Heating water in a copper pipe from a wood oven

[mattsawyer]

I'm a newbie here and it has been a long time since I did complex science and I would be grateful for any help and expertise on my problem.

 

I am looking to build a wood burning pizza oven in the middle of no where with no services/water/etectricity etc and hope to use it to also heat some water. My design basically is to sandwich a 10m coil of 10mm diameter copper pipe in the floor - sandwiched between two layers of firebricks. I have looked elsewhere on the net and coiling a copper tube either inside the oven or within the walls of the oven doesnt seem the best idea due to the problems of direct heat on a copper pipe, corrosion and oxidisation etc.

 

I think that sadwiching the coil between firebricks would give a more even distribution of the heat. I have three thicknesses of firebrick and will be doing some experimentation to see which would work the best above and below the copper pipe. This will tell me the temperature of the non-heat side of the brick.

 

The temperature of the wood burning/pizza ovens can get up to 800'c. The hot water system would be an unvented system as i think there is the likelihood of rapid boiling/superheated steam accumulation and the subsequent risk of explosion in a sealed system.

 

My question is - assuming that I can have a constant temperature on the copper pipe, how can I work out the speed of flow of water through the pipe to have a temperature of say 90'c (the termerature is not critical) water coming out. I cannot regulate the temperature of the oven but I can regulate the flow of water to ensure it doesnt boil. As I have to transport the water to the site myself, I want to try and work out if I would need 1 litre, 10 litres, 100 litres or 1000 litres. The wood burning ovens can stay hot for upto 10 hours. If the water ran dry and the copper pipe was exposed to 200'c heat for several hours would this cause any long term problems ?

 

Are there any easy to use graphs showing heat loss over time from copper to water for varying temperatures ? I assume that at 400'c that the heat transfer would be far faster than at 200'c

[CaptainPanic]

First of all, building your own water heating is always risky. If you have a closed system, you risk the buildup of steam, and therefore you risk a steam explosion. This is a serious hazard. It's not straightforward to build a control system. If your water flow is too low, you get steam. Too high, and you have cold water. But the main problem is that your heating source is not constant. The heating power of a fire can easily vary by a factor 2, if not more. Therefore, your water will sometimes heat up by just a few degrees, and sometimes by a lot more... unless you can actively control your water flowrate.

 

Anyway, let's crunch some numbers:

As a first estimate, you can assume your heat transfer coefficient (U) is 50 W/m²K to 1000 W/m²K. It's a broad range, and the actual value depends on the design. Firebricks are insulators, so it depends how deep you bury the coils into those bricks, if you would want to do that at all.

 

In the calculation below, I take a couple of shortcuts to save time. There are better ways to calculate this, but they also take more time (which I don't have today). This will give you a first impression.

 

So, since 10 meters of 10 mm pipe has a surface area (A) of 0.31 m². Let's sasume you place your coils at a place where the temperature never exceeds 100 C (that's the safe thing to do). Let's also assume your water comes in at 20 C, and leaves the coil at 80C. The average temperature difference between the wall and the water inside the pipe is (dT) is 50C. The power (P) is then:

 

P = U*A*dT = 50*0.31*50 = 785 W (low value)

P = U*A*dT = 1000*0.31*50 = 15700 W (high value)

 

Assuming you want to heat up water from its inlet temperature (T_in) of 20 C to its outlet temperature (T_out) 80C, that means you need a 60 C increase. We want to find the flow (F). We know that the Cp of water is 4180 J/kgK.

 

P = F*Cp*((T_out-(T_in)), so:

F = P / Cp*((T_out-(T_in)) = 785 / (4180*60) = 0.003 kg/s, or about 0.19 liter/minute (low value).

F = P / Cp*((T_out-(T_in)) = 15700 / (4180*60) = 0.063 kg/s, or about 3.8 liter/minute (high value).

 

It should be noted that the high value is so high that you would be actively cooling down your oven, and you should burn more wood to heat that much water, or risk that your pizza never gets hot enough.

 

Also, cooling your oven that much can affect the draught of the chimney, and therefore mess up your oven.

 

Personally, I would see if there are some commercial (=tested!) solutions for your problem. Off the top of my head, sauna heaters often have hot water reservoirs. A reservoir does not heat up so quickly, and will almost never boil completely dry, which increases safety. Also, your hot water will be available after the fire went out. It might be a starting point from a practical point of view.

[random]

I would copy the design of an outdoor wood furnace , your pipe diameter seems pretty small compared to these systems using a water jacket and 3/4 inch pipe to carry the hot water quite a distance without losing much temperature. A radiator for these systems would also allow you to use your oven as a safe heat source for any shelter you have. only you would need a 12 volt pump system or propane generator etc. to provide electricity for the pump.

[Enthalpy]

Because can't adjust the oven's heat to your water needs, I feel better to have a separate water boiler. Heat it with wood as well.

 

Then corrosion won't pollute your food and is less of a worry; it may still pollute your water. Many water boilers have copper pipes (they release dirt but survive for long). Stainless steel would resist better and may be cheaper.

 

Could you maybe find a trashed water boiler - meant for gas for instance - and burn wood below it? It would already have an efficient heat exchanger, and if your scrap dealer isn't too expensive, you can replace the heater when wood fume makes it unusable.