Heat gain calculations

[Stephan_K]

Though I'd give you boiler buffs a shot at this one. I need some help figuring heat gain from a coil I'm going to put in my current wood stove. Here are the details of my project.

I'm working on storing heat in a water storage tank, size of this tank has yet to be determined as I don't know exactly what kind of energy I will have available to dump in it. What I'm trying to calculate first off is the amount of energy I can get from a copper loop placed through the burn chamber of my existing wood burner. Specs are as such for this leg of the heat generation:

wood stove is a Buck Stove 41000 BTU 63% eff. I'm figuring that there will be 65000 BTU of heat/hr in the burn chamber.
loop that I plan to put in it consists of 1/2" x 16' copper holding 3.75 pounds of water (1 1/2 pints)
Flow rate 1/2 GPM and fluid temp input range should be 80 - 120 degrees

I know the gain will vary somewhat with the input temp but I'm having one hell of a time trying to figure out what kind of a gain this coil is going to produce period. Can anyone throw me a little nugget of help on how to figure on how much of that 65000 btu's this coil will absorb at any flow rate ?

 

[Stephan_K]

Bueller ?, Bueller ?, Bueller ?... Anyone ? ;o}

Anyone got an educated guess ?

 

[heaterman]

I wouldn't advise running copper tube or anything else containing water directly through the firebox of a wood stove..........Unless of course you wouldn't mind if it forcibly removes itself from your house someday.

What usually/always happens at some point in time is the copper or steel will become fouled with mineral deposits from the water and eventually plug off. When this happens it is a very short time until the coil relieves itself of the pressure buildup in a most inconvenient manner. Many times involving walls of your house and maybe the neighbors too depending on how much steam energy is stored in the trapped water.

 

[Stephan_K]

I wouldn't advise running copper tube or anything else containing water directly through the firebox of a wood stove..........Unless of course you wouldn't mind if it forcibly removes itself from your house someday.

What usually/always happens at some point in time is the copper or steel will become fouled with mineral deposits from the water and eventually plug off. When this happens it is a very short time until the coil relieves itself of the pressure buildup in a most inconvenient manner. Many times involving walls of your house and maybe the neighbors too depending on how much steam energy is stored in the trapped water.

Looking at using a closed glycol loop for this... How do boilers avoid this issue ?

 

[heaterman]

I wouldn't advise running copper tube or anything else containing water directly through the firebox of a wood stove..........Unless of course you wouldn't mind if it forcibly removes itself from your house someday.

What usually/always happens at some point in time is the copper or steel will become fouled with mineral deposits from the water and eventually plug off. When this happens it is a very short time until the coil relieves itself of the pressure buildup in a most inconvenient manner. Many times involving walls of your house and maybe the neighbors too depending on how much steam energy is stored in the trapped water.

Looking at using a closed glycol loop for this... How do boilers avoid this issue ?

There are some boilers actually made in a similar fashion having water filled tubes run directly above the burner flame. There are however major differences between those and what you are planning. Tube size, automatic and immediate flame shutdown, flow activated switches. high temperature safeties that kill the burner if temps get too high, etc etc. come to mind quickly. Far different than a loop of tube with no guarantee of circulation and no means to kill the flame should something go wrong.

 

[Stephan_K]

I wouldn't advise running copper tube or anything else containing water directly through the firebox of a wood stove..........Unless of course you wouldn't mind if it forcibly removes itself from your house someday.

What usually/always happens at some point in time is the copper or steel will become fouled with mineral deposits from the water and eventually plug off. When this happens it is a very short time until the coil relieves itself of the pressure buildup in a most inconvenient manner. Many times involving walls of your house and maybe the neighbors too depending on how much steam energy is stored in the trapped water.

Looking at using a closed glycol loop for this... How do boilers avoid this issue ?

There are some boilers actually made in a similar fashion having water filled tubes run directly above the burner flame. There are however major differences between those and what you are planning. Tube size, automatic and immediate flame shutdown, flow activated switches. high temperature safeties that kill the burner if temps get too high, etc etc. come to mind quickly. Far different than a loop of tube with no guarantee of circulation and no means to kill the flame should something go wrong.

Ya... I've thought long and hard about the safety issues of what I'm thinking of doing. I do have a number of options to implement that would prevent a catastrophe so that really isn't a huge concern at this point. Right now I'm just trying to calculate somewhat close the thermal gain possible from such a setup or how to calculate it more importantly. From my best guess I'm thinking only a 15 degree or so increase is about all I'll see. Until some kind of formula to follow can be provided... this leg of the overall project is a research project !

 

[heaterman]

Go from here.

1BTU raises 1 pound of water 1 degree. If you add that btu to the water over an hours time, you have one btu per hour.

What you are asking though is nearly impossible to figure because the rate of input will vary over the course of an hour.

Look at it this way.

say your circulation rate is 1 GPM, thats 8.33 pounds per minute, since your btu input is rated / hour you need to figure pounds per hour so 60 minutes x 8.33=499.80......figure 500#/hour. If the input 50,000tbu's in that hour you will raise that 500#s of water 100* (50,000/500=100) If the flow rate is half that the temp rise will double.

This help with the formula does not mean I give your project my blessing..........I'm just sayin'

 

[Stephan_K]

Go from here.

1BTU raises 1 pound of water 1 degree. If you add that btu to the water over an hours time, you have one btu per hour.

What you are asking though is nearly impossible to figure because the rate of input will vary over the course of an hour.

Look at it this way.

say your circulation rate is 1 GPM, thats 8.33 pounds per minute, since your btu input is rated / hour you need to figure pounds per hour so 60 minutes x 8.33=499.80......figure 500#/hour. If the input 50,000tbu's in that hour you will raise that 500#s of water 100* (50,000/500=100) If the flow rate is half that the temp rise will double.

This help with the formula does not mean I give your project my blessing..........I'm just sayin'

That formula is the easy one within the overall problem and I can't wait to use it... But before I can use it I have to figure how much of the 65,000 BTU's produced in the fire box will be absorbed by the coil ? There is no way ALL of the heat created will go into the coil only. I'm assuming there is no magic formula for this part of it and most likely a trial and error solution collecting input and output T data.

And to gain a little more of your blessing... ;o} If I actually do anything with my EXPENSIVE wood burner in this regards, I'll avoid putting holes in the fire box for a coil and do so in the flue, where for one it is much cooler and not near hot enough to boil it and can be dampened down completely if necessary, secondly putting holes in a $10 piece of stove pipe sits much easier with my mind than does doing the same with a $3K double walled stove. And thirdly I will probably gain more heat from a much larger up to 150' coil I can get in the flue versus the small 16' one in the fire box.

All being the same... I still need to figure possible heat gain for the coil no matter how much heat is actually available weather it be in the box, flue or anywhere else. Like i said though, probably going to be a trial and error solution in any case.