# Thread: BTU's needed to heat air

1. ## BTU's needed to heat air

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?

2. 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.

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

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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.

5. 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.

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Here's what you need to calcualte this yourself:

http://en.wikipedia.org/wiki/File:Ps...eaLevel-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

7. 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.

8. Originally Posted by David Keller NC
Here's what you need to calcualte this yourself:

http://en.wikipedia.org/wiki/File:Ps...eaLevel-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.
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?

9. 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.

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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.

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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?

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.

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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.

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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.

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Originally Posted by Leo Zick
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.

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Originally Posted by Gordon Peery
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!

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