A Possible Vulnerability, in our Above-Ground Telephone Cables

When I was a young teenager, I sometimes spoke to tech professionals, who were working on power-lines and/or telephone cables, the latter of which were strung above-ground from the usual telephone poles. Sometimes, those tech professionals were disposed to answer my curious questions.

What above-ground telephone cables had or have, is refrigeration stations at some of their connection-points, that refrigerate air to “-20⁰C”, which also makes the air very dry, and then to feed that air into the cables in compressed form. The purpose of this exercise is to prevent moisture buildup inside the telephone cables, that have hundreds of wires, if not thousands of wires.

Assuming that such a unit is being used, the question remains unanswered of how it’s supposed to work, if the outside air temperature is below -20⁰C. If the process continues, then air will be fed into the cables at a higher temperature than the ambient temperature, at which point technically, the air being fed in is also moister, than the saturation point of the ambient air. (:1) What could follow, is ice build-up in the cables, and, when the temperature outside rises suddenly, the ice can melt.

I’m not sure what the exact conductivities are, but think that liquid water conducts better than ice, so that liquid water can cause shorting of the telephone wires inside the cables. I suppose that if the ambient air stays warm long enough, continued feeding of cold, dry air into the cables can dry out the cables again…

(Updated 1/20/2019, 7h40 … )

(As of 1/16/2019 : )

But I think that this could be one way in which our infrastructure is finally not designed to handle Climate Change. And it may be affecting me as a DSL user. Because of Climate Change, we’re seeing more continued stretches of time, during which the outside temperature is -20⁰C or colder. Tonight, January 16, is supposed to be yet another night during which this happens. (:2)

When I was a young teenager, we also had occasional days, when the thermometer dipped below -20⁰C. But those were exceptional days, while in the modern era, this weather comes in prolonged spells.

I suppose it might have been well if Scientists had had the foresight to equip these refrigeration systems with a ‘shutoff’, so that If the ambient temperature dips below -15⁰C or something like that, the unit just stops feeding air of any sort into the cables.

(Update 1/17/2019, 7h35 : )

In fact, something which nobody may be monitoring, is whether all those refrigeration-stations are actually reaching -20⁰C. Just as it goes with household air-conditioners, those stations could eventually suffer from refrigerant loss, which means that they won’t be able to achieve anything colder than -10⁰ – -15⁰ C. And, while it was true in my youth that even for-profit companies would spend the time and money to check each station periodically, the economic realities today could be such, that these stations run unattended for years, meaning that the telephone company may not even know how well each one is running.

And this may also present a reason for the company, not to put one.

What this would mean is, that the telephone cables could start icing up when the temperature outside drops to -15⁰C, and not even at -20⁰C.

(Update 1/17/2019, 14h20 : )

I suppose that one question which some readers might have, would be How such a refrigeration unit can collect moisture from the air, at -20⁰C, when all water would be ice. And the best answer I can think of would be, that such units ‘cycle’. In other words, such a unit would need to turn itself off periodically, so that an electric heating element can turn on, and heat up the block on which the ice collects, so that the water can run off in liquid form.

I think that a more important question would be, what such a unit does with the run-off, especially since, liquid water being drained into a sub-freezing exterior would freeze in the wrong places. And the answer, for hypothetical operation at external temperatures below freezing, would be, There could be a collection container, with another heating element, this time an element which applies gentler heat 100% of the time. And this tank could convert all the water into vapour, which could get vented into the exterior air, through a hose.

But when I was a young  teenager, I never even bothered to ask the telephone technician, whether these “Air Dryers” were in fact meant to operate, when the external air was below freezing. I just accepted the sparse information which he had to give me.


 

(Update 1/19/2019, 13h00 : )

1: )

For the sake of brevity, I made some unspoken assumptions about how such a unit would work. What some readers might ask, who do not have a Scientific background, could be, ‘How can such an air-dryer feed compressed air into the cable, which is warmer than the exterior temperature, if the unit does not have a heating component?’

The fully-correct explanation would be, that the unit must at first compress the ambient air, and would then refrigerate the resulting compressed air, ostensibly to “-20⁰C”, at which point some moisture would be collected from it in the form of ice. After that, the dryer air would be fed into the cable, with whatever amount of pressure it has left.

When one compresses a gas, the initial result is adiabatic compression, which means that the gas has also become warmer, or hotter in some cases, than it initially was. It’s only after the compressed gas transfers heat to some type of exterior surface, that the compressed air can be brought down to the ambient temperature again, or to a lower temperature, at which point the compression has become diabatic.

So yes, just because it’s compressing air, such a unit could be feeding air into the cable, at a higher temperature than it first had.

And in practice, all adiabatic compression of gasses, is partially diabatic, because some loss of heat following the high-end of the compressor, is unavoidable.

(Update 1/20/2019, 7h40 : )

2: )

A different type of question which the subject of this sort of air-dryer might suggest is, ‘Should it not be more-important, that any refrigerant loop can fail, during the hottest Summer day, when the outside temperature reaches +35⁰C?’

And my thoughts on that subject suggest that indeed, any sort of refrigerant loop is most likely to fail under such conditions, in addition to which the refrigerant compressor can burn out. But I’ve been focusing my attention on the possibility of ‘partial refrigerant loss’, the consequences of which might not be as bad, when the telephone cables are actually +35⁰C warm.

When dried air at any temperature different from the ambient temperature enters a cable, then it will only travel a few feet, before its temperature adjusts to that of the cable. As long as that temperature is actually higher than the temperature with which the air entered, there will be no condensation or crystallization of ice.

It’s at any point in a pipeline, where the temperature of the air decreases, that condensation will take place… This would be the reason for which the concept was, to cool the compressed air below ambient temperature, before allowing it to enter the cable. That way, moisture will either condense or crystallize in the air-dryer and not in the cable.

As long as the compressed air is only slightly colder than the cable, before it enters, there should be fewer problems inside the cable. And this is still likely to be the case with a compromised refrigerant loop, when the cable is +35⁰C warm.

Dirk

 

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