Objective:
To Calculate “Wastepaper requirement” and “Pulper Feed Conveyor
speed” for Paper recycling pulping plants.
Inputs
required:
Wastepaper Bale Length (m)
Wastepaper Bale Weight (ton)
Wastepaper Moisture
(%)
Machine Production (tpd)
Reel moisture (%)
Fiber and other mass losses in the process (%)
In order to calculate the feed conveyor wastepaper input, the
required bone-dry pulp needed to make paper, and the losses in the recycling
plant and machine to be calculated.
Formula-1 (Calculating bone-dry Machine production)
Pb = Pa x (1 - m)
Where
Pb - Bone-dry machine production (bdtpd)
Pa – Air-dry machine production (adtpd)
m - moisture in paper at reel (%)
Example:
Consider…
Machine production, Pa = 800 adtpd
Reel moisture, m = 8% = 0.08
Machine Production in bone-dry = Pb = 800 x (1 –
0.08)
Pb = 736 bdtpd
Formula-2 (Calculating Wastepaper requirement)
a) Calculating Wastepaper required bd-tpd
CPb = Pb ÷ (1 - L)
Where
CPb – Wastepaper feed onto the conveyor (bdtpd)
Pb - Bone-dry machine production (bdtpd)
L – Losses (mass) of fiber and other waste materials in process from recycled
paper (%)
Example:
Pb = 736 bdtpd
Assume...
9% mass loss in recycling plant
1 % mass loss in Paper machine
hence total mass loss, L = 10% = 0.1
Therefore
Wastepaper to be loaded onto the conveyor (bone-dry) = CPb =
736 ÷ (1 – 0.1)
CPb = 817.78 bdtpd
Yield of recycling plants – It varies
based on the type of paper/board we make and type of wastepaper used. Each type
of recycled wastepaper contains different non-fibrous foreign materials which
is unwanted for papermaking. Hence, we need to consider those losses while
designing the plant and also calculating the input wastepaper feed rate onto
the pulper feed conveyor.
b) Calculating Wastepaper required ad-tpd
Wastepaper bales are loaded at air-dry condition onto the pulper
which contains moisture.
Let us assume Wastepaper bales contains average moisture of 12%
CPa = CPb ÷ (1 - m)
Wastepaper to be loaded onto the conveyor (air-dry) = CPa = 817.78 ÷ (1 - 0.12)
CPa = 929.30 adtpd
c) Calculating Number of Wastepaper bales required
With the air-dry tons per day requirement, we can calculate number
of bales required as follows
Let us assume average bale weight, BW = 700 kg = 0.7 ad
ton
Number of bales required = Air-dry bale input rate ÷ Average bale
weight
= CPa ÷ BW
Number of bales required per day = 929.3 ÷ 0.7 = 1328 bales per
day
Formula-3 (Calculating Pulper Feed conveyor speed)
Loading of bales onto the conveyor – bales to
be loaded keeping its length direction along the direction of conveyor in order
to pack more on the conveyor.
Sc = (CPa x BL) ÷ (BW x 1440)
Where
Sc – Speed of the conveyor (m/min)
CPa – Wastepaper feed onto the conveyor (adtpd)
BL – Wastepaper bale length (m)
BW – Wastepaper bale weight (ad ton)
Example:
Consider...
BL = 1800 mm = 1.8 m
Bw = 0.7 ad ton
Sc = (929.3 x 1.8) ÷ (0.7 x 1440)
Sc = 1.66 m/min
Conveyor Motor:
a) Motor is
designed for the maximum load and maximum speed; however, it is normally
operated in a range of minimum to maximum.
b) Conveyor
Vendor generally offers the required Gearbox and Motor Power as the components
for calculation involves conveyor design parameters.
Power
(kW) = Torque (Nm) x Speed (rpm)
c) Torque is
important factor in sizing the motor and gearbox. The conveyors are designed
for the full paper load and stop/start from rest position. Torque components
includes Weight of the bales loaded on the conveyor + Weight of the conveyor +
Gravity + Conveyor Frictional factors. Higher the load on the conveyor, higher
the torque required.
Wastepaper Conveying system:
a) Bale loading
conveyor – Bales are loaded onto this conveyor to measure the weight of the
loads automatically using load cells. In some cases, this conveyor is
installed in such a way that the conveyor slat chain top is matched with floor
level in order to push the bales using forklifts from nearby storage
b) Wire
cutter – installed at the end of bale loading conveyor; is
equipped to remove the bale wire and wound as coil automatically. Manual wire
removal involves significant time and hazardous
c) Bale breaker – installed
after bale loading conveyor to break-off the bale to make it shredded and loose
to ease pulping and to avoid whole bale bombarding the pulper rotor while
falling from feed conveyor into pulper
d) Pulper feed conveyor – feeds the pulper
Factors to be considered:
a) Wastepaper
bales are loaded onto the conveyor in such a way that more bales are packed
with available conveyor width; loading direction is chosen to suit the feed conveyor
direction to pulper.
b) Based on
the production grade bale nature varies like for Kraft pulp making, old
corrugated container (OCC) and little mixed waste bales are used; for deinked
pulp, old newsprint (ONP), Office waste etc are generally used. Hence,
depending upon the nature of wastepaper bale, the weight and dimensions of the
bale varies and accordingly the bale parameters to be used to calculate the wastepaper requirement and conveyor speed.
c) Consistent
bale loading on the conveyor gives homogeneous consistency in the pulper output
and also easy to control the consistency. In some plants, conveyors are
operated with VFD (variable frequency drives) which is looped with pulper
outlet stock flow consistency control. This is to control the stock consistency
by adjusting the pulper conveyor speed.
