Material Temperature Effect on
Pneumatic Conveying
Guest article by Paul Solt, Pneumatic Conveying Consultants
Printer friendly PDF
 In
many pneumatic conveying systems, the temperature of the material
being conveyed can be overlooked, however, under some circumstances,
a serious problem can be encountered if we ignore this temperature.
Pickup Velocity
Many systems are designed with a
conveying air pickup velocity. If we assume that the air will be
heated by the material, and thus expand, and we can use less air, we
are wrong. At the pickup point the air is the air temperature and
the velocity must be calculated without any consideration of the
material temperature.
Air Volume Changes Due To Material
Temperature
If the material being conveyed is
very hot or very cold, and the conveying distance is significant
enough that the air is heated or cooled by the material, ultimately
we would come to an equilibrium temperature. But this requires time
for this heat transfer to take place.
If you are calculating the time for
the heat transfer, don’t make the mistake in assuming that the
airflow over the particle is equal to the pickup velocity. This is
only true for the first foot of conveying. As the material
accelerates to its conveying velocity, it will probably approach
about 80% of the gas velocity (depending on particle size and
density).
As Temperature Changes - Velocity
Changes
If we ignore this affect of material
temperature, the calculation of the conveying system may be very
undersized.
For example, when conveying hot ash
at about 400 degrees F, the air supplied in a pressure system might
be between 100 and 200 degrees F, depending on the air source and
distance from the feed point. We may have selected a volume of
conveying gas to result in a pickup velocity of 3600 feet/minute
with 150-degree air. If this air now is heated to 300 degrees F, the
volume will increase by the absolute temperature ratio of 760 / 610
= 24%. This increase in velocity will increase the air friction
losses and thus increase the system operating pressure or require
the reduction in capacity.
Now that wasn’t so serious, but in
some system we have designed where the material being conveyed is in
the range of 800 degrees C (1472 F) this creates a major
consideration in the design of the system.
Solution For High Temperature
Materials
It would appear that the simple
stepping of the conveying line (increasing the diameter of the pipe
along the conveying line) would solve the problem, and it very well
might. But remember now that you have designed a system to work with
hot material. What happens on start up if the material isn’t hot?
Will the system work?
Usually (hopefully) during startup
the capacity is lower and thus the operating pressure is lower,
resulting in sufficient air velocity to convey the material through
the system. Frequently, a startup time, or warm up time, must be
considered when the system has been idle for a period of time.
Unusual Application - Ice
A design was required for the
conveying of ice!
In the fishing industry, when fishing
ships are being unloaded, the fish are elevated through the use of a
bucket elevator and dropped into the storage bin. In this case they
wanted to mix ice with the fish as they were deposited.
An ice making machine was obtained,
which created a sheet of ice, about 6’ wide and bout 0.5” thick. As
the ice extended off the end of the conveyor portion of the machine
it would break off, by it’s own weight and drop into a funnel type
feed. This ice was then conveyed to the fish storage tank, and blown
in at the same time the fish were being unloaded, packing the fish
in ice.
The only unusual design consideration
for this system was cooling of the conveying air to as low a
temperature as practical, and then designing the system for an
almost constant temperature conveying.
Once again I will say, “Anything can
be conveyed, if you have the money!”
For more information contact our
author:
Mr. Paul Solt
Pneumatic Conveying Consultants
529 South Berks Street
Allentown, PA 18104-6647
Phone: (610) 437-3220
Fax: (610) 437-7935
Email: pccsolt@enter.net
Web site:
http://www.powderandbulk.com/pcc
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on our On-Line Help Forum. For
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