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heating tanks with a slow combustion heater.
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Author:  monya [ Aug 27th, '08, 19:11 ]
Post subject:  heating tanks with a slow combustion heater.
Today we had a wet back installed in a large slow combustion heater in the shed. There are 5 x 2000 litre tanks and a 2000 litre sump/swirl seperater and a pressure filter we made ourselves. We are pumping the water thru the stainless steel pipes in the heater and back to the sump. The temperature gain is largely dependant on flow rate. I just can't get my head around whether it is better, worse or no different to pump twice as much water and get half the gain at each pass or halve the flow for twice the gain per pass. for example. At 2 litres per minute, the water is going in at 18 and coming out at 29, and 4 litres per minute (approx,) it is coming out at about 23. My thought is to pump slowly until the temp gets to 22 or 23, then adjust the flow so it goes in at 23 say and comes out at 25 to compensate for the loss you would expect as the water gets pumped around. Or would it be better to up the flow so the water is coming out at 24 or 25 and leave it there? :?:
the aim is to get the tanks sitting at 22 for the 9 months of the year that the ambient sends it below 15. The wood heater negates the need for sunshine as we can tend to have a week of rain and constant cloud cover at any time. And it works in the dark too :roll:
Author:  Stuart Chignell [ Aug 27th, '08, 19:21 ]
Post subject:  Re: heating tanks with a slow combustion heater.
Pumping more slowly means that your pumping costs would be smaller.
Author:  Outbackozzie [ Aug 27th, '08, 19:33 ]
Post subject:  Re: heating tanks with a slow combustion heater.
Yep + 1 for a smaller pump running constantly
Author:  Dufflight [ Aug 27th, '08, 19:37 ]
Post subject:  Re: heating tanks with a slow combustion heater.
Do you have pictures. :D

Can this system work on thematic shipon. Man I hope that is a real word. What I mean is heat rises is there a way to let this get the heat into your system. Even if the hotter water has to go into a holding tank before it is mixed into you tanks.
Author:  Outbackozzie [ Aug 27th, '08, 19:41 ]
Post subject:  Re: heating tanks with a slow combustion heater.
As long as the holding tank is higher than the wetback it would work.
Author:  monya [ Aug 27th, '08, 20:00 ]
Post subject:  Re: heating tanks with a slow combustion heater.
We have thought about including a header tank... that is how the et back on our house solar water works- no pump... but the pump is only going to use about 15 watts so the power use side of it is negligible and the cost of setting up the header tank would be considerable.

I will take pics.
Author:  RupertofOZ [ Aug 27th, '08, 20:27 ]
Post subject:  Re: heating tanks with a slow combustion heater.
Quote:
I just can't get my head around whether it is better, worse or no different to pump twice as much water and get half the gain at each pass or halve the flow for twice the gain per pass. for example.


Had the same dilemna with the solar array/controller Stu....in the end I settled on the default differentials....

Translated.... low flow, twice the gain per pass.... worked best in my situation.
Author:  Sleepe [ Aug 27th, '08, 20:54 ]
Post subject:  Re: heating tanks with a slow combustion heater.
Monya

How are you going to handle crud in the pipes esp at the hottest point and how do you turn it off without water boiling in the heat exchange?
Author:  heka [ Aug 27th, '08, 21:46 ]
Post subject:  Re: heating tanks with a slow combustion heater.
Hi Monya,
I'm using a GRUNDFOS circulation pump to pump water through my underfloor heating system in the house. It has 3 speed settings. I found the slow speed works best.
In summer I use the same pump to circulate water through the solar hot water heater. A temperature switch (JAYCAR kit)
switches the pump on (55C) and off via a relay (JAYCAR kit ) depending on temp. I have had the pump for 17 years. It works at temperatures 95C+.
DON'T FORGET to install pressure relief valves, one near where the water enters the 'wet back' and another where the pipe comes out.

heka
Author:  ruins [ Aug 27th, '08, 22:06 ]
Post subject:  Re: heating tanks with a slow combustion heater.
+ 1.5 for solar pumps
Author:  Tony From West Oz [ Aug 27th, '08, 22:45 ]
Post subject:  Re: heating tanks with a slow combustion heater.
The thermo-syphon system will work well in this application, providing the wet back is lower than the lowest water level (top of water must be above wet back) If possible, have the heating stove lower than the bottom of the water tank for greater flow thru the wet back, to increase the flow rate.

A pump can be used to assist the thermo-syphon, but should not be necessary, unless you need a high flow rate to cater for the volume of water to be heated.

With heat exchangers, the efficiency is measured by the heat transferred per minute of operation. Some factors which affect the efficiency are:
  • Temperature difference - if you have a roaring fire, the water will be hotter than a smouldering fire,
  • flow rate - how much water is pumped thru per minute, and
  • residence time - how long the eater is in the heat exchanger.
Thus, for a constant temperature difference (water input temp : fire temp), more heat will be transferred to the water if more water is pumped through it. This will result in a lower outlet temperature as the water is not in the heater as long, but more heat will be transferred.

If you control the fire to control the temperature, that will allow you to stop the pump (if equipped) when the fire is low or out, and run the pump to maximise heat transfer when the fire is burning.

Regards,
Tony
Author:  monya [ Aug 28th, '08, 08:20 ]
Post subject:  Re: heating tanks with a slow combustion heater.
Just to cvlarify, the water is going in one end and out theother- not a heat exchanger. For solids we will just flush out the pipes at a higher flow every now and then. The wet back is flat across the top of the fire box, so no low points for the shite to settle hopefully. cheers for your comments.
Author:  Dufflight [ Aug 28th, '08, 08:25 ]
Post subject:  Re: heating tanks with a slow combustion heater.
Any chance of a picture. I keep imagining something different when I hear wet back.
Author:  monya [ Aug 28th, '08, 09:21 ]
Post subject:  Re: heating tanks with a slow combustion heater.
a few pics. The biofilter we built is out of a pickling vat. the water in pipe goes right to the bottom of the drum. The water out ipe is at the top so the water is forced out thru all the bioballs. So far it seems to be working well. To backflush it, we simply reverse the taps and the black pipe is to waste ie the olive grove.

you can see the pipes in the heater... the water in has the tap on it. Obviously, the pump is chronically oversized for the application :roll:

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wetback 001.jpg
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Author:  Uncle Yarra [ Aug 28th, '08, 09:50 ]
Post subject:  Re: heating tanks with a slow combustion heater.
Losses will increase as temperature increases - in the connecting pipes is your concern, you want to heat up the water, not the air.
Better to have a lower temperature difference in those pipes so less loss occurs.
(this assumes that the system is tightly coupled to the sink [FT water] so doesn't need a large delta T to operate.)
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