# Thread: Formula to cool water with air

1. ## Formula to cool water with air

I need to determine how much Air is needed to cool a copper tube. The air will be blowing on the outside surface of the tube as there will be water running through the tube.

The starting temperature of the water will be 70°C and the goal end temperature is 35°C...all done in less than 2 minutes. I already know that it will take ~250 W to do this; however, how would I determine the amount of air needed? Am I looking for the velocity of the air to cool it? If so, what equation should I use to solve for this?

I won't be able to have room temperature water available to cool the solution, neither will I be able to use ice. This needs to be repeated on command, so I'll need a fan that will kick on to cool the piping as the 70°C starts flowing through.

As you can see I'm quite rusty on my heat transfer and thermodynamics. It doesn't help that I can't find my textbook from when I was in school either!!

2. What does this tube look like. In order to do this , you need some minimum length and surface area of the tube, more like a heat exchanger;

The tube is placed in the block (similar to a heat exchanger). It is shaped in a labyrinth manner right now, but obviously can be changed and can be run outside the block and on its own.. Right now the tube is ~3 feet long with 3/8" diameter.

Originally Posted by zeke
What does this tube look like. In order to do this , you need some minimum length and surface area of the tube, more like a heat exchanger;

4. Originally Posted by Vig16
the tube is ~3 feet long with 3/8" diameter.

Surface area.
Thermal conductivity per second/minute/hour.
V1 T1 = V2 T2 and V will be constant.

You should not need any "text books."

Dave

5. Thanks, Dave!!

Originally Posted by PinkertonD

Surface area.
Thermal conductivity per second/minute/hour.
V1 T1 = V2 T2 and V will be constant.

You should not need any "text books."

Dave

6. I would buy a heat exchanger from a mfr who could advise you better. This is not simple.since the conductance at the surface of the block/tube depends on air flow and the internal film coefficient depends on water flow rate, etc.

7. Zeke,

Not sure why, but I guessed this was non-critical application since he was trying to just cool the pipe. Another alternative could be a transmission cooler from an Auto wrecker and a couple of muffin fans. That's what I use on one of my Lasers here. Total cost about 20-bucks.

Dave
Generally, I will not give you the answer to your question, but I **will** guide you into discovering how to solve this yourself.

8. Dave,

Since he mentions cooling water from 75 to 35 deg C , it looks like a heat exchanger problem, not merely cooling pipe
He's pretty inexperienced and I wanted to show that it is not a trivial problem.

9. hi the amount of heat is to be extracted is 250w, and temperature limits are 70 to 35
u can use the formula " Q=ha(t1-t2)"
where q= amount of heat transferd in ur problem it is 250
h=convective heat transfer co efficient of air
a = surface area
t1= intial temperature of the air
t2 = final temperature of the air

with that u will know the value of t2

then substute it in Q=mc(t1-t2)
where m= mass of air required
c= sp.heat of air
from this u will get mass of air

10. Thanks for your reply! I still have a question though. If the temperature of the environment is ~23°C, I still don't know what my t1 and t2 are. Wouldn't they be the same?

Originally Posted by harsha.mech360
hi the amount of heat is to be extracted is 250w, and temperature limits are 70 to 35
u can use the formula " Q=ha(t1-t2)"
where q= amount of heat transferd in ur problem it is 250
h=convective heat transfer co efficient of air
a = surface area
t1= intial temperature of the air
t2 = final temperature of the air

with that u will know the value of t2

then substute it in Q=mc(t1-t2)
where m= mass of air required
c= sp.heat of air
from this u will get mass of air

11. Originally Posted by Vig16
I still don't know what my t1 and t2 are. Wouldn't they be the same?
Only if there was no heat transfer. Think about it. You want to transfer heat from one thing to another.

Dave
Generally, I will not give you the answer to your question, but I **will** guide you into discovering how to solve this yourself.

12. Originally Posted by Vig16
Thanks for your reply! I still have a question though. If the temperature of the environment is ~23°C, I still don't know what my t1 and t2 are. Wouldn't they be the same?
it is t2... with this u can get t1

13. Oh, OK...thanks everyone!

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