Re: Pneumatic Conveying, Saltation & Milling
Dear Kaos1,
From your description, I understand (or believe to understand) that you are referring to a vacuum system.
In the direction of the material flow there is:
-inlet strainer
-piping
-explosion valve
-rotary lock feeder
-5”conveying piping
-mill with internal screen
-piping
-2 cyclones in series (with rotary locks underneath?)
-vacuum pump.
The screens are the initiators of conveying problems.
The vacuum pump is the clue to the detection of air flow through the system.
If the vacuum pump is a positive displacement pump, the pump vacuum is a direct indication of the air mass flow through the vacuum pump curve.
You could install a vacuum measurement to the inlet of the vacuum pump and when the vacuum is rising over a preset value, the rpm of the rotary valve is reduced.
If the vacuum pump is a centrifugal fan, then this method is not applicable, as a centrifugal fan drops drastically when the vacuum increases just a little bit.
A centrifugal fan is a bad choice anyway as an air mover in a pneumatic conveying system.
A ordinary measuring flange with pressure drop sensor is then an option.
have a nice day
Teus ■
Teus
My Pneumatic Coveying System Blows And Sucks At The Same Time
Dear Teus,
The system actually relies on two sources of air transport. The mill on one end provides pressure to the conveying line while on the other end is the bag house with a fan that draws, or provides vacuum, to the system. Maybe not the best set up but it does work.
You have the layout about right. The inlet to the mill is the area that only handles clean air so this is the area where I would like to monitor the flow.
The system layout would be more like this:
Inlet filter
Explosion valve
Mill - Separate inlet for material feed into the air stream controlled by the rotary valve
Mill exit begins the conveying system through two cyclones - w/ rotary valves
Bag house or as some here call it "the filter fan"
All of the air that passes through the system, passes through the mill screen. This helps in temperature control and conveying / cleaning the mill. As the screen is quite fine, 400um, when processing chemicals the air flow is reduced. Some chemicals affect this more than others.
Ideally, I need to be able to have the system monitor itself so that the feed to the mill is reduced for those chemicals that impede the air flow while milling.
Thanks for your thoughts but maybe I steered you in the wrong direction,
Kaos1 ■
Re: Pneumatic Conveying, Saltation & Milling
Dera Kaos1,
If the compressor, supplying pressure to the conveying system, is a positive displacement blower, then the pressure is an indication for the air/material flow resistance, in which the screen is incorporated.
The compressor pressure can therefore be used as a choking indicator.
Pressure based control of the rotary lock rpm is then an option.
The flow measuring orifice is also an option.
see: http://en.wikipedia.org/wiki/Orificeplate
The feeding rotary valve must be in good condition, because too much leaking here will destroy the pneumatic conveying operation.
Take care
Teus ■
Teus
No Compressor
There is no compressor in the system. The mill acts as a fan/blower. It moves large volumes of air while the fan on the bag house draws from the other end.
Due to the turbulent flow in the intake there is no optimal area to place a pitot. This is why I ask if there is a straightner, maybe a honeycomb matrix, that can establish a laminar flow in a short distance to utilize a pitot or maybe a magnehelic.
Kaos1 ■
Re: Pneumatic Conveying, Saltation & Milling
Dear kaos1,
A straighter (I am not sure what size) can also be made by yourself, using some thin plating.
Nevertheless, since the airflow is generated by the mill itself by rotation and a centrifugal fan, the vacuum of the fan is a good indicator of the airflow.
If the vacuum becomes too high, this is an indication that the airflow is reduced.
According the law of Bernoulli, the vacuum is low at high air flows and the vacuum is high at low airflows.
As said before a centrifugal fan is a bad choice anyway as an air mover in a pneumatic conveying system, due to the horizontal pressure region at low airflows.
Even the bag filters can cause a significant pressure drop and thereby airflow reduction when polluted.
As the airflow through the system is the same as the airflow from the filter fan outlet, this flow can also be measured there or on another location in the system. What is obvious, is that any measurement should not introduce extra pressure drop.
Take care
Teus ■
Teus
Re: Pneumatic Conveying, Saltation & Milling
Hi Teus,
I have thought about this and the opposite will occur upstream of the mill in our system.
If the vacuum becomes too high, this is an indication that the airflow is reduced.
According the law of Bernoulli, the vacuum is low at high air flows and the vacuum is high at low airflows.
The problem that we have seen in the past is that the screen in the mill, which all of the air passes, becomes partially occluded or totally blinded. The air upstream of the mill slows and stops. In this case the pressure will become the same as the room pressure. Most recently, we had the filters in the bag house clog and this reduced the air flow and we dropped below the saltation velocity. Well, you know what happened next. Now if the blockage occurred upstream of the mill air inlet, then the vacuum would increase between the mill and the blockage as the mill would be pulling against the restriction.
I am thinking this through and maybe using vacuum (delta P) could be used as if the pressure differential drops then this would be an indication that the screen is clogging and we could slow or stop the feed to the mill and allow the mill to clear itself if it can.
I think that this could be set up using a digital magnehelic with an output to the control system.
What do you think?
Thanks for your insight,
Kaos1 ■
Re: Pneumatic Conveying, Saltation & Milling
Dear kaos1,
vacuum (delta P) could be used as if the pressure differential drops then this would be an indication that the screen is clogging and we could slow or stop the feed to the mill and allow the mill to clear itself if it can.
should be:
vacuum (delta P) could be used as if the pressure differential (vacuum) increases then this would be an indication that the screen is clogging and we could slow or stop the feed to the mill and allow the mill to clear itself if it can.
this could be set up using a digital Magnehelic with an output to the control system.
Correct.
Success
Teus ■
Teus
Pneumatic Conveying, Saltation & Milling
Greetings,
I have a system which uses a mill and a bag house to provide the energy to pneumatically convey fine powders through a system of ~5" round pipe and dual cyclones in series. There is a vertical rise of about 12' in the system.
I would like to monitor the air flow in the system and there is a significant length of inlet which has two 90° bends, an explosion containment valve, and an inlet air filter. It is this area that I would like to capitalize on for monitoring as it is free of product. There currently is not a length of the 8 to 9 pipe diameters available to establish an area of non-turbulent air flow in which to stick a pitot tube. Are there air straighteners available to lower this requirement? I don't believe that I need a very accurate means to measure the velocity as we set it currently with a hand held vane anemometer held at the intake but I need it to be precise or repeatable.
The goal is to monitor the health of the milling process as some materials (of which there are over 60 different chemicals and many times more in mixtures) will occlude the screen in the mill causing reduced air flow or no air flow. I would like to monitor the air flow and tie it into the controller to slow or stop the rotary valve feeder to the mill to prevent over heating within the mill and saltation in the conveying lines.
Any ideas of the quickest, easiest, least expensive, most compact method (space is limited) to accomplish this?
Kaos 1 ■