Heater Element Corrosion

The cause appears to be hard water. Calcium and magnesium carbonates and sulphates, together with silica in the water, will coat any heating surface as they are inversely soluble with temperature. If there is a free surface anywhere within the circulation system, water vapour will evaporate from the top of it and the make-up water, probably very hard, will exacerbate the situation by adding minerals to the circuit that coat-out as a precipitate on the heating elements. Once that thermally-insulating layer has formed, the electric heater inside the elements will cause the casing to run hot, eventually causing the failure indicated.

Consider running the remaining and replacement heaters at a lower voltage as a short-term measure until the water supply problem is fixed.

Consider reducing the evaporation loss from any header tanks. A commercial polypropylene ball blanket is a nice cheap-and-nasty fix. Ping-pong balls might be a reasonable second choice, depending on temperature. Make sure the tank has a lid as well.

Provided the heated water is for non-potable use, as heated circulating water usually is, consider fitting a commercial water softener upstream in the make-up water supply so as to get rid of those nasty divalent ions and replace them with monovalent sodium and chloride ions: sodium chloride is highly soluble in water (ref: Dead Sea, Jordan and Israel) at most commonly-encountered temperatures and will not precipitate the way that calcium and magnesium salts do.

The elements, when removed, have an encrustation of calcium and are split.

You, capblanc, have just answered your own question. An electric heating element, immersed in water and supposed to heat that water, depends absolutely on heat transfer. At all times the metallic heating tube of the element MUST BE CLEAN of any deposit, sediment or whatever. Only then will heat flow from the element to the water as designed. As calcium deposits settle on the heating tube, they forms a insulating layer, decreasing the heat transfer gradient and the element begins grow hotter and hotter until it bursts. This is a common problem with immersion heaters heating hard water. Try to heat your water with steam.

I also believe there are electric immersion heating elements designed for hard water service, but sure I am not. Try to ask around, shop around.

The water is it fresh water? You may find other sheath material will hold up better. 316 SS or 304 SS.

What is the hot water being heated for. I have a wash water tank here about the same volume 60 gals has a 18k watt heater on it maintaining the water at 140 deg F. An increase in wattage could help. Element would have more surface area to heat water faster. Which would reduce cycle times. This may drop the surface temperature of the element so that you have a reduction in hard water salts build up PWSlack talking about. Can't stop it still have to have ours replaced due to similar failures. Get 3 t0 5 years though.

Thanks for the prompt replies,...unfortunately no help. The water is domestic hot water. This yacht lies alongside, and takes the same water as, several other super-yachts, also clients, who do not have this problem. The temp is pretty much a standard 60ºC/140ºF which is the compromise between minimal calcification while killing most bacteria and is still not so hot to be generally dangerous.

The elements are not massively caked in cal, that would be an obvious reason if the water was very hard. We had a lot of element failures until a massive desalination plant was installed on the island. The water now is quite potable. During the season the yacht will be on water from its own desalinator which is very pure, almost distilled quality. When they take water from the quay they do run it through a treatment plant but I don't know what is involved with that.

I have a suspicion that the cause may be not that the power heater is too low but instead that they have too much power/ sq mm of element surface. (I suppose I was looking for someone with specialised experience to confirm this).

PWS, running lower voltage is impractical, but changing for lower power elements is what I have in mind which is essentially the same thing. I'll try to find out what their treatment plant does, I suspect it is principally a softener.

Guest, see http://www.painc.com/corrosion.htm. Heating with steam is impossible however there is a possibility to heat with from generator heat rejection. The generators are close and are run throughout the season, which is when most heating is needed. (Crew + guests would be up to 16)

OzzB, Your suggestion seems to me to be the wrong way. If they up the power they will have even more localised (surface) heating which I think would exacerbate the problem. I think (when absolutely forced to) that having the elements on for longer at a lower surface temp may help the situation.

Thanks again, Chas

You seem to be adamant re the water quality being a cause, pure distilled water would not leave deposits ? Most elements I have seen have considerable deposit and have been running for years.

If it is not the water then the design of the elements must be the next area of concern, ie heating element underrated, or contaminated filling of the element. Insulation leakage problem between the the 3 elements although with earth leakage protection I could not understand how this would happen unless all three elements had identical leakage and cancelled out any earth return, very unlikely.

However you are having repeated problems so the element design should be top of the list of suspects.

A similar situation kept us busy in our solar water heaters with electrical heating element for inclement weather. We solved it with a heat exchanger -model jacket- (like a partial double wall or chamber) with separate overpressure safety valve. We have put the heating element in there and use glycol, as surrounding liquid - with small expansion tank) The glycol is always the same and not enough to kill the heating element. For completion we also have a sacrificial element in it - but that is different talk. You can consider a heat exchanger serpentine in it too. Perhaps, if your generator runs its cooling circuit through it and it is switched on all the time, you have established some economic recovery.

Doing some research for you on solutions via HotWatt expertise. Improved heater performance is the goal.

Hi Sensorcontrols,

Many Thanks, Performance in terms of longevity / low maintenance is the goal. If you come up with anything or have any good links I'd be very happy, as this problem has been an issue since the tanks were installed about 5 years ago

regards Chas

Hi Folk,

I put the same enquiry to a company called Elmatic and got the following response. It seems that the W/sq mm ratio is a likely culprit

"Unfortunately, this is a common occurrence with immersion heaters due to limescale attacking the elements.

There are a couple of suggestion which can improve the life of the element.

Firstly, not knowing who manufactured the element you are currently using, different manufactures use different

quality materials depending on their market they are supplying to. We see alot of "cheap" heaters which are aimed towards the domestic market,

and the manufactures tend to use cheap materials with a thin wall tube, to keep the cost down. However, these elements are not suitable for

many applications.

Here at Elmatic, we only make for the industrial industries and tend to use more expensive materials, knowing the more robust

Applications they are going to be used for. We tend to use harder grade materials and thicker wall thicknesses.

I suggest you try using a Incoloy 825 element which is a harder material and more suitable to aggressive fluids to be heated.

We can also us larger diameter elements which we manufacture to size.

In addition, by reducing the watts density of a element, the water will not attack the element so aggressively. Some element manufacturers

Use up to 90 watts per sq inch on the sheath which is very high. This is just to get the kilowatts in, in such a small area. This sometimes leads to

Elements being overloaded on the watts per sq inch, which means they do not last so long.

You do not have to reduce the wattage required. You need to find a way of getting more element length into the space required. By doing this

You reduce the watts density of the wire in the element and by doing this improves the element life.

Can you use a longer heater, or can we coil the element instead of using a u bend. I need to see a photo or have a drawing of the element

To help you decide this."

Something than can work too: a 230/240 element double the wattage and give it no more than 120 Volts. This will have a lower emission temperature, but sufficient to get 55- 60 degrees Celsius water temperature.

dvmdsc

no here to get 120V from! I think we will just go to a lower power as i know the boat hasnt suffered even with half the units out

Chas

What is your working voltage? Are you running it on AC or is it a DC element? If your supply is 220 Volts AC, a double wattage element will work too with 1 power diode in series. If you have e.g. 1500 Watts, buy a 3000 watts and put a 15 ampere diode in series. Make sure it can handle the voltage. The polarity doesn't matter. You will produce heat over a far larger surface in this case. and your element will stay less hot

You mentioned other local user do not have this problem. There is the possibilty that your tank has a bad ground and you are getting a small dc charge in the tank. This can lead to high deposition rates of calcium. some undersea structure apply a dc charge on their structures to cause a calcium deposition to increase the strength of their structures.

DVMDSC.... read my OP..... Delta 220Vac (unless you know any application for deltaDC?)

Silver Crow.. Tanks are grounded, we put the grounds in during the initial installation. Boat hull is steel. If there is any galvanic issue it can only be different materials, but this shouldnt be the case as all is stainless.

Chas

I have seen items which appeared to be well grounded to not be well grounded. A small amount of crud in a connection can act as a small diode creating a DC bias leading to all sorts of weird things. Two differnt metal put together can act as a diode. In this case I would thoroughly check the element to tank resistance and measure for a DC voltage on the element while it is powered up. I only mentioned it as a small possibility. It is an easy and quick check.