How to Calculate BTU's

[maksim]

I have one question. Im A Student in BTC and im having some problems on how to calculate BTU's For example.
"How many BTU's are required, to raise 2cu.ft steam from 35F to 320F." I need a step by step how to do it, I asked the intructor and he exmplained it differemtly harder, so i was hoping to have some hope here.. Thanks..

[cem-bsee]

state the problem EXACTLY.
list your known facts.

[key]

Originally posted by maksim
"How many BTU's are required, to raise 2cu.ft steam from 35F to 320F."

Does steam exist @ 35F? (only in a vacuum)

I think you need to know the weight of water in 2 cubic feet at a given temperature and pressure, then work your backwards or forwards.

[Edited by key on 12-01-2006 at 06:20 AM]

[propmanage]

I dont remember it has been 20+ schooling for HVAC After you figure it out give us ab update

http://www.massengineers.com/Documents/properties_of_steam_and_water.htm

[sskzekeman]

Originally posted by maksim
I have one question. Im A Student in BTC and im having some problems on how to calculate BTU's For example.
"How many BTU's are required, to raise 2cu.ft steam from 35F to 320F." I need a step by step how to do it, I asked the intructor and he exmplained it differemtly harder, so i was hoping to have some hope here.. Thanks..
If you are starting at 35F, then I assume it is water, if not, it is steam in a partial vacuum. And if it ends up as steam at 320F, then depending on the pressure it may be superheated.

If are talking atmospheric pressures then,you can either look up the enthalpy of water at 32F and superheated at 320F. I have tables ( Kenan and Keyes)which show enthalpy referenced to 32F so at 35F it is 3 BTU per lb and at 320F steam it is 1202 BTU per pound, the difference being 1199 BTU/lb. Since he wants it for 2 cu ft, then you get the density of the steam from tables 1/30.3 lb per cu ft.

Without tables you first get the heat to raise the water to the 212 boiling point temperature 212-35=177 BTU/lb, then add the heat of vaporization=970 BTU/lb and finally since the specific heat of steam is about 0.5 you get the heat added from saturated steam at 212 to superheated =0.5(320-212)=54 BTU/lb. Addind these 3 items you get 177+970+54=1201 BTU/lb which is close to the lookup method.
Hope this helps! Remember, this is for atmospheric pressures.

[cem-bsee]

WHY do the math?
let the student think thru the problem!

first, EXACTLY what is the problem?
then state the known facts
then decide which facts are important [my first EE prof would ALWAYS give too many facts= real world]