# Car AC theory question

On Wed, 13 Mar 2013 12:45:22 -0500, Vic Smith

It is not even that much convection. The fluids on both sides of the heat exchanger are both pumped.
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On Mar 13, 3:11 pm, snipped-for-privacy@aol.com wrote:

It's still convection even though the fluids involved air pumped.
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On 03/13/2013 10:45 AM, Vic Smith wrote:

because that's what it is, technically speaking. it's not convection in the traditional sense - it's forced transfer of a medium typically characterized by its mass. you've heard of air conditioners rated by their tonnage haven't you?
<http://easycalculation.com/weather/ac-tonnage.php > The heat exchange is nearly all conduction and convection
technically, it's not convection because that is what occurs unaided.

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WTF? That's got nothing to do with "mass transfer," which is a term used for other processes. I looked it up because I never heard the term used when talking about heat transfer. You won't find mass transfer conflated with heat transfer anywhere. Analogies only.

Kiss my ass with your condescending bullshit. . There's forced convection and natural convection. I was a trained Navy boilerman for 4 years, further studied heat exchange while in the Merchant Marine, worked in the power generating plants at U.S. Steel, had a stationary engineers license, and worked on plenty of car cooling systems. Nobody but you ever used the term "mass transfer" to describe a simple car cooling system heat exchange process. Only you. The term "mass transfer" is mostly used in chemical engineering. According to Wiki, "The driving force for mass transfer is typically a difference in chemical potential." Heat exchange is driven by temperature differences. You can study the details if you like. I'm done caring now.
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On 03/13/2013 07:17 PM, Vic Smith wrote:

why are you on a "tech" forum if you're not interested in using technical language? to put this another way, if people use language that you don't understand, why is it their problem not yours?

!!!

???
<http://en.wikipedia.org/wiki/Convection> > Analogies only.
no, science only.

it's not condescending, it's helpful to people on a "tech" forum with an interest in understanding what they don't know.

i don't know where you got that little nugget - you don't cite a link - but there's no "chemical" potential here.

i do like. that's why i have studied them. why someone who doesn't and hasn't thinks they're better informed is the confusing bit.

pretzel logic given that you took the trouble to tell everybody what you think.
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Mass is not being transfered across the system boundary. Only heat. Yes, water flows in the engine cooling system and air flows through the radiator, but no water is transfered into the air and vice-versa.

Yes, we've not only heard of it, but actually understand it. It has nothing to do with mass transfer. A ton in HVAC terms is the amount of energy it takes to transform a ton of ice at 32F into water. Again, no mass is transfered, it just melts.

That's just wrong.

You use Amazon as a reference for physics and engineering?

That's for sure!
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On 03/14/2013 06:01 AM, snipped-for-privacy@optonline.net wrote:

that's deliberate obfuscation. air [mass] is transferred within the greater body of itself. that's why "mass transfer" is used to describe the process which for free bodies is otherwise known as "convection" or forced systems, "advection".

!!!

look up the definition of "convection".

ah, so you're not serious. should have guessed.

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It's not obfuscation, it's basic physics.
air [mass] is transferred within the

BS. Convection is convection whether it's natural or forced. An example of natural convection is hot air rising above an electric stove burner. An example of forced convection is the air flowing through the car radiator. If we're all wrong, then maybe you can explain all the "convection ovens" being sold and used. I have two here in the house. Both have fans. When you select convection heating, the fans turn on to circulate air within the oven cavity. That is how a convection oven works. Very basic.

If you have some point, !!! doesn't make it. I would note that you accused me of obfuscation. YOU asserting that "tons" in an HVAC context has something to do with mass transfer, that it proves your point, now that's obfuscation. Tons in that context is a measure of heat energy, nothing more. I hope you accept that now.

I have, perhaps you should too. Here, it could not be spelled out any clearer:
http://en.wikipedia.org/wiki/Convective_heat_transfer
"Convection can be "forced" by movement of a fluid by means other than buoyancy forces (for example, a water pump in an automobile engine). In some cases, natural buoyancy forces alone are entirely responsible for fluid motion when the fluid is heated, and this process is called "natural convection." An example is the draft in a chimney or around any fire. In natural convection, an increase in temperature produces a reduction in density, which causes fluid motion due to pressures and forces when fluids of different densities are affected by gravity (or any g-force). "
It's particularly interesting that as an example, they actually use an auto cooling system. That should remove any doubt.

That link is to a book being sold on Amazon. It's a college engineering text book on heat and mass transfer. So, what on earth are you implying that picture of the book proves? That's a scientific reference? A book cover? Good grief.
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On 03/14/2013 10:35 AM, snipped-for-privacy@optonline.net wrote: <snip crap>

um, i'm not referring to the "picture" but the contents. but you'd recognize that if you weren't contriving to be so disingenuous.

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On 3/14/2013 10:02 AM, jim beam wrote:

...

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Not, hardly. Read the introductory chapter of the granddaddy of all transport phenomena texts, Bird, Stewart, and Lightfoot. It's preview is available at <(Amazon.com product link shortened)>
The first few paragraphs of Chap 0 and the table of processes on p. 4 (Table 0-2.1) (and where they're covered in the book) is sufficient.
This is a dual-loop, closed system forced convection heat transfer problem. No mass transfer in the analytic meaning thereof is occurring.
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On Thursday, March 14, 2013 3:03:17 PM UTC-4, dpb wrote:

To me it seems you are being unnecessarily pedantic. It appears that the c hemical engineering usage of "mass transfer" would not be correctly applied to this system. I'm guessing because I'm not a chemical engineer, I'm a m echanical engineer. Mechanical engineers do not have the same precise usag e standard and very well might talk about mass transfer in this problem. C ertainly mass crosses the system boundaries at a high rate.
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he chemical engineering usage of "mass transfer" would not be correctly app lied to this system.  I'm guessing because I'm not a chemical engineer, I 'm a mechanical engineer.  Mechanical engineers do not have the same prec ise usage standard and very well might talk about mass transfer in this pro blem.  Certainly mass crosses the system boundaries at a high rate.  - Hide quoted text -

Maybe we should back up to where this rat hole started. The discussion was about whether paint color of the radiator made a difference in heat transfer, which lead into a discussion of how heat is transfered from the radiator. We then had Jim Beam claiming that a car radiator does not transfer heat to the air via convection, that it works via mass transfer.
There are 3 modes of heat transfer, conduction, convection and radiation, correct? We're talking about how the heat leaves the car radiator. I say the vast majority, probably 90%+ is by convection, that is the air moving through the radiator. A small amount is by conduction, that is heat transfering from the radiator to the surrounding metal that it's touching, etc. And a small amount is leaving via radiation.
I think the essential hangup here is that JB refuses to accept that convection can be natural or forced.
Do you agree that convection is the predominant heat tranfer mode? Or do you agree with JB that convection is not involved? And if you agree that it's via convection, then I don't believe you'd find mechanical engineers approaching this as a mass transfer problem.
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wrote:

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the chemical engineering usage of "mass transfer" would not be correctly a pplied to this system.  I'm guessing because I'm not a chemical engineer, I'm a mechanical engineer.  Mechanical engineers do not have the same pr ecise usage standard and very well might talk about mass transfer in this p roblem.  Certainly mass crosses the system boundaries at a high rate.   - Hide quoted text -

I think he thinks (as I do) that convection refers to the buoyancy of a fluid due to temperature differences. Movement due to mechanical means is nothing to do with convection Ergo "forced convection" does not exist. A misnomer. Assisted convection exists. It's about terminology. A car "radiator" in fact radiates very little heat. Another misnomer. These terms arose historically when people had little understanding of what was going on and are best avoided as they confuse the simple minded.
I would say that the majority of the heat transfer in an automobile radiator is by conduction. The heat has to get from the water into the metal and from the metal into the air. Both air and water have to be moved mechanically because convection is negligable.
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at the chemical engineering usage of "mass transfer" would not be correctly applied to this system.  I'm guessing because I'm not a chemical enginee r, I'm a mechanical engineer.  Mechanical engineers do not have the same precise usage standard and very well might talk about mass transfer in this problem.  Certainly mass crosses the system boundaries at a high rate.  - Hide quoted text -

Anytime anyone thinks like you do, that's a pretty good indication that they are probably wrong. You've demonstrated that you're the village idiot over and over again. You just did it again. What you "think" convection means isn't relevant. How it's defined and used does.

Wrong.
A misnomer.  Assisted

Which you obviously do not understand and instead of educating yourself, here you are making a fool of yourself again.

That would be you.

Wrong yet again.
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wrote:

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that the chemical engineering usage of "mass transfer" would not be correct ly applied to this system.  I'm guessing because I'm not a chemical engin eer, I'm a mechanical engineer.  Mechanical engineers do not have the sam e precise usage standard and very well might talk about mass transfer in th is problem.  Certainly mass crosses the system boundaries at a high rate.  - Hide quoted text -

I see you were educated in n the hillbilly school of technology
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On Friday, March 15, 2013 3:15:19 AM UTC-4, harry wrote:

Well, that would be wrong, but it is an understandable mistake.
You're focusing on the movement of the fluid.
Convection refers to the movement of the HEAT, not the fluid. Moving air, water, or any other fluid can carry more heat away. As far as the heat transfer is concerned, what causes the fluid to move is not relevant.
Heat transfer class, at 0800 in the morning after I'd worked the night shift (yuck) covered free and forced convection, transient and steady state. Four chances to get it wrong.
In one sense, convection IS also conduction. The transfer of the heat from the exchanger metal to the boundary layer of the fluid is conduction. This is normally ignored as it is not the limiting factor.
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Wrong again. Fill a beaker sitting on a burner with water. Use a pipette to put a few drops of blue dye at the very bottom. Heat it and watch what happens. The blue dye water starts rising via CONVECTION.
Moving air, water, or any other fluid can carry more heat away.  As far as the heat transfer is concerned, what causes the fluid to move is not relevant.

ift (yuck) covered free and forced convection, transient and steady state.  Four chances to get it wrong.

No it's not.

the fluid is conduction.
That's true.

No, that's wrong too. It does radiate, it's just that if it's a car radiator, home heating radiator, etc radiation is not the main and most significant heat transfer mechanism. Convection is.
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On 3/14/2013 2:18 PM, TimR wrote:

It isn't unnecessary nor pedantic either one imo.
If mechanical engineers didn't have precise definitions/usage then they'd never get anywhere. That they do (get somewhere, that is) implies a rigorous set of definitions. :)
There's mass flow but not mass transfer at work here.
The key point though, that the dominant heat transfer mechanism is forced convection and the point to the naysayer was to show the table that there _are_ precise meanings for the terms.
(Strictly speaking I'm not a ChE, either, I'm NucE w/ ChE minor undergrad and Phys/NucSci grad... :) )
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The discussions have been interesting but I'm still wondering just how much, if any, the layer of paint reduces the effectiveness of the AC condenser, or the engine radiator. I have to think that adding a layer of anything to a "radiator" reduces it's ability to "let the heat out". But how much? Surely if you picture a tube type radiator, like some of the long looped tubes (without fins) used as power steering or oil coolers it would seem logical to think that if you put a nice thick coat of paint on them they would become a lot less effective as an oil cooler. If you add the fins, and then paint the whole surface area, same thing would seem like it would happen.
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On 03/14/2013 12:03 PM, dpb wrote:

are you for real? do you understand what you're reading? if you think that contradicts a single thing i said, you have a serious comprehension problem.

comprehension problem. there is one closed loop on an automotive engine coolant system. the other is wide freakin' open.
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