Car AC theory question

[trader4]

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.

[dpb]

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... :) )

[jim beam]

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.

no, it's suggested reading.

[jim beam]

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.

[trader4]

te:

Do you still deny that convection is the primary energy transfer mechanism in a car radiator?

Do you still deny that convection can only exist if it's natural convection? If the answer to the above is yes, please explain:

A - Convection ovens, which use a FAN . I have two here so I know they exist.

B- Why numerous references, some of which have been cited, talk about natural and forced convecton. I have yet to see your reference that says convection can only be natural, ie without a fan or pump.

C- Why engineers treat air to water heat exchangers using CONVECTION, not mass transfer

And citing a book cover on Amazon is not a scientific reference.....

[TimR]

Do chemical engineers consider mass flow and mass transfer to be very diffe rent things? If mechanical engineers do, then that's either a recent chang e or my memory has completely faded. I haven't actually done this calculat ion in a long long time, I became a suit.

But anyway. The very FIRST thing a mechanical engineer does in analyzing a n air to fluid heat exchanger, after drawing the system boundaries of cours e, is a Mass Balance. The second thing is an Energy Balance. The third th ing is an Entropy Balance. Normally all three are required for a solution. Sometimes the Entropy Balance is called the Availability Balance.

The mechanism by which heat is carried away from a car radiator is the flow of mass called convection. Convection can be forced or free in the mechan ical engineer's world, that may not be true for physicists, who knows?

I looked it up, my memory was correct. A car needs a good 3 tons of AC, mu ch like an average sized house.

[gregz]

All depends on vehicle size. Imagine a van. I don't think a small car needs more than 2 ton.

Greg

[gregz]

We most always had to use recirculate in the summer desert. Often barely comfortable.

Greg

[harry]

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.  

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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.

[harry]

rote:

These are misnomers. The terms have come into use through attempts by salespeople to differentiate between normal and fan ovens.

The correct term would be "forced air circulation ovens" Commercial ovens are identified as such.

circulation.html

Without a fan, the correct term is "passive/natural air circulation".

[Ashton Crusher]

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.

[Ashton Crusher]

I've been looking into adding a unit and have found that most of the underdash units are 15000 BTU. Some of the bus type units are 24000 BTU. I also found some info that suggests that most OEM factory units are around 22000 BTU's. I also found info on the ratings of compressors at various rpm. The smaller ones may only deliver 10,000 BTU's at 1000 rpm and max out at 20,000 at their peak. Some of the larger ones will peak at around 25000 or a bit more. So it looks to me like the most you can expect for a car is a bit more then 2 tons of capacity. Still, that's a lot considering it would cool a 1200 sf house.

[trader4]

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.

[trader4]

:

ferent things?

Why do you answer a question with a question and try to take this even further off point? I posed a few very simple, yes or no questions that go directly to the point of the discussion:

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.

Do you agree that convection is the predominant heat tranfer mode? Or do you agree with JB that convection is not involved?

The above is the core of the issue.

[trader4]

de quoted text -

Convection is the concerted, collective movement of ensembles of molecules within fluids (e.g., liquids, gases) and rheids.

The term convection may have slightly different but related usages in different scientific or engineering contexts or applications. The broader sense is in fluid mechanics, where convection refers to the motion of fluid regardless of cause.[2][3] However in thermodynamics "convection" often refers specifically to heat transfer by convection. [4]

Additionally, convection includes fluid movement both by bulk motion (advection) and by the motion of individual particles (diffusion). However in some cases, convection is taken to mean only advective phenomena. For instance, in the transport equation, which describes a number of different transport phenomena, terms are separated into "convective" and "diffusive" effects, with "convective" meaning purely advective in context.

Forced convection: In forced convection, also called heat advection, fluid movement results from external surface forces such as a fan or pump. Forced convection is typically used to increase the rate of heat exchange.

Moron.

[TimR]

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.

Of course, those who say a radiator doesn't radiate are correct.

[trader4]

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.

[The Daring Dufas]

such.http://www.powdercoating.romerpp.pl/kategoria/3_50_84/Oven_with_force...

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Let me take a shot at in a different way. If you have a gas/liquid the density of the g/l is determined by the size of the molecules. It seems to me that adding energy to a molecule would would increase its size and make it less dense than the surround molecules in your g/l. When there is gravity, the lighter molecules would tend to migrate to the top of a container and if collected and cooled to a temperature below that of your existing g/l, those cooled molecules would have less energy thus less density than the g/l and would tend to migrate toward the bottom of your container. If you are a PhD and this is wrong thinking don't howl at me because it's a SWAG made from observation and could be totally wrong and too simplistic. ^_^

TDD

[trader4]

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Yes, that's the essence of the idea of how fluids move via natural convection. Except that it's not the size of the molecules that changes, it's the spacing between them. The more energy they have, the more they are bouncing around, the more space between them.

[harry]

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