Powder Cooling Using Cooled Air

I do not know how much your powder is built up (spread out) on the conveyor, or if you can add or replace a step with a difference process. As you know as you near your approach temperature, that is the most difficult.

Just a thought, Have you thought of a type of refrigerated falling cooler. Where the power is dropped through a tall chamber (depending on the volume of powder even a refrigerated tube) that is refrigerated where you get a better surface area contact for heat transfer. and then pick up the powder at the bottom with another conveyor.

Of course filters have to be installed as well as rotary air lock valves.

The air lift conveyer is blow conveyer that blows powder up to 50m
i.e: in air lift every powder granule is 100% exposed to air
I think that's what you meant by the refrigerated falling cooler?

don't know how I missed that.

the news is what you already know.

It comes down to your dwell time is not long enough. And I can't think of a process that would be viable.

Could you lengthen your pneumatic conveyor to increase the dwell time. Just loop it back on it self.

Norm

That's the point of research.

Can't you guys recommend a certain type of chillers? or a certain supplier?

The Chiller I have now can't go to air temp less than 12C due to the following limitations:

-Chilled water temp can't get less than 4C
-Relative Humidity of air must be less than 0.6

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The point of a project is to teach you how to do research, not ask other people to do it for you.

Sounds like your just looking for ideas, so,......

Yes, one thing you can do is look at Freezer tunnels to be installed/replace your conveyor systems. Your conveyor systems are running pretty fast, so it may be a serpentine type systems to get your hold time..... problem is, there is quite a bit of air flow threw these systems. So you may have to talk to the OEM for a system designed around your needs.

Try Heritage Equipment, BCast Stainless Products - Heritage Equipment

They carry BCast Freezers.

You contact them, with the information you have, they will send you a survey to complete, and they can come up with a design.

Talk to some of the vortex tube manufacturers, they may be able to assist.

The make is irrelevant, Mildred. You have to define the cooling load in order to size it properly first.

I'd be looking to change the flow rate relative to the existing air flow first, to get powder that is within the temperature spec first, and go from there. Before you say that the line would be running too slowly, the powder that is coming off it at the moment is off spec. So get the temperature within spec first so at least you have kosher material. Then look at what you can do to recover to the correct flow rate afterwards.

You haven't described how the air lift works; eg., is it a recirculating type where the blower picks up air ducted from a cyclone, entrains the powder and conveys it back to the cyclone to be separated and the air repeats the process, or does it pick up and cool ambient air which is then released when the powder is dumped? And what is ambient temperature etc.?

Part of your problem is likely that this is a con-current flow heat exchange process. With the powder and air flowing in the same directions, the tendency is to equilibrate to a mean point between the two starting temperatures. To get it to cool an extra 10 C at the output point means you have to cool the input air OVER 10 C more than you are now.

If you can't do that, the best ways I can see for you to achieve this without adding a separate cooling vessel are:

1. Slow down the air flow. This likely means you will have to use larger ducting and possibly a more powerful blower for the system to convey the same amount of material, but the longer exposure time to more air will drop the "pinch point". You still have to make sure the material is well dispersed in the air flow (we are assuming that your design already has this, so that that isn't your problem). You will be cooling a larger volume air per unit of material moved.
2. Lengthen the air lift for added cooling time- a path of diminishing returns.
3. Make sure you have good air flow over the first (hot) belt conveyor. A chiller here may work wonders due to the high delta T, and can flow counter current- a much more efficient heat exchange process than the concurrent flow.
4. Install a chiller for the air over the final belt conveyor. One possibility, depending on your design may be to cool the air, pass it countercurrent over the final belt conveyor and then draw it into the air lift to convey the material back to the belt. This would allow you to use one cooling circuit.

I was going to suggest a second air lift, although it would require an additional blower. Keep the existing lift as is and add a second one of x length. Take the partially warmed air from the existing lift and use it in a 'pre-cooler lift' ahead of the existing one. This would give a quasi-counterflow arrangement. (Or cool the air again before the pre-cooler. (Yes, I know this is basically what someone else has proposed.) But we/OP can't really put forth a design without a heat balance.

It's hard to move forward not knowing how much heat (thermal mass) there is to be removed, the thermal properties of the powder, the particle to cooling air distribution/ratio and so much more.

agreed,

Normally you can derive/extrapolate it from the data, but....... the data is in a range.

and the amount of CFM's carrying the powder with a delta T of this air against the volume of product..

All what is given here are suggestions.... its up to the OP to do the foot work.

Just looking for something to indicate OP has enough information to proceed.

Since when has a lack or surplus of information prevented things from proceeding.

Ready, Fire, Aim!

I guess I'm just haunted by memories of energypassion97, and a few others.

Our business mantra at this research facility. What about it?

Full speed ahead!

I had had all of that calculations done already :D
I was haunting ideas or general product recommendations
I am just so limited in sharing data :)

It sounds to me like your chiller is already optimized for the desired final temperature but not for the amount of thermal energy needed to be transferred. Decreasing the air velocity will slow the rise and lengthen the cooling time but at a risk of not lifting the material. If you could do more cooling with either belt conveyor steps you might then be able to do a better controlled final cooling with the chilled air stream. A more thermally conductive belt material or low velocity cool air blowing down the conveyor belt path might be possible.

Out of curiosity, did this powder average size dimension change? A coarser grind would have less surface area to transfer heat to air. I understand cooling a coarser grind might be the point of the exercise but if not then finding out what changed in this presumably working system design is where I would start my search.

I am already considering the ideas you guys are sayingThe major limitations are:

- Belt speeds are rapid

- Speed of air on belts are limited to 1m/s

About the air lift:

it works by pulling up cold air coming from the chiller AHU then releasing it when the powder is dumpedair speed can't be less than what it is as the material won't be liftedMaximum length of the lift is already achieved

I was considering cooling the body of the lift itself ... but worrying about the efficiency of such a solution.

About the Conveyors:

one more limitation is the RH value of air on the conveyers

most of the cooling belt conveyors I have researched uses water to cool the belt from underneath or even refrigirent ... the RH value would increase and the powder is stacked ... which will damage the material

Using chillers on air inlets at belts is one of the most promising as I previously stated, I just have hard times finding a small sized chiller supplier that will do the job

Considering a material of high conductivity is a smart solution I think ... any recommendations for belts of high conductive material?

I am using rubber belts atm

and as an answer
No the granule size shouldn't change by any means ... that is a major limitation
I would have used screw cooling conveyors if it wouldn't and it would have been magic

Here's a suggestion I found outside of the box. Adding some sort of deflection (baffle, vanes, curves) could lengthen the pathway in the lift without changing the total height lifted by the air conveyor. This will require more air being moved at a higher velocity for the same material net flow rate. This might not provide a full 10 °C drop. Cooling the lift tube itself will probably not do anything since the air stream in contact with the pwder is where most of the heat transfer will happen. The tube will just be heating or cooling the air contacting it but I do not anticipate this to be a large percentage of your airflow.

Can you replace the first conveyor with a rotating tube that has a scroll on the inside? Run the powder up the inside and chill the outside.

You may be able to chill this tube by ducting your air-lift air back over the tube. It should still be cool enough to have a large enough delta to make a difference on the tube.

Good luck!

A 50' lift at 19m/s even allowing for slip means that the powder is in contact with the cooled air for less than 2 seconds. Enclosing the and outlet conveyor and ducting the lift exhaust air along the powder while it is traversing the conveyor would probably give better results than upgrading the chiller. It depends on the relative length of the conveyor. The lift exhaust is right next to the start of the conveyor, especially if you are using a cyclone to remove the air from the powder. Due to the much lower flow rate along the conveyor, any carry over fines would drop out onto the conveyor and increase your throughput. If that provides insufficient additional cooling, first try slowing the conveyor to give a longer contact time or, more expensively, pipe the cooled air back to the start of a similarly enclosed inlet conveyor.

Looks like one complex task you've got there. You probably want to bracket as many variables as you can. Get yer Physics out.

Could be there is no Specific Heat data on your granules and you'll have to determine your own.

The relatively obscure discipline of Pyrometry will no doubt give you some good insights
( enthalpy transfer and such ).

d

Am on it
You just can't imagine how complicated it seems :D

How sensitive is this powder to contact with liquids? If not a problem, you could spray with a refrigerant such as liquid carbon dioxide, assuming, of course the CO2 is recovered and sent back around for another trip. Actually, the powder might not even contact liquid CO2, but might contact dry ice, in such arrangement.

Another option is convert one of the belt conveyor systems to a long water cooled belt (probably the first one in the train), such as the ones offered by Sandvik.

Other option: enclose the highest temperature portion in a pipeline that has numerous carbon dioxide (or liquid nitrogen) nozzles to achieve the most rapid cooling in that portion of travel, so that the final approach temperature in the air lift is already a close shot.