# Thread: BTU's needed to heat air

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Originally Posted by 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?

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

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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...tricCharts.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.

3. Originally Posted by Leigh Betsch
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

4. 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

5. 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.

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Originally Posted by 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.
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.

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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.

8. 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.

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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.

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"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.

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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.

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Originally Posted by 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.
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.
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.

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.
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.

13. 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.

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"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.

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

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

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