| Topic: |
Science > Physics |
| User: |
"nzswede" |
| Date: |
28 Apr 2006 11:42:25 PM |
| Object: |
Heat Transfer In Air |
I was wondering if anyone knows how to calculate heat transfer in air.
What I'd like to find out is the temperature a certain distance away
from a heated pipe.
For example: If I heat up a 30mm pipe to X degrees. What temperature
will I get Y mm above it if the surrounding temperature is Z degrees
and there is no wind?
Cheers
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| User: "Jan Panteltje" |
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| Title: Re: Heat Transfer In Air |
29 Apr 2006 04:54:18 AM |
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On a sunny day (28 Apr 2006 21:42:25 -0700) it happened "nzswede"
<nzswede@yahoo.com> wrote in
<1146285745.895445.178110@y43g2000cwc.googlegroups.com>:
I was wondering if anyone knows how to calculate heat transfer in air.
What I'd like to find out is the temperature a certain distance away
from a heated pipe.
For example: If I heat up a 30mm pipe to X degrees. What temperature
will I get Y mm above it if the surrounding temperature is Z degrees
and there is no wind?
Cheers
Amusing is perhaps, that in a case like this, the time to try it (measure it),
versus the time to lookup the correct formula or theory, will, or SHOULD
have, some influence over what you are going to do to get the answer.
Especially with airflow added etc...
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| User: "nzswede" |
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| Title: Re: Heat Transfer In Air |
30 Apr 2006 04:53:07 PM |
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Thank you all for your comments. It seems like this is way more
complicated than I first thought.
This is just a little experiment I'm carrying out at home and it
doesn't have to be 100% accurate. What if we assume there is only
convection? Would it make it easier?
Please simplify any answer as much as possible since I'm not that
experienced in this area.
Thanks again everyone.
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| User: "Tony" |
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| Title: Re: Heat Transfer In Air |
30 Apr 2006 07:19:50 PM |
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Here's a rough rule of thumb electrical transformer designers used to
use. If the transformer surfaces are exposed to still air, the temp
rise of the transfomer is about 120 degrees C for every watt per square
inch convected away as heat. A fan forcing air across it can improve
that number by a factor of 10 or so.
The estimate works pretty well for heatr loss from hot water pipes and
electrical resistor temp rises as well.
You can even use it to calculate how long it's take that can of Coors
to cool if you put it outside on a cold night. This is a practical
application of skin effect and convective cooling!
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| User: "nzswede" |
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| Title: Re: Heat Transfer In Air |
30 Apr 2006 11:06:28 PM |
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Thanks Tony, this is interesting.
Let's see if I can get this right.
If 1 Watt heats up 1 square inch to 120 degrees C, then since I am a
metric guy this will be equal to 1 Watt heats up 6.45 square
centimeters to 120 degrees C.
I have a 100 centimeter copper pipe with a diameter of 3cm (Surface
area = 943cm2).
943/6.45 = 146.2 times more then 1 square inch.
If 1W / 6.45cm2 = 120 degrees, then I would need 146.2W / 943cm2 = 120
degrees (I hope I'm still correct)?
If these values are constant I can change (half the wattage) this to
i.e.
73.1W / 943cm2 = 60 degrees ???
If this is correct I have got half the answer I was looking for. Then,
what will the temperature be right above this pipe, let's say 50
centimeters? The pipe is hanging horizontally.
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| User: "Jan Panteltje" |
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| Title: Re: Heat Transfer In Air |
01 May 2006 04:47:51 AM |
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On a sunny day (30 Apr 2006 21:06:28 -0700) it happened "nzswede"
<nzswede@yahoo.com> wrote in
<1146456388.047002.180370@i39g2000cwa.googlegroups.com>:
Thanks Tony, this is interesting.
Let's see if I can get this right.
If 1 Watt heats up 1 square inch to 120 degrees C, then since I am a
metric guy this will be equal to 1 Watt heats up 6.45 square
centimeters to 120 degrees C.
I have a 100 centimeter copper pipe with a diameter of 3cm (Surface
area = 943cm2).
943/6.45 = 146.2 times more then 1 square inch.
If 1W / 6.45cm2 = 120 degrees, then I would need 146.2W / 943cm2 = 120
degrees (I hope I'm still correct)?
If these values are constant I can change (half the wattage) this to
i.e.
73.1W / 943cm2 = 60 degrees ???
If this is correct I have got half the answer I was looking for. Then,
what will the temperature be right above this pipe, let's say 50
centimeters? The pipe is hanging horizontally.
The problem (also with that transformer), is that in the real world
things are often in an enclosed space.
Transformers in electronic equipment for example.
In the question you ask:
Take a candle.
Hold your hand 10 cm next to it.
You do not burn.
If you did hold your hand 10 cm above it... not so good, do not do it,
you can boil water that way.
So the direction of the air flow COUNTS.
And in any enclosed space CIRCULATION of that air will start to happen (candle
is the air pump).
How the air circulates, what objects it touches in the process, the *thermal
resistance* of those objects to a cooler or warmer outside world, all have influence
on the temperature of that air.
Even if you did the most beautiful math simulation, and used a supercomputer,
you STILL would need to measure it to make sure your model is right.
'We have calculated it to be safe' will get you in trouble end endless reviews.
'We have measured it in worst case circumstances (specify range) to be OK',
will make you sleep better.
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| User: "Tony" |
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| Title: Re: Heat Transfer In Air |
01 May 2006 05:10:50 AM |
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You're on the right track. This approximation will suggest how much
power you'll have to supply in a circumstance where the heat loss is
mostly convective, but you are lots of bigger assumptions away from
estimating the temperature of the plume of air above the heat source.
The approximation is useful, for example, when choosing heat sinks for
components, for estimating how hot an enclosure will get given a
thermal heat load, that sort of thing. It will let you guess at the air
temp very near the surface as well. Knowing that and the thermal
capacity of air will give you a guess as to what effective volume of
air has to moving to maintain those boundary conditions near the pipe.
How that plume diffuses is way past any rule of thumb I know.
In real life if your estimate of the maximum air temp is too high, just
add fins to the pipe if you are in a circumstance where a given amount
of power has to be given up to the air, or add insulation if the goal
is reduce heat loss.
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| User: "PD" |
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| Title: Re: Heat Transfer In Air |
29 Apr 2006 08:43:05 AM |
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nzswede wrote:
I was wondering if anyone knows how to calculate heat transfer in air.
What I'd like to find out is the temperature a certain distance away
from a heated pipe.
For example: If I heat up a 30mm pipe to X degrees. What temperature
will I get Y mm above it if the surrounding temperature is Z degrees
and there is no wind?
Cheers
Oooh, way complicated. That's because there is more than one process
involved.
a) Convection
b) Radiation
c) Conduction
There are expressions for all three, but which one is dominant (if any)
depends on many parameters.
As an amusing exercise on this score, it is useful to try to estimate
how much a fireplace *accelerates* room cooling on a winter day.
PD
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| User: "" |
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| Title: Re: Heat Transfer In Air |
02 May 2006 05:57:44 PM |
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PD wrote:
nzswede wrote:
I was wondering if anyone knows how to calculate heat transfer in air.
What I'd like to find out is the temperature a certain distance away
from a heated pipe.
For example: If I heat up a 30mm pipe to X degrees. What temperature
will I get Y mm above it if the surrounding temperature is Z degrees
and there is no wind?
Cheers
Oooh, way complicated. That's because there is more than one process
involved.
a) Convection
b) Radiation
c) Conduction
There are expressions for all three, but which one is dominant (if any)
depends on many parameters.
As an amusing exercise on this score, it is useful to try to estimate
how much a fireplace *accelerates* room cooling on a winter day.
I actually did this, er, um, experiment last winter. The house cooled
by one degree Fahrenheit after running the fireplace for, oh, about an
hour. (At least, as measured by the thermostat in the hallway...)
Trying to explain this cooling to my wife, her reply: "Well, it's
night time, so of course the house is cooling down..."
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| User: "mushtaq" |
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| Title: Re: Heat Transfer In Air |
30 Apr 2006 04:34:58 PM |
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assum the process mainly as convection,,, the radiation factor can be
included in it,,,negelc conduction
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| User: "cnctut" |
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| Title: Re: Heat Transfer In Air |
02 May 2006 01:51:01 PM |
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nzswede wrote:
I was wondering if anyone knows how to calculate heat transfer in air.
What I'd like to find out is the temperature a certain distance away
from a heated pipe.
For example: If I heat up a 30mm pipe to X degrees. What temperature
will I get Y mm above it if the surrounding temperature is Z degrees
and there is no wind?
Cheers
nzswede--
Recommend you Google "Coefficient of Heat Transmission" to get you
started. Also, any older books on steam or hot water heating will help
greatly. A good one is "Air Conditioning, Heating and Ventilating"--War
Department Education Manual EM 978 by J. Ralph Dalzell and Charles L.
Hubbard--publisher American Technical Society Aug 1944.
Roughly speaking--
Heat given off by 30MM pipe (BTU's/hr) = K x (temperature hot surface-
temperature of surrounding air)
where K=coefficient of heat transmission for your pipe--probably around
1.95-1.70 depending on pipe length.
Once you have BTU's/hour you can approximate the temp rise you desire.
Sorry, I know this doesn't completely answer your question, but should
get you started with some approximations.
Good luck,
Tut
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