Conveyors: fluid current – Processes
Reexamination Certificate
2000-02-10
2002-02-26
Ellis, Christopher P. (Department: 3651)
Conveyors: fluid current
Processes
Reexamination Certificate
active
06350086
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an apparatus and method for establishing the proper gas flow to unload efficiently and effectively materials from a storage location using a pressure conveying or vacuum conveying system. The present invention is ideally suited to set proper gas flow in conveying systems used to unload friable, abrasive, or degradable materials from tank trucks.
BACKGROUND ART
Plastic pellets are commonly transported from the facility at which the plastics are manufactured to bulk plastic consumers using a tank truck. Referring to
FIG. 1
, a common transport system is a tank truck T, or bulk truck, which has conical hoppers H to store the plastic pellets (not shown). To unload the trucks 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 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 one manufacturer. The tractor portion of the truck is assembled from many components manufactured by different companies, such as the transmission, blower, and filters. The bulk trailers, likewise, may have different pipe sizes, piping arrangements, valves, filters, and, optionally, coolers. Thus, bulk trucks lack any standard arrangement in the industry and may 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 and has 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 pressure in the pipe. System pressure results from the resistance of the system to the flow of gas and the entrained product. That is, resistance to the flow of air and product creates a pressure differential between the two ends of the convey line. As such, the more pellets that flow through the pipe at one time, the higher the resistance and thus the higher pressure required to maintain the flow. 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 pellets is determined by the allocation of the pressure resource shared between the density of the pellets flowing in the pipe and their velocity. 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 system 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
TACKY
VELOCITY
WITHOUT
PSI WITH
PRODUCT NAME
TEMP. (F.)
(FPM)
COOLER
COOLER
PET (Solid State)
265
3600
11
12
PETG (Glycol Modified PET)
180
3500
6
12
PET (CHDM Modified)
255
3400
10
12
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 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
TAKE-OFF
ENGINE SPEED (IN RPM) FOR (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-L9
GD-L12
GD-L13
DRUM-607
DRUM-807
DRUM-907
BLOWER SIZE AND MANUFACTURER
TABLE 3
POWER
TAKE-OFF
ENGINE SPEED (IN RPM) FOR (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-L9
GD-L12
GD-L13
DRUM-607
DRUM-807
DRUM-907
BLOWER SIZE AND MANUFACTURER
TABLE 4
POWER
TAKE-OFF
ENGINE SPEED (IN RPM) FOR (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-L9
GD-L12
GD-L13
DRUM-607
DRUM-807
DRUM-907
BLOWER SIZE AND MANUFACTURER
TABLE 5
POWER
TAKE-OFF
ENGINE SPEED (IN RPM) FOR (3500 FPM) AT 6-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-L9
GD-L12
GD-L13
DRUM-607
DRUM-807
DRUM-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 lines 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 excessive product damage—the most common complaint from bulk plastics customers—and less than optimum unloading rates. Accordingly, there is a need in the art to allow operators to determine easily and accurately the correct
Dailey Ronald K.
Dibble Merton L.
Stanley Robert R.
Dillon, Jr. Joe
Eastman Chemical Company
Graves, Jr. Esq. Bernard J.
Wood, Esq. Jonathan D.
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