Home Handyman Water Preheating System

[SID66]

Good day all,

 

I was looking into this topic and found this site, I have obviously regiatered and look forward to new ideas.

 

Here is one to mull over- Interupt the cold water suppy to the geyser (at ground level) and run a pipe into a 200l (44Gallon) drum, run another pipe(outlet) from the drum back to the origional feed to the geyser.

 

Firstly the water (in this country) is colder coming out the ground than the air above the ground. Thus the water now housed in the drum will warm up to ambient temperature and saving you having to pay to heat it by those few degrees.

 

Secondly- as you cut the grass, pile the cuttings around the drum. The "rotting" cuttings will generate heat that will save you even more.

 

Any comments?

[CaptainPanic]

It'll work. It's a principle that is called "heat storage".

The best thing to do is to look up "heat storage" in your phone book, or using Google. There may be companies that install this professionally. Those installations, while more expensive, will be a lot more efficient, and will avoid the next couple of dangers:

 

- Make sure to have sufficient surface-exchange area in the drum for it to be efficient.

- Also, does it ever freeze? If it does, you absolutely do not want the pipes to be exposed to the outside air (insulation is probably not enough). They will freeze, and then possibly break.

- Obviously, you should pay a lot of attention to the new connections in your water supply. Do not let any contaminations get into them (especially not from that drum, which will eventually have contaminated water).

 

You may also consider a solar boiler, which reduces your heating bill even more, because it is able to heat up water to temperatures well above ambient.

[SID66]

Thanks for that - interesting when you put things in the air for others to comment.

 

In the coastal regions of South Africa it rarely goes below 15 deg C so freezing is not likely.

 

Not sure on the relevance of your first danger point (surface area) - I will be using the standard 200l/44gal drum as a reservoir with garden refuse around it and the sun shining on the exposed sections. Thus making your danger point 3 moot as there will be no connections within the "compost".

 

For info - I am using a stainless steel drum (I believe was origionally used to supply syrup to Coca Cola) and have had it standing full of water for a few days to dissolve any remaining syrup, I will empty it before I pipe it into the closed system.

 

The water coming out the ground is approx 19 deg C, My geyser is set at 55 deg C. My aim it to increase the t of the water entering the geyser to 25 deg C or higher.

I will obviously have to paint the drum black and will put a wire "cage" around it for the grass cuttings. The drum will stand in the sun for approx 8 hours of the day.

 

I have done some calcs to get the resultant t of the mixed water after using half the hot water but am short the formula to work out how much electricity it will take to heat up 200l of water by 1 degree C - help on this would be greatly appreciated. ( have forgotten my high school science)

[Schrödinger's hat]

Water requires roughly 4 joules (actually 1 calorie if you want to look up more exact values) to head 1 mL 1 degree C

So 200L is 800kJ/Degree

You generally pay for electricity in kWh (one of the sillier units that people use, although not as bad as my pet hate kWh/year :/) so divide by 3600 gives 0.2kWh/degree

[CaptainPanic]

A rough value for commercial water/water heat exchangers is a heat exchange coefficient of 1000 W/m2K. But that's a value you only achieve if both sides of the heat exchanger are flowing (and turbulent).

 

Since your water in the barrel will be practically stagnant, you will be significantly lower. I would guess only 100 W/m2K (note: do not use this to do calculations).

If the pipe gets dirty, this number will drop. If the water flowing through the pipe is not turbulent (you cannot check that, but you can calculate the Reynolds number), it will be lower.

 

If you have somehow managed to squeeze 10 meters of piping into the barrel, and that pipe is 2 cm diameter, you have 0.63 m2 of surface area on your pipe.

If the pipe is smaller, this number will obviously drop.

 

Let's also assume that you have a 20 degrees temperature difference between fresh water, and water in the barrel.

Obviously, if the temperature difference is smaller, the number will be lower.

 

Then the heating power for your water is: P = 100 W/m2K * 0.63 * 20 = 1200 W

 

Showers typically use 10 liter per minute... or 0.17 kg/s of water. So, you can increase the water temperature by about 1200 / (4200*0.17)= 6 degrees, which is considerable.

 

Please note: this is just an estimate, and most likely a high-estimate. I did not look up ANY numbers, especially the heat exchange coefficient can be very different (!), and I don't know how practical it is to put a 10 meter, 2 cm diameter pipe in a barrel.

Edited May 12, 2011 by CaptainPanic