System and method of pumping a constant volume of powder

An apparatus and method for pumping a constant volume of powder to a powder spray gun includes a powder pump which meters the powder into a stream of air so that a constant volume of the air entrained powder is distributed to the spray gun. The invention also discloses a pressure equalizing system that prevents powder leakage across a powder metering system that meters powder from a reservoir into an outlet tube carrying the constant velocity stream of air. A control system is disclosed which adjusts the rate at which powder is metered into the outlet tube so that the mass flow of powder exiting the outlet tube remains constant.

RELATED APPLICATIONS 
This application relates to Assignees' co-pending U.S. application Ser. No. 
08/336,469 filed Nov. 9, 1994 and entitled POWDER COATING GUN MOUNTED 
DIFFUSER AND AIR COOLED HEAT SINK IN COMBINATION WITH LOW FLOW POWDER PUMP 
IMPROVEMENTS 
FIELD OF THE INVENTION 
This invention relates to the field of powder pumps for pumping powder to 
powder spray guns. More particularly, this invention relates to an 
apparatus and method for pumping a constant volume of powder at a constant 
velocity to a powder spray gun. 
BACKGROUND OF THE INVENTION 
In powder coating systems, a jet pump or ejector is conventionally used to 
aspirate powder from a powder container or hopper and to transfer the 
powder through an outlet conduit to a spray device, i.e., a powder spray 
gun of the type disclosed in U.S. Pat. No. 5,056,720 ('720), assigned to 
Nordson Corp., the assignee of the present invention, which patent is 
incorporated in its entirety herein. The ability of a pump or ejector to 
control the flow rate of the powder is very important in order: a) to 
deliver the powder smoothly to the spray gun without surging or pulsing 
effects; b) to control the velocity at which the powder exits the spray 
gun; c) to insure that the air entrained powder is well dispersed in the 
air stream when it enters the charging or pattern forming structure in the 
spray gun; d) to minimize wear of the structural components of the gun; 
and e) to minimize impact fusion of the powder with the structural 
components of the spray gun. At present, powder pumping equipment attempts 
to accomplish these operating requirements incorporated varying tradeoffs 
and met with varying degrees of success. 
A conventional system for pumping air entrained powder from a container to 
a spray gun is illustrated in FIGS. 5 and 6, and primarily discussed 
beginning on column 6, line 47 to column 9, line 54, of U.S. Pat. No. 
4,987,001 ('001), assigned to Nordson Corp., the assignee of the present 
invention, which patent is incorporated in its entirety herein. A primary 
flow of air directed into pump 114 through an injector nozzle forms an air 
jet which creates a suction at a powder inlet. The suction at the powder 
inlet draws fluidized powder from a powder container 100 into pump 114 
where it mixes with the air jet. The resulting air entrained powder is 
propelled through a venturi throat of a pipe 116 to a spray gun. Varying 
the air flow through the injector nozzle 12 controls the suction and the 
volume of powder delivered to the spray gun. The air entrained powder can 
then be directed through an air amplifier 117 which injects a secondary 
flow of air to increase and precisely control the velocity of the air 
entrained powder flowing through outlet pipe 116. After the air entrained 
powder travels for a distance (usually about 4 to 12 meters) through 
outlet pipe 116 to the powder gun, the powder can separate from the air 
stream for various reasons, such as the inertial separation effects of the 
bends in the conduit. To obtain a uniform spray pattern and achieve high 
electrostatic charging levels, the powder must be redispersed in the air 
stream before charging or pattern forming occurs. This redispersion can be 
accomplished by either adding additional air to promote strong turbulence 
and mixing, or mechanically inducing turbulence. 
In some applications, such as when the powder stream is subdivided and 
distributed through multiple tubes of a triboelectric charging gun of the 
type described and illustrated in U.S. Pat. No. 4,399,945, assigned to 
Nordson Corp., the assignee of the present application which patent is 
hereby incorporated by reference in its entirety, the powder must be 
thoroughly redispersed in the air to insure that the powder is evenly 
distributed in the flow passage at the point of subdivision. Good results 
have been obtained with either air jet diffusers, as presently used in a 
Tribomatic II.RTM. gun manufactured by Nordson Corporation of Amherst, 
Ohio and described in U.S. application Ser. No. 07/956,615, filed Oct. 5, 
1992, which is also hereby incorporated by reference in its entirety, or 
with porous diffusers as shown in the '001 patent. 
A number of serious shortcomings result when a powder spray gun with an air 
jet diffuser is operated in conjunction with a pump having a secondary air 
flow injected at a downstream location, as previously discussed. First, 
there is an excess in both the volume and velocity of the air entrained 
powder being sprayed from the gun which lowers the overall coating 
efficiency and increases the overspray being generated and the amount of 
recycled powder introduced into the system. Second, the addition of a 
diffuser at the pump increases the control devices to three, i.e., two 
sets of controls, one for each of the primary and secondary air flows at 
the pump, and a third set of controls for the air flow through the 
diffuser mounted to the gun. Besides the extra cost associated with the 
additional set of controls, the adjustment of the three sets of controls 
to obtain the optimum settings is difficult and time consuming, especially 
for an inexperienced operator. Third, certain types of pattern forming 
elements and some powder charging schemes are not practical without a high 
degree of powder dispersion at the gun. 
Therefore, there is a need for a practical and easy to use system for 
pumping air entrained powder at a low flow rate to an electrostatic gun so 
that the powder formulation can be effectively sprayed to apply a uniform 
coating on a workpiece. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an apparatus and method 
for pumping a constant volume of powder to a powder spray gun to obviate 
the problems and limitations of the prior art systems. 
Yet another object of the present invention is to provide a powder pump and 
method of operation which meters powder into a constant velocity stream of 
air so that a constant volume of air entrained powder is delivered to the 
powder spray gun. 
Still another object of the present invention is to provide an apparatus 
and method for metering powder into a constant velocity stream of air so 
that the resulting air entrained powder is conveyed at a constant velocity 
to a powder spray gun. 
Yet another object of the present invention is to provide an apparatus and 
method for equalizing the pressure across a powder metering system which 
meters powder from a reservoir into a constant velocity stream of air 
flowing through a conduit tube so that a constant volume of air entrained 
powder exits the conduit tube for delivery to a spray gun. 
Another object of the present invention is to provide an apparatus and 
method for metering powder from a reservoir into the outlet tube carrying 
a constant velocity stream of air which includes a system for equalizing 
the pressure between the reservoir and the outlet tube and air isolation 
device in the reservoir to prevent air leakage out of the inlet of the 
reservoir. 
According to the invention, a constant volume conveying method and system 
for pumping a constant volume of air entrained powder to a spray gun 
includes a cylindrical outlet tube having an inlet receiving a constant 
velocity of pressurized air at one end, an outlet discharging a constant 
volume of the air entrained powder at an opposite end, and a powder inlet 
connected to a reservoir of powder disposed between the inlet and the 
outlet. The reservoir includes an inlet section and a funnel shaped 
section secured to and disposed directly below the inlet section. A powder 
pickup tube has an inlet section at one end inserted into a container of 
air entrained powder and an outlet opening at the opposite end for 
discharging the air entrained powder from the container into the inlet 
section of the reservoir. A powder metering system secured to the outlet 
of the reservoir includes a multi-vane, helical rotary lock disposed below 
the funnel shaped section for metering the air entrained powder through 
the outlet tube's powder inlet and into the constant velocity air stream 
flowing through the cylindrical outlet tube to provide a flow of air 
entrained powder at a constant volume and constant velocity to the spray 
gun. 
Further in accordance with the invention, a constant volume conveying 
method and system includes an expansion nozzle for directing a constant 
volume of air into a cylindrical outlet tube. The expansion nozzle is 
connected at one end to a source of compressed air and at the opposite end 
to the cylindrical outlet tube. The powder inlet in the cylindrical outlet 
tube receives coating powder from the reservoir which in turn is 
constantly being filled with coating powder from a container of coating 
powder. The powder from the reservoir is then metered by the metering 
system into the cylindrical outlet tube. 
According to the invention, the constant volume conveying method and system 
includes a pressure equalizing system for equalizing the pressure between 
the reservoir and the cylindrical outlet tube to balance the pressure and 
prevent powder leakage from the outlet tube and into the reservoir. The 
pressure equalizing system includes a tube interconnecting the cylindrical 
outlet tube and the funnel shaped section of the reservoir so that the 
pressure in the funnel shaped section is substantially equal to the 
pressure in the outlet tube. The pressure equalizing system further 
includes an isolation device mounted within the reservoir to substantially 
reduce any air leakage from the funnel shaped section and out of the inlet 
section of the reservoir. In a first embodiment, the isolation device 
includes an impeller with vanes that seal against walls of the inlet 
section of the reservoir. In a second embodiment, the isolation device 
includes two spaced slide gates which reciprocate to open and close the 
powder flow path through the reservoir. In a third embodiment, the 
isolation device includes two spaced ball valves which rotate to open and 
close the powder flow path through the reservoir. In a fourth embodiment, 
the isolation device includes two valve plates with out of phase holes 
which rotate to open and close the powder flow path through the reservoir. 
The invention also includes a controller method and system which operates 
the powder metering system in response to a sensor in the reservoir that 
measures the density of the powder in the funnel section and adjusts the 
amount of powder metered into the conduit tube to feed a constant volume 
of powder into the cylindrical outlet tube. The controller system also 
includes a capacitor sensor in the conduit tube to measure changes in the 
density of the powder flow therethrough and to adjust the powder metering 
system so that a substantially constant mass of powder flows through the 
conduit tube.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to FIG. 1, there is illustrated a schematic of a constant 
volume conveying system 10 for conveying air entrained coating powder to a 
powder spray gun 12. The system 10 includes a fluidized bed powder 
container or hopper 14 which is constructed in accordance with the 
principles set forth in Assignees' co-pending U.S. application Ser. No. 
08/336,469 and described in detail below. Fluidized powder is transferred 
from container 14 into a reservoir 16 with a powder metering system 18 at 
the outlet for metering powder into the powder inlet 25 of a cylindrical 
outlet tube 20 of constant volume conveying system 10. The powder mixes 
with a stream of substantially constant velocity air flowing through 
outlet tube 20 and is discharged as an air entrained powder from an outlet 
23 into a delivery conduit 22, typically a flexible hose, which in turn 
transfers the powder to powder spray gun 12, such as a tribocharging or 
corona charging gun. The stream of air entrained powder from system 10 is 
electrostatically charged in the body of the gun and then sprayed from the 
spray nozzle section (not shown) of gun 12 towards an article being 
coated. While a tribocharging gun is described herein, a conventional 
corona-discharge electrode arrangement can be substituted. 
A principle feature of the invention relates to the ability of constant 
volume conveying system 10, as illustrated in FIG. 1, to provide a soft, 
controllable, powder spray pattern. In this embodiment, the flow of 
compressed air from a supply (not shown) is transferred through an inlet 
orifice 24 to create an air jet in an expansion nozzle 26 which flows from 
expansion nozzle outlet 27 and into outlet tube 20. Expansion nozzle 26 
has an approximate included angle "b" of less than about 19 degrees and 
preferably about 17 degrees. The included angle "b" is selected to prevent 
flow separation and turbulence in the air entrained powder flow through 
cylindrical outlet tube 20. That is, the air jet is expanded in expansion 
nozzle 26 to provide a smooth, conveying air flow through outlet tube 20 
into which a very "rich" powder/air mixture (high ratio of powder to air) 
is metered by powder metering system 18. The rich powder/air mixture only 
requires a small volume of conveying air flow to thoroughly mix the 
powder/air with the constant velocity air flow from expansion nozzle 26 so 
that a substantially constant volume of air entrained powder flows through 
delivery conduit 22 at a substantially constant velocity. 
In one operational constant volume conveying system, the conveying air 
flows through an inlet orifice 24 having a diameter of about 0.060 inches. 
For an outlet tube 20 having a 0.50 inch inside diameter, an air volume of 
about 2.0 cubic feet per minute (cfm) causes the air entrained powder 
metered onto outlet tube 20 to have a velocity which is close to optimum 
velocity, i.e., about 1500 feet per minute. To reach this velocity, about 
50 psi is needed on the upstream side of orifice 24. Since the 
backpressure in outlet tube 20 is typically less than 1 psi, the pressure 
drop across orifice 24 is the primary factor in controlling the flow rate 
of the air through outlet tube 20. Thus, a constant pressure drop across 
orifice 24 creates a flow of air entrained powder with a substantially 
constant volume and velocity irrespective of the volume of powder in the 
air flow or the length of tube 20. 
Two important considerations in the operation of the constant volume 
conveying system 10 are: a) selecting the air flow across inlet orifice 24 
so that the pressure is just enough to overcome the back pressure in tube 
20; and b) injecting fluidized coating powder into the constant velocity 
of air flowing through tube 20 with the minimum accompanying volume of air 
to maintain the volume of air entrained powder flowing through tube 20 as 
nearly constant as possible. 
FIGS. 1-3 illustrate an effective technique and structure by which powder 
can be injected into the air stream flowing through outlet tube 20 of 
constant volume system 10. A reservoir 16 of air entrained powder is 
disposed directly above outlet tube 20. Reservoir 16 has an inlet section 
29 at the top end for receiving air entrained powder and a powder metering 
system 18 at the bottom for dispensing the air entrained powder into the 
stream of air flowing through tube 20. Powder metering system 18 includes 
a multi-vane, helical rotary lock 30, as shown in FIG. 3. The powder is 
metered into the stream of constant velocity air flowing through 
cylindrical outlet tube 20 by the rotation of rotary lock 30. A connection 
shaft 32, which is part of a drive system 34, is secured at one end to the 
impeller of rotary lock 30 and at the other end to a power source, such as 
a stepper motor 36. The volume of powder metered into outlet tube 20 is 
controlled by the rotational speed of rotary lock 30. 
Being that the volume of powder metered by the rotary lock 30 is a function 
of its bulk density, it is advantageous that lock 30 be supplied by a 
source of powder with a constant head height to keep the degree of powder 
packing relatively constant. A constant head height can be achieved by 
placing lock 30 directly below an outlet opening 38 of a funnel shaped 
well section 40, as shown in FIG. 3. The well section 40 is filled with 
air entrained powder which pours in under the influence of gravity from 
inlet section 29 of reservoir 16. Inlet section 29, in turn, is filled 
with air entrained powder delivered through a powder pickup tube 42. 
Pickup tube 42 can be connected at a lower end 44 to a source of air 
entrained powder, such as a powder fountain 46 located in fluidized bed 
powder container 14, as discussed below. 
Powder fountain 46, as illustrated in FIG. 2, includes a flow inducer 48 to 
provide a rich mixture of powder with minimum air volume. The inlet 
section 50 of flow inducer 48 has a flared opening and is located beneath 
the surface 52 of air entrained powder 54. Flow inducer 48 has a nozzle 56 
with an air inlet 58 positioned to direct air through an outlet opening 60 
and into the flared opening of inlet section 50 of tube 44. This flow of 
air draws fluidized powder 54 into inlet section 50 of tube 44 and carries 
the resulting mixture of powder and air to the outlet 64 of tube 42 for 
discharge into inlet section 29 of reservoir 16. 
An important requirement of system 10 is to minimize the volume of air 
needed to carry the powder from hopper 14 to the cylindrical tube 20. 
Therefore, the volume of air flow discharged through inlet 58 is adjusted 
to the minimum value needed to deliver a "rich" powder/air mixture into 
the air flowing through tube 42. By adjusting the pressure of compressed 
air through air inlet 58 into injector nozzle 56 of flow inducer 48 with 
an air regulator or valve (not shown), the volume of air entrained powder 
discharged into reservoir 16 can be controlled. It is noteworthy that 
since flow inducer 48 uses only a small volume of air, the velocity of the 
air entrained powder flowing through pickup tube 42 is low. Further, the 
velocity of the air entrained powder flowing through powder conduit 20 is 
also held to a constant, optimum value which minimizes wear and impact 
fusion on component parts of powder spray gun 12, as compared with prior 
art pumping systems. While a specific design of a flow inducer is 
described, it is within the terms of the invention to substitute another 
flow inducer design or other type of powder pump to deliver a rich mixture 
of powder, typically air entrained powder, with a minimum air volume to 
reservoir 16. 
The air entrained powder is discharged from powder container 14 into inlet 
section 29 of reservoir 16 so that the powder surface 70 in shaped funnel 
or well section 40 is kept at a fixed location and the powder density is 
kept substantially constant in the shaped funnel section. Any excess 
powder above surface 70 is drained out of well 40 through a weir (not 
shown). To insure that the powder density is kept constant, the powder is 
continually monitored by a capacitance probe 72 disposed in well section 
40. The capacitance of the air entrained powder is a function of the 
dielectric constant (permitivity) of the material surrounding it. As long 
as the permitivity of the powder is maintained at a constant value, the 
powder density remains uniform so that a substantially fixed mass of 
powder is discharged through the outlet opening 38 of well section 40. 
Rotary lock 30 also provides an air tight seal between a powder inlet 
opening 74 of outlet tube 20 and outlet opening 38 of well section 40. The 
seal eliminates air leakage from the high pressure air in conduit tube 20 
to funnel section 40 which could hinder the filling of the cavities or 
pockets 76 between helical vanes 78 in rotary lock 30, as shown in FIG. 3. 
Rotary lock 30 preferably incorporates an impeller 79 formed of a 
cylindrically shaped component 81 with a plurality of vanes 78 integrally 
connected and extending outwardly therefrom. Impeller 79 is preferably 
constructed of a compliant material, such as, for example, molded rubber 
or elastomer and incorporates shaft 32 extending axially tube 81. Vanes 78 
are preferably formed in a helical configuration to discharge a smooth and 
substantially continuous flow of air entrained powder through powder inlet 
opening 74 and into conduit tube 20. The vanes 78 can also be curved along 
the longitudinal axis, as shown in FIG. 5. While shaped vanes of different 
configurations can be substituted for helical vanes 78, conventional 
straight vanes, which are best used in selected applications, have a 
tendency to discharge discrete chunks of powder causing pulsed powder 
flow. It is within the terms of the invention to provide dividers 77', as 
shown in FIG. 5, between vanes 78. The dividers extend radially outward 
from impeller 79 to provide more precise control of the powder flow 
through inlet opening 74 and into outlet tube 20. 
While the rotary lock metering system 18 shown in FIGS. 1-3 is suitable for 
its intended purpose, its use in a commercial environment can necessitate 
the need to reduce the amount of pressurized air in cylindrical tube 20 
which leaks around vanes 78 and the ends of impeller 79 into funnel shaped 
section 40 of reservoir 16. Air flow in this direction reduces the 
volumetric efficiency of impeller 79 of rotary lock 30, and also reduces 
the accuracy with which metering system 18 delivers air entrained powder 
into cylindrical tube 20. Therefore, in the embodiment shown in FIGS. 2 
and 3, it is important to insure that the outer tips and ends of impeller 
vanes 78 are tightly sealed against the inner surface 80 of rotary lock 
housing 82. 
While the embodiment of the constant volume conveying system shown in FIGS. 
1-3 is operable, the embodiment of a constant volume conveying system 98, 
as shown in FIGS. 4-6, eliminates the majority of the maintenance and 
design problems described above and provides a more accurate and robust 
metering system. As described above regarding the embodiment of FIGS. 1-3, 
the embodiment of FIG. 4 includes a mechanical powder metering system 18' 
with a multi-vane, helical rotary lock 30'. Throughout the specification, 
primed, double primed, triple primed, and quadruple numbers represent 
structural elements which are substantially identical to structural 
elements represented by the same unprimed number. The powder is metered 
into the stream of air entering cylindrical outlet tube 20' by the 
rotation of rotary lock 30'. A drive shaft 32' is secured at one end to 
rotary lock 30' and at the other end to a motor 36'. The volume of metered 
powder is controlled by the rotational speed of rotary lock 30'. Since the 
volume of powder metered by the rotary lock 30' is a function of its bulk 
density, it is important that lock 30' be fed from a source of powder with 
a constant head height to control the "packing" of the powder. This 
requirement can be met by placing lock 30' directly below an outlet 
opening 38' of a funnel shaped well 40' section of reservoir 16'. The well 
40' is filled with powder delivered through a pickup tube 42', which in 
turn is connected to source of air entrained powder, such as a powder 
fountain, located in fluidized bed powder container. 
A pressure equalizing system 99 is incorporated in system 98' to prevent 
leakage from the tube 20' across metering system 18' and into reservoir 
16'. System 99 includes an isolation device 100 placed between the inlet 
of funnel shaped section 40' and inlet section 29' of reservoir 16'. The 
pressure equalizing system 99 includes a pressure equalizing tube 104 
interconnecting outlet tube 20' and funnel shaped section 40' to allow 
pressurized air in expansion nozzle 26' to flow into the funnel shaped 
well section 40' between the isolation device 100 and rotary lock 30'. 
With the addition of pressure equalizing tube 104, rotary lock 30' is in a 
pressure balanced condition with little or no differential pressure across 
impeller 38'. This pressure balanced condition results in vanes 78' of 
lock 30' requiring only a relatively good mechanical seal between the tips 
and edges of the impeller vanes 78' and the inner surface 80' of rotary 
lock housing 82 to prevent powder in funnel shaped section 40' from 
leaking between the clearances at the ends and sides of vanes 78' and 
inner surface 80'. Since an air tight seal between vanes 78' and inner 
surface 80' is no longer necessary, factors relating to the design and 
wear of impeller 38', which were thought to be a deficiency of the 
embodiment shown in FIGS. 1-3, are substantially reduced and the 
volumetric efficiency and precise metering of powder into outlet tube 20' 
are substantially improved. A feature of pressure balancing tube 104 is 
its location in outlet tube 20' upstream of lock 30' so that the air 
introduced into funnel shaped section 40' is free of powder. The flow 
capacity of pressure balancing tube 104 is also small compared to outlet 
tube 20' so that air flow through tube 104 has no significant effect on 
the flow velocity through tube 20'. 
Referring to FIGS. 4, 5, and 6, there is illustrated a seal design which 
reduces the need for close tolerances between impeller 38' and the rotary 
lock housing 82' to prevent leakage of air therebetween. As shown in FIG. 
5, one end of the drive coupling shaft 32' can have a rounded or conical 
tip 84 which is rotatably received within a blind bore 86 in end plate 88 
of rotary lock housing 82'. The opposite end of drive coupling shaft 32' 
can extend through an aperture in end plate 90 of rotary lock housing 82. 
As best seen in FIG. 6, a seal 92, preferably formed as an integral part of 
impeller 38', is located between impeller 38' and end plate 90 to provide 
self sealing against the interior facing surface of end plate 90. In this 
embodiment, impeller 38' is preferably constructed of a molded rubber or 
elastomer material. A second seal 94, such as an oil lip or Bal.RTM. seal, 
is mounted on shaft 32' and abuts against the exterior facing surface of 
end plate 90. 
A principle feature of this invention is the provision of a pressure 
balancing tube 96 having one end 97 tapped into outlet tube 20' at a 
location upstream of lock 30'. Tube 96 is preferably tapped into balancing 
tube 104 which in turn is tapped into outlet tube 20'. The opposite end 98 
of tube 96 is attached to the inlet of a pressure channel 99 located in 
end plate 90 and opening on the exterior facing surface thereof. The 
opposite end of channel 99 opens onto the interface of the aperture 
through which shaft 32' extends. The pressurized air from outlet tube 20' 
pressurizes the space in the aperture between seals 92 and 94. The result 
is to substantially prevent air leakage from the interior of rotary wall 
30' across the inner seal 92 because the pressure on either side of seal 
92 is substantially equal. 
As shown in FIG. 7, isolation device 100 can be constructed of an impeller 
108 having vanes 110 that form a mechanical seal with the inner surface of 
an insert 111 mounted between the opposite walls 112 and 112A of isolation 
device 100. Insert 111 has an inlet section 113 which funnels powder into 
an adjoining central section 115. Central section 115 has an arced surface 
that engages vanes 110 and forms a mechanical seal therewith. The angle of 
the arc of engagement is greater than the angle between two adjacent vanes 
110, as shown in FIG. 7, to insure that the powder is trapped in the 
pockets 120 therebetween as the impeller 108 rotates within insert 111. An 
outlet section 117 of insert 111 funnels the powder from central section 
115 into the funnel shaped section 40'. 
Isolation device 100 is connected by a drive shaft 114 to a control device 
118, such as a stepper motor. The volume of metered powder is partially 
controlled by the rotational speed of isolation device 100. The pocket 120 
between each of the vanes 110 is filled with powder delivered through a 
pickup tube 42'. Pickup tube 42' can be connected at a lower end to a 
source of air entrained powder such as a powder fountain located in a 
fluidized bed powder container, as described in the embodiment of FIGS. 
1-3. The mechanical seal of the ends of impeller vanes 110 with inner 
surface of central section 115 need not be an air tight seal because 
impeller 108 is not a precision metering device but merely used to deliver 
powder into the funnel shaped section 40'. The requisite characteristics 
of isolation device 100 is that it have a greater volumetric throughput 
than lock 30', that it achieve a reasonable but not air tight seal, and 
that the powder is transferred through it by gravity. It is also important 
that when funnel shaped section 40' is completely full of powder, 
isolation device 100 is of a design that continued operation does not 
force additional powder into section 40'. In some cases, a small amount of 
air leakage through isolation device 100 can have the beneficial effect of 
keeping the powder in reservoir 16' above isolation device 100 loose and 
free flowing. Therefore, in certain designs, it is beneficial to make such 
leakage deliberate and controlled in volume and location. 
Normally device 100 is driven from the same drive mechanism, ie. through 
gears or belts, as lock 30' and its increased throughput can be 
accomplished by higher speed and/or larger volume. A principle aspect of 
the invention, relates to a control system 122 which controls the rotary 
speed of rotary lock 30'. As previously discussed, there are two primary 
factors in controlling the amount of powder metered into conduit tube 20'. 
The first is the speed of rotation of rotary lock 30' and the second is 
the density of the powder. As discussed before, the speed of rotation of 
rotary lock 30' can be accurately controlled with a stepper motor 36' and 
the density of the powder, whose capacitance is a function of its 
dielectric constant, can be constantly monitored by a capacitance probe 
72' disposed in funnel shaped section 40'. Controller 122 is connected to 
sensor 72' by line 124, to stepper motor 118 by line 126, to stepper motor 
36' by line 132, and to a sensor 130 (such as a capacitance sensor 
disposed in conduit tube 20' at a location downstream of rotary lock 30') 
by line 132. In the operation of controller system 122, when the density 
of the powder in funnel shaped section 40' changes, such as by deaerating, 
sensor 72' measures the change in its permitivity and adjusts the speed of 
the impeller 76' to meter a constant mass of powder into tube 20'. 
Concurrently, capacitor sensor 130 measures changes in the density of the 
powder flow through conduit 20'. Controller 122 can compensate for any 
changes by varying the rotary speed of rotary lock 30' so that a 
substantially fixed mass of powder flows through conduit tube 20'. 
Controller 122 can also control the speed of rotation of impeller 108 and 
if necessary, turn impeller 108 completely off in the event it is driven 
by independent drive means and is supplying too much powder into funnel 
shaped section 40'. 
Referring to FIG. 8, there is illustrated an alternative embodiment of an 
isolation device 100" in reservoir 16" which includes two spaced slide 
gates 140 and 142 with a powder storage section 144 therebetween. Each of 
the slide gates 140 and 142 extend substantially perpendicular to the 
longitudinal axis 146 through reservoir 16" and have actuating arms 148 
and 150, respectively, which extend through the side wall 152 of section 
144 and outward therefrom. Slide gates 140 and 142 reciprocate to open and 
close the powder flow path through reservoir 16". Preferably, a motor or 
actuator 154 is connected by a conventional mechanical power transfer 
mechanism to arms 140 and 142 to reciprocate them back and forth as 
required. The actuator 154 can be controlled by a controller 122". 
In the operation of isolation device 100", slide gates 140 and 142 can both 
be opened during startup in order to charge funnel shaped section 40" with 
powder from tube 42". Subsequent to startup, gates 140 and 142 are moved 
sequentially so that one is always closed. This insures that the pressure 
in funnel shaped section 40", as established by tube 104", is always 
substantially equal to the pressure in conduit tube 20". 
Referring to FIG. 9, there is illustrated another alternative embodiment of 
an isolation device 100"' in reservoir 16"' which includes two spaced ball 
valve 160 and 162 with a powder storage section 164 therebetween. Each 
ball valve 160 and 162 has a bore 166 and 168, respectively, extending 
therethrough. Also, ball valves 160, 162 each have a pair of pivot pins 
170 and 172, respectively, which are pivotally secured in openings through 
side walls 174 and 176 of powder storage section 164. Ball valves 160 and 
162 are disposed within cylindrical inserts 178 and 180, respectively, 
which include a cylindrical, funnel shaped inlet section 181 and 183, 
respectively, that funnels powder into bores 166 and 168 of ball valves 
160 and 162, respectively. Ball valves 160 and 162 are mounted in central 
cylindrical shaped sections 185 and 187 of cylindrical inserts 178 and 
180, respectively, which are adjoined to inlet sections 181 and 183, 
respectively, at one end and to downstream cylindrically shaped exit 
sections 189 and 191, respectively, at the opposite end. Central sections 
185 and 187 have an arced surface that forms a mechanical seal with ball 
valves 160 and 162, respectively. The diameter of the portion of the inlet 
and outlet sections 181, 183 and 189, 191, respectively, which engage ball 
valves 160 and 162, respectively, is less than the diameter of bores 166 
and 168, respectively. This size relationship reduces any powder leakage 
from bores 166 and 168 so that the powder flow which funnels into the 
downstream cylindrically shaped exit sections 189 and 191, respectively, 
can be controlled. Also, this size relationship helps prevent powder from 
collecting between the inner surfaces of cylindrical shaped sections 185 
and 187 and ball valves 160 and 162, respectively. 
The ball valves 160 and 162 rotate to open and close the powder flow path 
through reservoir 16"'. Preferably, an actuator 182, such as a stepper 
motor, is connected by conventional means to pivot pins 170 and 172 to 
rotate the ball valves, as required. The actuator 182 can be controlled by 
a controller 122"'. 
In the operation of isolation device 100"', ball valves 160 and 162 can 
both be opened during startup in order to charge funnel shaped section 
40"' with powder from tube 42"'. Subsequent to startup, the ball valves 
160 and 162 are rotated sequentially so that one valve always remains 
closed. This insures that the pressure in funnel shaped well 40"', 
established by tube 104"', is always substantially equal to the pressure 
in conduit tube 20"'. 
Referring to FIG. 10, there is illustrated another alternative embodiment 
of an isolation device 100"" in reservoir 16"" which includes two spaced, 
rotary valve plates 190 and 192 forming a powder storage section 194 
therebetween. The valve plates 190 and 192 are circular and are secured at 
their centers to a shaft 200 which is parallel to axis 146 through 
reservoir 16"". Plates 190 and 192 extend outward from reservoir 16"" 
through sealed slot openings 196 and 198, respectively, in the side wall 
199 of section 194. Each valve plate 190 and 192 has a through opening 202 
and 204, respectively, located to one side of shaft 200. Valve plates 190 
and 192 are rotated by shaft 200, respectively, through slot openings 196 
and 198 in side wall 199. Valves plates 190 and 192 are disposed within 
cylindrical insert elements 201 and 203, respectively, which include upper 
cylindrical, funnel shaped inlet sections 205 and 207, respectively, that 
funnel powder into openings 202 and 204, respectively, of valve plates 190 
and 192. Insert elements 201 and 203 also lower cylindrical shaped exit 
sections 209 and 211, respectively, for directing powder into funnel 
section 40"". The upper and lower sections 205, 207 and 209, 211, 
respectively, form a mechanical seal with valve plates 190 and 192, 
respectively. 
Valve plates 190 and 192 are secured to shaft 200 so that through openings 
202 and 204 are out of phase, i.e. preferably about 180.degree. apart. It 
is within the terms of the invention to vary the angle with which openings 
202 and 204 are set with respect to each other as desired. As shown in 
FIG. 10, whenever valve plate 190 is open, i.e. the flow path through 
opening 202 into powder storage section 194 is open, the valve plate 192 
is closed, i.e. the flow path through opening 204 out of powder storage 
section 194 and into funnel section 40"" is closed, and vice versa. 
Preferably, a motor or actuator 206 is connected through a conventional 
drive mechanism (not shown) to shaft 200 so as to rotate the shaft, as 
required. The actuator 206 can be controlled by a controller 122"". 
In the operation of isolation device 100"", valve plates 190 and 192 can be 
rotated continually during startup in order to charge funnel shaped 
section 40"" with powder from tube 42"". Subsequent to startup, the valve 
plates 190 and 192 are rotated at a rate so that the pressure in funnel 
shaped well 40"", established by tube 104"", is always substantially equal 
to the pressure in conduit tube 20"". 
It is apparent that there has been provided in accordance with this 
invention an apparatus and method for pumping a constant volume of air 
entrained powder to a powder spray gun to obviate the problems and 
limitations of the prior art systems. According to the invention, the 
powder pump meters powder into a stream of air so that a constant volume 
of air entrained powder is directed to the spray gun. The invention also 
discloses a pressure equalizing system that prevents powder leakage across 
the powder metering system which meters powder from a reservoir into a 
conduit tube. A control system is disclosed which adjusts the rate at 
which powder is metered into the outlet tube so that the mass flow of 
powder exiting the outlet tube remains constant. 
While the invention has been described in combination with embodiments 
thereof, it is evident that many alternatives, modifications, and 
variations will be apparent to those skilled in the art in light of the 
foregoing teachings. Accordingly, the invention is intended to embrace all 
such alternatives, modifications and variations as fall within the spirit 
and scope of the appended claims.