Rimming again

Errrmm I think in the UK you will find the term 'rimming' is used in a very different way...

John.

Not only in the UK!

"Across the pond" (in the US), that word often connotes "something completely different" as well... (to paraphrase a UK group from the 70's...)

That is a term probably known "around the world", which happens also to be a synonym.

Hey there.. i do know about the other use for the word "rimming"

Unless someone out there has already solved this problem, which is what we are hoping for, it is probably time to get your calculus + boundary layer + rotational dynamic hats on.

If we think of the problem on a vertical flat plate first that is continuously going up: water runs over the flat plate, it wants to fall under gravity but is prevented from doing so by the shear in the boundary layer. The velocity profile of the water will be linear over that short distance (more or less for an ideal fluid and lets initially assume so for ease of calculation) in proportion to the distance from the plate. This gives a kinetic energy to the water which is combatting gravity i.e. potential energy gain. Given the speed of the plate, a thickness of water can occur that has a positive velocity i.e. it follows the plate albeit with a lag. The viscosity of water can be used to obtain the velocity profile based on the speed of movement of the water over the plate (or plate over water). The critical speed is when the water does not move, on average, across the section of water. Anything above this will allow the rimming to occur.

Now it becomes a rotational calculus problem. Or can we take the worst case of the cylinder where it is vertical? If this is the worst case, then the problem is already solved based on the vertical plate. One limit on the thickness of water is the critical depth. I don't fully understand this concept, but I think it is limit on the depth of the water at any given speed and slope. It is generally used for channels and weirs but we have used it for measuring the depth of rainwater flow off a large roof.

The other limiting factor is the centripetal acceleration which, if the speed is too great, will throw the water off the roller. Probably not an issue if the speed is likely to be less.

Hope this helps.

Thanks for the answer.

I guess I will have to do some work on this.

I will give you a little background info.

The drum is heated with steam, so it can heat up paper running over the drum as it rotates.

The drum is filled with steam at about 10 bar and the steam condenses to form a layer of water approx 2,5 inch. I suspect that the water is "rimming" as we get up to speed and therefore insulates the steeldrum from the steam, and the result is decreased heattransfer.

thanks again.

""Rimming" is when the water is lifted by cetrifugal force and forms an even layer around the roll."

The high speed rotation of the roll creates a centrifugal force which will throw the water off of the drum.

"The roll in my case has a 15" diameter and about 2,5" of water in the bottom(as "illustrated") the speed of the roll is maximum 400m/min"

Capillary forces and adhesive/cohesive forces tend to wet the drum. The material of the drum will determine whether the water will wet the surface or not and whether or not you so called "rimming" will occur.

What it the application?

Why is wetting of the drum needed?

I don't think you can ever get an even layer of liquid around the inside of the drum ever, unless you get out of our gravitational force field.

Just add up all the forces on your system and you find that gravity is always going one way so at the top you have centrifugal force minus gravity as the total whereas in the bottom you have centrifugal plus gravity as the total force.

The layer in the bottom will always be thicker.

Case491, i know that a completely even layer is not possible because of gravity. but it is possible to get close.

Maybe i have formed my question incorrect.

This layer of water i am talking about is unwanted, because it insulates the steeldrum from the steam and thereby reducing the heattransfer. The problem is that i can not get enough heat to the paper and i think this layer of water could be causing the problem, so i am trying to find out at what speed this layer is formed to run some test near that speed. increasing the wrap of the paper on the drum does not seem to solve the problem. and i have no way of increasing the amount or pressure of steam put into the drum.

Thanks again

You problem has therefor nothing to do with what you call "rimming" (god just feel dirty typing the bloody word) but with condensation. Your environment outside the drum is much colder I suspect then the steam inside. You try to heat the paper like in a colander machine for water marking and such.

I think the cylinder you are talking about may be of the wrong design for this purpose as you should have the steam go through small pipes in the cylinders wall much like a heat exchanger has over the whole inside. Higher pressure may also be needed to keep the steam from condensating on the surface. Maybe you might even need super heated steam instead of just steam.

That sounds more sensible. There's probably not enough surface area for "plain steam" to overcome the heat loss PLUS, the condensation is a problem as he now says, however, superheated steam is dangerous.

I wonder if it would be possible to heat the roller some other way, either resistance heating or even induction? These would be much easier to regulate as well.

"superheated steam is dangerous"

So are busses, trains and lots of other things. If this application needs super heated steam he will have to use it. If it is dangerous he will have to satisfy local health and safety regulations. Not up to us to decide if the world can or cannot use something.

Often these questions also depend on what is available. If he is in a papermill he might have oodles of steam already. If so then the conversion to superheated steam are less expensive than any other form of heating.

point taken....

It sounds like this fella is "experimenting" though. If there is process steam available, hopefully it is very clean and demineralized etc., Steam often has high maintenance issues with it.

The way this post started out, it reminded me of a client of mine who had repeated failures of steam kettles in a commercial kitchen. These well built units were pitting through just above the water or condensate line. Factory reps couldn't figure it out but the factory stopped honoring warranty.

http://www.armstronginternational.com/files/common/allproductscatalog/cg-37.pdf

using steam heated rotary cans is a long time practice in paper and textile industries

the can rotates and should be fitted with a rotary bearing in the end thru which a tube connected to a steam trap would be attached - the drum should be able to turn at any speed and still drain

you are correct in assuming the liquid will drastically drastically reduce the heat transfer coefficient

http://www.kadantjohnson.com

This company has done extensive testing on rotary joint / syphon setups for paper dryers. I,m sure someone there could help with your problem. Yes, to maximize heat transfer you must minimize the depth of the condensate on the dryer ID.

Thanks.. I´m sure they can help me.. I have however come up with a basic formula o get close enough.. rimming is starting to occur around 300m/min.

Thanks Again,

Kasper