how much w/cm²

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how much w/cm²

Postby Fredistiller » Sun Dec 03, 2017 9:01 am

Hi,

I run with gas in the garage but want to go electric for my new still. Heating elements are hard to find in my country. I can find one completely SS with 12.5W/cm² (for water). If I want lower like 5w/cm² they make it with brass screw plug and I don't want brass. If I want one with SS screwplug they ask me more than three time the price!! :thumbdown: .

So my question is: is 12.5W/cm² OK for a boiler I would use only for spirit run? or is it too much and is there an security issue? Security is one of the reason I want to quit gas. And as I would, in a close future, have a room inside the house, I want to be sure about safety.

Thanks!

Fred.
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Re: how much w/cm²

Postby still_stirrin » Sun Dec 03, 2017 10:05 am

Here's a guideline:
A ULWD 5.5kW 240VAC wavy element has a watt density of 50W/sq.in. (7.8W/sq.cm)
A LWD 4.5kW 240VAC foldback element has a watt density of 150W/sq.in. (23.25W/sq.cm)

Either of those would be good choices for a boiler. So, a 12.5W/sq.cm element would be great.
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Re: how much w/cm²

Postby Kareltje » Sun Dec 03, 2017 10:25 am

I just measured my element: radius is 8,5 mm, length is 90 cm, so in total 24 cm2. According to the friend that built the still, it is 2,1 kW, so 87,4 W/cm2. But while running I measured an uptake of about 3,3 kW per hour. The other element is a bit smaller, both in size and in power.
I stripped some birdwatchers with them without any problem.

By the way: I did not ask my friend how he built his still, some 20 years ago. But knowing his style of working I guess he used a second hand element from a washing machine or something like that. Or he asked a friend mechanic for some spare ones.
(Hij en ik wonen in het noordelijke van de twee kleine koninkrijken.)

Edit: compared to the figures of still_stirrin these calculation seems 10-fold too high. What am I missing?? :?: :?:
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Re: how much w/cm²

Postby Odin » Sun Dec 03, 2017 10:48 am

Kareltje, your wattage per cm2 is quite high. OP, you will be more than fine.

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Re: how much w/cm²

Postby Kareltje » Sun Dec 03, 2017 12:24 pm

Odin wrote:Kareltje, your wattage per cm2 is quite high.

Clearly. But what did I do wrong????
Luckily I get corrected. :oops:
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Re: how much w/cm²

Postby BugHunter » Sun Dec 03, 2017 12:45 pm

Let's see, the circumference is 8.5mm * 3.14 = 26.69mm or 2.669 cm

2.669cm * 90cm = 240.21 cm^2

So it looks like you multiplied mm times cm to get cm^2, which got you off by a factor of 10.

2100W / 240.21cm^2 = 8.74 w/cm^2 which sounds pretty reasonable.
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Re: how much w/cm²

Postby Odin » Sun Dec 03, 2017 12:46 pm

If it is that close to 8 w/cm it is perfectly fine.

Regards, Odin.
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Re: how much w/cm²

Postby Kareltje » Sun Dec 03, 2017 4:01 pm

BugHunter wrote:Let's see, the circumference is 8.5mm * 3.14 = 26.69mm or 2.669 cm

2.669cm * 90cm = 240.21 cm^2

So it looks like you multiplied mm times cm to get cm^2, which got you off by a factor of 10.

2100W / 240.21cm^2 = 8.74 w/cm^2 which sounds pretty reasonable.
Oh fuck!! That is the problem!!

I found 8.5 mm makes 26.69 mm circumference but took it for mm2, so made it 0.2669 cm2.
How stupid! :oops: :oops:
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Re: how much w/cm²

Postby Kareltje » Sun Dec 03, 2017 4:08 pm

But then again: I trusted my friends build, so I did not question his choice of element. But why is this parameter of interest?

I had to clean the elements when I bought the still, but is this not more caused by the contents of the still than by the power?
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Re: how much w/cm²

Postby Odin » Sun Dec 03, 2017 5:46 pm

If you run dirty washes, sugar rich washes, or do on the grain distilling (very) high w/cm2 can give you scorching easier.

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Re: how much w/cm²

Postby Kareltje » Mon Dec 04, 2017 6:28 am

OK, I see. Haven't done that yet with my elements, so did not think of it.
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Re: how much w/cm²

Postby Fredistiller » Mon Dec 04, 2017 12:59 pm

Nice to see how you can calculate it. Never thought about it.... :thumbup:

We always talk about W/cm² for dirty wash and scorching. But if I understand well, there is no issue using a higher one for a spirit run then. Is that right?

Odin wrote:Kareltje, your wattage per cm2 is quite high. OP, you will be more than fine.

Regards, Odin.


What do you mean by OP please?

(kareltje: ik las je zinnetje in het nederlands en mijn grijze cellen waren zoals: bergepen vs klopt hier iets niet...zeer grappig!)
Last edited by Fredistiller on Mon Dec 04, 2017 1:07 pm, edited 1 time in total.
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Re: how much w/cm²

Postby Odin » Mon Dec 04, 2017 1:05 pm

Original Poster.

Odin.
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Re: how much w/cm²

Postby Fredistiller » Mon Dec 04, 2017 1:08 pm

Odin wrote:Original Poster.

Odin.

Thanks Odin

And so it answer my question as well :mrgreen:
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Re: how much w/cm²

Postby still_stirrin » Mon Dec 04, 2017 2:34 pm

Fredistiller wrote:...if I understand well, there is no issue using a higher one for a spirit run then. Is that right?

Yes and no. Theoretically as the watt density increases, you get more and more like a point source of heat. That can cause localized boiling points on the element (or in the boiler). The result could reduce the life of the electric element (resistive element burnout).

To decrease boil time, more heating element surface area is desireable over a higher watt density. It would have more element surface area to transfer heat to the liquid it’s immersed in. Bubbles will form along the element, coalesce and rise to the surface.

So, the “issues” with higher watt density elements are: 1) usually they have a lower wattage rating, 2) usually they have less surface in contact with the conducting liquid, 3) usually the internal resistance per lineal inch of the element is higher (conducts less current at rated voltage), and 4) with a higher rsistance per linear inch/cm of length they will get hotter along the length and this leads to burnout.

But....your 12.5 W/sq.cm element is very appropriate for a boiler.
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Re: how much w/cm²

Postby Fredistiller » Tue Dec 05, 2017 3:12 pm

Thanks for the replies everyone!

This forum is and stays a very good community... :clap:
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