Conveyors: fluid current – With load braking or retarding means
Reexamination Certificate
2001-12-20
2002-11-19
Ellis, Christopher P. (Department: 3619)
Conveyors: fluid current
With load braking or retarding means
Reexamination Certificate
active
06481935
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an apparatus and method for establishing the proper gas flow to efficiently and effectively unload materials from a storage location using a pressurized or vacuum conveying system, respectively. The present invention is ideally suited to set a proper gas flow in conveying systems used to unload friable, abrasive, or degradable materials from tank trucks.
BACKGROUND OF THE INVENTION
Plastic pellets are commonly transported from the facility at which the pellets are manufactured to bulk plastic consumers using a tank truck. Referring to
FIG. 1
, a common transport system will include a tank truck T, or bulk truck, which has conical hoppers H used to store the plastic pellets (not shown). To unload the truck T, a gas stream, usually air, is directed through a pipe, called a convey line C, below the conical hoppers H. The pellets are introduced into the gas stream by pressurizing the tank trailer TT and opening a hopper valve HV that separates each hopper H from the convey line C.
Still referring to
FIG. 1
, a positive displacement blower B, which is usually located on the tractor portion of the truck, drives the airflow in the convey line C. The blower B generates an air velocity proportional to the speed of the truck engine (not shown). The air velocity at the product pick-up point should only be great enough to entrain the solids dependably so that minimum damage is done to the solids and the unloading rate is maximized for the allowable convey line pressure.
The unloading process, however, is complicated by the fact that bulk trucks vary in performance and design, even among trucks made by the same manufacturer. For example, and as known, the tractor portion of the truck is assembled from many components manufactured by different companies, such as the transmission, the blower, and the filters. The bulk trailers, likewise, may have different pipe sizes, piping arrangements, valves, filters, and, optionally, coolers. Thus, bulk trucks lack a standard arrangement in the industry and may thus have any of a number of blower models and gearing ratios between the engine of the truck and the blower.
Further complicating the process, each different type of plastic pellet transported by the trucks has a unique entrainment velocity as well as an optimum conveying velocity for both acceptable convey rate and product degradation. The speed of the positive displacement blower, which is established by the speed of the truck engine through gearing with a predetermined power take-off ratio, determines the amount of air moved and, therefore, the gas velocity for a given pipe size.
In addition to blower speed, the velocity of the airflow is also a function of the air pressure within the pipe. System pressure results from the resistance of the system to the flow of the gas and the entrained product. That is, resistance to the flow of air and the entrained product creates a pressure differential between the two ends of the convey line. As such, the more plastic pellets that flow through the pipe at one time, the higher the resistance and thus the higher the pressure required to maintain the airflow therethrough. However, the pressure in the system compresses the air, and the resulting reduction in gas volume tends to reduce gas velocity. Accordingly, at a constant airflow, a higher pressure results in compression of the air, causing the air velocity to be lower.
The actual convey rate of the plastic pellets is determined by the allocation of the pressure resource shared between the density of the pellets flowing in the pipe and the velocity of the pellets. Thus, the operating pressure must be known or assumed before it can be determined what flow of gas will produce the desired velocity. The prior art systems do not adequately address these engineering considerations to unload bulk solids from tank trucks as efficiently as possible with the least possible damage to the material.
In the prior art systems of unloading bulk trucks, the driver first selects an operating pressure based on an acceptable temperature and the system pressure. The driver then selects a blower speed by setting the engine speed to move the amount of air that will produce the optimum velocity for the product to be handled at the desired operating pressure. To assist the operator, tables are available to select the desired pressures and velocities for the system. For example, Table 1 lists some appropriate conveying velocities and pressures:
TABLE 1
PICK-UP
PSI
PSI
PRODUCT
TACKY
VELOCITY
WITHOUT
WITH
NAME
TEMP (F)
(FPM)
COOLER
COOLER
PET (Solid State)
265
3600
11
12
PETG
180
3500
6
12
(Glycol Modified
PET)
PET (CHDM
255
3400
10
12
Modified)
PE (LDPE)
210
4000
8
12
PE (HDPE)
255
3700
10
12
After the operator selects the proper velocity and unloading pressure from the appropriate table for the product to be moved, he or she then determines the proper engine speed based on the blower frame and the power take-off from another table. For example, Tables 2 through 5 below illustrate engine settings for popular blowers driven by three different power take-off ratios to produce three different velocities at specific operating pressures for a few plastic materials:
TABLE 2
POWER
ENGINE SPEED (IN RPM) FOR
TAKE-OFF
(3500 FPM) AT 6-PSI
RATIO
IN 4″ UNLOADING LINE
1.23
934
764
556
1747
918
593
1.41
815
666
485
1524
801
517
1.6
713
587
428
1343
706
456
GD-
GD-
GD-
DRUM-
DRUM-
DRUM-
L9
L12
L13
607
807
907
BLOWER SIZE AND MANUFACTURER
TABLE 3
POWER
ENGINE SPEED (IN RPM) FOR
TAKE-OFF
(3700 FPM) AT 10-PSI
RATIO
IN 4″ UNLOADING LINE
1.23
1281
981
803
2103
1229
884
1.41
1053
855
699
1833
1071
770
1.6
927
752
651
1615
943
677
GD-
GD-
GD-
DRUM-
DRUM-
DRUM-
L9
L12
L13
607
807
907
BLOWER SIZE AND MANUFACTURER
TABLE 4
POWER
ENGINE SPEED (IN RPM) FOR
TAKE-OFF
(3600 FPM) AT 12-PSI
RATIO
IN 4″ UNLOADING LINE
1.23
1291
991
813
2113
1239
894
1.41
1063
865
709
1843
1081
780
1.6
937
762
625
1625
953
687
GD-
GD-
GD-
DRUM-
DRUM-
DRUM-
L9
L12
L13
607
807
907
BLOWER SIZE AND MANUFACTURER
TABLE 5
POWER
ENGINE SPEED (IN RPM) FOR
TAKE-OFF
(3400 FPM) AT 8-PSI
RATIO
IN 4″ UNLOADING LINE
1.23
1056
878
630
1804
975
650
1.41
921
766
549
1574
851
567
1.6
812
675
484
1387
750
500
GD-
GD-
GD-
DRUM-
DRUM-
DRUM-
L9
L12
L13
607
807
907
BLOWER SIZE AND MANUFACTURER
After considering the applicable charts and starting the blower at the tabulated speed, the operator then adjusts the trailer valves to develop the desired operating pressure.
As one skilled in the art will appreciate, the listed tables are illustrative, and many more tables would be required to include all possible combinations and factors relevant in determining the proper unloading airflow velocity. For example, additional charts would need to address filter pressure drop and unloading pressure for each truck, in addition to more tables showing other combinations of unloading line power take-off ratios for different blower sizes and manufacturers.
In actual practice, however, the operators do not always wade through the numerous charts to tabulate the appropriate speed. Instead, engine speed is often set at the discretion of the truck driver at a value that he thinks will unload the truck at the highest rate. The driver seldom knows what blower model or power take-off ratio is installed on the truck, and he often lacks any specific training or any written procedures for loading and unloading the products. Further exacerbating the situation, it may be counterintuitive to some drivers that using a lower blower speed can actually result in unloading a bulk truck quicker than a higher blower speed. The unfortunate result of using a less efficient higher blower speed to convey plastic pellets is the generation of more fines and streamers when unloading the pellets. Another consideration is that compression of the air also increases the air temperature, which may result in product damage unless the maximum pressure is limited or a gas cooler is added.
The result of the prior art practice frequently is excessi
Dailey Ronald K.
Dibble Merton L.
Stanley Robert R.
Dillon, Jr. Joe
Eastman Chemical Company
Ellis Christopher P.
Graves, Jr. Esq. Bernard J.
Wood, Esq. Jonathan D.
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