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Author Topic: Corrosion in plate heat exchanger  (Read 7979 times)
ecogen
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« Reply #15 on: July 20, 2011, 01:03:14 PM »

The reason I mentioned the wire brush was not because of surface damage but because it forms anodic iron on the surface of the stainless steel and will initiate immediate surface corrosion. This is why stainless steel wire brushes must be used.
Surface roughness and corrosion resistance are related, but of minor relevance in this case.
Place a magnet on the boss. If the attraction is strong similar to that of steel then its not annealed austenitic stainless. Compare the magnetic property of the boss to the . Does the boss show any difference in any magnetic attraction to that of the plate material? This is not definitive, as cold working changes the magnetic properties. Assuming the plates to be 316, a lower grade of stainless will be less noble / more anodic in contact with a higher grade and more prone to corrosion. This may be comparing apples to oranges, but I have put a rare earth magnet to a Bowman brazed plate 316 H/Ex. Both plates and Boss non magnetic.
If you were to show me the before and after corrosion photos, with the S/S H/Ex blanked out. Top of my material list would be cast iron.
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TimSmall
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« Reply #16 on: August 20, 2011, 08:44:28 PM »

FYI, I suspect this pic is of the same (or a very similar) model of exchanger - it's never been used, and was sitting in my (fairly dry) basement for a couple of years, til I noticed that it looked like this.  I suspect from the look of your one Wookey that I'll now not bother doing much with it!

It was originally destined for a project almost identical to Wookey's, but the relative which it was bought for keeps umming and ahhing about the idea - fortunately by the look of it.  Time to find a different make/model I think.


* corrosion-on-zilmet-vaillant-FPE.JPG (73.92 KB, 1024x382 - viewed 585 times.)
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wookey
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« Reply #17 on: August 20, 2011, 11:36:01 PM »

That is indeed extremely similar , if not identical (I'd have to check the number). It does strongly suggest that  only the 1/2" side should be used for DHW.  I could do that.

Alternatively does anyone know of reasonably-priced PHEs with nice threaded bosses like this? I actually found it quite hard to find such a thing. Most combi PHE do not have threaded boses. Most PHE that do have threaded bosses are quite expensive, or excessively large for the max 2kW solar trasnfer.
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Wookey
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« Reply #18 on: August 21, 2011, 09:49:36 AM »

I'm still searching for a reasonably priced PHE with threaded connections, but am considering this one:

http://www.ebay.co.uk/itm/Stainless-Steel-Heat-Exchanger-10-Plates-ca-22-kW-/270793678539?pt=UK_Home_Garden_Hearing_Cooling_Air&hash=item3f0c8f9ecb

I think the photo is generic for all sizes they sell. This particular one is 10 plates, 29.4mm high - How does this compare dimensionally to the one you have been using Wookey?

What I am unsure of, is the best method/type of fitting to use to connect to the threaded connectors........
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« Reply #19 on: August 21, 2011, 11:02:31 AM »

That exchanger looks reasonable, I think.

I think the best sort of connection to go for is the style used by washing machine hoses - i.e. a brass fitting with female parallel BSP thread, which compresses an EPDM washer between a flat "shoulder" in the fitting and the heat exchanger boss when tightened.  I think (but I'm not entirely sure) that BES part 17613 is what I have in mind.  I think screwfix may also stock them as "female couplers", but they don't really give enough detail.

Maybe best to contact the ebay seller and ask if they would supply fittings suitable to connect their exchanger to 15mm copper tube (I'm assuming that 15mm copper is used in germany, I may be wrong).

Tim.
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al_uk
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« Reply #20 on: August 21, 2011, 11:03:04 AM »

That PHE looks very similar to the one I've got. I ended up using flexible hoses from screwfix to connect, as I couldnt find anything else that would fit. The problem with the hoses is that they are not full bore, and don't look great. I'd be interested to know if you find any better fittings in either copper or plastic.

I've had mine fitted for a few months now. I suppose I'd better check it for corrosion.

My thread is here

http://www.navitron.org.uk/forum/index.php/topic,13123.0.html
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« Reply #21 on: August 21, 2011, 11:48:30 AM »

That PHE looks very similar to the one I've got. I ended up using flexible hoses from screwfix to connect, as I couldnt find anything else that would fit. The problem with the hoses is that they are not full bore, and don't look great. I'd be interested to know if you find any better fittings in either copper or plastic.


I would be concerned using these flexible connectors at high temperatures i.e. solar side >100 degrees C - not sure how long they would last before the hose failed.

The search continues - have the solar panel sitting in my garage, don't want to start the installation until I have the PHE problem solved!


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« Reply #22 on: August 21, 2011, 05:14:27 PM »

Tim, I don't think there's a shoulder inside 17613. I reckon you you want 17643 (4/5/6/7).
But I found that the awkward part is that tap conncetors expect a particular ID and the PHE has a smaller ID than normal for the 3/4" thread so 3/4" BSP tap connectors don't fit without cutting off the bit that goes inside the pipe (so that the washer can actually be compressed. (I realise a picture would help here, but hopefully you can work out what I mean).
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Wookey
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« Reply #23 on: August 21, 2011, 05:40:03 PM »

Are the plates corroded  or just  the connection?


The earlier  ebay link phe is from Wiltec. I've used a couple of their heat exchangers.

You can purchase  big 160kw alfa laval units  here.....
http://www.eco-equipped.com/alfa-laval-brazed-plate-heat-exchanger.html
Includes the insulation  casing  and reasonably priced also.

« Last Edit: August 21, 2011, 05:51:16 PM by Solal » Logged
TimSmall
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« Reply #24 on: August 21, 2011, 07:22:02 PM »

Tim, I don't think there's a shoulder inside 17613. I reckon you you want 17643 (4/5/6/7).

Sounds plausible.  I'll see if I can find the one that I think I have here to illustrate - originally from Screwfix I think.

Tim.
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« Reply #25 on: June 03, 2017, 07:27:47 PM »

So, 6 years later I come back to this issue. My solar setup seemed to be being rather inefficient this spring, with not as much hot water as I was expecting. I noticed that the pump was working in the morning, but was cutting out when the system got properly hot. So I put in a new pump - it's done 9 years. But the tank still wasn't getting hot.  At this point I realised that the thermosiphon wasn't circulating, and also that I had a leak from the upper PHE spigot.

So, looks like it lasted almost another 6 years before springing a leak. And has probably been slowly bunging up for quite some time, (we had noticed degraded performance for a year or so at least).

I now can't get either speedfit connection off the PHE - the nuts seems quite solidly corroded on. But the blockage turns out not to be in the PHE, but in the short horizontal bit of pipe, and is made of what looks like blue calcium carbonate, so that's hard water deposits, but with coppery bits in?

I'm probably going to have to get a new PHE as I can't fix the leak without getting the leaking spigot off and that may well destroy the PHE, and then I'll probably  find there isn't enough material left to get a seal against anyway.

I was hoping to have got round to fitting a whole new tank and setup by now, but what with one thing and another don't seem to have got round to it.
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Wookey
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« Reply #26 on: June 10, 2017, 09:35:12 PM »

Perhaps not relevant as we are in a relatively soft water here in wales, but I have used several of the following make of plate heat exchangers in industrial settings with no issues. They are usually the same boss size for both sides

http://www.ebay.co.uk/itm/Stainless-steel-PLATE-HEAT-EXCHANGER-NORDIC-TEC-25-65kW-Ba-12-INSULATION-BOX-/252813982017?var=&hash=item3adce33941:m:m2HP_EZMaDw4nkEJhoSan-Q

I usually buy them direct from the manufacturer (LEDNET) in Poland, but ebay might be your best bet.

As they are used in and industrial environment, they have a LOT of water going through them... Hot side is around 70 Dec C, with cold mains on the exchanger side. around 7000 Litre per day of fresh water going thought them. The hot side is permanently pumped 24/7, with a flow of around 4-8000 litre per hour IIRC.

I've also used with chilled water (5 Deg C) to mains water.

They are connected with brass fittings and copper pipe. I tend to used CXFI (compression female connectors) with fibre washers and fernox water hawk, or similar jointing compound.

I took a replaced two heat exchangers last year for much bigger ones they had seen around 2 years of industrial usage, and no signs of any sort of corrosion or anything.


I've noticed over the years' that any leaks on the hot water side does product a nice build of crust around the leak quite quickly. I've always assumed that's it due to the large volume of 'fresh' hot water we get travelling around the system every day (about 30,000 litres).


Hope this might be of help
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« Reply #27 on: June 11, 2017, 01:38:23 PM »

Wookie, I know folk laugh at my calcium carbonate slurry magnet (low voltage set up) I bought years ago ...but it works.

We have same (ish) water as you being in close proximity, did you fit one on the incoming supply ever?

When you have resolved your current issue I'd recommend one still as a preventative bandaid.

Mine has always worked a treat, & whilst I could not tell if there had ever been a sacrificial anode in my he tank (looks plausible) there is negligible calcification in my pipes nor hot water tank, the crust it slowly removed over a two year period & ejected was considerable,  & my shower head only requires a thumb nail on it every 18 months or so (had 2x proper cleansings in its life just for fun in a decade).

If calcium is a problem compounding other issues worth adding maybe!? (I seem to recall you tried one some years ago though)!?

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