Conveyors: fluid current – With means to control conveying fluid or movement of load in... – Control of load input or output
Reexamination Certificate
1998-05-21
2001-01-23
Ellis, Christopher P. (Department: 3651)
Conveyors: fluid current
With means to control conveying fluid or movement of load in...
Control of load input or output
C406S014000, C406S039000, C406S093000, C406S176000, C406S180000, C406S185000, C406S194000, C406S195000, C406S046000
Reexamination Certificate
active
06176647
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an instrument for measuring the mass flow rate of powder conveyed together with a gas, a constant powder feeder, which is easily manufactured, operated, maintained, and its operational efficiency is highly enhanced by applying the said instrument, and a high-performance electrostatic powder coating apparatus in which the constant powder feeder is utilized.
2. Background Art
Conventionally, constant powder feed systems for pneumatically conveyed powder as shown in FIG.
4
and
FIG. 7
of the accompanying drawings are known as being used for accurately feeding an expensive powder to each of several to several tens of apparatuses, with a relatively small amount of constant flow rate, e.g., several tens to several hundreds of grams per minute, as in the case of powder feed to powder coating apparatuses, thermal spraying apparatuses and the like.
FIG. 4
shows one example of a so-called volumetric type constant powder feeder, wherein powder
102
in a hopper
101
is fluidized by means of compressed air
104
through a porous plate
103
and fed by a screw feeder
106
provided at the bottom of the hopper
101
to an injector
113
where the powder material is fed to a powder gun by means of the injector
113
.
At this time, the feeder is able to feed powder with a volumetric flow rate (Fv) represented by the following equation:
Volumetric flow rate
Fv=AR
(cc/min.)
The equation is based on a constant which is determined by the volume of a screw, that is, [an effective volume per one pitch of the screw]=A (cc), and [the motor
108
rotation speed]=R/min. However, the feeding rate essentially required in the powder coating process is not a volumetric flow rate (cc/min.) but a mass flow rate (g/min.). Therefore, it is common to presume the mass flow rate by reflecting the actual bulk density S of the powder in the inside
107
of the screw feeder
106
(hereinafter referred to as feeding bulk density) using the following equation:
Mass flow rate
FM=SAR
(g/min.)
Then, the mass flow rate is presumed by graduating the feeding rate indicator
110
on the motor rotation speed.
For the volumetric type constant powder feeder, it is an important point to avoid the influence of the powder level in the powder hopper upon the feeding bulk density S. In the embodiment shown in
FIG. 4
, a powder level controlling device
105
is provided for this purpose. Besides the above-described one, there are some other methods used for the purpose of insulating the influence of the powder level upon the feeding bulk density, but most of them adopt a method for fluidizing powder in a hopper. This situation is the same with respect to volumetric type constant powder feeders which utilize a table feeder, an eccentric pump, a roll with grooves and the like as a feeding means.
The loosened bulk density of a fresh powder that largely affect on the feeding bulk density S varies in the wide range of 0.4 to 0.75 g/cc depending on the kind of the powder. It sometimes varies by 3 to 5% by the production lot even in the same kind of powders. The feeding bulk density S is also affected by 5 to 15% depending on the fluidizing air flow rate. The reason for this is, besides the overall problem of the powder hopper, an air flow resistance that causes fluctuation by several percentage depending on the location of the porous plate
103
in case a plurality of screws are provided to one powder hopper. Since this situation directly affects the feeding bulk density, it necessiates checking the feeding bulk density for each screw, or in other words, an adjustment of the rotation speed for each screw in relation to the rotation speed and the on-site measuring of the feeding rate, as a constant feeding apparatus which is required to have the flow rate accuracy of ±2.5%.
In addition, the bulk densities of the fresh powder
111
and recovered powder
112
sometimes varies in the range of 0 to 25%. This situation directly affects on the feeding bulk density when the powder is recycled as common in many powder coating lines. Moreover, since the effect of the mixing ratio of the fresh powder and recovered powder is revealed through the transfer efficiency of the coating line, it is very difficult to maintain the accuracy of the feeding bulk density, or the accuracy of the feeder within ±2.5% especially in a multi-purpose coating line in which a variety of objects to be coated are handled, wherein the transfer efficiency thereof tends to vary.
The mechanism and problems of the volumetric type constant powder feeder are summarized in FIG.
5
.
FIG. 6
shows the relation numerically. When the screw rotation speed is set to be constant, the relation between the volumetric flow rate and mass flow rate varies in a wide range of the area indicated between the lines
115
and
116
. When taking into consideration of the influence of the state of fluidization and transfer efficiency of the coating line, probably the values vary in a wider range of the area indicated between the lines
118
and
117
. For the purpose of determining the feeding bulk density that prescribes the volumetric flow rate, i.e., the relation between the rotation speed of the screw and the actually required mass flow rate, there is nothing for it but to regulate the actual flow rate of the practically used powder for each feeder, or for each screw in case of a multiple feeder, at a working site.
While a long period of working time and proficiency are required for this regulating work, it is not easy to always maintain a feeding accuracy of about ±2.5%. Thus, it is the first crucial problem common in volumetric type constant powder feeders. The volumetric type constant powder feeder, which can not be operated without regulating each apparatus individually depending on the practical working conditions on-site, can be hardly recognized as an industrial measuring and controlling equipment, therefore the creation of a new system that enables a direct detection and control of the mass flow rate of powder has been waited.
The screw feeder
106
as an apparatus having a volumetric type constant powder feeding function (hereinafter referred to as a volumetric type constant powder feeder) shown in
FIG. 4
has such as complicated structure that the screw feeder requires dismounting and disassembling for having a cleaning at the time of color change of the powder together with the hopper. This requires a long period of working time and many hands. When a prompt color change is required, spare machines have to be prepared, which calls for a large amount of investment of equipment. The said difficulty in dealing with color change is the second problem of a volumetric type constant powder feeder. The same is true with respect to other systems which utilize a volumetric type constant powder feeder other than a screw feeder.
The third problem of the volumetric type constant powder feeder is that the screw feeder shown in FIG.
4
and other volumetric type constant powder feeding apparatuses are always large in size, heavy in weight, and expensive without exception. Furthermore, the space between neighboring volumetric type constant powder feeding apparatuses becomes so large that hoppers are required to be strongly constructed in a square shape with a high accuracy. As hoppers tend to be expensive, heavy and large-sized, they are inconvenient to carry, thereby hinder the operations such as color change at working sites. It is also practically impossible to streamline the coating process inexpensively by adding the above-mentioned volumetric type constant powder feeder to the hoppers of already installed powder coating facilities.
FIG. 7
shows another example of the conventional art other than the above-mentioned volumetric type constant powder feeder. A desired powder feeding rate is obtained by the steps comprising: flowing a measuring gas
114
at a constant velocity into a measuring tube
120
by means of a nozzle
121
; introduc
Ellis Christopher P.
Price Heneveld Cooper DeWitt & Litton
Rid Corporation
Shapiro Jeffrey A.
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