25 September, 2009

Preparation of a stainless steel coil for the Russian stove

(This document was generated using Google Translator and then correcting some errors from the original spanish article).

Recently I showed you how to get better performance from a wood stove, using a Masonry Heater.
Yet I realized I could get even more performance (the accumulator design is not very efficient), and could benefit even more from the heat from the flames by adding a similar coil placed on the other stove my parents to recover more heat for other uses.

Underfloor heating
Hopefully, this floor was rebuilt in 2003 I had the great idea of putting underfloor home heating.

(of which I only keep this photo) made with copper pipes covered with dough and small amount of cement to make it loose and allow expansion of the tubes.
The technique of using the floor instead of radiators is the best way to heat your home for several reasons:
  • Being a greater heat-emitting surface the water temperature can be much lower so the energy efficency  is between 15 and 20% higher than with other systems.
  • By the same reason, solar energy systems can be used to transfer heat energy from the sun roof to the rooms of the house, for its performance at low temperatures (around 20 to 40 º C).
  • The comfort is higher, since heat is evenly distributed and from the bottom up ... it is a pleasure going barefoot with the flagstones at 25 º C! Soil is also the heat accumulator, thus heating it for several days, we will be warmed up for two days.
  • We eliminated the pesky radiators, usually placed next to windows covered with curtains so the losses through the glass are accentuated. All this is avoided.
  • When working at low temperatures we can use cheaper plastic materials and is often used PE.
The disadvantages of this system are almost nonexistent. It may be mentioned that requires more work and labor (if done at the moment of building the house could be even cheaper) and that if there is a leak it is difficult to repair (could involve a large surface lift) and it is inappropriate to use soil with low transmission of heat as wood.

On my system I should have isolated the soil below the radiant tube, since underneath is the rock of the mountain to which is transmitted 30% of the heat and taking longer to reach temperature. The good thing about this is that if the soil is heated to 40º C (which requires several loads of wood in the stove), the soil will remain hot for 24 hours ;-).

This system was in operation a year with a gas boiler, now I will use to circulate the water, activating the pump with a thermostat, but this I will discuss in another post.

Building the stainless steel coil.
When I thought I decided to build in steel because it would resist the pressure if the water pump is stopped and it allowed me much more freedom in the way than if it was made of copper.
But it requires some skill with a welding machine with electrodes, which in another entry will discuss his techniques and tricks.

Before you start anything you need to plan, at least the basic operation of the system so we have clear ideas.

Given the interior design of the stove installed, what interests us is not to obstruct the placement of firewood or diminish space. More on this, the more surface contact with the flames and smoke we have, energy efficiency will increase.
We must also take into account section of the tube and the power it can absorb the heat radiators or floor that to be heated, in this case the soil absorbs all the energy that let you have a mass absorber (as I said did not isolate the base of the building), and very efficient as the coil is not going to have problems of excess temperature in the system.
Basically, I raised the coil according to the following schema, which is commonly used to capture heat in the solar thermal collectors. We force water to pass through the central tube of smaller section from one corner to another:

I also wanted to use the heat of the smoke and flames when the fire is intense, so I extended the coil to the top of the fireplace opening, removing the element that diverts the smoke (and whose function is to improve performance of the stove):

However I had the problem that could not directly take the tube out (and avoid accumulation of air) in that area, so I had to divert a little elbow down the exit tube of hot water.
Esa parte quedará por la mitad de aire (dependiendo de la fuerza de la bomba, que podría irlo arrastrando en forma de burbujas), pero si queda horizontal no quedará ningún tubo vacío, lo que tenemos que evitar ya que se recalentaría pudiendo llegar a hacer vapor de agua (que luego eliminaría poco a poco el autopurgador de aire de la caldera). That part will be for half the air (depending on the strength of the bomb, which could drag as bubbles), but if it is horizontal then it will not be any tube with air, which we must avoid as it can possibly do water vapour due to overheat  (which the selfbleed will gradually eliminate from the boiler).

I also removed the reinforcement plate that carries such a fireplace, separate fund to protect the chimney from excess heat, and took the measure of the bottom where will be the grid tubes.

Note: Although the image are already charged, water holes should not be done until we have finished coil, with which we will mark the exact position as we have remained in place.

This time the chimney is already embedded, so to put tubes in and out was a bit complicated, the distance from the street through the stove is 30 cms. If it was an external stove like the one I prepared with the first copper coil, it would be easier to splice tubes.

Other formats of coils
The coil can be prepared and placed in numerous ways, but the ideal is in contact with flames, to force the flames to cross it.I've seen coils as a V-shaped grills where the logs were placed and both the fire as flames contact the tubes. Others simply put it under the logs, but have lower yield because it cooled the embers and didn't use the heat of the flames and smoke rising.

Having the shape in the head, we start to prepare the widest part, which will be on the side of the back of the stove, which is not bothersome.

To begin we take the measure of interior space where the coil will go and draw the rectangle in the work zone.
We put the tubes at presentation with veneers (also stainless) that I will use to better capture and transmit heat to the tubes, and mark the place where we make the cut or hole to splice the lower grille section tubes. The wider tubes are 25  mm and the narrowest of about 15 mm. in diameter with a wall of 2 or 3 mm (the more better resist pressure wall).

To join the pipes, once cuted to desired size, instead of making holes with a drill, I cut just a little with the Rotaflex and a grinding disc.

First tip (giving a bit of solder just to keep the pieces in place) and then finish welding, and clean the scum with a pickaxe to verify that we have been holes or pores. Otherwise, what I'm doing to prevent that the material falls down, is not doing continuous welding, but to touch, and with the clamp at a low angle to avoid the force of the material provided then underlying the area and lift.

Thus pore until closing the hole. Through this work we will be taking lots of practice with the welder. It is advisable not to try to do this if you do not practice with the machine and welding electrodes, pores get free solders is not small feat.

Once I have prepared the bottom, we take steps to how it should be the top, leaving room to maneuver in and out the coil into the stove.

The chimney is close to the outlet of the smoke. By making the coil more closely in that area, causing the water to flow faster (water tends to go where it costs less to go, where lower friction), but as will be the area where the flames hit with more energy, we are concerned that water will cool more, so it is even positive.

En la siguiente imagen le pego un par de cortes para ponerle otro tubo en medio. In the next image I hit a couple of cuts to put another tube in the middle. Ya va cogiendo forma. It will take shape.

Prepared the top, we can join it to the bottom, we can help by putting the two sides in the chimney to see what is bent. One can see that is not in the same plane as the bottom of the shape of the mouth of smoke.

Now you only need to place the inlet and outlet pipes. As discussed, the output is more complicated since we can not practice in this case bore directly on the stove and the bottom (a tube would be so long that we could not get the coil), then the stove is already built and we can not access the rear part.

Colocamos a presentación y marcamos dónde tenemos que practicar los agujeros. We place presentation and mark where holes have to practice.

Making holes in water pipes

First I made cuts with a cutting disc and 125 mm Rotaflex., And retired with a chisel the pieces where the pipes go.

Aquí se puede ver cómo quedó una vez colocado el serpentín. Here you can see how he was once placed the coil.

With an electric hammer pierced brick wall from within, and then finished enlarge them a bit from the outside.

Y finalmente y tras varios ajustes en la longitud de los tubos, presenté la parrilla de tubos. And finally, after several adjustments to the length of the tubes, I presented the grid of tubes.

Checking the coil

With tubes of suitable length, we proceed to make the thread with a sweep of 1 / 2 inch (the diameter is usually used in tubes of 22,5 mm.). In my case, the tube was not far, so I had to give more in diameter with a bit of soldering and then arranging the Rotaflex to finally threading.

In theory, the heating does not need more than 1.5 kg.pressure and at home we have 7 kg., so I connected to a tap, plug the other end with a plug (in this case is a plugin I made to remove the air valve with a bike) and see if pores were left where he could lose water.

Closing the leaks
Indeed, there were left two pores, that closure is necessary to clean up a little with Rotaflex to give more strength to the weld.

Once settled, the re-test for two hours at least three times the pressure you have when you work to ensure it will stand.

Completing the coil
All that was missing was the coil increased surface contact with flames, so I thought welding pieces of sheet 3 mm. thick, which had in the junk store.
I put them flat on the bottom and inclined in the vent for not closing the passage of smoke and that the actual movement of the flames and smoke coming out "caress" the sheets, taking cold.

It is important not to complete welds in sheets, to leave room for expansion. However, we must weld almost all of the contact zone (to improve heat transfer), in strings of 3 cm with 2 cm separation between them.

I just had to connect the piping to the radiant floor heating to prove it worked. In this case put elbows of steel (not stainless.) I should change once proved that it works for the same or another material that will not rust (for corrosion where different materials meet).

I helped me with a key dog (named after the dog-head ;-)).

In the next image, at night, are seen PE tubes I'm using to test the coil, and are still not secured.

I cut the original pipe and spliced heating tubes 90 degree elbows bronze and stainless steel 1 / 2 inch and two piers between them.

Purgando el aire del circuito Purging the air from the circuit
This is the old boiler Fagor pilot flame, uncovered. I put a switch bypassing the engine to start it to test (at the bottom of the image).

Before lighting the fire is to remove the air, so that (apart from gradually eliminated by the trap of the heater) the engine comes with a screw can be loosened a bit, and while pushing air will work passing through it.
When no longer the distinctive sound of bubbles in the circuit, or no longer hear air out, and we adjusted the pressure to 1.5 bar, and we can start the fire ... What nerve!

At first it costs to take heat, everything is very cold and the water from the wood is condensed in the coil, falling on the fire. Slowly takes strength and the water begins to get warmer and warmer. 

After an hour the soil temperature begins to rise slowly, from 15 to 20 º C and the outlet temperature from the coil reaches nearly 40 degrees C. .. It is a real success!

The next day took a picture of the coil blackened shows that the coldest parts tar and smoke have been stuck, however the plates are practically free, the effect of pyrolysis (to catch fire a lot of the clean temperature).

The accumulation of resins and dirt is not a problem, because when they reach a certain thickness (about 3 mm) are burned. Because of this same plates are great for improving the transfer that is lost through the insulation of the soot in the tubes.

I just have to put the thermostat to activate the motor itself when you fire, a UPS in case of blackouts ...

The perfect stove
Thanks to this coil I can get even more performance from the oven, with a single load of firewood per day have all the hot house (45 m2) with tiled floor warm.
It is a truly enjoyment, reducing wood consumption reduce the hours devoted to light and chop wood.
I love it when a small investment of effort I see a great impact on improving quality of life. As Hannibal Smith from Team A would say, I am delighted that the plans go well! (although many adults took the smoking habit for their fault...)

Now for the next project!

In addition I have also thought about harnessing the power of this stove to preheat the hot water from a tank of 150 liters, because although the sun can heat something in the Basque country the sun can be seen rather little in winter, and since there to make fire nearly every day, will save a lot of gas if I manufacture a homemade heat exchanger so that part of the water temperature going to the reservoir by natural circulation.
As winter is just around the corner, the project of manufacturing a solar thermal collector simple and cost will have to wait, get free ACS is going to be easier and faster to finish ;-).

Stay tuned!

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