Author Topic: Measure limescale thickness through hot resistance of heating rods?  (Read 1100 times)

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Offline Apfelmus

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Hi all,

I'm trying to divise a method of computing the thickness of limescale on the heating rods of my vapour generator...

Problem: The vapour generator is a 6kW 3-phase model, has approx. 3 litres of water inside, water is let in whenever water level drops, and vapour comes out towards the shower. Inevitably, these heating rods get clogged with limescale. The controller forces regular de-calcariciations that I followed as in the manual. One day I had a problem with the pump that empties the boiler to allow the procedure to start. So I had to dismantle everything (a half-days job) and to my surprise I found out the heating rods were clogged by 5-10mm of limescale!

After de-clogging everything the boiler works again, and heats up a good deal faster. Now, given the vapour generator is very badly accessible, and the heating rods inside need compete dismantling to see the limescale: is there a way of indirectly measuring the thickness of the limescale on them?

My idea:
- the heating elements have a given cold resistance
- they heat up under full power (240VAC for each of the 3 rods).
- the "hot" resistance for given conditions (voltage, water temperature) can be measured (ammeter and ohms law)
- when the limescale builds up this acts as an insulator. The heating element will get hotter under otherwise same conditions (voltage, water temperature).
- measuring the "hot" resistance will lead up to a higher value when the clogging increases

Question:
- anybody who tried that?
- can this work, or is the difference of hot resistances too small to be measured reliably?
- if it works, what would be the effect of 1, 2, 5 or 10mm of limescale on the heating rods hot resistance?

The heating rods get hotter when clogged, so there is a drop in power, making the heatup slower. Maybe its easier or more reliable to measure the time the water takes to heat up and calibrate this time to limescale thickness?

cheers
Martin
 

Online daqq

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #1 on: March 27, 2021, 11:55:57 am »
Quote
- can this work, or is the difference of hot resistances too small to be measured reliably?
I'd guess you can make an estimate off an overall temperature, there will be a correlation, not sure what the resistance difference will be though. Depends on the material. That said, with sensitive enough instruments (and 4W measurements) you can measure the change of a few ppm.
Quote
- if it works, what would be the effect of 1, 2, 5 or 10mm of limescale on the heating rods hot resistance?

The heating rods get hotter when clogged, so there is a drop in power, making the heatup slower. Maybe its easier or more reliable to measure the time the water takes to heat up and calibrate this time to limescale thickness?
I would concentrate on the time-temperature plot rather than the static resistance/temperature since it will eventually equalize. But if you have a good thermal connection to the water, you'll get a slow temperature ramp up. If you have a poor connection (limescale), you'll get a very fast temperature ramp up of the heaters. Measuring the time constant will (I'm guessing) be more telling and less dependent on the initial temperature of the water and so forth.
« Last Edit: March 27, 2021, 11:57:51 am by daqq »
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Offline beanflying

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #2 on: March 27, 2021, 12:24:50 pm »
Same Same but different coffee machine boilers (2-5kW typical) run into the same issues even with good pre filtration including Resin Ion bed filters (generally 30-50PPM TDS water) . Some sideways discussion for you below.

I doubt you will be able to detect a change in resistance as even with calcification you will still have some non exposed element and the deposition varies across the element.

What you should know over time is when to pull them out for a clean. The Coffee Industry tend to ignore them until they die but if I am doing a refurbish or the machine is in one disaster area (700-1000 PPM TDS) they get ignored. Cleaning material is a soak in near boiling concentrated Citric Acid solution which works as well or better than commercial products and it is cheaper. Vinegar can be used but in your coffee water NO :P In the local area on the horrid Bore Water every 2 years is what I recommend and these machines will run 8-10 hours a day 6-7 days a week.

You could look at routine dosing of it as well the issue with this if you don't do it regularly you tend to get chunks breaking away rather than suspended particles Coffee Boiler cleaner search Typically this works best if you dose a makeup tank and run it through over some time.

Heatup time could be used - maybe, but Environmental temperatures will play a part and unless you gather data over say a year then drop it out for a clean and do it again you won't gain much useful data.
« Last Edit: March 27, 2021, 12:27:12 pm by beanflying »
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Offline f4eru

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #3 on: March 29, 2021, 07:15:24 am »
you probably can detect this:
- resistance can be used to measure temperature.
- temperature shoots up faster if insulated.

But :
- power suply voltage varies, has to be compensated for
- resistance age, drifting and bringing in unwanted changes.

Test if you can make it work.

Alternatively, use this to inspect faster :
https://www.amazon.fr/s?k=usb+endoscope&__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3%95%C3%91&ref=nb_sb_noss_2

Offline Apfelmus

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #4 on: April 02, 2021, 05:26:26 am »
Hi all,

thanks for your input. Indeed, there are some options.

One that does not work: the small camera. Although the heater has a hole on top where I have to put the acid into for regular decalcarications, its above of a metal sheet inside the unit, and the heating rods are below. Invisible. This is to avoid that boiling water is projected into the vapour outlet - same thing as the vapour dome on top of steam locomotives. The camera would have to travel 20cm to the right in a tight space and then turn downwards and reverse direction...

Basically resistance monitoring by monitoring voltage and current during heatup could be working, problem is which resistance (i.e. at what moment during heatup) to pick for comparisons. Perhaps easier to monitor is the heatup time of the water inside (say from 30 to 90°C), it will get longer with more limescale on the rods. Difficult thing in all cases is to calibrate the process, to end up with a table that gives me thickness as a function of, say, increase of heatup time. I would have to do the inspection of the rods regularly, which I want to avoid...

I remember having seen a page where they quoted numbers like "1mm thickness leading to 10% decrease of efficiency". Not really usable numbers (what efficiency??), but I wonder if there is actually some data available as to how the heating of the rods changes when limescale thickness increases.

Maybe someone has heard of such data?

cheers
Martin
 

Offline daniel444

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #5 on: April 02, 2021, 05:52:21 am »
what about a simple cheap amp meter on the supply of the machine

when its clean it should draw more amps because the water is cooling the elements

when it is filled with scale the elements will draw less amps because the resistance will go up , because the elements get hotter , because they are insulated , maybe

or you could use distilled water
 

Offline Kleinstein

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #6 on: April 02, 2021, 06:06:17 am »
The limescale would not change the efficiency very much: electric energy is till converted to heat by nearly 100% efficiency.
The matrial for resistive heaters usually use a material (e.g. NiCr or similar) with low TC - this is to avoid local hot spots. So it is not clear that a different temperature would really show as a significant resistance change.

The limescale cause the heater to run hotter and this way reduce the durability. Another point is that the steam production can change from more contineous to more fluctuating.
 

Offline RoGeorge

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #7 on: April 02, 2021, 06:13:48 am »
The thickness of the limescale is irrelevant.  Might be nice to know it, but that's not the point.

Diluted vinegar is suppose to break the limescale.  There are dedicated cleaning formulas, too, usually expensive, but some diluted vinegar should be about as good as the commercial lime scale remover.

Search for how to clean boiler limescale (without opening the boiler).

Offline beanflying

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #8 on: April 02, 2021, 06:14:51 am »
SNIP

or you could use distilled water

Distilled water is a bad idea. As you reduce the Ions in the water it goes looking for things to strip new ones from if you make it 'to clean'. Distilled is sub 1PPM TDS and a lot better than that depending on process.

Back in my former real job time one of my Customers was having an issue with a large Reverse osmosis plant and more specifically the pumps and really specifically the Brass impeller nuts on an otherwise stainless steel pump (Grundfos jacketed submersibles). Every 4-6 months each one of the 5 pumps needed to be taken out of service and have all the bronze parts replaced due to 'chemical attack' from the water. I didn't spec the originals in use but I put a replacement proposal together for them for three Marine Grade Bronze piston pumps plus a spare including drives which was accepted as a cost and quality improvement. Their detection equipment and analysis was actually picking up the Brass from the pumps in their sub 0.1PPM water. I had a very nice overseas holiday thanks to this sale and bonus my boss hated paying to me ;D

In the coffee industry we stick to around the 50ppm TDS partly for taste with Espresso and more like 100 to a bit more for brewed if it was just a case of keeping the gear free from 'gunk' then you would go to more toward say 10ppm as a balance between attack and gunk.
« Last Edit: April 02, 2021, 06:18:09 am by beanflying »
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Offline beanflying

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #9 on: April 02, 2021, 06:17:36 am »
The thickness of the limescale is irrelevant.  Might be nice to know it, but that's not the point.

Diluted vinegar is suppose to break the limescale.  There are dedicated cleaning formulas, too, usually expensive, but some diluted vinegar should be about as good as the commercial lime scale remover.

Search for how to clean boiler limescale (without opening the boiler).

'Breaking the scale' is asking for issues vinegar or citric or commercial compounds when used in situ with existing scale, it creates hard lumps that get stuck where you least want them. If you start from clean or near clean then regular dosing is ok as it breaks down more to a suspended sludge at worst.
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Offline Apfelmus

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #10 on: April 02, 2021, 07:14:27 am »
'Breaking the scale' is asking for issues ... it creates hard lumps that get stuck where you least want them...

Thats maybe what is happening in my unit.

I have this steam generator since approx 10 years. After 6 or 7 years with regular use  (twice a week on average for 30 minutes or so) one of the three heating rods triggered the "earth leakage circuit breaker" (Fehlerstromschutzschalter), a 30mA type. I took the unit apart and discovered a huge and irregular limescale buildup that, at some places, completely clogged the space between the heating rods! And that was although I always followed the decalcarisation process that is forced by the controller.

I cleaned the rods, bathing and brushing them for days in citric acid. Built everything together and the circuit breaker remained silent... I suppose the initial problem was due to one of the rods getting too hot internally.

Now, after 4-5 additional years, the circuit breaker triggers again. I made a "manual" decalcarisation with much more citric acid than normally used, but without dismantling the unit - so no mechanical action directly on the rods. That seemed to cure the problem, but I wonder how long it will take to come back.

cheers
Martin
 

Offline Apfelmus

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #11 on: April 02, 2021, 07:27:19 am »
Here are two pics I took in 2017, when I dismantled the unit. Enjoy.... :-(

[attachimg=1]
[attachimg=2]
 

Offline beanflying

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #12 on: April 02, 2021, 07:37:08 am »
Looks nice and crusty  ;)

Before and after. Would you drink a coffee made from the cloudy before shot :P This was done with a full strip and clean in a bath due to the level of gunk. This was only on a fancy home unit but fed with rubbish water.

Was trying to find one of the real horror shows but most of them are somewhere on a backup CD or six.

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Offline daniel444

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #13 on: April 02, 2021, 08:31:41 am »
how long are you soaking it in citric acid solution  for ? 
are you heating the citric acid solution in the boiler ? 
 

Offline beanflying

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #14 on: April 02, 2021, 08:40:28 am »
The question on using any descaler be it citric or commercial is how bad is the problem to start with. The two shots above went into a stainless pot and were brought up to a simmer then left for a few hours then rinsed and in that case the boiler went back in for a second go as it was still caked but the pipes and solenoids etc came up fine.

For casual/regular descaling it is fine generally to simply run it through the boiler and you will put any scale into suspension then a proper flush is needed afterward. In a commercial setting for an industrial boiler generally they run dosing pumps and continually feed chemicals into them to stop/reduce the problem.
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Offline Apfelmus

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Re: Measure limescale thickness through hot resistance of heating rods?
« Reply #15 on: April 03, 2021, 03:20:02 pm »
In my vapour generator the controller makes the descaler simmer for 12 hours, then flushes once and thats it.

Problem, as can be seen, is that the descaling action wasn't strong enough. Thats where I'm back to my initial problem: how to tell how bad it is, or if at all, without dismantling the whole installation.

cheers
Martin
 


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