Effect Of Vibration On Pneumatic Conveying.
A product of 5 microns size can easily be conveyed by a properly designed system. This system is hopelessly inadequate , from your description. Vibration should form no part of the design.
What is the product transfer rate?
What provides the conveying air, the mill, a separate compressor or an exhaust fan?
What is the airflow rate, the ducting diameter, design conveying velocity?
Is there any moisture involved?
Michael Reid. ■
Re: Effect Of Vibration On Pneumatic Conveying?
Yes it is an extremely poor design. Unfortunately I have been called in many years (decades) after the system was put together.
The mill is a Hosokawa Air Classifier Mill. There is a classifying fan on the mill itself, but the main circulating fan is a 3 kW, 236 l/s 3,000 rpm centrifugal fan.
The product transfer rate is vague but a 2,500 litre batch is processed in around 2 to 3 hours.
The ducting is 250 mm diameter.
There is no moisture involved, however some of the bulking agents can "melt" on impact. The mill rotor operates at 3,000 rpm. The circulating nitrogen enriched air enters the mill at around 20 deg C, and exits the mill at around 40 deg C. After passing through the bag filter to collect the milled product, there is a air:water heat exchanger to cool the air. While it is essentially a dry product, the air is not dried, and with relatively high relative humidity, there is a reasonable amount of condensate that drains from the heat exchanger after the circuit has been started, and there have been instances when the heat exchanger has filled with condensate to about 1/3 of the height of the unit because of humidity. It is essentially a closed loop, however it is not sealed. The argument used by the original designers was to run the system without any product in the mill for say 5 to 10 minutes to let the heat exchanger drop the water from the air stream, then start the rotary valve discharging the product from the primary blender into the mill. Of course, that is not necessarily done now.
I am looking at reducing the number of 90 degree bends as they are all short radius bends. There is very little that can be done with a major redesign as my client is not in a position to completely shut things down and essentially knock things down and start again. They also have a tendency to make changes to get things working for the production throughputs to be met, and then they call me in, sit down and justify what they have done. They are not always able to do this.
Thanks. ■
Vibration And Pneumatic Conveying.
236 L/s in a 250 dia. duct gives 4.8 m/s. The system ought to be designed for 20 to 22 m/s (i.e., 120 dia.) to avoid duct blockages, especially if there is moisture involved.
The client needs to spend some money and redesign the system. You may need a larger fan motor than 3kW to make it work.
Michael Reid. ■
Re: Effect Of Vibration On Pneumatic Conveying?
An update.
As a first step. we are smoothing out the ducting. Unfortunately engineers tend to like everything to be at right angles. Some freak out when they have things that head off at an angle in other than an ortho plane. This is what I have proposed as a first step. The conveying speed is similar to another production line that is designed with more generous bends and not as many 90 degree bends.
This should eliminate the need for vibrators on the ducting which has seen material dropping out of the air stream and flushing back into the mill. This drops the throughput dramatically. This may also drop the peak temperature The vibrators have also been shaking the ducting to bits...literally. Flange welds have been cracking on a regular basis. This gives you an idea as to the severity of the vibrations.
This will be done at the next shut down and then if there any further changes they will be a future project.
Thanks. ■
Re: Effect Of Vibration On Pneumatic Conveying?
Just thought I'd keep you posted.
I redesigned the ducting so that there were not as many 90 degree bends, and that there was a more gradual transition from the inclined duct. The horizontan section was also eliminated so the changes in direction were more gradual.
The result was amazing. The vibrating motors are no longer there, and the material is flowing a lot better. They were processing around 500 kg per hour through the line, mainly because there was material dropping out of the stream where the abrupt bends were creating turbulence. The vibrating motors were keeping the ducts clear (sort of), but the powder was then flushing back into the mill.
After the changes, the same product is running at 1500 kg/hour, and no vibrating motors. The production had to be stopped last Friday because they could not organise trucks to take the finished product away!
This coupled with a significantly redesigned bulk bag discharger has seen a huge improvement in production. The old discharger was taking around 15 minutes to load a bag. This has reduced to 2 minutes. The original bag massager has been replaced with a new design that actually works, so after 15 years (yes, they have put up with it for that time), they no longer need to keep prodding the bag with broom handles to get the material to flow.
About the only issue with the massager is getting the massager plates to synchronise. The system uses compressed air and so is difficult to synchronise. You adjust it so that the plates work together, but then the bag is not placed dead centre and so one plate sees a greater load and the lighter loaded plate moves more. I think that this is partly due to the fact that operators are so used to the old system, they are having a hard time adjusting to a system that actually works. ■
Effect of Vibration on Pneumatic Conveying?
G'day,
I am currently working on a project where product is reduced in size down to around 5 micron in an Air Classifier Mill. The product aspirated by the classifier is fed into a duct that takes it up to a bag filter over a re-blender.
The product being milled is essentially a refined mineral clay used as a bulking agent in herbicide production.
The duct from the mill to the dust collector has an inclined length around 3 m long that is at an angle of 30 degrees to the horizontal. It is not ideal, but building constraints don't allow it to be steeper. At the moment apart from the initial bend which is a 60 degree short radius bend, all the other bends (3 of them) are 90 degree short radius bends. I have come up with an alternative arrangement that uses 3 45 degree long radius bends and straight duct with just one 90 degree long radius bend which should reduce the losses in the ducting.
The problem is that at the moment the inclined duct ends up with product dropping out of the air stream and the slope is too shallow. This saw operators take to the duct with rubber mallets and while this cleared blockages, it also collapsed the duct to the point where it has affected air flow through it. The other thing that has been done is to fit a compressed air vibrator to the duct, however this is being operated as long as the process operates. The vibration is so severe, I could not get a clear slope reading off a spirit level.
When I was on the site taking measurements to model the duct and come up with the alternative layout of the ducting, I was surprised by the amount of vibration in the duct. Now while the inclined duct is not blocking, I wondered if the vibration would have an adverse effect on the ability of the air stream to convey the product. The vibrators might be causing some of the product to drop out of the air stream and end up back at the start. Product does get through, but is it operating efficiently?
So, as the subject asks, what (if any) is the effect of vibration on pneumatic conveying? I have suggested my client use a timer and say have a 5 second pulse delivered every minute or two. This would clear any product that might drop out of the air stream, but still get the majority of the product through.
Any thoughts?
Thanks. ■