Nov 19, 2013 at 9:14 am #1310001
Can the top of an aluminum-flashing windscreen do double-duty as a pot support? It does on the caldera cone, yes, but what about a straight-sided, roll-up windscreen that would contact the bottom of the pot? I have a wide-bottom, short-sided pot–more like a pan– I'd like to use with an efficient (slow, therefore not oxygen devouring) up-burning alcohol stove. (It's the Zelph Simmerlyte.) Would a cylindrical windscreen with holes at top and bottom work as both screen and support, do you think?
Clearly the windscreen would not shield the pot from heat-robbing wind, as a caldera cone would. I might forfeit that, for the simplicity of being able to roll up the windscreen/support, and have it take a minimal amount of storage space around the inside of the pot during transport.
Part of my question is: how important is it to have the bottom part of the pot sitting within the windscreen, as is done traditionally?Nov 19, 2013 at 9:23 am #2046155
@bzhayesLocale: So. California
It certainly can work, but you will loose all of the efficiency you gained by going with a wider pot. Less of the pot will be exposed to the hot combustion gases (meaning less area to transfer heat to the liquid), and more of the pot will be exposed to the ambient air (more heat loss to the environment).
I would definitely look into adding an insulating sleeve around the sides of the pot.Nov 19, 2013 at 12:09 pm #2046212
@b-g-2-2Locale: Silicon Valley
Yes, this can work, but you need to be careful and keep it low for stability.
I scaled down the idea and made one out of a tealight cup as the low-pressure alcohol burner (2 grams) and a thin strip of aluminum as a pot support (2 grams). The pot was an aluminum cup.
–B.G.–Nov 19, 2013 at 12:52 pm #2046225
@edvinLocale: Gothenburg, Sweden
I would design it so there's a gap between the windscreen and the bottom of the pot, not just some holes close to the top. Otherwise most of the hot gasses from the stove would never reach the bottom of the pot.Nov 19, 2013 at 1:20 pm #2046227
@retiredjerryLocale: Oregon and Washington
You could have slits at the top to let hot gas through and stay close to pot
Or you could have half circles – like take your existing screen and cut off right through the circular holesNov 19, 2013 at 1:23 pm #2046228
What will happen if you have a 5.5" dia. pot and a 5" dia.> windscreen/pot support that is 2" tall and use a StarLyte burner as the heat source? The pot is 2.5" in depth and holds 2 cups of water with plenty of head room. the windscreen will fit nicely inside the pot.
The windscreen design is such that you have plenty incoming and outgoing air holes. What happens to the heat coming out of the top holes…..it rises up the sides of the pot where the pot absorbs the heat because the surface of the pot is much colder than the rising heat…..right? The pot will continue to absorb heat until the water begins to boil…right? If you have ever watched steam coming off the top of your pot you can see the air currents swirling inward over the top of the pot….right? Do we have the pot completely surrounded by heat…yes, the same as the Caldera Cone.
Will a breeze make a difference on it's efficiency….yes! How much????? depends….we have variables:-)
Just today I was able to see Hiram Cook has some videos on his test of the StarLyte without windscreen and with it under a Caldera Cone. Some interesting info in the videos.
If the links don't work you can see the videos at my websiteNov 19, 2013 at 1:24 pm #2046229
Interesting, Edvin. So the hot gasses would not "pile up" on the center bottom of the pot and finally push themselves out to the sides (as I thought); you're saying that much of the hot gas would be exiting prior to getting to the bottom of the pot, because the top row of holes are somewhat lower than the bottom of the pot. If that's true, then it's a seriously flawed design idea.
I suppose the easy way out, here, is to use a wider windscreen, and to put two titanium skewers perhaps an inch down from the top of the windscreen and rest the pot there. And eliminate the top row of holes. That would drop the pot down "into" the windscreen and even catch the heat as it curls around the edges of the pot.
Dan: I'm having a hard time understanding…?Nov 19, 2013 at 1:28 pm #2046230
@jamesdmarcoLocale: Finger Lakes
There are a couple things that do not favor doing it that way, but it can certainly be done.
You would loose the airflow around the sides. As the heat shoots around the corners, it sets up a small eddy current that will prevent residual heat from warming the sides efficiently, hence the water inside. Of course, this makes the case for a rounded bottom to help minimize that. But, a more bowel shaped bottom also decreases surface area for adsorbing heat. A full heat screen will trap the residual heat, forcing it to warm the pot more evenly along the sides as well as the bottom. Don't forget we are tlaking only after the heat as done its job on the bottom, so, this is strictly for efficiency.
Radiative heat loss was previosly discussed. An insulating blanket would help with that.
Wind will also drive heat away from the pot. While you do have the benefit of heating the bottom, the heat being blown away from the pot will likely yield poor efficiency in any sort of breeze.Nov 19, 2013 at 1:46 pm #2046236
@retiredjerryLocale: Oregon and Washington
I think "flawed design" is over stating it. It might take 10% or 20% more fuel.
The hot gases flow out of the holes and are directed away from the pot. Better to have the hot gases flow up the side of the pot, like James and others said.
I have two windscreens, one with holes like yours, another that better directs gas up sides of pot, it uses about 10% or 20% less fuel. But that's un upright canister which will probably be different than alcohol stove.Nov 19, 2013 at 1:50 pm #2046238
@b-g-2-2Locale: Silicon Valley
"But, a more bowel shaped bottom …"
I'm not sure that this is anatomically correct.
–B.G.–Nov 19, 2013 at 2:32 pm #2046251
@hikin_jimLocale: Orange County, CA, USA
"Straight sided" (cylindrical) windscreens can in fact do a good job. Here's a Bobcat system from Flat Cat Gear.
And indeed, you can balance things on top, like this pan.
Would it be more efficient to have the pot reside below the rim? Yes, particularly if you can get the hot gasses from combustion to flow up the sides of the pot as they exit, but the functions of a windscreen are first to block wind and second to trap heat. Your windscreen with top mounted pot will take care of those two functions.Nov 19, 2013 at 3:47 pm #2046265
Laminar Flow Photos by Tony(Schlieren photographry)
Take a look at these photos to see how heat surrounds a pot to isolate it from ambient air temperatures:
Laminar Flow versus Turbulent Flow
I know a lot of you have seen the discoloration of the Caldera cones that show 1/2 of the upper portion of the cone has more discoloration towards the opening where the pot handles are. It's my theory that the boundary layer is much greater/thicker around the top of the cone. Some photos show the discoloring like a large ring 2 inches from the top of the cone. That shows you the heat distribution inside the cone.
Tony's photos clearly show that the heat coming up from around the pot goes strait up not outward and away from the pot.Nov 19, 2013 at 4:25 pm #2046278
@jamesdmarcoLocale: Finger Lakes
Dan, I would add that the velocity of heated gasses, or heat density if you will allow, is less along unrestricted sides. As you can easily see in your photos, the layer of heat along the sides is much less than the bottom, due to the velocity of the heated air. Unless this is broken up and contained next to the side walls, you can loose a significant percentage of otherwise usefull heat as it speeds by the pot. You can easily see the edge "bubble" I was speaking of in the third (flat sided pot) and fourth pics, though somewhat after the fact, as with most turbulence. This tells me that a wind screen about 2/3 of the way up a pot would be nearly as effective as a full screen.Nov 19, 2013 at 4:34 pm #2046285
@ckrusorLocale: Northwest US
Dan, I would agree that the hot air will tend to cling to the pot as it rises. But even a gentle breeze will blow that column of air away from the sides of the pot. When I have boiled water without a windscreen, I've noticed that even moving my body while tending the stove produces enough air disturbance to cause boiling water in the pot to stop boiling and then resume when I hold still.
I would guess that I have used a stove in air still enough to permit that kind of air flow on fewer than 5% of the trips I've taken. That kind of air flow is just not relevant to backpacking except under conditions that are unusual in most places.Nov 19, 2013 at 5:01 pm #2046294
That Bobcat windscreen is just about the tightest cylindrical windscreen I've ever seen. Usually cylindrical screens are recommended to stand off from the pot by 1/2 inch or so.
During one experiment I cinched up a cylindrical screen modestly close around a pot (using a high-oxygen-requirement SuperCat) and snuffed out the flame entirely. This, despite plenty of holes in the bottom of the screen. So I'm not sure how the Bobcat can get so tight to the pot, and still allow the stove to breathe. Anyone know?Nov 19, 2013 at 5:29 pm #2046300
Looks like the Bobcat has holes at the top, so tightness shouldn't be an issue.
If it Does, snuff, or restrict to stove, just start adding holes.Nov 19, 2013 at 5:57 pm #2046311
@dwambaughLocale: Pacific Northwest
I like the "cylinder with a couple tent stakes" to get the pot down into the windscreen a bit. I've been cooking up (no pun) this idea to make slots for the fasteners so the screen can be squeezed into a smaller coil and dropped into the pot. I thought Chicago screws would be easy— like the ones used for binding papers. Whip it out of the pot, letting it spring open, slide the stakes in the sides, drop it over the stove with a priming pan/reflector and flame on :) I've used simple aluminum flashing for the screen, BTW.
I'm sure your fixed cylinder would work, it's just a matter of efficiency. I do like the simplicity and stability of a fixed tube. As others said, some slots or vents of some kind at the top edge would allow the flames to caress the sides of the pot. I like the idea of scoop shaped vents: you can buy "knockout punches" for punching holes in an electronic chassis if you want something that will give a clean cut. They can be cheap on eBay.Nov 19, 2013 at 7:59 pm #2046346
Greg: My confusion continues, because when I had my windscreen reasonably tight, it was barely covering the base of the pot. On the Bobcat above, the holes are cut in the screen high up, and much of the pot is covered. All else being equal, my system would have had more breathability. The only way I can make sense of it, is that the stove used by the Bobcat is not a huge oxygen sucker (like the SuperCat is known to be). I'm fairly certain my SuperCat would snuff with a Bobcat screen.
I suspect that slower burning alk stoves have two advantages: greater fuel efficiency, and less of a need for air quantity (allowing tighter, more windproof setups).
Dale: Scoop shaped vents seem like a winner of an idea! The windscreen wouldn't roll as tight, but neither would you have to worry about wind blowing through the holes as much (if at all). By "scoop shaped" I assume you mean louvered vents like this?
EDIT: just realized what you meant, Dale. You were referring to "crenellated" cuts along the top, like a castle tower.Nov 19, 2013 at 8:16 pm #2046351
@jonfongLocale: FLAT CAT GEAR
It's magic (and a lot of HARD WORK). Best regards – JonNov 19, 2013 at 8:39 pm #2046358
@dwambaughLocale: Pacific Northwest
Okay, crenellations are typically straight sided on a castle wall, seeing that it's probably easier to cut and set the stones. Let's say semi-circular crenellations.
Louvers could be very interesting. The cheap trick there would be to cut the edges and fold them over 45 degrees or so. It would be easy to tweak them for testing.
Going from there, a windscreen doesn't need to be round. Imagine a hex or octagon with tops and/or bottom flanges spread for airflow and some extra stability too. They could be bent alternately too. This is stuff you could do with basic hand tools and a vise with some plywood in the jaws. This could take all winter :)Nov 19, 2013 at 8:40 pm #2046359
…and the appreciation for the design of the Caldera Cone continues to grow.
Sometimes things aren't quite as easy as they look.
Go Rand!Nov 20, 2013 at 3:38 am #2046423
@newtonLocale: Southeastern Louisiana
It worked for me in testing using my tray type Esbit stove.
This was a test at home and in controlled conditions. To date I have not used this set-up in the field. I did however try to simulate windy conditions using a box fan and only saw flames licking out of the top exhaust holes.
Most of my alcohol stoves serve as their own cook pot support. I do however see the possibility of a windscreen / cook pot support working with an alcohol stove that doesn't support the cook pot.
Your stove will have to breathe so the ability of your screen / support to "draft" will be important as will the height of your cook pot above your stove.
FWIW I've found that holes do not work as well as vents / cut-outs on the bottom of a windscreen. The same may be true for the top of a windscreen used as a cook pot support.
Be careful and enjoy the experimentation. As I've learned with alcohol stoves there are no guarantees of performance or safety with a new or adapted design.
Newton ;-)Nov 20, 2013 at 4:15 am #2046424
Years ago when I hiked the A.T. I used a standard MSR windscreen with two aluminum gutter nail stakes driven through it as a pot stand and wind screen. It worked great and I believe was quite efficient as the flames went all across the bottom of the pot then were directed up the sides. There is a small photo of it about half way down this page.Nov 20, 2013 at 7:28 am #2046452
What kind of can are you using for the Pot in your pic?Nov 20, 2013 at 8:10 am #2046461
Dale, a while back I started working on a louvered windscreen. I was amazed how well it works. It's work in process.
Click on the large black square, it's a photobucket video made at night in my garage. It starts out by viewing the underside of the stove (StarLyte) and pot thru a sheet of glass. I am observing the flame pattern as it swirls around under the bottom of the pot. No flames are able to pass out and beyond the bottom edge of the pot. I then go from under the sheet of tempered glass to the top of the windscreen and remove the pot to show how the flames are swirling around. Lots of stuff going on there….turbulence and centralizing of the flame.
A small 15 gram stove with pot support would be ideal for what Delmar wants to do (sorta) He has the burner for simmering. I have a batch of them waiting for me to install the ss pot supports.
The louvered windscreen, a StarLyte stove and Foster's pot make for a good kit. I will make them available after the first of the year.
The use of crenelation pot support worked well in the Ti windscreen I designed a while back in this forum
Here is a combination square windscreen and storage container:
Square Windscreen/pot support and storage container
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