Btu of Natural gas

galvatech,

More information is needed in order to fully answer your question:

What is your "ambient temperature"???

What is the moisture content in your "ambient air"?

("Dry" air weighs 0.075 Ibs. per cu. ft., and the amount of water vapor present in the air to be heated will markedly affect the # of Btus required)

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Is 850 cubic feet your initial or ending volume???

According to Charles' Law

and the ideal gas law

A gasses' pressure, temperature, and volume are directly interrelated...

if pressure is held constant, then the volume is proportional to the temperature -

As an approximation, air expands roughly 1/500 of its volume for each degree of rise in temperature from 32° F.

Also for approximation purposes, 1 (one) Btu will raise the temperature of 55 cu. ft. of air one degree F.

Therefore, for rough approximation purposes, and ignoring the air's initial moisture content, and the expansion of the air, you will need to provide roughly 15.5 Btu of natural gas for each degree (F) rise in temperature over the ambient temperature.

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Just my $0.02...

the ambient temperature is 68 F. The temperature we want to reach is about 250 F and the space we are going to heat is a tank size 50' long, 6' wide and 8' deep which is 2400 cubic feet. I am sorry I wrote 850 cubic feet. The tank is open from top. The air is very moisturous.

Thank you very much. So, can we calculate how much Btu will be needed to heat it to maintain 250 F one hour?

What I am basically doing here is blowing air through heat exchanger into the tank. We are trying to dry the steel parts after its dipped into pre-flux+rinse(water) tank. The air will be blown onto the steel to dry it in an eclosed tank of 2400 cubic feet. The top surface has to be open for us to load the steel parts from top using a crane.

The Btu/hr available is 273210.9 Btu/hr. I calculated this using formula Cpx(T1-T2) and the formula for the mass flow rate. I wanted to find out if this is enough to heat the air to dry the steel in the tank of 2400 cubic feet volume using above mentioned principal.

now you want to evaporate water. That takes 1000 BTU/lb of water you need to evaporate.

do you think I would need a condensor to keep the water vapour away?

It is a open top tank, so it will most probably escape. Well, you can always recycle the water.

But having a condenser, you will have to cool the air, which will mean that you will need to reheat them up again.

Why do you need such an exact equation to dry stuffs?

So! You want to make a bloody great hair dryer? Yes?

Stu.

Seems the real question is about how the dry off some kind of steel parts from some kind of aqueous bath. LOTS of unknowns to consider:

Temperature of the parts; amount of liquid (water?) clinging to the parts;shape of the parts; surface area of the parts; void volume; weight of parts; air flow volume,relative humidity of incoming air; temp of 'ambient' air. The real challenge is how to distribute the heated air so all the liquid gets evaporated. One cannot do a simple heat balance because the drying rate varies as more liquid evaporates and recesses are left(much reduced surface area for evaporation.) Typically 4-10 times the 'theoretical' material and heat balance air is required to dry such a non-uniform stack of solids. Drying rate is predicated on the heat and material driving forces--sensible heat and relative humidity difference between the hot air and the liquid surface. Hot saturated air does not evaporate any more liquid.

Yeah, who knows. The poster could have gave a more detailed application he has in mind, but a question asking how much BTU is needed to heat up a box? It sounds like homework.

Maybe we can convince him that the application is not efficient in term of drying steel slabs.

I am sorry I didnt come back to you all soon. The project is to do with the galvanizing plant. We have acid tanks where we pickle or clean the rust of the steel and then dip the steel into Zinc kettle where the actual galvanizing of the steel takes place. Now, after pickling the steel is wet and if we dip wet steel into the galvanizing tank we experience splatters and thus to avoid splatters we need to dry the steel before we dip into the zinc kettle.

We have furnaces heating the zinc kettle and the by-products are passed through the flue to the chimney going out to the atmosphere. The flue gas temperature is about 900F. Now the real project we are thinking of doing is to use this heat to dry the steel coming out from the pickle tanks. Diverting this flue gases through the heat exchanger and then blowing air through it to dry the steel. We are not actually blowing the flue gases on the steel because doing so we may contaminate the whole atmosphere by CO gases. The flue gases decipating heat will be used to pass through the heat exchanger and from there back to the chimney. A blower will blow air through heat exchanger on the steel slabs to dry the steel. The question is that will this heat be enough to dry the steel slabs? Is this a good idea? Will this work?