Why is hot dry air impossible/impractical?

Posted by Green Xenon [Radium] on September 28, 2008, 11:17 pm

On Aug 9, 8:02 pm, .p.jm@see_my_sig_for_address.com wrote in
http://groups.google.com/group/alt.hvac/msg/9646d0d0d4cfa110?hl=en :

> Think of this - stand next to a campfire on a cold night. up
> close = toasty warm. 10 feet away = cold.

The overwhelming majority of heat emitted from a fire, is convective
heat, not radiant heat. There is some radiant heat but it is very
small compared to the convective heat.

> That is radiant heat in
> action.
> The power falls off as the square of the distance. "radiant
> cooling', if there were such a thing ( there is not, just as there is
> no such thing as 'cold', there is only 'absence of heat' ) would work
> exactly the same.

Um, the human body can give of heat via radiation as well as
conduction and convection. If you put your hand near a piece of
extremely cold metal, you'll feel a perceptible amount of cold even if
you don't touch the metal. This is an example of radiant cooling.
There is a sharp difference in temperature between your hand and the
cold metal. Physics wants to equalize the temperature and will attempt
in whatever way possible to do so. If you are not touching the metal
[a painful conductive cooling], then the next option to equalize the
temperature is for your hand to emit IR radiation and warm the metal.
In this case, your hand is the thermal radiator. Your hand emits
radiant heat toward the cold metal.

To your hand, this is radiant cooling. For the cold metal, it is an
example of radiant heating, because the IR radiation from your hand
will warm up the metal.

> And when you heat a house, you do NOT want 'dry air'.

Why not? What's wrong with dry convective heating?

> Even
> if you could have it, which you can't.

What makes hot dry air impossible?

Posted by on September 28, 2008, 11:31 pm

On Sun, 28 Sep 2008 20:17:18 -0700 (PDT), "Green Xenon [Radium]"

>On Aug 9, 8:02 pm, .p.jm@see_my_sig_for_address.com wrote in
>http://groups.google.com/group/alt.hvac/msg/9646d0d0d4cfa110?hl=en :

>> Think of this - stand next to a campfire on a cold night. up
>> close = toasty warm. 10 feet away = cold.

>The overwhelming majority of heat emitted from a fire, is convective
>heat, not radiant heat. There is some radiant heat but it is very
>small compared to the convective heat.

Not unexpectedly, you missed the entire point.

>> That is radiant heat in
>> action.
>> The power falls off as the square of the distance. "radiant
>> cooling', if there were such a thing ( there is not, just as there is
>> no such thing as 'cold', there is only 'absence of heat' ) would work
>> exactly the same.

>Um, the human body can give of heat via radiation as well as
>conduction and convection. If you put your hand near a piece of
>extremely cold metal, you'll feel a perceptible amount of cold even if
>you don't touch the metal. This is an example of radiant cooling.
>There is a sharp difference in temperature between your hand and the
>cold metal. Physics wants to equalize the temperature and will attempt
>in whatever way possible to do so. If you are not touching the metal
>[a painful conductive cooling], then the next option to equalize the
>temperature is for your hand to emit IR radiation and warm the metal.
>In this case, your hand is the thermal radiator. Your hand emits
>radiant heat toward the cold metal.
>To your hand, this is radiant cooling. For the cold metal, it is an
>example of radiant heating, because the IR radiation from your hand
>will warm up the metal.

>> And when you heat a house, you do NOT want 'dry air'.

>Why not? What's wrong with dry convective heating?

>> Even
>> if you could have it, which you can't.

>What makes hot dry air impossible?

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Posted by on September 29, 2008, 4:55 am

>The overwhelming majority of heat emitted from a fire, is convective
>heat, not radiant heat. There is some radiant heat but it is very
>small compared to the convective heat.

What fire temperature makes them equal?

Nick

Posted by Green Xenon [Radium] on September 29, 2008, 1:24 pm

On Sep 29, 1:55=A0am, nicksans...@ece.villanova.edu wrote:

> >The overwhelming majority of heat emitted from a fire, is convective
> >heat, not radiant heat. There is some radiant heat but it is very
> >small compared to the convective heat.

> What fire temperature makes them equal?

Huh?

Posted by on September 29, 2008, 1:49 pm

On Mon, 29 Sep 2008 10:24:26 -0700 (PDT), "Green Xenon [Radium]"

>On Sep 29, 1:55 am, nicksans...@ece.villanova.edu wrote:

>> >The overwhelming majority of heat emitted from a fire, is convective
>> >heat, not radiant heat. There is some radiant heat but it is very
>> >small compared to the convective heat.

>> What fire temperature makes them equal?

>Huh?

Don't bother, Nick. He's denser than you are :-) He thinks
radiant heat transfer is a bi-directional force.

--
Click here every day to feed an animal that needs you today !!!
www.theanimalrescuesite.com/

Paul ( pjm @ pobox . com ) - remove spaces to email me
'Some days, it's just not worth chewing through the restraints.'
'With sufficient thrust, pigs fly just fine.'
HVAC/R program for Palm PDA's
Free demo online at www.pmilligan.net/palm/
Free 'People finder' program now at www.pmilligan.net/finder.htm

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