Further to Dr Henwood’s comments on Angel Hair, I would like to add the following…
Dr Henwood is correct that it is typically formed due to high temperatures in the conveying line.
I would comment that a good conveying system design utilizing good conveying parameters can be employed to eliminate the creation of angel hair, therefore eliminating the need for any kind of angel hair trap. Traps can be helpful in collecting the angel hair that is created, but why not investigate and potentially eliminate the basis for the angel hair creation?
Most conveying in the plastics industry is known as “Dilute Phase conveying”. This typically represents that the conveying line is approximately 10-15% material concentration at any given moment.
In the designing of a conveying system, there are limited factors that can contribute to a safe or unsafe conveying situation.
Velocity is everything in conveying.
If the air is moving too slow, the air stream will not have enough velocity to pick up the material, and the material will literally “fall out” of the air stream, causing plugging and slugging in the lines.
If the air stream is moving too fast, the material being conveyed will “slide” along the elbows in the system, and cause friction. With friction comes heat, and with heat comes angel hair.
If the air is moving even faster, the particles of material will not have the ability to travel as fast due to the conveying resistance, and again the material will literally fall out of the air stream.
Therefore, when designing any successful conveying system, VELOCITY is the key component to the design.
Some basic terms for reference…
PICK UP VELOCITY – THE SPEED OF THE AIR WHERE THE MATERIAL ENTERS THE CONVEYING STREAM – TYPICALLY LISTED IN FEET PER MINUTE
TERMINAL VELOCITY – THE SPEED OF THE AIR WHEN THE MATERIAL REACHES THE DESTINATION – TYPICALLY LISTED IN FEET PER MINUTE
INCHES HG – TERM OF MEASUREMENT FOR VACUUM
PSI – TERM FOR MEASUREMENT FOR PRESSURE
AIR VOLUME – THE AMOUNT OF AIR GENERATED BY THE VACUUM OR PRESSURE DEVICE THAT GENERATES THE CONVEYING STREAM – TYPICALLY IN CUBIC FEET PER MINUTE
When we at Wittmann Battenfeld Canada (ROTOLOAD) do conveying system calculations, we begin with calculating the air volume required to get the desired conveying rates. We then determine the pressure requirements for a given system.
Once we have these two parameters, we can calculate the necessary conveying line diameter to get a safe conveying velocity.
If a system application requires 12” hg to achieve the desired results, this amount of 12” hg is only required where the material enters the conveying pipe. The closer the material being conveyed gets to the required destination, the vacuum required drops, and therefore the air expands, therefore increasing velocity.
I would comment from my experience that the necessary PICK-UP velocity for typical roto powder is something in the range of 2500 – 3000 feet per minute.
If we can design the system with this low pick up velocity, we can essentially be sure that the terminal velocity is in the range of 5000 – 6000 ft per minute. The closer that we can keep the terminal velocity to the lower end of that range, the more confidence that we have that we will not create angel hair.
The point, to be very clear about, is that it is necessary to ensure that the VACUUM readings on the system equipment are maintained at the required levels.
As the VACUUM drops off, the air expands, and therefore by nature will create a higher velocity.
The above diagram is a hypothetical example of the way that a conveying system functions.
If for a 200 ft conveying distance, you start with 10” HG of Vacuum at the destination end, and about 150 cfm of air at the starting point, the initial conveying air speed should be about 3000 ft/minute. As you get closer to the destination the vacuum required to get the material the balance of the distance drops off because there is less resistance as the remainder of the distance becomes smaller. The expanding air in a fixed pipe diameter increases the volume of air, thereby increasing the air velocity which in turn increases the powder velocity
As this vacuum drops off, the result is air expansion and as the air expands in a fixed diameter conveying pipe, the velocity goes up.
I would welcome the opportunity to explore this with anyone. Should you be experiencing angel hair issues, typically with some smart system modifications, they can be reduced or even eliminated from the process. Sometimes, the solution could simply be an inexpensive pulley change!
Rob Miller began in the Plastics Conveying industry in 1986. With Nucon Systems Inc. Nucon Systems ONLY manufactured Plastics Resin Conveying systems. In 1999 Nucon Systems was purchased and became part of the global Wittmann Group of companies. Over the last 33 years, and 5000 systems, Rob worked through the various positions of the company including Service, Conveying System design, Project Engineering, Sales Engineering, Sales, Sales Management, VP Sales and Engineering. He has been the President since January 2000.
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