[vhfpmr (OP)]

About 13 years ago (so my recollection of the details is a bit hazy) I inflated a cycle tyre quite rapidly after mending a puncture using a Zefal plastic pump similar to this, which has a small threaded brass bush for the hose moulded into the end of the cylinder.

Whilst I don't recall the end of the pump getting any hotter than usual, on this occasion the brass bush got hot enough to soften the plastic until the pressure (~80-90 psi) pushed the bush out of the end of the cyclinder. What intrigues me is why the heat was so localised, if the whole end of the pump had softened to that degree it would have inflated like a balloon and burst. The outer surface of the bush was heavily knurled to give it a key, so it wasn't going to come out lightly without the plastic softening.

Zefal describe the plastic as a 'technopolymer resin', so there's no prospect of looking up the melting point, but some plastics have an upper 'useable' temperature below 100C. It seems possible that the heat was getting funnelled toward the outlet along with the air, and it must be travelling at quite a high speed, so 'wind warming' would be quite significant. Pressing a finger over the outlet, I can burn it in a couple of strokes of the pump without the cyclinder end even getting warm.

Any thoughts?

[alancalverd]

The compression will have been almost adiabatic, and the maximum temperature rise will have been at the sharp end of the pump since that is where the compressed air resides. As the tyre inflates, so the residual air at the sharp end increases in both temperature and pressure and you have to work even harder to shove some of it into the tyre  and the rate of temperature rise increases. As you say, there will also be some frictional heating as the air flows through the narrow nozzle, depending on the speed of pumping.

[paul cotter]

There is another factor: brass is a good conductor of heat while plastic is not. The brass fitting will get hot first but if you keep it going the whole pump will turn to goop.

[Petrochemicals]

Duff manufacture? A bit of heat on an old pump, it is most likely to fail under pressure. Expansion rates, glue aging, plastic degredation, localised heating.

[vhfpmr (OP)]

As you say, there will also be some frictional heating as the air flows through the narrow nozzle, depending on the speed of pumping.

That's another point I hadn't considered, what I meant was the reduction in the effective thermal resistance of the air film due to wind speed, as in 'windchill'.

There is another factor: brass is a good conductor of heat while plastic is not. The brass fitting will get hot first but if you keep it going the whole pump will turn to goop.

This is the line I was thinking along, but I can't make it happen with plausible looking data. I've tried modelling the thermal time constant, but the problem I'm having is that the thermal resistances of the plastic and the brass are both neligible compared to what I estimate the air film resistances to be. I'm using a figure of 0.13Km^2/W for the resistivity of the external film, where the air is still, and I have some faith in that, because I've measured it, and got a result within about 12%. I also use it in my model of the house and it's heating system, and get results that match measured data very well.

I'm a lot less sure of the inside film though, and that's what's insulating the pump components from the air temperature. I'm not sure that the only data I've found is very relevant as it's specified at quite low air speeds. The two figures I have are:

0.044Km^2/W at a wind speed of 7.5mph
0.03Km^2/W at 15mph

So I tried those figures as they are, and found that not only is the time constant is implausibly slow, taking >5mins to approach quilibrium, but the plastic is getting hotter than the brass:


Film Hi.png (24.84 kB . 932x796 - viewed 225 times)

As I've estimated the speed in the pump outlet at nearer 50mph, I arbitrarily reduced the internal film resistances tenfold and ran the model again. This gives a much faster rise time and higher temperature, but the outside film resistance is still allowing the plastic to get hotter than the brass:


Film Lo.png (23.04 kB . 932x796 - viewed 249 times)

Duff manufacture? A bit of heat on an old pump, it is most likely to fail under pressure. Expansion rates, glue aging, plastic degredation, localised heating.

There's no glue, the bush is moulded in, and I don't know of any reason why the melting point of plastics would reduce with age.

[evan_au]

due to wind speed, as in 'windchill'

I thought 'windchill' (as used on the nightly weather forecast in some countries) refers to the fact that wind makes water evaporate faster from human skin, making the weather feel colder than if the air were still.

However, a plastic and brass pump does not evaporate water from its surface, so 'windchill' would not have an effect like on humans (or at least, not the same effect).

[Bored chemist]

Valves typically have a spring loaded "stopper" of some sort held by a spring and pushed aside by air pressure.
It's easy enough to see how such a system could start to oscillate.
And if that happens the resilient stuff- typically some sort of rubber- which forms the seal with be squeezed + released very rapidly.
Each time that happens some energy is dissipated in it as heat.
That could be enough heat to locally melt the plastic.

[Petrochemicals]

There's no glue, the bush is moulded in, and I don't know of any reason why the melting point of plastics would reduce with age.

I would have thought that there was some sort of adhesion encourager, plastics and metal do not really form much of a bond. I would have thought some mechanical moulding would be present that would have failed under ageing and stress from pressure and heat.

You are only softening the plastic, iron is still solid when the balcksmith hits it.

[vhfpmr (OP)]

due to wind speed, as in 'windchill'

I thought 'windchill' (as used on the nightly weather forecast in some countries) refers to the fact that wind makes water evaporate faster from human skin, making the weather feel colder than if the air were still.

However, a plastic and brass pump does not evaporate water from its surface, so 'windchill' would not have an effect like on humans (or at least, not the same effect).

If I hang up a hot object surrounded by air that's cooler, the object will heat the layer of air in immediate contact with it (transferring some energy to the air in the process), but since air is a very good thermal insulator, the warmed film at the surface of the object slows down further heat transfer to the air. Convection removes the thin film of air in contact with the surface, which then gets replaced by more cold air which has to be heated up again, but this happens only slowly.

That's in still air, but in moving air the insulating film gets carried away more quickly, so the heat is lost from the object more quickly, and faster still when the air moves faster, but it is a law of diminishing returns: there comes a point where further increases in wind speed make little difference.

The same effect occurs in reverse if the object is cooler than the air: it will warm up faster if the air moves: 'windwarming' if you like. So 'windchill' is a bit of a misnomer, what's really going on is an acceleration of heat transfer regardless of which direction it travels: an effective reduction in thermal resistance of the air-object interface, hence the term 'film resistance'.

None of this requires any evaporation, which is a separate effect. If I blow on molten solder it will solidify quicker, and it's not because the solder's evaporating, and not because my breath is colder than the room air.

Valves typically have a spring loaded "stopper" of some sort

Schraeder valves do but Presta don't, and the valve is at the opposite end of the hose from the union in the end of the pump cylinder where the overheating occurred anyway.

I would have thought that there was some sort of adhesion encourager

There was:

the bush was heavily knurled to give it a key, so it wasn't going to come out lightly without the plastic softening

[paul cotter]

Yep, I could agree with that. In electronics it is common to have a fan blowing on a heat sink to aid thermal dissipation.

[Bored chemist]

I thought 'windchill' (as used on the nightly weather forecast in some countries) refers to the fact that wind makes water evaporate faster from human skin, making the weather feel colder than if the air were still.

I think that when people are talking about windchill, they are expecting to be fully dressed.
The heat loss is by advection.

[Petrochemicals]

There was:

the bush was heavily knurled to give it a key, so it wasn't going to come out lightly without the plastic softening

Something like

https://www.loctiteproducts.com/en/know-how/fix-stuff/metal-glue.html

[vhfpmr (OP)]

Yep, I could agree with that. In electronics it is common to have a fan blowing on a heat sink to aid thermal dissipation.

Indeed. The latent heat of evaporation magnifies the windchill effect, but it's not the cause of it.

I think that when people are talking about windchill, they are expecting to be fully dressed.

If you're insulated by enough clothing the windchill effect will be negligible, because the outer surface of the clothing will be at virtually the same temperature as the air. There is no wind chill on an object that's the same temperature as the air passing over it.

Most of the algorithms I've seen for calculating the equivalent windchill temperature are based on bare skin, because otherwise the answer depends on how much insulation the clothing provides.

advection

Today's new word.

Something like

https://www.loctiteproducts.com/en/know-how/fix-stuff/metal-glue.html

Eh?

[Bored chemist]

If you're insulated by enough clothing the windchill effect will be negligible, because the outer surface of the clothing will be at virtually the same temperature as the air. There is no wind chill on an object that's the same temperature as the air passing over it.

The same applies to the evaporation of sweat.

[alancalverd]

I think that when people are talking about windchill, they are expecting to be fully dressed.

Sort of. Wind chill, when reported as a number in a meteorological prediction, is the still-air temperature that produces the same effect on exposed skin as the actual ambient, so it is determined by temperature, velocity and humidity. As you say later, if you are wearing your full survival kit with a face mask and goggles, the wind chill should be nil because you have no exposed skin.

Not to be confused with forced convection, which will cool (or heat) you eventually, whatever you are wearing.

[vhfpmr (OP)]

Windchill wins my award for the vaguest, least comprehensive entry I've ever found in the OED.
Definitions of words aside, if an object is at a different temperature to the air surrounding it, there will be a transfer of heat, and if the air moves the transfer will be faster.

This guy seems to regard windchill and forced convection as same effect, different context, this paper he cites talks about bare skin and risk of frostbite, and models radiation but not evaporation, but then this paper they cite refers to windchill in the context of clothed skin and quantifying an adequate level of clothing.

[alancalverd]

That's another point I hadn't considered, what I meant was the reduction in the effective thermal resistance of the air film due to wind speed, as in 'windchill'.

The wind speed through the nozzle decreases as the tyre reaches its final pressure. If the pump barrel is 2.06 cm diameter and the stroke is 30 cm, the first stroke displaces about 100 CC of air at ambient pressure, but the final stroke has to compress the air to about 1/6 of its initial volume before it can flow into the 90 psi tyre and the final flow velocity is thus 1/6 of that of the initial stroke.

What is happening meanwhile is that each stroke is adiabatically compressing air (and thus raising its temperature) towards the nozzle. Hence the nozzle and immediately surrounding plastic heat up more rapidly than the rest of the barrel. The nozzle will feel hotter because the metal has a lower specific heat capacity and higher conductivity than the plastic which may absorb the same amount of heat but won't become as hot or feel as hot on the outside. .

[vhfpmr (OP)]

the metal has a lower specific heat capacity and higher conductivity than the plastic

But that's what I was modelling above, and it's not giving me results that look very convincing as an explanation for the failure.