Expansion percentage?

[windyknight]

My batch solar water heater is almost complete but for reasons i wont go into i have an open top tank in the loft with no easy way of putting an overflow pipe to it.  So, if tank is 4 ft tall and 18 inch dia how much empty space at top do i need to accommodate heating water from say 45deg.F to say 150deg F (8-70 deg C)?

I've managed to incorporate a small expansion vessel in heating loop though I suppose copper tubes etc will expand when hot to give a bit more room so its only tank I have to sort now.

keith

[Bruce S]

Keith;

   I'm pretty sure that a good rule of thumb would be to allow 10% for expansion.

Heard it on a PBS show.

Hope that Helps;

Bruce S

[wdyasq]

http://tinyurl.com/awqe3

First hit a good-un

Ron

[nothing to lose]

"but for reasons i wont go into i have an open top tank"

Well don't go into it if you don't want, but I have a question on it. When you heat the water and you have an open tank will you have humidity problems?

Just thought I'd ask. To keep static shock to a minimum here I place a coffee can of water behind the wood burner. It does not get too hot, but enough to put some moisture back into the air and we now seldom get static shocks like we used to.

So if you have an actual open tank, geuss that's what you meant, and you heat water to

near 150F as you say, is humidity and evaporation going to be a problem for you?

My water near the wood burner never gets near that hot, and it does evaporate. Don't remember how often I fill the coffee can normally. Have not needed the water can yet this year, just now really starting to need a fire.

Just thought I'd mention the humidity in case you hadn't thought of it and do have an open tank.

[GaryGary]

Hi,

Expansion volume is calculated as:

dV = (expn coef)(Initial Volume)(Temp Change)

For your case:

Expansion coeficient for water = 210E-6

Initial Volume = (4ft)((1.5ft)^2)(Pi/4)= 7.06 ft^3 or 53 gallons

Temp change = (150F-45F)1.8 = 58C

So, dV = (210E-6)(53 gal)(58C) = 0.64 gallons of expansion between 45F and 150F

Since each inch of your container holds a bit over a gallon, it seems like having one inch of expansion space at 45F would still leave about 1/3 inch at 150F.  This assumes everything is perfectly level and nothing moves -- a bit more space might be prudent.

One other thing is that the container itself also expands and makes more room for the water -- so the above calculation is conservative -- how much depends on what the container is made from.

Another consider is that if it ever freezes it will expand a lot more during the phase change.

Stuff here:

http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Thermal/ThermExpan.html

Gary

[windyknight]

Thanks Gary,et al,

Reckoned on an inch from gut feel so it's nice to have some maths/physics to validate decision! Need to minimise it (ie Not go 6 inches to be safe) cos i built the heat exchanger too big (before i thought about expansion) and didnt want hottest part not actually in the water!

Thanks k