As appeared in January 2012 PBE
Copyright CSC Publishing
What factors should I consider
when selecting a discharge for
my mixer?
he most common types of discharges are slide gates, butterfly and plug valves, and drop-
Tbottom units. To choose the best discharge, pay close attention to your material. Slide
gates and butterfly valves are mounted below the mixer bottom, leaving a dead spot that fills
with the first ingredient that comes through, and that ingredient will remain there during processing. The best way to eliminate this problem is to use a drop-bottom or a plug valve.
If a dead spot is tolerable, you should next consider the amount of material segregation
on discharge. If you’re discharging into a larger vessel, a heap will develop on the bottom
and the larger particles will roll off the heap. This roll-apart segregation can be reduced by
either filling a smaller container fast or by using a drop-bottom and slug-loading a hopper.
A drop-bottom can trap air under the material that will puff up through the material and
carry the lighter particles with it. To eliminate this, try using a plug discharge that reduces flow.
If you use a hopper with one side that’s nearly vertical, this will allow the material to slide
down one side.
Plug valves don’t close against material flow, but continue working for long periods.
If you must control the flowrate, use a butterfly valve or slide gate under the plug valve.
Bliss Flower, president, A & J Mixing International, 800-668-3470
o select a suitable discharge valve for
Tyour mixer, first determine whether
metering from the vessel or frequent
cleaning and sanitizing is required. Next,
determine whether a dead spot can be tolerated where the curved vessel transitions
to the flat valve-mounting flange, since
material can reside here during the blending cycle without being picked up by the
agitator.
• Knife or slide gates and butterfly
valves are a good choice for product
metering because they easily close
against material flow. They’re also
relatively easy to clean, especially
sanitary designs that disassemble
easily, but they require a flat mounting flange that creates a pronounced
dead spot where unblended material
can reside.
• Plug valves and drop-bottoms fit
flush against the vessel bottom’s curvature and virtually eliminate dead
spots. The primary drawback is that
their support housing and linkages
may be difficult to clean. Plug valves
are also less effective than knife gates
and butterfly valves at interrupting
material flow, making them more
suitable for metering and discharging
the entire batch.
• Spherical disc or dome valves have a
minimal dead spot, making them a
viable alternative to knife and butterfly valves. They’re also effective for
metering and are available in easyto-clean designs. Spherical disc
valves tend to be more expensive
than the other designs, however, and
may require special mounting.
• Ball valves are a good choice for slurries and emulsions as they seal positively against liquid flows.
It’s important to determine whether
your application requires manual or electronically controlled solenoid operation
and if the valve’s sealing materials are
compatible with the materials being handled and meet any necessary sanitary requirements.
Steve Knauth, marketing manager,
Munson Machinery, 800-944-6644
very mixing application is different, but, in general, answer the following questions to
Edetermine the best discharge for your mixer: Is it a batch or continuous mixer? Is the
downstream equipment batch or continuous? Is there a pressure difference between the
mixer and the downstream process? What are the material’s physical characteristics?
What is the discharged material’s temperature? Are there any special requirements for
the materials of construction? Does the discharge device need to provide any metering or
dosing capabilities? What is the required flowrate?
George Schlager, managing director, AVA-Americas, 704-248-2767
www.powderbulk.com
hoosing the best discharge depends
Con the next step of your process. If
you’re discharging into a container, it’s
simple to connect the blender to a gravity
discharge. If you’re discharging to a
screener or grinder, you’ll need a controlled discharge rate via a conveyor. You
can use a rotary or screw feeder for shortdistance conveying, or a pneumatic conveyor can be connected to the blender
discharge for long-distance conveying.
The conveyors usually need to be cleaned
between material changeovers.
Rather than employing conveyors
and other equipment, consider using a
container blender, in which the material is
blended in a removable container. The
container is removed from the blender
when the blend is complete and is moved
to the next step or to storage. The container is discharged using a simple docking station, which can feed the materials
via gravity or at a controlled rate. When
the container is removed from the
blender, a new container with unblended
powders is inserted and the blending
cycle begins again. Container blenders
don’t require cleaning between blends
and are completely sealed after batching.
Larry Guenther, sales,
Custom Powder Systems,
417-868-8002
neumatic batch blending incorporates
Ppulses of compressed gas into a coni-
cal hopper bottom to homogenize contained ingredients. If you’re using a
pneumatic blender, consider an outlet
valve arrangement that allows partial
batch discharge. This arrangement allows
reblending, which minimizes material
segregation during discharge.
Mike Weyandt, corporate sales
manager, Nol-Tec Systems,
651-780-8600
Equipment suppliers are a valuable
source of information about equipment and processes. In light of this,
each month we ask suppliers a question of concern to our readers. Answers reflect the suppliers’ general
expertise and don’t promote the suppliers’ equipment. If you have a question you’d like suppliers to answer,
send it to Alicia Tyznik, Associate Editor, Powder and Bulk Engineering,
1155 Northland Drive, St. Paul, MN
55120; fax 651-287-5650
([email protected]).