To use it, choose a temperature along the x axis, and go up a vertical line until you hit the relative humidity of the air you're interested in heating (the relative humidities are the curved red lines on the chart). Estimate the enthalpy of the air you're starting with (the black diagonal lines sloping from right to left, in this chart given in SI units). Draw a horizontal line to the right (for heating), until you're over the final temerpature. Estimate the enthalpy of this air as you did with the begining condition.

The difference between the two enthalpies gives you the amount of heat you must apply to the air at a given humidity to raise it to the final temperature. Note that this is based on the weight of the air. Naturally, as it heats it expands, so the actual volume goes up, and you will need to convert the SI units to get BTUs. Generally, 1064 Joules = 1 BTU

[Leigh Betsch]

Thanks for the info. I thought I should do some calc's to see if it made any sense at all to exhaust the air. I bought a used 1.5 hp DustKop dust collector last summer at a garage sale but I have never installed it. It was designed to pull metal dust, I doubt the cyclone is much good or wood dust and I doubt the airflow is good enough to overcome the sp of some good filters. But I got to thinking if I duct it outdoors and don't use the cyclone or filters maybe I could generate the airflow and reduce the sp and get it to do a decent job on wood dust. So on one hand I could buy a $1900 new system that would work, or on the other hand maybe I could get buy with the old. I can toss in a Delta 1.5 hr blower into the mix as a booster also. I'm doing the duct work calc right now to see want my cfm and sp requirements are. All kinda depends on how long it would take to eat up $1900 of natural gas.

[Leigh Betsch]

Here's what you need to calcualte this yourself:

http://en.wikipedia.org/wiki/File:PsychrometricChart-SeaLevel-SI.jpg

Note that this is based on the weight of the air. Naturally, as it heats it expands, so the actual volume goes up, and you will need to convert the SI units to get BTUs. Generally, 1064 Joules = 1 BTU

That's a nasty chart to be springing on a guy while he's not drinking.:eek:
But as I read it my starting enthalpy is about zero because it is so cold. Assuming a 20* ending temp my ending enthalpy is about 25 j/gm dry air, at approx 15% RH. With 1064 J per BTU that gives .234 BTU/gm dry air. So what is the conversion of BTU/gm dry air to BTU/cubic foot of air? Or am I really missing something here?

[David G Baker]

I did okay with high school algebra but that was too many years ago so instead of high math I use the trial and error method with a little common sense tossed in for good measure. If I like the results I go with it.
I haven't looked up the word "enthalpy" but there must be some kind of cure with all the new advances in medical science. :D

[Josiah Bartlett]

If you could build a rudimentary air-to-air heat exchanger to recover some of the waste heat from the outgoing air you could really save a lot of energy that way. It would have to be something that wouldn't clog with dust easily, though.

[Leo Zick]

I'm trying to calculate how much it would cost if I exhusted my dust collector to the outside rather than filtering and returning the air to the shop. Does anyone know how many BTU's it takes to heat one cubic foot of air 1 degree?


this seems to be going into the complicated zone to me. im a dumb engineer so i need to keep things simple.

you want to exhaust air, and are concerned that the cost of heating fresh air is more than the recirc cost, correct?

lets get one assumption out of the way - are you, or would you be bringing fresh air into your shop? i think its safe to assume you havent enclosed your space (which you didnt mention, is it in a house, separate, etc etc) so well that youd be created some huge vacuum in the space,or that youd need to supplement with outside air.

my guess is the makeup air from surrounding rooms or whereever is tempered enough that a few degree temp rise isnt going to break the bank.

[Leo Zick]

the more complicated answer, btw, is to calculate it. you cant raise a cubic foot of air by 1degF, but ~0.24BTU raises one lb of air by 1degF.

if you have a dedicated heater for the space, you can measure airflow cfm and degF in front of and behind the heating coil, and measure how many watts or amps the heater uses, then calc the BTUs being used in each scenario. but this assumes you are only serving this space, have some return air to the unit, etc etc.

[Gordon Peery]

Another factor: The cost of Enthalpy Therapy! I checked the yellow pages (what's that?): $185. per hour, not including the down time while you're at the Doc!

Lots of good variables have already been listed. Some others: the BTU per hour loss thru floor, walls (special consideration for windows & doors, and ceiling. Also, heat loss is so greatly affected by the temperature differential: outside temp and the various temperatures of the air in the space (60 at floor level, 80 or 90 at ceiling) as well as the "R" factor (resistance to heat transfer) of the floor, walls & ceiling of your shop. And, let's not neglect to factor in the average "Temperature Degree Days" of your zip code. (40 year records courtesy of USGS or NWS).

All things being equal, If you choose to exhaust that dust/air to the outdoors, you should count on (1) wearing very thick longjohns, & windbreaker, and (2) paying dearly, your attorney to defend you against your neighbor's lung cancer lawsuit!

In my opinion, if you are a lifelong smoker who inhaled, I would invest in a very good dust filtration system. If not, spend $350. on a unit for your drum sander and lots of Kleenex when using the planer, routers, shaper, drill press, etc.

Gordon


P. S. The HVAC (heating, ventilation, air conditioning) folks probably will want double time to calculate your problem.

[Frank Hagan]

this seems to be going into the complicated zone to me. im a dumb engineer so i need to keep things simple.

you want to exhaust air, and are concerned that the cost of heating fresh air is more than the recirc cost, correct?

lets get one assumption out of the way - are you, or would you be bringing fresh air into your shop? i think its safe to assume you havent enclosed your space (which you didnt mention, is it in a house, separate, etc etc) so well that youd be created some huge vacuum in the space,or that youd need to supplement with outside air.

my guess is the makeup air from surrounding rooms or whereever is tempered enough that a few degree temp rise isnt going to break the bank.

That brings up another issue; if you put the shop into a negative situation, and it starts pulling air from other sources, you could cause the natural draft vents on appliances such as water heaters, wall heaters, etc., to reverse and spill "products of combustion" into the space, killing you. You do save some money for your heirs.

[Leo Zick]

If not, spend $350. on a unit for your drum sander and lots of Kleenex when using the planer, routers, shaper, drill press, etc.



welcome to SMC, be prepared to get bashed on that comment!

[Danny Dore]

I'm trying to calculate how much it would cost if I exhusted my dust collector to the outside rather than filtering and returning the air to the shop. Does anyone know how many BTU's it takes to heat one cubic foot of air 1 degree?


Hi everyone this is my first post so bear with me. I'm new to woodworking but HVAC is right up my alley.

Leigh,

You need to calculate the amount of heat it will take to heat the air coming into the space and not the air you are exhausting. the calculation for this is simple. BTU's= CFM x 1.08 x change in temperature. If the air is coming in from outside in the winter at 20 degrees and you want to heat it to 70 then your change in temperature is 50deg. From this point if you use electricity you divide the BTU's by 3413 to get the KW of heat required.

Hope this helps
Danny

[David Keller NC]

Leigh - Part of the issue with the psychometric chart I linked to is that it's set up for near-normal room temperatures and pressures, so it's a bit tough to use for heating up very cold air.

But from the standpoint of conversion, remember that a gas expands as its heated, so 1 gm of dry air at 0 degrees farenheit occupies a lot smaller space than air at 70 degrees farenheit, given the same pressure of 1 atmosphere.

But what you could do is to make your calculations based on the starting point, in which case you use the ideal gas law to calculate the density of air (lbs/cubic foot, for example) at a given temperature and pressure.
The formula is:

PV=nRT, where P=pressure, V=volume, R=universal gas constant, and T=temperature in absolute scale (in other words, 0 degrees celcius is 273.15 Kelvins - you plug in the 273.15 into the equation, not 0). Here's a link to this on Wikipedia:

http://en.wikipedia.org/wiki/Ideal_gas_law

This is called the "Ideal Gas Law" and isn't realy a law, it's an approximation, but it's very, very close to the measured answer under the conditions that you're interested in.

BTW - nothing truly has "zero" enthalpy, unless it's at absolute zero temperature. Here's a link to a low-temperature psychometric chart in English units, so you don't have to do the conversions:

http://www.me.umn.edu/courses/me4131/LabManual/AppGPsychrometricCharts.pdf

If you're not up for this (it's admittedly a fairly complex thermodynamics subject for the layman), I may have time to do a few calcualtions for you this afternoon - ping me by PM.

[Rod Sheridan]

Maybe a safe estimate would be 10*F and 65% RH in the winter.


Hi Leigh, sorry about asking for relative humidity, when I saw the 10 figure my brain was in metric, and thinking summer time humidities.

0.02 BTU's will raise 1 cubic of air 1 degree F (Aproximately)

So if you have 600CFM of exhaust air and a temperature delta of 60 degrees F

600CFM X 60 X 0.02BTU/CF = 720 BTU/minute

720BTU/minute X 60 = 43,200 BTU/Hour

if you're using electric heat, that's about 13,000 Watts.

As others have mentioned, you'll need an opening to the outside so that you don't create a net negative pressure in your house which will be a serious safety issue if you have fuel buring appliances such as water heater/furnace etc.

Regards, Rod

[Jeffrey Makiel]

Nice job Rod.

A simple formula for heating (also called 'sensible heating') is:
BTU/hr = 1.08 x cfm x (Ti - To)

where
Ti=temp inside degree F
To=temp outside degree F

You need not be concerned with humidity for a home/shop heating calculation. Therefore, you don't need a psych chart unless you have specific humidity concerns to deal with (like a surgical operating room, or operating a kiln) in which you must have a humidfier.

-Jeff

[Leigh Betsch]

I like simple things. Rod and Danny have nice simple formulas. I don't need extreme accuracy just a general conclusion that it is a good idea or dumb idea to exhaust outside. So if I understand correct:
Assuming a 1200 cfm exhaust and 50 degree temperature change
Rod's formula: 1200*.02* 50=1200 BTU/min*60=72,000 BTU/hr
Danny's formula: 1200*1.08*50=64,800 BTU/hr
So worst case (72,000 BTU/hr)/3413 = 21.10 KW/hr
at $.10/kw = $2.11/hr

So if I wire my DC to start and run only when my woodworking equipment is running and if my equipment run time is 1/2 hr per weeknight and 6hrs per weekend it would cost about $17.94 per week to exhaust the DC to the outdoors.

So if my math is correct it would make economical sense to look into the other factors that people have brought up, including the safety concerns.

My shop is not part of my house and my nearest neighbor is 1/2 mile away so I probably have a few advantages along this line.

I'll probably give it a real world test this weekend, hook up my DC and see if the shop gets cold, and see how much the furnace runs.

[Leo Zick]

I like simple things. Rod and Danny have nice simple formulas. I don't need extreme accuracy just a general conclusion that it is a good idea or dumb idea to exhaust outside. So if I understand correct:
Assuming a 1200 cfm exhaust and 50 degree temperature change
Rod's formula: 1200*.02* 50=1200 BTU/min*60=72,000 BTU/hr
Danny's formula: 1200*1.08*50=64,800 BTU/hr
So worst case (72,000 BTU/hr)/3413 = 21.10 KW/hr
at $.10/kw = $2.11/hr

So if I wire my DC to start and run only when my woodworking equipment is running and if my equipment run time is 1/2 hr per weeknight and 6hrs per weekend it would cost about $17.94 per week to exhaust the DC to the outdoors.

So if my math is correct it would make economical sense to look into the other factors that people have brought up, including the safety concerns.

My shop is not part of my house and my nearest neighbor is 1/2 mile away so I probably have a few advantages along this line.

I'll probably give it a real world test this weekend, hook up my DC and see if the shop gets cold, and see how much the furnace runs.

per my post above, these both assume you supply 100% outside air. thats a pretty impressive HVAC system you have, if you can modulate OA through your supply.

[Sonny Edmonds]

Simply stated, you are going to need a heater the size of a jet engine to heat air as fast as it will exit your shop with a total loss system.
Just for a round figure, say you are moving 1200 CFM (cubic feet a minute). You 'll need to heat the incoming air from 10 degrees F, to what? 60-70 degrees? Just so it can be lost outside again?
Nope, not a good idea. Not a good idea at all.

[Leigh Betsch]

I'm going to borrow a meter to measure air velocity and static pressure this weekend from a HVAC contractor buddy of mine. This should help me see just what my used (cheap) cyclone system in capable of. If it has a bit more capacity than I think it has perhaps I will be able to install a filter system and return the air to the shop. I also have a Delta blower that I can probably add in to boost the airflow. Sonny I've read your posts about your dirty air booster but I haven't seen the answer I'm looking for. When you add a booster in series into the system are the cfm air lows additive? Can you add the two CFM ratings together to get a system cfm rating? What happens to the static pressure calc? If I can jury rig some piping hopefully I can just take some measure met this weekend and get some empirical data.

[Frank Hagan]

Maybe you can rig a type of heat exchanger to have the exhausted air warm the incoming air. Building a "duct within a duct" type of system, you could have the incoming air surround the duct with the air being exhausted and recover some of the heat.

[David Keller NC]

"I'll probably give it a real world test this weekend, hook up my DC and see if the shop gets cold, and see how much the furnace runs."

Leigh - This is, by far, what you want to do. Despite my penchant for enthalpy calculations, direct experimentation is always the "proof of the pudding". The equations mentioned by Jeff and Danny, are, by the way, approximations based on the psychometric chart and some simplifying assumptions, one of which is that you're heating/cooling air in a return system within a home, so the air density and relative humidity can be considered relatively constant.

All this not withstanding, I think you're still going to conclude that you get a really ugly draft when you turn on a full-exhaust DC system. Regardles of whether your HVAC system can keep up, there's just something about getting a shot of 10 degree F air on your shins coming from under the door that's mighty unpleasant. If you're not using this system with a wide-belt sander or random orbit sander, you could easily construct a filter out of ordinary furnace filters. Such a filter has extremely low pressure drop across it, and would be greatly sufficient to catch the sort of chips you'd get from a planer, jointer, and the large dust particles from a table saw.

Just my opinion, but I personally think expecting any central DC system to catch dust from a random orbit sander is not the way to go. A simple craftsman shop vac from Sears with a supplemental HEPA filter is really cheap (about $60 with the filter), and will actually clean your shop air, in addition to catching the dust (and most ROS have dust ports that hook right up to the 1-1/2" hose). And you can roll it outside to open, empty and clean the filter - something that you can't do with a central DC system. Opening one of the central DC systems that's been used with a large sander seems to always dump large amounts of very fine dust in the air.

[David Giles]

Just crack open an outside door or window, then turn on the dust collector. The suction will pull cold air from the door into the machine, collect the dust and exhaust it back outside. This keeps you from expelling all of the warm air in a room.

Purely theory, since I live in South Texas.

[Sonny Edmonds]

I'm going to borrow a meter to measure air velocity and static pressure this weekend from a HVAC contractor buddy of mine. This should help me see just what my used (cheap) cyclone system in capable of. If it has a bit more capacity than I think it has perhaps I will be able to install a filter system and return the air to the shop. I also have a Delta blower that I can probably add in to boost the airflow. Sonny I've read your posts about your dirty air booster but I haven't seen the answer I'm looking for. When you add a booster in series into the system are the cfm air lows additive? Can you add the two CFM ratings together to get a system cfm rating? What happens to the static pressure calc? If I can jury rig some piping hopefully I can just take some measure met this weekend and get some empirical data.

Leigh,
It isn't a matter of the cost of the system, it's a matter of the air being removed from your shop has to be replaced.
I did that in my old shop to calculate my actual CFM of collection at each machine on my system. The only way to really see how much actual CFM was being moved at each machine. (IE: TS was 356 CFM)
It was really interesting, but in the real world, of very little value to others because of all the variables that could be involved.
That system, BTW, was a total loss type of system. Just like what you have in mind.
My current shop has an internal system. That is, all air in the shop is contained and filtered when any of the systems is used. So my dust collector collects, processes, filters (.5 micron), and returns the air to the shop.
I set it up that way for a couple of reasons.
1. It would have been ugly to put the system outside.
2. Because I had/have delusions of grandeur of one day having a heated/cooled shop. :D
Yeah, like that will ever happen!
Nobody is saying you can't have a total loss system and heat the make-up air. 200 CFM out has to be made up somehow. And that make-up air has to be heated if you hope to maintain some semblance of warmth in the shop.
And the more that is being pumped out, the more that has to be heated coming in.
And there becomes a point of extreme cost to heat the make-up air. Your HVAC buddy will concur. Just as this State licensed HVAC buddy is trying to tell you.

Of the two choices, my opinion is to use a total loss system. But it isn't a practical choice.
So I use a closed system which circulates the air in my shop, be it 200 CFM, or 1200 CFM.
Where I get uncomfortable is when the fumes from the CA glue get to burning the carp out of my eyes and I have to open the big door. But that only happened once. :eek: :o

Booster blowers are not so much additive to the volume (CFM) as they are to increasing the speed of the air moving through the systems piping.
Once you get your booster blower inline with your main DC, you can control it separately from the main if you desire. But put your hat in a safe place when you turn on the booster blower in the line. :eek:
They run in series. The line going to your main DC has the booster blower with it's discharge connected to the line going to the DC, the suction of the booster goes to your area of collection.
In my case, I have my 50-760 Delta blower under the TS out feed table. Under there, it connects to 4" pipes going to various places, TS, RT x2, and an auxiliary gate for roll up machines, and my down-draft table.
It kicks up the suction at these areas to an incredible amount.
Here is a picture of my old shop and a booster blower that handled all of the shops collection. (That is, all of the DC air past through it)
The booster blower runs with the main collector running. Effectively making it a true two stage system.
Try it, you'll like it. ;) :)

[Leigh Betsch]

Thanks a bunch guys. I'm an engineer so before I do something I've just got to run some calculations. Dave Keller, thanks for the help, I would love to spend some more time to understand in depth what I'm trying to do, I think you could teach me a lot, I like the theoretical applied to the the real world. But most of all I like empirical data derived from experimentation. So I need to experiment with a system that is realistic for my shop. So what that means is, I've run enough calculations, nice and simple, thanks Rod and Danny, I received a fan curve from the DustKop mfg today so I've calculated my available CFM, FPM, and SP, for my proposed system. I'm running right on the boarder line of having enough air flow and air velocity to make the system work, unless I eliminate the sp loss due to the filters and cyclone which means a total loss system directly vented outside. I believe Sonny and others that a total loss system just doesn't make sense, even if the cost of reheating the air isn't too bad, the air loss needs to be made up and a normal HVAC system just doesn't have the recovery time. In my case I have two adjacent relatively unused heated rooms with a volume almost as large as my shop, where I could pull heated air from and I think I could find a way to minimize the drafts.
So, what to do, where to go from here... I have a thought that since both of the DCs that I own now are probably too small, I will couple them together to overcome the cyclone and filter resistance and thus create a closed system with the return air vented back into the shop. I plan to borrow the necessary instrumentation this weekend and take enough measurements to see what the real world is giving me. I will also give the total loss system a dry run and see if I notice the effect of purging all the warm air outdoors or if my heated space (20x90) is large enough to absorb the cold air. By the way David Giles, I did consider ducting cold air to the equipment as make-up air but I think the cost of the duct work would be prohibitive. My suspicion is that I will end up with a hybrid system that allows for recirculating the air during the heating and cooling season (shop is air conditioned in the summer) and also allows to direct vent the rest of the year, thus saving the filters and reducing the noise in the shop from the air thru the filters during the good months.

[David Keller NC]

"But most of all I like empirical data derived from experimentation."

Hmmm - Despite having a PhD in Chemical Engineering, and sitting through more than my share of heat transfer classes, I agree with you here. There are simply too many variables to gain more than a rough idea from calculations, which is why companies like Boeing still run wind-tunnel tests, in spite of a staff of highly educated aerodynamics professionals with enormous computing power and extremely expensive finite-element analysis software.

I'm quite serious about using furnace filters to construct your dead-end filtration after the cyclone. In spite of the large crop of "new-fangled" HEPA cannister home woodworker dust collection systems, I find that my old fabric-bag system catches nearly all of the dust from my machines in my shop, with the exception of the super-fine dust from a random orbit sander, and for that I simply use the cheap craftsman shop vac with the small HEPA filter. Works great, and is dirt cheap.

[Sonny Edmonds]

20 X 90! Jeese!
So, you have a golf cart to get around in there?
Hello!Hello!Hello! Hello! Hello! Hello! Hello! :D

Einganear...
Well, no wonder you ponder something to death then.
Have fun! ;)

[Leigh Betsch]

Well after a day of Einganearing I have the makings of a system that should do what I need. I borrowed a air velocity meter and used it to evaluate various configurations of my dust collector system. And along with an Amp Probe to measure motor current I found a configuration that with a bit more tweaking should take care of most of my issues. I think I'll go with hooking my 1 1/2 hp DustKop and my 1 1/2 Delta in series, with 8" main trunk line and 6" drops to the machines. By doing this I can put 1100 cfm right at the machine hood, with 5700 ft/min air speed. Right now I am testing using the original Delta bag as the filter resistance, so when I build my filter unit I probably will reduce some of the cfm. I have tuned the prototype system to pull about 3.5 amps (220 volt) on the 3phase DustKop and 14.5 on the 110 Delta motor. I have a little upside capacity on the DuctKop (nameplate amperage is 4.2) but the Delta is pretty much maxed at 14.5. The Delta will pull 18 amps with out a load and the DustKop will pull about 6 amps. I'm trying to run the amperage right up on the high nameplate limit but not go over so I don't damage the motors.
Like I said before the DustKop unit was made for metal grinding dust so I'm not sure how well the cyclone will work for wood dust. The fan is in front of the cyclone which presents problems with wood chunks going thru the fan before the cyclone. I put the delta fan after the cyclone. If I remember correctly with both units running I get about 4000 f/m and 1300 cm right at the unit. My numbers might be a little off, I don't remember the exact numbers right now. But the really good news is that I can see my way to a system with enough air flow and hp to overcome the resistance of a decent filter so I can vent into the shop, and I shouldn't need to exhaust outdoors. Thus ends the "BTU's needed to heat air" problem that I was trying to work out. So tomorrow I start to build the filter system. Since I'm not confident in the DustKop cyclone separating ability, I'm thinking of exhausting the air from the DC into a plenum, with a clean out on the bottom. This should slow the air speed down and let the left over chips/dust settle out before going into the filter. I have a bunch of 3M Filtre furnace filters, couple of 95% industrial air handler filters, and (best of all) two large 2' x 4' powered Heppa .3 micron filters out of a clean room. Each of these filters has a 650 cfm internal fan to move the air through. So tomorrow I'll calculate the plenum size needed so the air speed drops, figure out how to mount the furnace prefilters, then the 95% industrial filters, and then how to get the heppas mounted inside the shop.
I'm also going to look into building a cyclone so I can put it in front of the DustKop fan and eliminate the wood chunks going thru the fan problem, that I see coming.
Seems like a lot of work but I've got about $400 invested and a Oneida system is going to run me $1900 by the time I get it here. And this is a lot more fun that writing a $1900 check.

[David Keller NC]

I suspect you're going to find that your set-up doesn't have enough "oomph" to run HEPAs as dead-end filters. They take a lot of power to run, and again, unless you're doing a lot of ROS sanding, you just don't need it for bandsaw, tablesaw, jointer and planer dust and chips.

[Leigh Betsch]

Actually, I hooked up one of the heppas to the system on Sunday afternoon. The input air velocity didn't change much, about 1500 cfm, right at the opening of the DustKop fan. The air speed going out the heppa was really to small to measure accurately. When I kicked on the heppa blower, along with the DustKop and Delta, I could measure about 2100 cfm on th filter output, but still the velocity was very low and I doubt I got an accurate measure. The Delta motor was pulling about 17 amps but before I could experiment with choking down the input air from the DustKop, my amprobe quit on me, so I couldn't continue to measure the amperage and see what the heppa output would have been with the delta running closer to 14 amps. Now I didn't have any input piping on the input side of the dust collector so it was just running free and I know I wont get that kind of cfm when I get everything connected. I don't really know what to expect when I get the piping all installed and the pressure losses start to add up. There isn't a fan curve for a system with two fans, (plus Two powered heppas) all in series so I'm on my own trying to guess what will happen when I get every thing hooked up.
I built the discharge plenum tonight, installed two furnace filters, one on top of the other to act as pre-filters to the heppas, I've got room for a whole stack of furnace filters if that is looking like the way to go. Then I pulled out what I thought was going to be industrial air handler filters from the storage side of the shop and discovered that they are actually heppa filters also (.3 micron, 99.97% efficient, 1" wc at 1080 cfm). So I have the filter end covered, just gotta hope that I don't choke the unit on the input side.
I'm thinking about building my own duck work out of mdf, try to save a few bucks. It would be rectangular duct. I found an on line calculator that calculated the equivalent rectangular and round duct sizes. With out a fan curve and being able to predict what the pressure drop will be through the duct work I'm a bit hesitant to just buy duct. I want to maximize duct size to minimize the pressure loss but don't want to get too large of duct and loose too much air velocity. So if I build my own rectangular duct I can always take it apart and make it smaller if I can't keep the air speed up.
A lot of work but what else is there to do in SD in the winter?

[David Keller NC]

Leigh - I'm having a hard time believing that you can make MDF ductwork cheaper than 5" PVC pipe. MDF if relatively inexpensive, but that's really stretching it. Wouldn't it also be really heavy?

I'll admit that I'm a bit surprised at cfm measurements when you have your HEPAs in-line. Generally, when I've measured it before in industrial applications, there's roughly triple the airflow with single-stage HEPA filters in/out of the process stream. I've tried pleated non-HEPAs in the same application, and that resulted in about a 1/3 reduction in airflow between the filters in/out of the airstream.

[Leigh Betsch]

You could be right about the air flow restriction with heppas. I may have been loosing more airflow out the bottom of my cyclone that I thought, I didn't measure the leakage. It was a jury rigged set up so I could be introducing more error than I thought. The heppa I was using is a MAC 10 FFU (Filter Fan Unit) by Envirco, rated at 650 cfm at 90 ft/m and .44 in w.g. 2 ft x 4 ft. with a 650 cfm airfoil fan.
I'm going to try a CamFil Mirotain filter in the final construction, cause I have a couple and then I can save my powered units as room scrubbers, which is what I use them for now.
MDF for ducts, well maybe it isn't a good idea but I'm thinking 1/4 mdf rectangular ducts could offer an advantage in being cheaper, and easier to attach down pipes to. Just bore a 6" hole and hook a flange any place you want a drop.

[Frank Hagan]

MDF for ducts, well maybe it isn't a good idea but I'm thinking 1/4 mdf rectangular ducts could offer an advantage in being cheaper, and easier to attach down pipes to. Just bore a 6" hole and hook a flange any place you want a drop.


I have the same idea floating around in my head, using MDF or any other suitable material (hardboard, etc., perhaps with one plexiglass "wall" for visibility, etc.) You could make longer sweeps for ells and do some other "custom" things, but you also have to be able to take it apart to clear any obstructions, etc.

[Leigh Betsch]

I hadn't thought about taking it apart to clear jams. Maybe the bottom would have to be screwed on. I've been thinking that I could pull the air out the side rather than the bottom so I wouldn't get dust falling down into the drops, similar to Sonny's horizontal hook up only with fewer pipe fittings. I'm thinking the drops could either be standard HVAC 30ga Borg pipe or maybe custom bent PVC, heat and bend into sweeping arcs. This would eliminate the cost of hoses. I like the idea of a plexiglass window although I would go with Lexan, much, much tougher. It would get very expensive to make an entire side of Lexan.

[David Keller NC]

Leigh - Your post cleared up a few questions - I was thinking in my head you actually talking about a true ductwork out of MDF, now I'm thinking you're talking about making a plenum out of it, and attaching drop hoses - that makes a lot more sense. I can imagine that cutting and fitting all the curves for ductwork out of MDF would be a major pain in the tail.

But since you're thinking of this, I've another suggestion. While unconventional, if you've a friend or relative in the HVAC business, you might ask whether they use/make 6" plenums out of sheet metal, and whether they sometimes have misfits that they can't use (made a bit too short, or the wrong shape, etc...) and that just get thrown out.

A plenum made from sheet metal would have a couple of advantages that I can think of - extremely lightweight, both compared to MDF and PVC, and generally spark-proof, so long as it's grounded.