Why is hot dry air impossible/impractical?

Posted by Green Xenon [Radium] on September 29, 2008, 8:41 pm

On Sep 29, 10:49 am, .p.jm@see_my_sig_for_address.com wrote:

> He thinks
> radiant heat transfer is a bi-directional force.

It is a bi-directional force. It is possible for a colder object to
cool a hotter object via radiation. What makes you think it isn't?

In the above scenario, the hotter object will warm the colder object
by emitting IR radiation. Why do you say this is impossible, when it
clearly does happen in everyday life?

If you put your hand close to cold-metal [without touching it], the
cold you feel is a result of your hand's temperature decreasing. This
decrease in temperature is the result of your hand transferring heat
to the cold metal via electromagnetic radiation.

Posted by on September 30, 2008, 7:56 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?

>Huh?

The formulas are well-known. If a room is 70 F and the fire is Tf,
the room gains Qc = 0.19(Tf-70)^1.33 Btu/h-ft^2 by convectiion and
Qr = 0.1714x10^-8((460+Tf)^4-(460+70)^4) Btu/h-ft^2 by radiation.

What are Qc and Qr when Tf = 100, 1000, and 10,000 F?

If Qc = Qr, what's Tf?

Nick

Posted by old and grunpy on September 29, 2008, 8:43 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.

WRONG!!!!!!

>> 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 Green Xenon [Radium] on September 29, 2008, 8:54 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.

> WRONG!!!!!!

Convection is what causes the fire to point upwards. Hot air travels
up via convection. Put your hand below or on the side of a flaming
candle, you won't feel much heat [esp. below the below the flame].
However, if you place your hand above the candle, you get a painful
amount of heat. This is because most of the flame emits heat via
convection. Only a trace amount of the fire's heat is emitted through
radiation.

Posted by old and grunpy on September 29, 2008, 10:02 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.

>> WRONG!!!!!!

> Convection is what causes the fire to point upwards. Hot air travels
> up via convection. Put your hand below or on the side of a flaming
> candle, you won't feel much heat [esp. below the below the flame].
> However, if you place your hand above the candle, you get a painful
> amount of heat. This is because most of the flame emits heat via
> convection. Only a trace amount of the fire's heat is emitted through
> radiation.

Well I am not to argue with you, however any benefits that you are getting
from fire is radiant heat what ever goes up it is waste and yes it is
definitely hot strait above of it however I would not call it convection
heat.
Convection heat is consider your base board water or electric as long
you don't have forced air over but natural convection.
Tony

Page 2 of 2 << first < 1 2

Why is hot dry air impossible/impractical? - Page 2