[Petrochemicals]

As you have demonstrated an electrical system of heating is unfeasably complicated and therefore expensive to be retrofitable and therefore maintainable.

Actually, I've demonstrated that an electrical system of heating is feasible and what for others was too complicated to design in more like 200 years since the first wire cable bridges https://en.wikipedia.org/wiki/Suspension_bridge#Wire-cable is for me something I can do in a week.

Engineering designs are de rigueur full of lists of numbers in mind-numbing detail. It's what makes a good design foolproof not "complicated".

A terrible design plan that was no plan at all would leave lots of decisions to the construction workers to choose an arbitrary length, a thickness, a material, a strand, a voltage or a current and that would be a recipe for disaster because without the detail they don't know what to do, even where to begin.

Architects can't simply pass an artists impression to labourers and invite them to have a go.

You could fit external heating covers easier than teflon coated strands, they would be easier to engineer around and retrofit, to replace one is going to be a hasstle. But lets say you can run the national grid through the existing cables, each cable in entirity is insulated.  Its the only way to make it uncomlicated and reliable enough. Each cable is going to need to be heated to keep its temerature above freezing.

You will need

Temperature difference
Rate of loss
Ammount of steel
Ammount of water to be heated

Then you will need :

the steel cross section (run it up and down alternate cables, you would not achieve the required resistance with square metres of steel)
Resistance heating ammount
Voltage
Length


See whether it will add up.

[Peter Dow (OP)]

Spraying PTFE all over the bridge is a non-starter.

They paint bridges...

Painting the Forth Road Bridge is a never ending dangerous, labour-intensive job. I don't want to inflict the same burden on the Queensferry Crossing whose stay cables are supposed to be maintenance-free.

The PTFE sheaths to isolate the strands don't add a weak-point but we'll be sure to test your "weak-point" theory in the laboratory, making such adaptations as are necessary to prove that strength is maintained.

In the very real sense that PTFE isn't weaker than steel.

The PTFE-coating per se doesn't need great strength. The PTFE-coating serves to lubricate the wedges in compression against the glass fibre cloth. The compressive strength is provided by the glass fibres, which are  stronger in compression than many grades of steel.

So you and I know that covering the ice-accumulating surface of planes with PTFE doesn't work.

I  have never tried it, have you?
I'm guessing that neither of us has, so your claim that "you and I know that..." is not true.
Why say it?

Presumably, if PTFE was a perfect solution for anti-icing aircraft then it would have replaced all other methods. "PTFE doesn't work perfectly" is a reasonable deduction for a reasonable person to make.

Actually, I've demonstrated that an electrical system of heating is feasible and what for others was too complicated to design in more like 200 years since the first wire cable bridges https://en.wikipedia.org/wiki/Suspension_bridge#Wire-cable is for me something I can do in a week.

So, what you are saying is that 200 years of experience says we don't need it.

The ice-fall on the Queensferry Crossing which closed the bridge for the best part of two days is the recent experience which says that we do need a good solution.

Time to close the thread then

If you are bored with this thread it doesn't mean that others are. I'm having a whale of a time with this thread! 

[Peter Dow (OP)]

You could fit external heating covers easier than teflon coated strands, they would be easier to engineer around and retrofit, to replace one is going to be a hasstle.

Thanks for that suggestion.

But lets say you can run the national grid through the existing cables, each cable in entirity is insulated.  Its the only way to make it uncomlicated and reliable enough.

Heating with DC means that each of the 55 to 109 strands in each cable will have be electrically isolated from other strands. The strands are already isolated as shown in this cross-section model.

The only issue with completing the isolation of the cable strands from each other is where they are attached to the anchor head, where the insulation is stripped off and it is metal strand compressed by metal wedges into a metal anchor head housing cone - that's the bit that needs to have insulation added - I suggest by adding Teflon/PTFE-coating glass fibre fabric sheaths, illustrated in this diagram coloured red.

Each cable is going to need to be heated to keep its temerature above freezing.

0°C should be sufficiently warm for the temperature of the cable surfaces - higher is OK, lower would not be OK.

You will need

Temperature difference
Rate of loss
Ammount of steel
Ammount of water to be heated

Then you will need :

the steel cross section (run it up and down alternate cables, you would not achieve the required resistance with square metres of steel)
Resistance heating ammount

The maximum rate of heat loss is a key design objective which determines all the other heating current calculations - I have suggested 318 Watts/m² of stay pipe surface area would be plenty for all eventualities of temperature and amount of water/snow/ice that needs to be heated.

The strand resistance I have assumed to be 0.001137 ohms per metre, following a calculation I made last year.

Voltage
..
Length

The voltage and power I have quoted per metre and to calculate for each cable I will need the length data for each cable which will be specified in the Queensferry Crossing engineering design plans which I have requested sight of.

The strands have quite a variety of lengths and current requirements and whilst all will require to be paired to facilitate current flow in both directions, some of the lowest-power, shortest strands could have up to 3 sets of strand pairs to form one heating circuit.

See whether it will add up.

It adds up.

[Bored chemist]

Painting the Forth Road Bridge is a never ending...

No it isn't.
Were you being wrong about the road bridge
https://www.scotsman.com/news-2-15012/forth-road-bridge-to-get-first-paint-job-since-1964-1-4669130
or the rail bridge?
https://www.bbc.co.uk/news/uk-scotland-edinburgh-east-fife-14789036

Thanks for that suggestion.

You are welcome

There are a few ways I know of that they use for deicing planes.
Inflatable rubber bits, trace heating

[Bored chemist]

"PTFE doesn't work perfectly"

Is not the same as

So you and I know that covering the ice-accumulating surface of planes with PTFE doesn't work.

And pretending it was is a fallacy.
https://en.wikipedia.org/wiki/False_dilemma

[Petrochemicals]

Loading insulation is a no no. Incredibly complicated. You just seem to be adding complication to a simple idea of adding an exterior heated sheath.

[Peter Dow (OP)]

"PTFE doesn't work perfectly"

Is not the same as
Quote from: Peter Dow on Yesterday at 22:33:29
So you and I know that covering the ice-accumulating surface of planes with PTFE doesn't work.

Here is a quote which puts some numbers on how good anti-icing chemicals are.

Their innovation, described in the Journal of Materials Chemistry, is a gel-based, soft coating made out of PDMS (polydimethylsiloxane), a silicone polymer gel with already widespread industrial use. Their experiments were supported by careful analysis of ice adhesion mechanics.

The performance measure of de-icing coatings is called ice adhesion strength -- the shear stress necessary to remove ice from a surface -- and is measured in kilopascals (kPa). Kota's group demonstrated ice adhesion strength for their coating of about 5 kPa. By contrast, soft coatings available on the market have ice adhesion strength of about 40 kPa (lower is better). Other types of de-icing coatings made of rigid materials like Teflon typically perform at around 100 kPa.
https://www.sciencedaily.com/releases/2016/11/161117150436.htm

So they reckon their PDMS anti-icing agent is 20 times more slippy than Teflon.

But Teflon/PTFE is not the anti-icing agent of choice, for a spray, it seems.

Teflon/PTFE is maybe better suited for a more durable non-stick surface on frying pans and glass fibre fabric?

All those sprays will wear off and have to be reapplied every so often.

[Peter Dow (OP)]

Loading insulation is a no no.

Spark plugs are made of aluminum oxide ceramic insulator which is loaded by the compression of the ignited fuel air mixture.

That "loading insulation" is a yes yes if you want to travel anywhere using a petrol engine.

Incredibly complicated. You just seem to be adding complication to a simple idea of adding an exterior heated sheath.

An exterior heated (or non-stick) sheath would have complications of its own - large surface area - 55,000 m².

70,000 metres length of cables with an average diameter of, say, 250mm,
 check my sums yourself https://vodoprovod.blogspot.com/p/area-pipe.html

[Petrochemicals]

Loading insulation is a no no.

Spark plugs are made of aluminum oxide ceramic insulator which is loaded by the compression of the ignited fuel air mixture.

That "loading insulation" is a yes yes if you want to travel anywhere using a petrol engine.

compressive loading tension loading, as seen in such things as bridges. Obviously electrically and thermally they work as sparkplugs

Incredibly complicated. You just seem to be adding complication to a simple idea of adding an exterior heated sheath.

An exterior heated (or non-stick) sheath would have complications of its own - large surface area - 55,000 m².

70,000 metres length of cables with an average diameter of, say, 250mm,
 check my sums yourself https://vodoprovod.blogspot.com/p/area-pipe.html

It will be roughly the same area ? Slighly bigger, but being as you will not be heating the solid mass of the cable, considerably more efficient. Still a technical maintenance and expense nightmare.

[Bored chemist]

So they reckon their PDMS anti-icing agent is 20 times more slippy than Teflon.

Then use it on the bridge.
Problem solved.

[Peter Dow (OP)]

So they reckon their PDMS anti-icing agent is 20 times more slippy than Teflon.

Then use it on the bridge.
Problem solved.

It is important to understand the science of these coatings before attempting to use this technology:

Instead of using fluorine atoms for repellence like many successful hydrophobic penetrating sealers (not super hydrophobic), superhydrophobic products are a coating—they work by creating a micro- or nano-sized structure on a surface which has super-repellent properties.
These very tiny structures are by their nature very delicate and very easily damaged by wear, cleaning or any sort of friction; if the structure is damaged even slightly it loses its superhydrophobic properties.[citation needed] This technology is based on the microstructure of the hairs of a lily pad which make water just roll off. Rub a lily leaf a little and it will no longer be superhydrophobic. Unlike a lily leaf, which can heal and grow new hairs, a coating will not do this.
As a result, unless advancements can resolve the identified weakness of this technology its applications are limited. It is used mainly in sealed environments which are not exposed to wear or cleaning, such as electronic components (like the inside of smart phones) and air conditioning heat transfer fins, to protect from moisture and prevent corrosion.
https://en.wikipedia.org/wiki/Superhydrophobic_coating#Applications

So on the bridge, such a coating would presumably quickly be weathered off and need reapplying, which is hard and dangerous enough to do once, but to have to keep doing it, would be a pain.

This new paper suggests mixing the anti-icing liquid with a resin to make a more durable anti-icing coating.

Facile One-Step Method to Fabricate a Slippery Lubricant-Infused Surface (LIS) with Self-Replenishment Properties for Anti-Icing Applications
https://www.mdpi.com/2079-6412/10/2/119/htm

Which would be great if it solved the durability problem, but it is possibly too early for this product to have got to market yet.

It sounds like it is worthy of research but I think it is for someone else to advocate for this anti-icing coating solution.

I will promote my electrical heating solution and we will see which (or both) will find favour with the bridge owners.

[Peter Dow (OP)]

Teflon/PTFE-coated glass fibre fabric sheaths to electrically isolate the strands from the anchor head.

I've modified my OP with the following additional ..

Such sheaths would likely not be available as an off-the-shelf product in the required dimensions, though general purpose PTFE-coated fibre glass cloth is commonly available and this expandable E-glass sleeving, expands from a relaxed internal bore of 15mm to a maximum bore of 38mm and insulates to 500V when not expanded, which is a useful size while relaxed to accommodate the strand and while expanded to accommodate the wedges.

The insulation should cope with the highest DC voltage of about 100 Volts, used to power the longest and highest heating capacity factor strands, albeit that this sleeving is inappropriately resin-coated and would therefore likely require to be custom adapted, the resin cleaned off and re-coated with PTFE, tested and proved in the laboratory. Perhaps wrapping the wedges in PTFE thread seal tape is all that is required to supplement the product as supplied for satisfactory performance? A promising avenue for research.



DC Power Supplies
Not forgetting DC power supplies and I have noticed a comprehensive range of 3kW to 10kW DC power supplies here that I think will do nicely, an average of about a dozen power supplies per cable (more for the longer cables, fewer for the shorter cables), about 3500 power supplies required to de-ice all 288 cables.



Where to store the cable power supplies?
Let’s examine the option of storing the cable heating power supplies in the towers, racked next to the anchorages of the cables which they will be heating. There might just be enough room to squeeze in another half a tonne of power supplies for the 4 cables per floor (assuming their racks are securely attached to the tower walls), 12 tonnes worth of power supplies for all 24 floors per tower, for all 3 towers!

Even at 94% efficiency for switch mode power supplies, each tower’s cable power supplies could be generating at most about 0.4 MW of waste heat energy. A new massive extractor fan fitted into the roofs of the towers would be required to cool the inside of the towers while the DC power supplies are heating the cables.

Considering how cramped the insides of the towers are already, the daunting cooling problem, not to mention the risk of a tower fire destroying all of a tower’s power supplies at one time, it looks to be much the better option to install the cable power supplies on the deck, next to the deck anchorages to allow them to be supplied with power.



The stay cables penetrate the surface of the deck, as can be clearly seen in this next photograph, taken during construction.



Therefore best access to the anchor heads, to attach the cable heating power supplies, may be from inside the deck, where the power supplies themselves should be stored too.
...

Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of the towers, in between the open stairways and scaffolding.

[Peter Dow (OP)]

I've modified my OP again with the following additional ..

DC Circuit Diagrams
Locating all the electrics at the deck anchorages, while leaving the strands earthed at the tower anchorages, offers advantages for design, development, installation, commissioning and servicing.


Circuit Diagram – 2 heating strands, 1 power supply


Heating strands pair current balance detector

The window detector circuit compares the isolated power supply’s potential with respect to earth to detect the expected balance of current and voltage in the heating strands pair. If an imbalance fault develops then the safety switch is used to cut the power.

[Bored chemist]

So on the bridge, such a coating would presumably quickly be weathered off and need reapplying, which is hard and dangerous enough to do once, but to have to keep doing it, would be a pain.

If you knew it wouldn't work, why did you bring it up?

[Bored chemist]

Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of the towers, in between the open stairways and scaffolding.

How about the waste heat from the PSUs?

[Peter Dow (OP)]

So on the bridge, such a coating would presumably quickly be weathered off and need reapplying, which is hard and dangerous enough to do once, but to have to keep doing it, would be a pain.

If you knew it wouldn't work, why did you bring it up?

Did I? I thought I brought up my electrical heating proposal and I was responding to other people's alternative suggestion of anti-icing coating, quoting references to support my responses.

[Peter Dow (OP)]

Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of the towers, in between the open stairways and scaffolding.

How about the waste heat from the PSUs?

What preceding part of my very post you were replying to, didn't you understand?

I had just explained that the waste heat from the PSUs is too much to store them in the towers, so the PSUs have to go on the deck.

Want to try reading that again?

Where to store the cable power supplies?
Let’s examine the option of storing the cable heating power supplies in the towers, racked next to the anchorages of the cables which they will be heating. There might just be enough room to squeeze in another half a tonne of power supplies for the 4 cables per floor (assuming their racks are securely attached to the tower walls), 12 tonnes worth of power supplies for all 24 floors per tower, for all 3 towers!

Even at 94% efficiency for switch mode power supplies, each tower’s cable power supplies could be generating at most about 0.4 MW of waste heat energy. A new massive extractor fan fitted into the roofs of the towers would be required to cool the inside of the towers while the DC power supplies are heating the cables.

Considering how cramped the insides of the towers are already, the daunting cooling problem, not to mention the risk of a tower fire destroying all of a tower’s power supplies at one time, it looks to be much the better option to install the cable power supplies on the deck, next to the deck anchorages to allow them to be supplied with power.



The stay cables penetrate the surface of the deck, as can be clearly seen in this next photograph, taken during construction.



Therefore best access to the anchor heads, to attach the cable heating power supplies, may be from inside the deck, where the power supplies themselves should be stored too.

[Bored chemist]

Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of the towers, in between the open stairways and scaffolding.

How about the waste heat from the PSUs?

How about the waste heat from some of the PSUs?

[Peter Dow (OP)]

Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of the towers, in between the open stairways and scaffolding.

How about the waste heat from the PSUs?

How about the waste heat from some of the PSUs?

Winter-long heat for inside the towers is needed more often, less intensely and when free surplus wind power is available than the fewer times when the PSUs are needed on demand, paying full price electricity to provide a more intense heat for the cables.

The PSUs in the tower anchorages would be high up the tower and can only heat the top of the tower without a lot of forced recirculating of heat.

So trying and failing to use some PSUs to heat the tower would unnecessarily complicate the design to no good purpose.

[Peter Dow (OP)]

Bridge Stay Cable Electrical-Heating Strand Calculator

I’ve written this spreadsheet to calculate the specifications for the electrical power supplies to heat the strands of stay cables of the Queensferry Crossing or of other bridges prone to icing with similar cables of an arbitrary number of strands up to 109 strands.

This is a screenshot – click to view a high-resolution image.


Download QFC Electrically-heated Cables.
It’s an Excel .xlsx file but it was developed in Google Sheets (for free!). I don’t have Excel on my computer so it has never been tested in Excel. If it doesn’t work on Excel then try uploading it to Google sheets. That should work but any problems let me know

Email: peterdowaberdeen@gm…….

and if necessary I can let you share my Google sheets version with viewer access which I can authorise to any Gmail account and then you can copy it directly into your Google sheets file space.

Instructions
Select a strands sheet with the closest number of strands to the cable you want to calculate for, choosing between 45, 55, 73, 91 or 109 strands. Best to duplicate the sheet for back-ups.

The strands arrangement is represented by coloured cells where the cell formula calculates that strand’s heating capacity.

To re-arrange and / or change the number of strands, first drag off the strand circles overlay image then add strands by selecting an existing strand cell, Edit Copy (ctrl-C) then select another coloured cell for the new strand and Edit – Paste special – Paste formula only. To remove a strand just delete the formula from its coloured cell.

To use the Strands calculator spreadsheet with data for another bridge, you will have to edit or create a new “Cable Data” sheet with the new data and modify the strands calculator sheets’ (hidden) cell X61 so that it fetches the cable data from the new sheet with the appropriate table range.

All my Queensferry Crossing blog posts.