adiel
Junior Member
Posts: 119
|
Post by adiel on Sept 27, 2015 22:29:35 GMT -8
i wonder if it is right that peters top gap (2* sys diameter) is needed because of the not hot chimney at first and in a situation of a bypass in the first bell (20 cm from the riser) the top gap can be a ring of csa. mmm. thoughts?
|
|
|
Post by satamax on Sept 27, 2015 23:27:22 GMT -8
Well, so far, i've always used a big top gap on all the experiments. Exept on the range rocket. Which still had about system size. But is plugged into an 11m chimney.
I realy would like to know if it's possible to reduce the top gap, as my next system will have a 158 heat riser, under a 277cm wooden floor, and i don't want to kneel to feed it. And i don't want to burn the joists above!
|
|
adiel
Junior Member
Posts: 119
|
Post by adiel on Sept 28, 2015 2:40:22 GMT -8
max i've seen your range stove and it looks amazing. do you have a bypass and a downdraft there? if so, what happens to the smoke backs when you switch to downdraft? i built a batch box ( actually it was my first install of one) in a place that the chimney is always pulling and i made there a system size top gap (ianto style) and it is working very good.
|
|
|
Post by satamax on Sept 28, 2015 4:46:20 GMT -8
Well, nothing happens, as I have soo much draft. 11m chimney. But the box is too small, as well as the top gap, the height of the heat riser too. And the bypass gate is too small too I think. I have a tiny soot accumulation in the ducts. But I think that's because of too short a heat riser . I even added a downdraft plunger tube in the chimney with no ill effect whatsoever.
|
|
|
Post by satamax on Sept 28, 2015 22:41:07 GMT -8
Adiel, i rechecked the range rocket. The top gap is 6cm. For a 12cm riser.
|
|
|
Post by peterberg on Sept 29, 2015 1:51:20 GMT -8
Max and Adiel, your findings are in line with mine. See my answer about a cook stove. In general, I'm inclined to think we can regard this minimum as half the riser's diameter or, cross section area divided by circumference multiplied by 1.5 or 2. And you'll need a potent chimney stack plus about 80 C minimum in there to run it well. This could be the recommendation in cook stoves, heaters are different animals with another purpose in my opinion.
|
|
|
Post by satamax on Sept 29, 2015 3:28:20 GMT -8
Ok Peter. I think it's hard to find the "free flow" point in a batch.
We all know that if the ring projection of the riser up to the top above should be at least 1.5 the CSA for a J tube. 3 times is better.
The problem with the batch is the need to keep the velocity of the gases. If they crash onto the top plate above the riser, that induces turbulences. Which impedes with the free flowing of the gases. The only example of a tight top gap i have is that range retrofit. And it buggs me onto the next heater, as i need the top gap to be as small as possible, to lay some mass above the "barrel"; two of the sides will be free, back,one side, bottom and the top will have mass around. If things go as planed. But it would be nice to be able to keep at least 18cm above, for a fire gap, below the joists.
|
|
adiel
Junior Member
Posts: 119
|
Post by adiel on Sept 29, 2015 9:27:38 GMT -8
i really believe, after seeing it in a few stoves (without a testo) when the chimney is good and the mass is not too long the top gap can be small. max - can you make a bypass right next to the riser? also - i believe that a riser end shape as a funnel (wide) will help the top gap to be small. and actually maybe for a cookstove it is better to have a funnel riser to have large cooking area? peter - what do you think - to make the effort and cast a funnel riser before you arrive?
|
|
|
Post by peterberg on Sept 29, 2015 11:53:41 GMT -8
This is a very playful idea. The best shape is probably the one like a trumpet. Fiddling with the shape of the riser is tried before, but only cross-section-wise, nothing in the realm of changing shape length wise.
|
|
|
Post by cfrith on Mar 13, 2016 12:51:51 GMT -8
Peter, max, adiel, has anyone tested this further or has anyone come to a conclusion on the top gap I currently have a j in my green house now but would like to convert to batch and only have 7 ft clearance from ground and was hoping all would fit and use my current thermal mass but would modify if necessary.
|
|