[Greenbuilding] thermosiphon questions
Frank Tettemer
frank at livingsol.com
Sun Nov 2 18:54:26 CST 2014
Just to add to Norbert's thorough thinking, I agree on the 1" dia.pipe
as being relevant, given that your rise to run ratio is way less that 1
to 2.
I've worked on three homes that use an ESSE cookstove, (must be similar
to your Ireland description, Norbert.)
In one home, the stove is on a lower level, and heats radiators in
distant rooms that are only five steps up. This one stove heats 2,400
sq, ft. of well insulated home, through direct radiant heating when in
the kitchen, air circulation heating, and water circulation heating
through three iron radiators.
It does this all using gravity flow circulation, which draws the cold
water return from the rads, right back down to the stove. Total height
is only four feet. from bottom of stove to bottom of radiator. The
furthest rad is about 45 feet away from the stove, with a 60 gallon
storage tank somewhere in middle of the supply run. Works good, with 1"
copper, and high temp (Boiler pipe insulation) wrapped on both lengths
of pipe. However, they have a sensor and some automatics involved, to
deal with too high temperatures. There is a pump that starts a side
circulation loop, that dumps the excess heat outside under the ground,
until the tank temperature returns to under 60C..
Considering the caution needed in dealing such high temperatures, there
was some additional safety added. The problem to solve was that with no
fire going, there is no circulation. So the beginning of the fire
building in the stove, with no circualtion, the water begins to boil in
the stove's heat exchanger. On the output pipe, on the side of the Esse,
is mounted a short leg of pipe with a large brass Maid'O'Mist air
evacuator valve. This lets out any air or steam that develops during the
morning's initial wood fire, that heats the pipes up near boiling
temperatures, just before the beginning of a good circualtion flow. This
brass Maid'O'Mist hisses and sputters for a few minutes each morning,
letting air out, until a reasonable flow develops in the circulation loop.
Not that you'll need that, but I included the detail FYI, in that the
Esse cook stove is not considered just a cooker and a water heater. The
heat exchanger along side the firebox is so efficient at drawing heat to
the water that the company describes this stove as a Boiler. Thus, it
needs to have overheating conditions dealt with right from the get go,
through careful consideration and thoughtful design.
Frank
On 02/11/2014 11:34 AM, Norbert Senf wrote:
> Hi Reuben: That's not to say that it can't be done. In Ireland, I saw
> wood fired cookstoves with a water jacket apparently running a bunch
> of radiators, pretty much at the same elevation and with no apparent
> circulation pump. Wish I'd had time to investigate further. We have
> one client who has a masonry heater with a water coil in the firebox,
> thermosyphoning into a regular hot water tank behind the heater, only
> slightly elevated off the floor. Works fine. With the large horizontal
> run, your enemies are friction and heat loss. You can insulate the
> heck out of the supply line, and use a large diameter pipe to cut the
> friction. If you can afford the loss, leaving the return line
> uninsulated would give you more temperature differential for
> thermosyphoning. Studying how those systems in Ireland work, would be
> useful...............Norbert
>
> On Sun, Nov 2, 2014 at 9:21 AM, Reuben Deumling <9watts at gmail.com
> <mailto:9watts at gmail.com>> wrote:
>
> Thanks, Norbert. That is what I'm fearing.
>
> On Sun, Nov 2, 2014 at 6:06 AM, Norbert Senf
> <norbert.senf at gmail.com <mailto:norbert.senf at gmail.com>> wrote:
>
> Nick's result was around 3/4", but I believe in his example
> the height was 16'.
> Off the top, 35' is a very long run, and the rise is low. You
> definitely want to minimize friction - I'd go to 1" smooth
> (copper) pipe, minimum, as a precaution.
> With the situation you describe, I would not expect much heat
> transfer................Norbert
>
> On Sat, Nov 1, 2014 at 6:10 PM, Reuben Deumling
> <9watts at gmail.com <mailto:9watts at gmail.com>> wrote:
>
> Is that what Nick's numbers suggested? I tried to follow
> but got lost somewhere.
> The tubing is 3/4", the run is 35', the rise is 5' from
> top of tank to top of tank. The water temperature from the
> originating tank (heated by solar thermal panels) can be
> expected to be 120F - 150F. House pressure is I think 60psi.
>
> On Sat, Nov 1, 2014 at 1:20 PM, Norbert Senf
> <norbert.senf at gmail.com <mailto:norbert.senf at gmail.com>>
> wrote:
>
> In other words, the pipe should be 3/4" or
> larger.......................Norbert
>
> On Sat, Nov 1, 2014 at 3:46 PM, <nick_pine at verizon.net
> <mailto:nick_pine at verizon.net>> wrote:
>
> One way to design a thermosyphoning system:
>
> 1. Calculate the required water flow,
> approximately G = 0.04A gpm for an A ft^2
> collector, eg G = 1.28 gpm for a 4'x8' collector.
>
> 2. Calculate the available thermosyphoning head,
> Ha = H(62-61.71)/144 = 0.002H psi, based on a 10 F
> temp diff at 100 F and an H foot height
> difference, eg Ha = 0.002x8 = 0.016 psi with H = 8'.
>
> 3. Calculate the minimum supply and return pipe
> inside diameter d in inches, given their total
> length L in feet.
>
> http://www.calctool.org/CALC/eng/fluid/hagen-poiseuille
> says dP = 32MuLV/d^2, where V is a velocity and d
> a diameter. A G gpm flow rate makes V = cG/d^2 for
> some constant c, ie dP = kLG/d^4 for some constant k.
>
> Plugging L = 8' and d = 1" and G = 1.28 gpm into
> the website makes dP = 0.001775 psi =
> k8'x1.28gpm/1^4, so k = 1.73E-4.
>
> In the example above, d = (kLG/Ha)^0.25 =
> (1.73E-4x16'x1.28gpm/0.016psi)^0.25 = 0.69" for L
> = 16'.
>
> Nick
>
--
Frank Tettemer
Living Sol ~ Building and Design
www.livingsol.com
613 756 3884
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