C. M. Consulting
P.O. Box 407
Odell, Oregon 97044
C. M. CONSULTING
A Division of Cliff Mansfield Incorporated
Screenless Batch Plants
Increased Productivity, More
Mashing big rocks
into little rocks. Crushers do this job well, if maintained and operated
correctly. The products they give us are well suited to the task of building
roads and satisfying the bureaucrats with their specifications. Drum mix AC
plants use the stuff 'as-is' and don't seem to suffer unduly. So why do we run
this product over a set of screens and disassemble it at our batch plants?
In the old days the answer to that question was simple: It was the most
efficient way to ensure a proper blend to meet state specs. But times have
changed. With today's aggregates divided into different stockpiles, screens have
become redundant, their value reduced to the removal of oversize material. Worse
yet, in some older HMA facilities where the exhaust air and burner systems have
been maximized the screens can become a 'pinch-point' which actually slows plant
production rates through 'blinding'. Reversing the direction of screen throw
sometimes helps combat this situation, but in reality it can't add substantially
to the unit's flow capacity. The screens can also contribute to 'out-of-spec'
conditions through carry-over at higher production rates. Another consideration
is the amount of time the plant spends waiting for each bin to supply its
contribution to the material in the weigh hopper. This wasted time would be best
used in mixing-time.
Admittedly, conditions still exist which make screens desirable. A good
example would be when a particular plant is switching back and forth between
5/8" and 3/8" mixes to supply various customers in the same day. Another would
be when a contractor is forced to use 5/8"-0" rock to produce 3/8"-0" mix.
Eliminating the screens completely isn't practical in all cases. So
wouldn't it be nice if we could by-pass the screens when high production rates
are desired, yet use them at other times when the need arises?
We can, and it can be done relatively easily. There are two steps we need
1- We must know each feeder's output and be able to
control it precisely.
2- We need to build a mechanical device to divert the
aggregates around the screens while maintaining the ability to switch back.
We'll discuss these steps one at a time. Keep in mind that the purpose of
this article is to discuss ONE way to accomplish this task. There are many other
ways, each as valid as the next.
Step 1 consists of two parts, modifications for feeder control and the
addition of belt scales.
Not so many years ago feeder control meant running out of the control
room to the feeders and beating a gate or two up/down to adjust the feed rates
at our AC plants. Then we'd race back to the control room, hoping we got things
going at the appropriate rates for what we were doing. If we were blending sand
we prayed that we didn't change its ratio enough so that our 200 went out of
spec. Crossing our fingers, we chewed Rolaids and waited for the next sample
results to see how well we guessed at the gate settings.
It was a great day when management finally had mercy on us and invested
in a set of U.S. Vari-drives, or better yet some Woods DC motors and
controllers. We were on top of the world. Now we could actually control the rate
of aggregate feed from the control-room (no pun intended?).
Most of us were happy as the proverbial clam until the state threw us a
curve. New mix specifications decreed that we divide our previously single pile
aggregates into three or four separate sizes, depending on what state we were
in. 3/4"-0" became 3/4"-1/2", 1/2"-1/4", 1/4"-10 and 10-0". If we added sand we
had five feeders to contend with. We now had to blend these sizes together
correctly at the feeders, send them to the batch plant, take them apart over the
screens and then mix them back together again in the correct proportions.
Feeder accuracy wasn't so critical at first. Batch plants have reject
chutes. Whatever wasn't used in the mix was simply discarded, to be carted away
and stockpiled for some later use. When fuel and aggregate prices were low this
didn't matter so much, but in today's market attention to such things can often
keep a company operating in the black. As time has progressed it's become more
and more important to precisely control the outputs from the feeders.
Controlling the feeders electrically is the most efficient way to control
their outputs. In recent years the prices for electronic gear has seen a
downward turn and some exciting new technology has hit the market. Some of this
technology can be advantageous in our quest to control the feeders. AC frequency
controllers are one example. These devices control motor speed by varying the
frequency-- normally 60 hertz -- of the voltage we send to the feeder motors. If
we reduce the hertz to 35 cycles, the motor slows dramatically. Conversely,
increasing the frequency speeds the motor up. This allows for an infinite speed
range that is repeatable-- a must when calibrating your feeders. Older frequency
drives had a problem starting motors when set to lower cycles, but the newer
drives use 'full-torque-start' technology which addresses this problem.
Mitsubishi makes a solid state motor controller for 5 hp motors which
sells for under $900 and uses standard electric motors, unlike DC drives which
require expensive specialty motors. A standard 5hp TEFC (totally enclosed
fan cooled) motor and a TD3 speed reducer serves to drive each feeder nicely.
The company also makes units for 10hp motors which sell for around $1400. Square
D and Cutler-Hammer offer similar units but at a slightly higher price. These
units all utilize existing AC motor leads. Standard start/stop stations also
Variable speed feeders alone won't address everything the batch plant
needs to run in a screenless configuration. We must also duplicate the feeder
control systems used on drum plants. I.E.: We need to be able to control feeder
ratios through the whole range of speeds likely to be used. To do this we must
have a way to control each feeder's speed independently yet still be able to
control all linearly, which means each feeder's percentage of contribution to
the overall output must remain constant.
On most jobs we do this by regulating each feeder's speed with a 20k trim
potentiometer hooked to each unit's 0-10 volt DC reference signal. We then used
a signal isolation card, a 2k resistor and a 5k trim pot to control all feeders
together. The net result was that we could set our gates and belt speeds to
yield a specific rate, then ramp plant speed up or down while maintaining the
correct ratios from each feeder. We then added 'percent' meters to each feeder
for reference points and a fault alarm system which utilizes a piezoelectric
horn. A word of caution: Avoid the 200 decibel horn, in a small control room the
little devils are painful!
We hooked this system to the customer's existing 5hp U.S. Vari-drives
after locking each drive's mechanical speed system in the 100% position. As
these units fail in the coming years the company will replace them with much
cheaper TEFC motors and speed reducers.
Total cost for most jobs is under $5500.00. We are currently upgrading
the control circuits and panel to a system featuring manual push-push
potentiometers which should make things a little more user-friendly. This will
add another $4500.00 to the installation price but should contribute
substantially to the ease of operations. Additionally, the new pots will provide
a set-point that will be easy to return to as ongoing adjustments are made to
At this point you are probably questioning the value of such expensive
modifications, considering that once you get your batch plant 'balanced' you
don't rejecting an overt amount of material. This is a valid point. But the true
value of the aforementioned feeder modifications can be easily illustrated: When
your plant is running and you have time between trucks take a sample of feeder
output. Stop your feeders and conveyors under full-production, I.E. Fully
loaded. Take a belt cut sample and run a sieve analysis of it. You will find
that it closely resembles the mix design you are using. Variations can almost
always be traced to adjustments made to compensate for screen 'carry-over' and
attempts to compensate for bins rejecting.
A couple of important questions to ask yourself: How long did it take to
arrive at a perfect 'balanced' condition at the plant? How long will it take to
regain that balance when plant speed is increased/decreased? With the feeder
modifications discussed earlier, and the addition of a belt scale the answer to
both these questions is 'negligible'. These upgrades are a one-time expense. The
time wasted adjusting a plant for efficiency, and the materials rejected in the
meantime are an on-going expense incurred every time changes are made. Over time
this expenditure will far out-strip the cost of the feeder modifications.
The plant discussed in the feeder section had a Ramsey belt scale and a
10-201 Integrator on it prior to the feeder modifications. The plant's owner had
recognized that belt scales address an issue on a batch plant that cannot be
readily addressed any other way: They allow for precise monitoring of the sand
to 1/4"-0" ratio. Since the lion's share of our aggregate pay factor/penalty
comes from this area of our sample results it seems desirable to have as much
control of it as possible. Most batch plants use an 1/8" screen for #1 hot bin.
Since sand readily passes through this size, the screens have virtually no
effect on the ratio of sand in the mix. No amount of adjusting the batching
computer can remedy a problem related to the sand. It can only be addressed at
the feeders, and without precise controls, we're just guessing.
The installation of a set of belt scales is neither excessively expensive
nor difficult. They can be installed on your plant's collecting conveyor--
following the manufacturer's recommendations as to positioning. Or, better yet,
they can be installed on the incline conveyor which feeds the dryer. The incline
is usually more accessible and easier to work on, making it the logical choice.
Once installed, the scales can provide information formerly unavailable
at a batch plant such as actual feed tons-per-hour. To the alert operator they
can also indicate those irritating partial feeder blockages that flow alarms
won't catch. Additionally, they can provide information unavailable through any
other source, such as an 'aggregate used' tonnage at the end of the day. This is
used to track actual material wasted at the plant, which can be an eye opener.
Combined with the feeder modifications previously detailed, belt scales
bring an unprecedented level of controllability to the batch plant. A discussion
of feeder calibration procedures can serve to illustrate this point.
We will use the setup described earlier, installed on a StanSteel RM-80 7000#
batch plant, for this discussion.
The first step is to warm-up our belt scales. Start the scale conveyor
and allow it to run empty for about 30 minutes. While this is going on we need
to figure out the maximum tons per hour we might run. At 45 second batching
cycles a 7000# pug-mill will yield 280 tons per hour. Since we don't want our
feeders to limit plant speed, let's use 350 tons per hour for our maximum rate
Referencing the mix design for the job we find that we need three
aggregates: 5/8"-1/4", 1/4"-0" and sand. The ratios are 24%, 70% and 6%
To convert these %s to tons/hour:
5/8"-1/4": 350tph x .24 = 84tph
1/4"-0": 350tph x .70 = 245tph
sand: 350tph x .06 = 21tph
total = 350tph
Armed with these numbers we are ready to proceed. First, set the feeder's
Master Speed Control (MSC) pot to 100%. Then set each individual feeder's trim
pot to 50%. (This will give us room for adjustment later.)
Block off the dryer inlet and provide a path for the aggregate to escape.
Verify that you have 'zero' on your belt scale integrator. If so, start all the
equipment necessary to transport material from the feeders to the dryer. On
plants with electrical interlocks it will be necessary to defeat the one which
kills the feeders when the dryer stops.
When ready, start the sand feeder. Adjust its gate opening until the belt
scale integrator reads 21 tph or as close as possible to it. Record the reading.
Stop the feeder and allow the belts to clear. Verify that your belt scale
returns to zero, then restart feeder and double check the reading. Repeat this
procedure for each feeder. At 245 tph, the 1/4"-0" will probably require two
feeders. Some advice: This is a good time begin getting into the habit of
recording every detail of feeder settings so that a particular 'set-up' can be
repeated at any time.
Once you are finished with calibrating the feeders, it is time to set
total tons per hour. Since you can't start-up at 350 tph, you need to pick a
rate which suits your plant and the job you're mixing for. The RM-80 used as an
example throughout this discussion rattled along nicely at 180 tph with little
strain, so we used that number as a start-up point. We know that 100% will yield
350 tph, so we need to establish lower reference points to use in our daily
To calibrate total flow: Stop all rock flow and allow belts to clear. Set
the MSC pot to 50%. Start all necessary feeders and record tph displayed on belt
scale integrator. Turn the MSC pot to 75% and record this reading. Seek the
position necessary to yield your chosen start-up tph. When you find it, record
the percent on the MSC. Repeat these procedures to verify 'repeatability'. If
all is well you are ready to enjoy the benefits of your new feeder control
system. More importantly, your ready to build a screen by-pass to increase your
plant's production rate.
For this discussion we will again use the StanSteel RM-80 7000# batch
plant. Other plants may vary in configuration, but the basic idea is the same.
In order to by-pass the screens it is necessary to create a path for the
aggregate to travel from the hot stone elevator directly into hot bin #1. On
this plant we accomplished this by installing an air controlled gate into the
bottom of the feed chute between the hot stone elevator the screens. We
then fabricated a chute that entered 1 bin just below the screens, being careful
to maintain at least 45 degrees of slope. We lined the gate and both chutes with
Scandia steel. Dual air cylinders control the position of the gate which can be
operated either manually or automatically.
We added a contact block to the screen motor 'start' switch to provide
initial power for our solenoid control relay. When the screens are started and
the barrel switch is in the auto position, the relay sends power to a 'gate
closed' indicator light and fires the air solenoid which closes the gate. With
the screens off, the gate remains open. When the switch is in the manual
position the gate closes and a 'manual' indicator light glows alerting the
operator that the 'auto' system is deactivated.
Once the screen by-pass system is in place we will need to modify one mix menu
location in the batching computer so that it pulls all of its aggregate from hot
bin #1. Also, some plants use a screen 'interlock' which must be defeated to run
in a screenless configuration. Additionally, we need some way to control the
oversize aggregates that seem to find their way into the most carefully
controlled stockpiles. For this job a scalping screen over the incline conveyor
at the loading point works well. A 1" screen will keep your aggregate clean,
though a 1.5" may be required for higher production rates. A vibrating 'grizzly'
over the drier inlet also works, and costs considerably less. Remember: You are
not trying to control aggregate size, only remove errant boulders that have
found their way into your feeders.
Assuming that your burner and exhaust air systems have been maximized,
you should now be able to run at production rates previously only dreamed of. A
7000# pug-mill at 30 second batch cycles yields 420 tph. If you are dumping into
a slat conveyor you can shorten the wet-mix batch timing another 5 seconds and
use the conveyor as a mixer. A 7000# pug-mill at 25 second batch cycles yields
504 tph. Both these production rates assume that you are not waiting on the
aggregate to weigh up, and while other factors limit production rates the above
examples illustrate the potential for increased production. In reality, to run
at 30 second or faster batch cycles will probably require additional
modifications such as over-sizing the burner and installing a larger main fan,
since the real question is how much aggregate we can dry per hour. But by
eliminating the screens we have eliminated one 'artificial' bottleneck in our
HMA facility. The question now becomes: How fast do you want to go?
For additional information on this subject
or help with any problems encountered
contact Cliff Mansfield,
7:30am to 9:00pm Pacific Standard Time.
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