Wind chill (2 Viewers)

[jonandshell]

OK then..................

So after all the back-peddling, are we any closer to confirming that moving cold air freezes stuff faster?

Cold air moving over a surface conducts away heat faster than non-moving air right?
Other wise why would air conditioning units, refrigeration plants, car radiators etc. have fans fitted to them?

OK then, if you have a Brit van with water pipes aplenty hanging underneath, the stationary water within will lose heat to the surrounding moving air faster than if that air was stationary right? Thus freezing the water faster!

Of course this is assuming the air is already sub-zero!!!

So Dave was partly right! Give him a break! We ain't all perfect, not even in Norfolk!

 

[dave newell]

I have never knowingly offended any member of this forum.. it is not my style.. I think the thousands of posts I have made stand testimony to this. I go out of my way to be helpful wherever possible.. not to offend or slag people off.

I did not know who 'Diamond Dave' was before this thread.. I've never read Practical Motorhome or any other publication he writes in.. so my comment was made about an article in a magazine by someone I had never heard of. If I had known it was Dave I would have chosen my words more carefully.. as it was, I wrote the first words I thought of.

I make no apology for saying the article was bollox.. but, I do apologise to Dave for calling him a 'so called expert' .. for clearly he is an expert in his field.. and well respected by many.. but again I stress, this was not known to me before this thread so my comment was not about or directed at Dave but to an unknown (to me) writer.

I hope that settles it, I hate when people fall out over mistakes.. as I said.. we all make them, ..... it's a chill wind that blows nobody any good!

Apology accepted Jim:thumb: buy you a pint some time

D.

 

[scotjimland]

Apology accepted Jim:thumb: buy you a pint some time

D.

cheers Dave..

just to reiterate, I had no idea it was you .. :Blush:

no hard feelings ..

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[dave newell]

cheers Dave..

just to reiterate, I had no idea it was you .. :Blush:

no hard feelings ..

No hard feelings, life is too short:Cool:

D.

 

[madbluemad]

So were not gonna have to go to marriage guidance councilling about this. Phew, thank goodness.
Jim
:thumb:

 

[Touchwood]

OK then..................

So after all the back-peddling, are we any closer to confirming that moving cold air freezes stuff faster?

Cold air moving over a surface conducts away heat faster than non-moving air right?
Other wise why would air conditioning units, refrigeration plants, car radiators etc. have fans fitted to them?

OK then, if you have a Brit van with water pipes aplenty hanging underneath, the stationary water within will lose heat to the surrounding moving air faster than if that air was stationary right? Thus freezing the water faster!

Of course this is [HI]assuming the air is already sub-zero!!![/HI]

So Dave was partly right! Give him a break! We ain't all perfect, not even in Norfolk!

With that assumption, absolutely correct.

I was upset to see my post leading to dissension between forumites, I never dreamt that it would, and I'm very glad to see it's all been sorted in an adult and sensible fashion. Reinforces my belief that this is a great forum

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[dave newell]

With that assumption, absolutely correct.

I was upset to see my post leading to dissension between forumites, I never dreamt that it would, and I'm very glad to see it's all been sorted in an adult and sensible fashion. Reinforces my belief that this is a great forum

Not that I want to drag this out particularly but there is a bit I don't quite fully understand and perhaps you could explain it for me. What causes carburettor icing?

D.

 

[Gooney]

Courtesy of Wikipedia

Carburetor, or carb icing, is an icing condition which can affect any carburetor under certain atmospheric conditions. Carburetor icing occurs when there is humid air, and the temperature drop in the venturi causes the water vapor to freeze. The ice will form on the surfaces of the carburetor throat, further restricting it. This may increase the Venturi effect initially, but eventually restricts airflow, perhaps even causing a complete blockage of the carburetor. Icing may also cause jamming of the mechanical parts of the carburetor, such as the throttle, typically a butterfly valve.
:thumb:

 

[Touchwood]

Not that I want to drag this out particularly but there is a bit I don't quite fully understand and perhaps you could explain it for me. What causes carburettor icing?

D.

A bit out of my comfort zone here Dave, I'm pretty sure the following explanation has the basics correct, but someone with a better understanding of how carburetors work may be able to give more defiinition:

Whenever a liquid changes to gas it needs an input of heat energy to do so - this is the latent heat of evaporation. In a carburetor the petrol is changed to a gas and mixed with air (I think there's a venturi which expands the fuel) This process takes heat from its surroundings, thus cooling them. The air also is cooled, and at lowish temperatures this cooling can lower the temperature of the water vapour in the air below freezing - hence the forming of ice in the venturi which can block the carburetor.

So - low temperatures and relatively humid conditions will increase the potential.

Trust this helps?

Mike

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[scotjimland]

A bit out of my comfort zone here Dave, I'm pretty sure the following explanation has the basics correct, but someone with a better understanding of how carburetors work may be able to give more defiinition:

Whenever a liquid changes to gas it needs an input of heat energy to do so - this is the latent heat of evaporation. In a carburetor the petrol is changed to a gas and mixed with air (I think there's a venturi which expands the fuel) This process takes heat from its surroundings, thus cooling them. The air also is cooled, and at lowish temperatures this cooling can lower the temperature of the water vapour in the air below freezing - hence the forming of ice in the venturi which can block the carburetor.

So - low temperatures and relatively humid conditions will increase the potential.

Trust this helps?

Mike

I'm treading carefully here ..

good explanation .. :thumb:

other examples, the ice that forms on the nozzle of an aerosol can.

Ice forming on an LPG bottle when gas is being used.

Fridges use the same principal ..

 

[dave newell]

My understanding was that it is caused by the venturi effect, in essence as the air passes through the venturi (reduced diameter section of the carb) its velocity increases and pressure decreases and as the pressure decreases so does the air temperature (Hooke's law I think? but I'm happy to be corrected). Under certain ambient conditions ( below 8 degrees C ambient air temperature and more than 50% relative humidity from memory) the temperature of the air passing through the venturi can drop below freezing point. After a short while this will lead to the carb body dropping to zero or below and the moisture in the humid air condenses on the cold carb body then freezes. As the ice crystals accumulate they will do two things:
1, the accumulation of ice will reduce the venturi throat diameter.
2, the ice crystals can buiild up on the fuel jets and restrict the fuel flow.

Symptoms of carb icing are:
rough running and misfire ascompanied by lousy fuel consumption and loss of power.

As I say this has always been my understanding, I had never considered the effect of the fuel changing state but then I'm not sure it does that in the carb and inlet tract. I'm happy to be corrected on this if anyone knows better.

D.

 

[Jaws]

Other were also rude and offensive but you got in first!

D.

See.. this is what I get for not being diligent

A chance for me to be my normal, gobby, opinionated rude self and I completely missed it.. Bugger !!!!!

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[simsy56]

My understanding was that it is caused by the venturi effect, in essence as the air passes through the venturi (reduced diameter section of the carb) its velocity increases and pressure decreases and as the pressure decreases so does the air temperature (Hooke's law I think? but I'm happy to be corrected). Under certain ambient conditions ( below 8 degrees C ambient air temperature and more than 50% relative humidity from memory) the temperature of the air passing through the venturi can drop below freezing point. After a short while this will lead to the carb body dropping to zero or below and the moisture in the humid air condenses on the cold carb body then freezes. As the ice crystals accumulate they will do two things:
1, the accumulation of ice will reduce the venturi throat diameter.
2, the ice crystals can buiild up on the fuel jets and restrict the fuel flow.

Symptoms of carb icing are:
rough running and misfire ascompanied by lousy fuel consumption and loss of power.

As I say this has always been my understanding, I had never considered the effect of the fuel changing state but then I'm not sure it does that in the carb and inlet tract. I'm happy to be corrected on this if anyone knows better.

D.

Carb icing is particularly dangerous on aeroplanes, the use of carb heat cannot be stressed enough, no carb heat on vehicles though.

Craig

 

[Jaws]

Oohh yes there is !!
I am sure there are many other vehicles fitted with them but the old Fiat Panda's fitted with the F.I.R.E. engine had one fitted, and they are very common on bikes !

 

[dave newell]

I'm beginning to think I should gracefully leave the stage. I of course meant Boyle's law not Hooke's which is to do with spring rates:Blush:.

Carb icing is the reason most cars have an air pipe to the intake system coming from near to the exhaust manifold. Interestingly SU and Stromberg carbs (constant depression type) don't (as far as I know) suffer from icing because the needle in the jet is almost constantly moving and keeps it clear of ice forming.

D.

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[shunter]

I had an old V Dub a long time ago and the carb kept freezing up one really bad winter and a old fitter told me to put some meths in the tank cos it will mix with water but not petrol it did work

 

[GJH]

I'm beginning to think I should gracefully leave the stage. I of course meant Boyle's law not Hooke's which is to do with spring rates:Blush:.

Carb icing is the reason most cars have an air pipe to the intake system coming from near to the exhaust manifold. Interestingly SU and Stromberg carbs (constant depression type) don't (as far as I know) suffer from icing because the needle in the jet is almost constantly moving and keeps it clear of ice forming.

D.

Off the Hooke only to get into Boyling water Dave?

 

[Touchwood]

My understanding was that it is caused by the venturi effect, in essence as the air passes through the venturi (reduced diameter section of the carb) its velocity increases and pressure decreases and as the pressure decreases so does the air temperature (Hooke's law I think? but I'm happy to be corrected). Under certain ambient conditions ( below 8 degrees C ambient air temperature and more than 50% relative humidity from memory) the temperature of the air passing through the venturi can drop below freezing point. After a short while this will lead to the carb body dropping to zero or below and the moisture in the humid air condenses on the cold carb body then freezes. As the ice crystals accumulate they will do two things:
1, the accumulation of ice will reduce the venturi throat diameter.
2, the ice crystals can buiild up on the fuel jets and restrict the fuel flow.

Symptoms of carb icing are:
rough running and misfire ascompanied by lousy fuel consumption and loss of power.

As I say this has always been my understanding, I had never considered the effect of the fuel changing state but then I'm not sure it does that in the carb and inlet tract. I'm happy to be corrected on this if anyone knows better.

D.

OK, instead of relying on my sometimes haphazard understanding of natural processes I've tried to research this a little, and it would appear Dave that you are correct, there is a cooling effect associated with the venturi, quite separate to the evaporative cooling from the petrol converting from liquid to gas.

I found the following "explanation" of this venturi effect cooling - without more time to study and digest all I can say is that although all the words make sense I'm left none the wiser - maybe it'll make more sense to you.

"Maybe this will help. Pressure is the force per unit area acting on a
surface. Since force is a vector, then pressure is also technically a
vector. The pressure on the walls of the venturi is the pressure
exerted in a direction transverse to the direction of flow of the gas.
However, the pressure of the gas along the line of motion is higher
when looking upstream. The former is called the static pressure, and
the latter is called the velocity pressure. By comparing the static
pressure to the velocity pressure you can actually determine the
velocity of the gas through any part of the tube. If there weren't a
continual drop in velocity pressure along the line of motion, then
there would be no flow.

In order to put you on an eye-to-eye level with the moving volume of
gas let's place you right in the gas stream, moving along with it. From
this new perspective the temperature of the gas is much lower because
the molecules have less KE wrt you. Since the temperature is lower,
and since the volume and density remain unchanged through this
transformation, then the pressure must also be lower according to the
Ideal Gas Law. You are now experiencing the static pressure of the gas,
the same pressure experienced by the walls of the venturi tube. The
tube, however, oddly experiences a higher temperature than you do. This
happens for the following reason: The KE of the molecules of gas is
higher wrt the stationary walls, and temperature is a measure of
average KE per molecule of an ideal gas, thus the temperature of the
gas wrt the walls must be higher than it is wrt you. The change in
momentum per second (force) is however the same as that experienced by
you per unit area, thus you and the wall are experiencing the same
static pressure. It is this trick that the venturi plays on the walls
of the tube that produces the venturi effect.

Now if you suddenly come to a stop within the tubing, the gas molecules
will begin striking you with a much greater force (greater change in
momentum) because wrt this stationary frame the molecules have a
greater energy, which translates to greater speed, and thus to a
greater momentum in that direction. The temperature of the gas also
seems to be quite a bit higher, so that what was room temperature gas
entering the stream may even scald you as it passes by and around you.
You've seen pictures and/or footage of the Shuttle during reentry where
the heat shield is glowing hot.

Conversely on the back side of you the velocity pressure is lower than
static pressure. The temperature will thus seem to be a bit lower in
this vicinity than room temperature, if there is laminar flow. With a
5 degree nozzle at the outlet, the surface of the venturi will
experience this lower velocity pressure to some extent since it faces
upstream to an extent, and thus it will experience the lower
temperature associated with the reduced velocity pressure.

Any water vapor present in the moving stream can easily drop in
temperate enough to condense into droplets and even freeze. With a
little turbulence these may even accumulate on the walls.

I think that this is probably the particular cooling effect that you
had in mind.

I've caught some flak over this description in the past by those who
only understand the process in terms of equations, but unless I
acknowledge any technical errors made within, then just ignore any
conspiracy theories that these people generate.

Richard Perry "

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[dave newell]

Thanks for that Touch wood, its re-assuring that I still know what I thought I knew. If you know what I mean

D.

 

[jhorsf]

Dave Broken Link Removed Broken Link Removed

 

[dave newell]

dave View attachment 11700 View attachment 11701 :rofl1:

???

D.

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[JJ]

An On topic joke for which I apologise in advance...

"What am I mummy?"

"you are a polar bear"

"Are you sure mummy?"

"Yes, I am sure..."

"Why are you so sure mummy?"

"Because you have white fur... and big feet... you are a polar bear"

"You are absolutely certain I am a polar bear mummy?"

"I am absolutely positive you are a polar bear."

"Mummy...."

"What now my darling?"

"If I really am a polar bear mummy..."

"You are, believe me...."

"...then please tell me..."

"Tell you what?"


























"Why am I so bloody COLD?"


JJ :Cool:

 

[wivvy's dad]

As you say, just a load of bollox from an 'expert'

That's why we have so many 'experts' on here

 

[Road Runner]

I have no interest in reading all the thread but congratulate Dave Newall for instantly apoligising when seeing his first view was wrong.

In my book an easy mistake to make and had to get very technical to prove it wrong:Wacko:

Good on you Dave:thumb:

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