I was
speaking with a colleague last month that told me a customer of his, a large
bagged mulch producer, purchased a large star screen this year and started
screening his mulch to size rather than grinding it all. The customer said
basically, screening is the best thing since sliced bread and that the fuel, grinding
and hauling costs he is saving is more than he imagined. He said to refer
anyone to him that is contemplating screening rather than grinding.
Hello?! What have I been saying for five plus years now? I
even coined the phrase, “Screen more, Grind Less.” I sell grinders, but I can’t
talk enough to sell the difference in operational cost now that fuel is $4 a
gallon as a norm.
So what does screening really save you? Here are a few big
cost savers:
·
Fuel- Obviously the biggest cost difference. 2
gallons an hour for a screener compared to 50 gallons an hour in some grinders.
Save $150-$200 an hour? Who doesn’t want that?
·
Wear- Small screens in a grinder equals more
wear and more wear parts
·
Production- Depending on what you are screening and
grinding and what with, you could produce more yards an hour with a screen
·
Since you are grinding less, you have more grinding
time available for projects and contract jobs. With less grinding needs, that
means less overtime for the operator AND your mechanic.
So, what screen do you need and what do you need to do?
Star Screen- Best
Option. Today’s star screens are not like the star screens of old. Try one. Sizing
is determined by the size of the stars, the spacing between the stars and the
speed of the star’s rotation. With an electric motor drive, the range of
product size is incredible. A standard deck could potentially screen from ¼” to
1 ¼” and any size in between. You would need about 8 trommel drums to produce
the same range of product sizes. Hydraulic drive machines still offer
adjustability, but nowhere near the electric drive units. Being able to adjust
at basically 1/8” increments, you can maximize production, maximize removal of
finished product size materials from feedstocks and produce a perfect quality
end product.
Trommel- Good
Option. Everyone has a trommel or you can rent one tomorrow. Screening with a
trommel works great for wood, compost, mulch and dry materials. The one issue
is that you will get some spearing of oversize materials with a trommel drum.
How do you avoid this and produce a quality product?
·
Use a trommel that has a level, horizontal drum
with spiral flighting inside to roll the materials over the hole in the screen
rather than dropping it through like a trommel drum that is on a decline from
front to back.
·
Use a smaller screen, at the end of the trommel
drum, than is at the beginning and middle of the trommel drum. Only 50% or less
of the material being screened makes it to the back of the drum, therefor
creating more opportunities for larger size material to make it through the
hole. By downsizing the hole in the screen at the end, a smaller sized material
will be produced.
·
Increase the trommel hopper speed so that some
of the finish size material does make it into the overs. This will help reduce
the amount of oversize product in your finished materials.
·
Increase the trommel drum speed so that some of
the finish size material does make it into the overs. This will help reduce the
amount of oversize product in your finished materials.
Flat Deck Screens-
If you have one and nothing else, it’s worth a try. These are the most
difficult machines to use with lightweight materials.
·
Bigger is better- The longer the deck and the
wider the deck, the better.
·
Use a Larger Hole- Because of the angle of the
deck, you can use a larger hole size than the actual product size you would
like.
·
Adjust the throw- Depending on whether you would
like the material to roll across the hole or bounce more and drop through the
hole, adjust the screen box vibration for more or less throw.
·
Feed Speed- Just like the trommels, increase the
hopper speed to put more material on the screen deck and limit the spearing or
decrease speed to allow more material to drop through the hole.
This should give you a good start at trying screening over
grinding no matter what screen you have.
Here are a few points to consider or issues you will
discover:
1.
Obviously, different materials will give you
different size materials with the same screen on a grinder. On a recent demonstration,
once ground logs, brush and landclearing debris produced 50% to 66% of finished
mulch size material when screened through a star screen. But the once ground
pallets only produced about 33% finished product size material. Consider using
a different size screen for your grinder on the initial grind with the
materials you are grinding. This will maximize your screening production.
2.
Since you
are screening after the initial grind and not regrinding, you are naturally
going to have less fines in your product
3.
For those that are currently just dropping out
the fines from their mulch with a screen, consider using a double deck screen.
Then you could grind with a larger screen and scalp a small percentage of overs
while you are dropping out the fines. This will reduce fuel and grinder wear,
produce fewer fines, reduce product loss and decrease the bulk density which is
great in the bag and on the truck.
4.
Depending on the size of your grinder and screener,
you may be able to grind right into a screen because you are only screening out
50% or so of the material.
Finally, before you say you don’t
have the money for a screener let’s do the math:
·
Regrinding mulch uses 100% load on any engine.
If it doesn’t blow up, it will use max fuel. A 12 cylinder grinder will be in
the 50gph range. An electric generator screen will use about 2gph.
·
Since these are max ratings and 48 gph is the
difference, we will use only half, 24 gph, and we will consider equal
production for this exercise. These are just rough numbers but easy for you to
sub in your own true costs, estimates, etc.
·
24 gallons per hour at $4 per gallon is $96 per
hour in savings. Do you regrind 1000 hrs. a year? That’s $96,000 a year. If you
only reground 500 hrs. it’s still $50,000 and we cut our fuel savings in half
for this example. If we used the full 48 gallons that would be $200,000 a year.
This doesn’t include any wear parts or other operational costs like the $300 a
week in air filters.
Use your numbers and calculate
what kind of savings you can assume.
So, you already have the money, but instead of paying a
Lender, you are paying your Fuel Man, Parts Man, Service Man, Operator………….
Questions?
Dave Whitelaw- Grinder Guy
grinderguy@askthegrinderguy.com
