Corrosion Protection for Hot Water tank

[frogeyepete]

Hi, I am putting together a solar water heating system in Spain (using the trial and error/hit & miss system). I have a very nice flat panel, which is approximately 1.25m x1.25m. Mounted on a steel structure over my parking area, pointed due South and angled at approximately 45 deg. I want to use an indirect heating system and have purchased a second hand steel tank and made a copper coil to go inside it. There is some corrosion in the tank which I will clean out with a wire brush on a flexible from my drill. The questions I would like some help with are:

1) Having cleaned out the tank, what type of paint should I use to protect the inside of the tank?
2) If I add glycol to the water in the tank will it help to reduce the possibility of corrosion?

The tank is 300 litres capacity. The contents of the tank will circulate through the panel using a 12v DC pump. The coil is approximately 3 meters of 9mm OD copper tube. I want to run the water that I use in the house through the coil and hope that it will pick up enough heat in the tank to provide hot water for showers etc. The design of this coil is where the trial and error comes in. There must be is a calculation that would give an approximate surface area, but have no idea what it is. Any help gratefully appreciated.

[LucMan]

Hi, I am putting together a solar water heating system in Spain (using the trial and error/hit & miss system). I have a very nice flat panel, which is approximately 1.25m x1.25m. Mounted on a steel structure over my parking area, pointed due South and angled at approximately 45 deg. I want to use an indirect heating system and have purchased a second hand steel tank and made a copper coil to go inside it. There is some corrosion in the tank which I will clean out with a wire brush on a flexible from my drill. The questions I would like some help with are:

1) Having cleaned out the tank, what type of paint should I use to protect the inside of the tank?
2) If I add glycol to the water in the tank will it help to reduce the possibility of corrosion?

The tank is 300 litres capacity. The contents of the tank will circulate through the panel using a 12v DC pump. The coil is approximately 3 meters of 9mm OD copper tube. I want to run the water that I use in the house through the coil and hope that it will pick up enough heat in the tank to provide hot water for showers etc. The design of this coil is where the trial and error comes in. There must be is a calculation that would give an approximate surface area, but have no idea what it is. Any help gratefully appreciated.

Why not just use a corrosion inhibitor additive? http://www.rhomarwater.com The only reason to use glycol would be to protect against freezing.
1/2" id copper (13 mm) would be a better choice for your coil. It has a larger surface area and would have less restriction.

[Mike90250]

steel and copper are always going to want to react in water. So you have to use a corrosion inhibitor in the water.

Paint is a no-no, it will flake off, and clog things.

[inetdog]

Be careful where you run the piping for the coil onto the tank that you do not create a place where the (galvanized? steel?) tank and the copper plumbing are in direct contact. That will minimize the electrogalvanic corrosion.

As for the coil area calculation, first figure out the length of the copper pipe in the coll and then multiply it by the surface area per unit length of the pipe:

Area per unit length of pipe, where D = outside diameter of the pipe: A = 2 pi D.

[frogeyepete]

Solar Heating tank

Thank you for the replies. The tank is just mild steel. I was going to buy a second hand stainless one, but missed the chance. I just paid 40 Euros for the carbon steel one that I have, and if the system works I guess it would be worth paying a couple of hundred for a stainless steel one. If any paint is going to come off and clog the system, perhapts I should just forget about that and put in corrosion inhibitor. For the coil calculation I was thinking that if the water in the tank is at 80 deg C, and the water enters the coil at 20 Dec C what temperature will it reah by the time it gets to the other end. It will obviously depend on flow rate.

[J.P.M.]

Thank you for the replies. The tank is just mild steel. I was going to buy a second hand stainless one, but missed the chance. I just paid 40 Euros for the carbon steel one that I have, and if the system works I guess it would be worth paying a couple of hundred for a stainless steel one. If any paint is going to come off and clog the system, perhapts I should just forget about that and put in corrosion inhibitor. For the coil calculation I was thinking that if the water in the tank is at 80 deg C, and the water enters the coil at 20 Dec C what temperature will it reah by the time it gets to the other end. It will obviously depend on flow rate.

Flow rate is one of about 6 or 8 major parameters and any number of smaller ones. Cold side outlet temps. will be impossible to predict with the information provided other than to say they'll be different than the20 deg. c. you mentioned cold side inlet. How much is f(all those parameters I mentioned).

[mapmaker]

Area per unit length of pipe, where D = outside diameter of the pipe: A = 2 pi D.

please elaborate on that formula... could it be off by a factor of 2? --mapmaker

[Robert1234]

Returning to the original issue, consider utilizing a magnesium anode rod in your tank. Cheap and effective.

[LucMan]

Thank you for the replies. The tank is just mild steel. I was going to buy a second hand stainless one, but missed the chance. I just paid 40 Euros for the carbon steel one that I have, and if the system works I guess it would be worth paying a couple of hundred for a stainless steel one. If any paint is going to come off and clog the system, perhapts I should just forget about that and put in corrosion inhibitor. For the coil calculation I was thinking that if the water in the tank is at 80 deg C, and the water enters the coil at 20 Dec C what temperature will it reah by the time it gets to the other end. It will obviously depend on flow rate.

I would make the coil as large as possible to still fit the inside the tank. Most soft copper coils are 60 ft long. It is highly unlikely that you will be able to get a consistant usable hot water temperature ( 120 degrees F) from your settup as you are just getting a single pass through the heat exchanger. Piping the the outlet of the heat exchanger through a tempering valve, then through your existing water heater would cut down on your energy consumption and give you usable HW at the faucet. The first slug of water could be extremely hot as it has been sitting in the coil at max temp easily scalding the user
To many variables in a single pass HX solar setup to make any predictions as to performance, but all energy absorbed will be a help.
A system utilizing a domestic HW storage tank would be more desirable for usable HW.

[inetdog]

please elaborate on that formula... could it be off by a factor of 2? --mapmaker

Um... Let's see. "I before except after C" and yes, 2 pi r = pi D.

[elisagrace]

If you will paint the tank it will not go for longer time and with time it will flake off. The best option in such case is to use corrosion inhibitor additive. The use of glycol is only a safety against freezing. http://www.greatbasinindustrial.com/...storage

[elisagrace]

Paint will not be suitable in such case as with time it will flake off. Four years back I have got stainless steel water tank from Great Basin Industrial. Six months back I also faced corrosion problem in the tank. After discussion with them, they told me to go for corrosion inhibitor additive. With the application of this additive, corrosion problem is over.

[frogeyepete]

Wow! Time moves on. The truth is that I dumped the mild steel tank and bit the bullet by buying a purpose made solar tank (glass lined) for a couple of hundred Euros. I got it going earlier this year and it seems to work quite well. The system includes a pressure reducing valve for the water that is pumped through the panel. The electric pump works off a couple of photovoltaic panels (I installed three originally but this was too powerful). The pump operates when there is fairly strong direct sunlight, so I am reasonable confident that I am pumping through a hot panel. There is a 500W immersion heater that is controlled by a thermostat, and is on a time switch (so it is switched off during the day). I have basically avoided any clever control systems (which could be a mistake on my part). The main thing is that it works, and the system is replacing two electric water heaters that had a maximum consumption of 3.5kW!!! In Spain we have a contracted maximum power supply and there is a fixed daily rate associated with whatever level you have decided to have. The solar heating not only saves energy, it helps to minimize the level of contract that I need to have.

[peakbagger]

I would still install a magnesium anode in the tank. Cheap insurance. Tanks will last a long time with anodes.

[J.P.M.]

I would still install a magnesium anode in the tank. Cheap insurance. Tanks will last a long time with anodes.

I'd not be surprised if the tank is already so equipped. In any case, if present, I'd check them every couple of years few years. They are after all sacrificial and are supposed to disappear over time.