# btu math help

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• 10-07-2012, 09:08 AM
oliesteveo
btu math help
If 11532 btu's were removed from 62 lbs of water at a 72 degrees f water temp , what would be the new temp of the water ?
Homework Question....appreciate some help with a formula.
Thanks
• 10-07-2012, 09:42 AM
R Mannino
It takes 1 BTU to raise 1 pound of water 1 degree F.
• 10-07-2012, 09:50 AM
oliesteveo
understand the definition, still confused on how to answer the question.
Thanks for the response
• 10-07-2012, 10:39 AM
jpsmith1cm
Either there is a typo here or you're getting into phase change, too.

You're removing eleven thousand, five hundred and thirty two BTUs from sixty two pounds of water at seventy two degrees?

I typed the full words out to prevent confusion.

I ran a little math and this doesn't add up to a typical problem.

Are you adding or removing heat?

Also, moving thread to tech to tech. AOP is for equipment questions.
• 10-07-2012, 10:53 AM
tedkidd
Quote:

Originally Posted by oliesteveo
If 11532 btu's were removed from 62 lbs of water at a 72 degrees f water temp , what would be the new temp of the water ?
Homework Question....appreciate some help with a formula.
Thanks

11532/62=186 per pound. 40 gets to freezing.

How many btu to freeze a pound? probably more than 146, so I'd say new water temp 32f.
• 10-07-2012, 10:54 AM
oliesteveo
yes sir...removing 11532 btu from 62 lbs of 72 degree f water there is a change of state latent and sensible i think
here is another one 13080 btu were removed from 10 lbs of water at 218 degree f what would be the temp of the water

Thanks for the help
• 10-07-2012, 10:58 AM
jpsmith1cm
Quote:

Originally Posted by oliesteveo
yes sir...removing 11532 btu from 62 lbs of 72 degree f water there is a change of state latent and sensible i think
here is another one 13080 btu were removed from 10 lbs of water at 218 degree f what would be the temp of the water

Thanks for the help

water at 218???

or STEAM at 218?
• 10-07-2012, 11:01 AM
jpsmith1cm
Quote:

Originally Posted by tedkidd
11532/62=186 per pound. 40 gets to freezing.

How many btu to freeze a pound? probably more than 146, so I'd say new water temp 32f.

This is it. You'll wind up with a water/ice slurry at 32 degrees.

it will take 7440 BTU to drop the 62# from 72 to 32 (186 * 40) = 7440 This leaves 4092 extra BTUs for latent conversion

You'll wind up with about 28# of ice by the time it's all done.
• 10-07-2012, 11:02 AM
rundawg
Quote:

Originally Posted by oliesteveo
If 11532 btu's were removed from 62 lbs of water at a 72 degrees f water temp , what would be the new temp of the water ?
Homework Question....appreciate some help with a formula.
Thanks

Formula: BTU's = Lbs x Deg x SH (specific heat of material)

Note: specific heat of water is 1.0.

So with a little Algerbra: Btu ÷ (Lbs x SH) = Deg.

Now to calculate Btu's required to lower temp from 72ºf to 32ºf (ICE), or 40ºf.

62 lbs x 40 deg x 1 SH = 2480 btu's

11532 - 2408 = 11408 btu's left at 32ºf

Change of State - Latent heat of Fusion

144 btu/lb to change liquid to ice : 62 lbs x 144 btu's = 8928 btus

Now we have used : 2480 (lowering temp to 32ºf) + 8928 (change of state) = 11408 btus

11532 - 11408 = 124 btus left

Note: specific heat of ice is 0.5

124 Btu's ÷ (62 Lbs x 0.5 SH) = 4 degs

32ºf (ice) - 4ºf = 28ºf (final temperature)
• 10-07-2012, 11:23 AM
tedkidd
I converted to per pound because it was easier for me to follow, would have converted back at the end had I known its only 144 to ice. Guess I assumed more because there is so much energy in condensation.

BTW, What is that exact number?

Did not realize its 1 btu to drop ice 2 f, so would said 30 and gotten the answer wrong even knowing cost of getting to ice. Thanks for the formula and explanation on specific heat!
• 10-07-2012, 11:32 AM
oliesteveo
change of state at 212 degrees f 970 btu at this point
• 10-07-2012, 11:41 AM
mgenius33
I remember in physics we had a question:
A round tub with 2' height, made of standard steel has a volume of 50 gallons at 50*, and it is filled with 15 gallons of 50*F water. How many gallons of boiling water would it take to over flow the tub, and at what temp would the water be? Answer to the nearest 0.001gal and 0.1*F.
• 10-07-2012, 12:04 PM
tedkidd
I'm sure it's not as simple as 35.001 gallons and 98.6, and am dying to see the formula...
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