# Car AC theory question

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• posted on March 15, 2013, 2:13 am

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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.....
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<%-name%>
• posted on March 15, 2013, 2:34 am
On Thursday, March 14, 2013 10:13:27 PM UTC-4, snipped-for-privacy@optonline.net wrote:

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.
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<%-name%>
• posted on March 15, 2013, 2:56 am

All depends on vehicle size. Imagine a van. I don't think a small car needs more than 2 ton.
Greg
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<%-name%>
• posted on March 15, 2013, 3:01 am

We most always had to use recirculate in the summer desert. Often barely comfortable.
Greg
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<%-name%>
• posted on March 15, 2013, 9:05 am
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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.
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<%-name%>
• posted on March 15, 2013, 12:11 pm
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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.
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<%-name%>
• posted on March 15, 2013, 7:26 am
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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. http://www.powdercoating.romerpp.pl/kategoria/3_50_84/Oven_with_forced_air_ circulation.html
Without a fan, the correct term is "passive/natural air circulation".
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<%-name%>
• posted on March 15, 2013, 12:28 pm

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http://en.wikipedia.org/wiki/Convection
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.
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<%-name%>
• posted on March 15, 2013, 2:42 pm
On 3/15/2013 6:28 AM, snipped-for-privacy@optonline.net wrote:

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
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<%-name%>
• posted on March 15, 2013, 2:58 pm
On Mar 15, 9:42 am, The Daring Dufas <the-daring-du...@stinky- finger.net> wrote:

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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.
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<%-name%>
• posted on March 15, 2013, 6:08 pm
On 3/15/2013 8:58 AM, snipped-for-privacy@optonline.net wrote:

That's what I was trying to remember. When energy is added to a molecule I believe it gains electrons which would account for the increase in spacing so the f/l itself gets less dense not the molecule. Hell, I wrote the original post after I had just awakened hoping my pain meds would kick in. I had a rough day yesterday since I figured out what was giving me chest pains, it was Naproxen Sodium. Now I must switch to another anti-inflammatory so I won't have so much trouble typing. o_O
TDD
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<%-name%>
• posted on March 15, 2013, 5:32 pm
On Mar 15, 1:09 pm, The Daring Dufas <the-daring-du...@stinky- finger.net> wrote:

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no it doesn't gain electrons. Where would these new electrons come from? The molecules just have more energy, so they move around more, increasing the seperation.

Uh oh. I hope you get that figured out...
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<%-name%>
• posted on March 15, 2013, 6:54 pm
On 3/15/2013 11:32 AM, snipped-for-privacy@optonline.net wrote:

It could be more stuff I forgot after all these years. I can't remember the specifics but I suppose I was thinking the change in energy level was caused by added electrons but now I seem to recall the added energy is simply winding up the molecule increasing the speed of particles and changing their orbit around the nucleus. Since stable identical molecules repel each other perhaps this increase in energy causes the increase in spacing. Heck it's been so many years since I was immersed in physics and I been busy making a living so I've used those brain cells for other things. It's a good thing I have Internet access, now I can go back and refresh my memory. I remember living in libraries but I can't sit still that long anymore. ^_^

Thanks, my physician friend who helps me is a cardiologist and we discus what works best since I despise drugs and having to take the darn things. I'll probably wind up on another anti-inflammatory at a lower dose that won't melt my brain. o_O
TDD
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<%-name%>
• posted on March 15, 2013, 3:38 pm
On Mar 15, 1:42 pm, The Daring Dufas <the-daring-du...@stinky- finger.net> wrote:

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Exactly correct Duf.
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<%-name%>
• posted on March 15, 2013, 6:15 pm
On 3/15/2013 9:38 AM, harry wrote:

Well, it's close, I forgot it's not the molecule itself that gets less dense but as Trader4 pointed out it's the spacing between the molecules that increases which makes the gas or liquid less dense. I was a physics major in college 40 years ago and I can't remember how to write a fancy calculation even if you put a gun to my head. ^_^
TDD
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<%-name%>
• posted on March 15, 2013, 3:36 pm
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I see you snipped the bit we are talking about.
http://en.wikipedia.org/wiki/Convection#Natural_convection Shitfer brains.
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<%-name%>
• posted on March 13, 2013, 2:02 am
Ashton Crusher wrote:

I had that discussion a few years ago with people in sci.physics about my house new outside a/c unit. The conclusion was that had I painted the outside unit black, the infrared radiation would have been infinitesimal compared to mass heat transfer due to air flow (as JB mentioned).
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<%-name%>
• posted on March 13, 2013, 2:10 am
Ashton Crusher wrote:

The paint keeps the aluminum from developing an oxide layer. Aluminum oxide is a good insulator. Usually the units get formed, brazed then tested. Then into an acid bath to clean off any oxide and prep the metal then into a dip tank to get a THIN coat of paint.
The plain aluminum ones usually have either clear or clear anodizing to keep them from oxidizing.
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Steve W.

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<%-name%>
• posted on March 13, 2013, 2:14 am
On 03/12/2013 07:10 PM, Steve W. wrote:

what you've just described is for copper, not aluminum. aluminum oxide is not a bad thermal conductor - it's why sintered alumina is used for spark plug insulators.

indeed.
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fact check required

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<%-name%>
• posted on March 13, 2013, 2:21 am
On 03/12/2013 07:14 PM, jim beam wrote:

clarification: spark plugs need good electrical insulators that are good thermal conductors. alumina is one of those materials.

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fact check required