Heat required to keep still bin from ice formation

[Parus50]

Hi guys,

Need your help to find the way to calculate amount of heat required to keep two uninsulated steel bins with surface of 790 m2 (8500 ft2) each from ice formation on internal wall. Bins are 19m (60’) high and diameter 14m (45’) located inside storage building which will be kept about 2° C (36°F) . Bins are half full of crushed ore with3% moisture content. Thickness of bin steel wall approx. 1”, temperature of ore unknown.
I thought to use unit heaters to keep lower portion of bin constantly warm and set of fans to push warm air down where conical hopper direct ore fall to conveyor.
My calculations using R13 for building wall and bin surface as partition walls with temperature inside around 0°C gives me about 480kw of heat required.
Heat transfer thru steel wall with ΔT only 1 degree C using Fourier's formula q = k A ΔT / s gives me 2.8 Mw of heat. I feel that something is wrong here and will appreciate your help and comments.

Another task is to calculate the temperature of ore extracted from underground T extracted approx. 15°C ( 5°F) traveling to storage bins on conveyor with outside T about -40°C(-40°F). Time of traveling approx.20 min.

Thanks in advance.
Cheers
Parus 50

 

[MintJulep]

Big bins inside a cold room --> lot's of heat needed.

With a -40 outside temperature and 2C interior temperature I'd guess that the inside surface temperature of the building walls are below freezing. So radiation is probably not negligible.

Electric or steam heat trace the bins, then insulate over.

 

[dvd]

Both the frosting of the bins and the temperature calculation seem difficult to nail down.

What aspect of the ice formation is worth spending that kind of money on? It sounds like an interesting problem to consider solutions to. If you could reduce the amount of moisture laden air that comes in contact with the wall, it would reduce the amount of ice formed. If you knew how much it would cost to de-ice, then you could look at ways to accomplish the same thing for less money. Would it be cheaper to move air instead of heat the bin wall? Is it possible to draw ambient air downward along the bin walls so that moist warmer air does not rise along them and form an ice layer? This seems like an area where you could run some calculations and convince someone to spend some money and then get fired. It seems like a reasonable thing to run some experiments, which is probably why you are asking the questions so that you know where to start. You could just do a section of bin to see what happens, and ignore the results at the edges.

I have always liked Chromalox, so I start there.

This may be interesting:

http://www.chromalox.com/case-studies/case-studies-detail.aspx?cs=287#.UqI6eCftOSo

And calculation based on this, but with the caveat that you also have mass flow in and out of the tank in the form of ore and some air that the ore carries with it:

http://www.chromalox.com/catalog/re...ns-and-Heater-Selection-Pipe-Tank-Tracing.pdf

How big is the particle size of the ore? and at what location on each piece do you want to know the temperature. It may not be uniform in temperature. You've got ore, you've got -40 temp, and since you're at a mine where it gets to -40, you've probably got 20 minutes to kill. You could buy a couple of data loggers and thermocouples and conduct some studies. Calculating the temperature might be a fun exercise that would impress the girls (or guys) up (assuming up, since it is springtime down) there, but it is going to be subject to the assumptions that are made.

 

[Parus50]

Hi all,
Thanks DVD for replaying.
Yes, its interesting problem when you do not need to provide answer and solution for a client quickly.
To answer your questions:
General idea is to create warm air layer inside storage building around bins to act as thermos to keep steel walls warm and prevent ice formation inside lower portion of bins.
-Its impossible to lower the moisture content in 360 ton/hour material going to bins.
- Heat tracing as you suggest is good only for power generating plans, as they probably do not pay for electricity, for others it is a dollar killer.
-I would recommend slow motion fans blow down to keep warm air at lower part.
-Ore size approx.150mm, I need to know average temperature of ore falling to bin. I hope it shall be above freezing.

My tasks is prevent ice formation and material freeze together and stick to the cold walls of bin, almost exactly as in Chromalox case study, but am not doing such exercise every day, my university books probably 3 times went thru recycling process.

Thais task is a fun, isn't it?
Cheers

 

[IRstuff]

SAE AIR 1168-4 might be of help; it's the SAE document for determining how to prevent ice and frost buildup. The general requirement for keeping, say, a windshield ice free requires ~2.5W/in^2 for ~150 kt flight speed.

The document doesn't specifically cover your case directly, but extrapolation of Table 3F-3 results in ~0.2W/in^2, which would result in ~250kW to keep your tanks ice free on the outside.

TTFN
faq731-376

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[dvd]

How fast do the bins ice up to the point of causing problems? Is it minutes, hours, or days? Is the icing issue related to interrupting the flow out of the bin? Is the hopper bottom the area where icing causes the problems, or the cylindrical bin, or both? An experiment with lining material might be interesting - stainless steel, or Tivar, or ? If the bin is flowing in mass flow, the ore would keep the ice formation minimized. What is the condition of the bin and hopper surface - beat to hell from hammering, or smooth? Does the bin flow well other than the problems due to cold weather?

Or maybe you should concentrate on keeping the ore temperature above freezing. Wildly assuming that the ore drops from 15C to 0C (you need to conduct an experiment to figure this out), and calculating how much heat input is required to raise 360 tph by approximately 5C comes up with about 250 kW, assuming specific heat of 0.1 kcal/kgC. Sounds difficult in reality, though due to no electrical utilities along conveyor route and exposure of the material to the environment inside of the conveyor covers.

 

[Parus50]

Hi,
Thank you guys for replaying even on weekends.

A few clarifications related to this task.
This is a new project, nothing is built yet, but client had previous bad experience of frozen ore in storage bins. It was quite a big problem in past, so this time we should avoid it.
The problem was when wet ore was stuck in lower conical part of bin and frozen together creating huge plug in hopper. We have no records how often it happened or how fast ice formation was built on side wall of bins. There wasn’t and will not be special mechanical devices to break down stuck material and open up free area to allow it drop down to conveyor. The problem was only in winter in really cold days what tells us it was when temperature of ore was 0 or bellow.

This project is different, different mine, conveyor length and time of conveying, different material flow.

Bin surface is smooth 60 deg. standard shape, I would not expect any troubles in normal day, but extreme of -40 may cause problems. Heat an ore on its way is impossible, too long conveyors and no one will do it because of cost associated.

Using IRstuff’s suggestion 0.2W/in2 gives me 500kW for both bins, close to my calculation of 480kW, but I would like to have some kind of formula to calculate it based on thermal properties of steel or ore.

Thank you again of all comments, I really appreciate this discussion.

 

[MintJulep]

Would it make sense to purposely keep everything below freezing rather than trying to warm it to some temperature above?

Ice that is already ice is slippery.

Jams happen when water transitions to ice and sticks stuff together.

 

[dvd]

I recommend that you also seek assistance at the bulk-online Forums:

http://forum.bulk-online.com/index.php

 

[Parus50]

Thanks for advice.
I never heard about bulk-online forum.
Cheers

 

[IRstuff]

"but I would like to have some kind of formula to calculate it based on thermal properties of steel or ore."

You could, but you would only get higher numbers. It wasn't perfectly clear whether you are attempting to warm up frozen ore, or simply trying to keep it from freezing. The 500 kW number is the latter, and since the ore is presumably not generating heat or cold, the material properties are, well, immaterial.

Only if you are trying to change the temperature of the ore would the material properties come into the picture. I think that in your original analysis, you slid into a transient analysis without noticing. Transient analysis would require some sort of time limit, i.e., 1 hr to warm up the ore from -20, or somesuch...

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[Parus50]

"It wasn't perfectly clear whether you are attempting to warm up frozen ore, or simply trying to keep it from freezing"

My aim only to prevent ore to be frozen and stuck in lower part of storage bin by transferring heat thru uninsulated bin wall and warming internal layer of the wall preventing ice formation. Ore in bins can be stored for 2-3 hours (not an issue) or for 2-3 days depending of concentrator needs and if it happened in very cold winter day(s) that piece of frozen ore may plug the chute completely.

 

[dvd]

I think that you should keep the ore moving when the temperature gets very cold. Design to bypass back to the bins, or to a cold weather bypass stack that you can reclaim later. And design the hopper for good mass flow which will keep the bin walls clean.

 

[IRstuff]

OK, but how much access to outside air does the interior of the bin get, i.e., conversely, how much or how fast is heat lost to the outside? If the bin is essentially a closed box, then the 500 kW is more than sufficient to keep the inside above freezing.

TTFN
faq731-376

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[Parus50]

Originally bins shall be completely enclosed inside storage building, that is why an idea was to create and keep a warm layer to heat bins and create “thermos” effect of keeping them warm.
Right now there is another option, to have bins partially enclosed (only lower part) and insulate upper part what is wrong from my point of view and will cause more problems with potential freezing of ore.

But there is a lot of other factors, initial temperature of incoming ore, time of storing, outside temperature, infiltration of building, dust control (removing of dust when bins are loaded with ore), and others. Too many factors to consider.

It will be nice if somebody had similar experience in past, but looks it’s not really common project.

Thanks

 

[chicopee]

A thought: about exploring the installation of vibrators mounted on the side if the bins before engaging in a heating system. Vibrators are installed on bins storing material that cling to each other or would be damp and clump up and stick to the side of bins. May be on frozen iron ore this could work.

 

[Parus50]

Hi all,

I would thank you for this discussion and help.
I made decision and got the final number close ~400-450 kw for heating including building losses and infiltration.
From various calculations and all kind of books, the best explanation I got from Mechanical Engineering Reference Manual for the PE Exam and Heat transfer Sections.
Great book and practical examples, straight to the root nothing extra.

Again thank you guys and have fun during Christmas break.

Best
Parus50