WO1995033574A1

WO1995033574A1 - Spray pressure control and method of operating for can coating system

Info

Publication number: WO1995033574A1
Application number: PCT/US1995/006757
Authority: WO
Inventors: Masafumi Matsunaga; Donald A. Chicatelli; Paul A. Armbruster
Original assignee: Nordson Corporation
Priority date: 1994-06-07
Filing date: 1995-05-30
Publication date: 1995-12-14
Also published as: AU2654195A

Abstract

A system and method of spray coating with individual spray coating guns mounted to an hydraulic circuit which supplies high pressure coating liquid from a pump and recirculates a portion of the coating liquid supplied to each gun through a restrictor in a circulation manifold to the pump. An adjustable pressure regulator reduces the pressure differential across the restrictor to reduce the amount of coating liquid recirculated to the pump without effecting the pressure of the coating liquid at each of the spray guns.

Description

SPRAY PRESSURE CONTROL AND METHOD OF OPERATING FOR CAN COATING SYSTEM

FIELD OF THE INVENTION

The invention relates to a method and apparatus for controlling the spray pressure of each spray coating gun used in a multiple gun spray coating system. More particularly, the invention relates to a method and apparatus for maintaining consistent hydraulic pressure for each gun of a multiple gun spray system used in spray coating surfaces, such as the inner surface of cans, while reducing the amount of coating being recirculated from each gun back through the system.

BACKGROUND OF THE INVENTION

Controlling and maintaining consistent inside-spray coating film weights is an important requirement of the can manufacturing process. Variations in coating film weights indicate a lack of adequate process control, and can lead to quality problems, such as exposure of the metal interior surface of the can. To avoid metal exposure and improve product quality, a heavier coating weight is applied at the expense of increased operating and product costs. Therefore, it is desirable to improve the control of the spray process so as to maintain coating quality while reducing coating film weight requirements, coating material costs, volatile organic compounds (VOC) emissions, and costs associated with environmental compliance. One primary factor that adversely effects spray coating film weights is fluctuations in fluid pressure. For example, reducing spray pressure from 800 pounds per square inch (psi) to 750 psi will reduce film weights by approximately five percent. Moreover, the effect of pressure variations on the coating film weight increases as the spray pressure decreases. While reducing film weights is very important, it is desirable to operate at low spray pressures because there is less overspray and bounce-back of the coating. That is, at low spray pressures, the process operates cleaner and more efficiently, which results in cost savings and improved productivity.

Pressure fluctuations are primarily caused by three factors. The first factor, caused by the pump stroke and actuation of valves in the pump, generates pressure variations ranging from 50 psi to 150 psi. Pressure variations of this magnitude are sufficient to effect coating weights.

The second factor is the effect of operating multiple spray guns in a single hydraulic circuit. Typically, five to ten guns and often more are used with a single pump, and each of the guns operates independently. When one of the guns begins spraying, it causes a momentary drop in pressure in the hydraulic circuit. The more guns that are operating at any given time, the greater the momentary drop in pressure. The resulting pressure fluctuation causes variations in the coating weight, which depends on the number of guns spraying at a specific time, and the location of the operating spray guns within the hydraulic circuit. The hydraulic circuit design is the third important factor that effects the consistency of pressure and coating weights. In the past, series plumbing was typically used in two-piece can coating machines. This type of circuit design has several drawbacks. The spray gun located closest to the pump usually operates at a considerably higher spray pressure than the spray gun farthest from the pump and the effect of pressure fluctuations due to multiple spray guns is significantly magnified. Variations in coating weights of ten milligrams from gun to gun are not uncommon with a series circuit. Parallel circuits offer better pressure control and less "noise" (or pressure drop) from gun to gun. However, pressure fluctuations are still noticeable, and result in coating weight variations.

With most inside-spray systems used in coating cans, fluid pressure is adjusted at a single location, i.e. the pump. Pressure adjustment at each spray gun is not possible, and the result is a different operating pressure at each spray gun. Also, fluctuations in the pressure caused by pump operation and opening and closing of other guns in the spray circuit, are not controlled or dampened.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and apparatus for controlling the spray pressure of each spray coating gun used in a multiple gun spray coating system and in a process for coating the inner surface of cans to obviate the problems and limitations of the prior art systems. It is a further object of the present invention to provide a method and apparatus for independently controlling the pressure at each gun of a multiple-gun spray system used in a spray coating process for spray coating surfaces to accurately control coating weights.

It is a still further object of the present invention to provide a method and apparatus for lowering spray pressures at each gun of a multiple-gun spray system used in a spray coating process for spray coating surfaces to provide a cleaner, more efficient operation.

It is yet another object of the present invention to provide a method and apparatus for reducing the amount of liquid coating material recirculated from each spray gun of a multiple-gun spray system to a supply pump without reducing the pressure at each spray gun.

In accordance with the invention, there is provided a multiple gun spray coating system for spray coating the insides of cans. The system includes a pressure regulated pump system for supplying high pressure coating liquid to an hydraulic circuit which in turn supplies the high pressure coating liquid to a plurality of individual spray coating guns. The hydraulic circuit also enables a portion of the coating liquid from each of the individual guns to recirculate to the pressure regulated pump system. The hydraulic circuit includes a plurality of pressure regulators, each associated with one of the guns, for controlling the pressure of the coating liquid delivered to each of the individual guns in order to minimize pressure variations within the individual gun. The hydraulic circuit includes a circulation manifold with a restrictor for controlling the amount of coating liquid recirculated from each of the guns to maintain a minimum circulation of the coating liquid through the spray guns. An adjustable pressure regulator is provided in the return line of the hydraulic circuit for reducing the pressure differential across the restrictor to reduce the amount of coating liquid being recirculated to the pressure regulated pump system without effecting the pressure of the coating fluid at each of the spray guns.

According to the invention, the restrictor in the circulation manifold has a fixed orifice through which the coating liquid being recirculated flows to the adjustable back pressure regulator which in turn controls the differential pressure across the circulation manifold. A valved device is connected to the outlet of the circulation manifold for directing the flow of recirculated flow to either a device for measuring the flow rate of the liquid flow through the orifice or to the pump system.

Further in according with the invention, the process of spray coating the insides of cans with a multiple gun spray coating system, comprises the following steps. High pressure coating liquid from a supply of coating liquid is pumped to a plurality of individual spray coating guns in an hydraulic circuit which recirculates a portion of the coating liquid from each of the individual guns to the coating supply. The pressure of the coating liquid delivered to each of the individual guns is controlled to minimize pressure variation within each of the individual guns. The recirculated portion of the coating liquid from each of the guns is directed through a restrictor to maintain a minimum circulation of the coating liquid through the spray guns. The pressure differential across the restrictor is lowered for reducing the amount of the coating liquid recirculated to the pump without effecting the pressure of the coating liquid at each of the spray guns.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and advantages of the presently preferred embodiment of the invention will become further apparent upon consideration of the following description taken in conjunction with the accompanying drawings, wherein:

Figs. 1A and IB, are together a schematic illustration of a spray pressure control system showing a pump supply connected to a hydraulic circuit with a number of individual spray guns and a recirculation path to return a portion of the coating material from each gun through a circulation manifold and a pressure regulator to the pump supply, in accordance with the invention; Fig. 2 is a side view of a circulation manifold having a three way valve connected to its outlet port; and

Fig. 2A is a side view of a plug containing a restrictor with a fixed orifice therethrough.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figs. 1A and IB, there is illustrated a multiple gun spray coating system 10 used for spray coating the inside surfaces of cans moving through a plurality of separate internal spray coating machines 11A, 11B, 11C, 11D (11A-11D) on a production line basis. Typically, in such equipment, the cans to be coated are carried by a conveyer system (not shown) through the separate spray machines (11A-11D) where the cans are held in position and rotated at a constant revolution per minute for a set period of time to control the thickness of the coating being applied. Coating system 10 includes a coating supply section 12 which supplies high pressure coating fluid to a plurality of individual spray guns 13A, 13B, 13C, 13D, 13E, 13F, 13G, 13H (13A-13H) through a parallel hydraulic circuit 14. Coating supply section 12 controls the pressure of the liquid coating material supplied to parallel hydraulic circuit 14. Hydraulic circuit 14 includes a plurality of separate hydraulic circuits 16A, 16B, 16C, and 16D (16A-16D) which control the pressure of each individual spray gun 13A-13H to minimize pressure variations between each gun of multiple-gun spray system 10 so as to maintain a consistent inside, spray coating weight of the coating being applied to the inside of cans moving through machines 11A-11D.

The coating supply section 12 includes a single electric pump 17, such as a 1200 psi pump, powered by a motor 18 through a gear reducer 19. A supply line 20 is connected at one end to a source of liquid coating material (not shown) and at the other end to a pump inlet line 22. A high pressure outlet line 24 from pump 17 is connected to a manifold block 26 having a pressure relief valve 28 and a pressure gauge 30 mounted thereon. A valved exhaust line 32 from manifold 26 is connected by flow line 34 to relief valve 28 and to a drain or collector 36 by flow line 38. An air bleed valve 33 in exhaust line 32 is used to relieve pressure in the high pressure outlet line 24 and to start pump 17 as conventionally known. A flow line 40 is connected through a three-way valve 42 to an adjustable back pressure regulator 44 which is used to set the pressure of pump 17 when valve 42 is in a first position so that the high pressure feed line 46 from manifold 26 is connected through flow line 41 to regulator 44. In this first position, regulator 44 reduces any excess pressure in feed line 46, i.e., above the amount for which regulator 44 is set, by directing excess flow through a flow line 50 to a three-way valve 52 which either recycles the excess to the pump inlet line 22 or to a drain or collector through drain line 54. When valve 42 is in the second position, line 46 can be flushed or bled through flow lines 41, 40 and 38 to a drain or collector 36. The outlet side of pressure regulator 44 is connected by a flow line 50 with a three-way valve 52 to either the pump inlet line 22 or to drain line 54 going to drain 36. As discussed below, a return line 55 from hydraulic circuit 14 is connected to flow line 50 so that the recycled flow can be directed back into the inlet of pump 17 or into drain line 54 as desired.

High pressure line 46 is connected by a three-way valve 56 to a high pressure supply line 58 through which liquid coating material is supplied to parallel hydraulic circuit 14 and ultimately individual guns 13A-13H. While a single coating supply section 12 is adequate for the present invention, it is also within the terms of the invention to provide an auxiliary backup coating section 12' that can be used to direct high pressure flow through flow line 46' to supply line 58 by repositioning valve 56 as needed, such as when coating supply section 12 is broken or malfunctioning. Throughout the specification, primed numbers represent structural elements which are substantially identical to structural elements represented by the same unprimed number.

Hydraulic circuit 14 includes a plurality of separate hydraulic circuits 16A-16D which control the pressure of each individual spray gun 13A-13H to maintain consistent inside, spray coating weight of coating material being applied to the inside of cans moving through machines 11A-11D. Each spray circuit 16A-16D is associated with one of the machines 11A-11D, respectively, and has two conventional spray guns, i.e., 13A,13B; 13C,13D; 13E,13F; 13G,13H, such as Nordson Model A20A spray guns manufactured by Nordson Corp. of

Westlake, Ohio, the assignee of the present invention. Since each of the spray circuits 16A-16D are substantially identical as to both construction and operation, only spray circuit 16A is described below.

A common high pressure supply line 60 of hydraulic circuit 14 directs coating material from high pressure supply line 58 into a flow line 62 having an isolation/shut-off valve 64. An in-line heater 66, such as the Nordson Model NH-4, from Nordson Corp. of Westlake, Ohio, the assignee of the present invention, can maintain the optimum coating viscosity by heating the coating material to a desired temperature regardless of ambient temperatures. Substantially reducing temperature fluctuations is an important control consideration because these fluctuations will effect the viscosity and flow of the coating material, which in turn can lead to variations in coating weights. While the use of a heater 66 to control temperature is preferably provided with the hydraulic circuit of the present invention, it is also within the terms of the invention to operate without a heater, particularly depending upon the nature of the ^' coating material.

The coating material from heater 66 flows into a flow line 68 with a shutoff valve 70. When section 16A is removed from service for reasons such as cleaning or repair, shutoff valve 70 can be closed and the flow is directed from flow line 68 and through a valve 72 in flow line 74 to a secondary return line 76 which is connected to a primary return line 78. Under normal operation, the flow of coating material from heater 66 flows through flow line 68 into a filter system 80, such as the Nordson Dual Filter Assembly, P/N 248328, from Nordson Corp. of Westlake, Ohio, the assignee of the present invention, which is designed for use on constant demand applications where it is impractical to stop production for filter cleaning or servicing and on applications with higher than average material contamination. The dual filter assembly 80 includes several three way valves (not discussed in detail) which can be opened and closed to enable one of the filters to be closed for servicing while the other is still in operation.

The flow of coating material exiting the filter assembly 80 is directed through lines 82 and 83 into fluid pressure regulators 84 and 84', respectively, which adjust the pressure to spray guns 13A and 13B, respectively, and enable independent pressure adjustment so that each spray gun can provide consistent coating weights. The pressure regulators 84,84', such as the Nordson Soft Seat Fluid Pressure Regulator, P/N 101 991 from Nordson Corp. of Westlake, Ohio, also reduce pressure fluctuations caused by pump 17 and the effect of any the other multiple guns 13A-13H in the hydraulic circuit 14. By insuring that the pressure differential across each regulator 84, 84' is set so that the input pressure is between about 100 psi and about 300 psi and preferably about 150 psi and about 250 psi and most preferably about 200 psi higher than the output pressure, the pressure fluctuation from spikes caused by pump 17 or the opening and closing of the other guns in parallel circuit 14 are substantially eliminated. That is, each pressure regulator 84 acts as a dampener as long as the pressure differential is maintained at the desired level.

A principle feature of this invention is the provision and construction of circulation manifolds 88A,88B,88C,88D,88E,88F,88G,88H (88A-88H) used in conjunction with each gun 13A-13H, respectively, to maintain a minimum circulation of coating material through spray guns 13A-13H at all times. Since the circulation manifolds 88A-88H are substantially identical, only manifold 88A, as shown in more detail in Figs. 2 and 2A, is described. Circulation manifold 88 includes an in-line filter 90A which is connected to the outlet of gun 13A by flow line 86 and to the inlet 92A of an orifice block 94A. Orifice block 94A has a holder or plug 96A, as shown in Fig. 2A, containing a restrictor 98A with a fixed orifice 99 therethrough. The returning coating flow circulates through the orifice 99 of restrictor 98A and out of openings 100A in holder 96A to outlet 102A of orifice block 94A. A three-way valve 104A is connected to outlet 102A and directs recirculated flow either to an outlet tube 106A which empties into a graduated collection cylinder 108 or to a flow line 110 which connects to secondary return line 76 through a shutoff valve 112.

The ability to direct flow into graduated cylinder 108 is needed to determine whether the orifice 99 is clogged. By measuring the amount of flow passing through the orifice 99 for a given amount of time, i.e. the amount collected in the graduated cylinder for a specified amount of time, the effective size of the orifice 99, i.e., the amount of blockage or wear of the orifice 99 can be determined. It is important to ensure that the orifice 99 is not blocked so that the back pressure on gun 13A does not increase and reduce the differential pressure across regulator 84 and allow pressure fluctuations caused by pump 17 and/or the effect of the multiple guns 13B-13H in the hydraulic circuit 14 to vary the coating weights being applied by gun 13A. In addition, flow line 110' from manifold 88B is also connected to return line 76.

Another principle feature of this invention is the provision of an adjustable back pressure regulator 114 in return line 55 to regulate the pressure of the recirculating return flow from primary return line 78 to line 50 and pump 17. The adjustable back pressure regulator 114, such as a Nordson Back Pressure Regulator, P/N 101 991 manufactured by Nordson Corp. of Westlake, Ohio, functions to raise the pressure of the primary return line 78 and decrease the overall differential pressure across each of the circulation manifolds 88A-88H without further reducing the orifice size of each circulation manifolds 88A-88H. The effect of the decreased pressure differential is a reduction of the amount of coating liquid recirculated to pump 17 without effecting the pressure of the coating liquid at each ot the spray guns. Typically, pressure regulator 114 is set to increase the downstream pressure in the return line 78 so that the pressure differential across the circulation manifold is about 150 to about 300 psi and preferably about 175 to about 225 psi, as compared to a pressure differential of about 750 psi in systems which did not have the downstream regulator of this invention. The advantage of decreasing the amount of coating liquid recirculated to the pump by the inclusion of a downstream regulator 114 is that it provides an inexpensive and effective technique of adding additional spray guns to a multiple gun spray coating system 10 having a pump that is already operating at its peak capacity for the reasons described below.

A typical example of two identical systems, with and without the inclusion of an adjustable back pressure regulator of this invention, follows to show the benefit of incorporating the back pressure regulator.

A coating supply section 12 includes a single electric pump 17, having a maximum pressure of about 1200 psi, is able to deliver 1.2 gallon per minute (gpm) of liquid coating material at a pressure of 1000 psi to a hydraulic circuit 14 which includes four separate hydraulic circuits 16A-16D with a total of eight guns 13A-13H. The pressure to each of the eight individual spray gun is set to about 800 psi by the fluid pressure regulator 84 associated with each gun so that 0.15 gpm of liquid coating material is delivered to each gun. Of this 0.15 gpm, 0.06 gpm of coating material is returned through the gun to supply section 12 and 0.09 gpm is emitted whenever the gun is actuated. It is extremely important that the pressure at each gun remains constant so that a constant amount of coating material is sprayed from each gun to apply a uniform inside, spray coating weight of coating material to the inside of each of the cans moving through the spray machines 11A-11D associated with the guns.

Each circulation manifold 88A-88H, used in conjunction with each gun 13A-13H, respectively, contains a restrictor, only manifold 88A with restrictor 98A is illustrated, with a fixed orifice 99 selected to maintain the recirculation of coating material, i.e. about 0.06 gpm, through each spray gun at all times. Whenever possible, the smallest practical orifice size is selected so that the amount of material being recirculated is kept at the minimum while insuring a constant return flow.

For example, if an orifice 99 having a diameter of .010 inches (calibrated for a flow rate of .05 gpm of H₂0 @500 psi) is selected, the flow rate with an upstream pressure of 800 psi and a downstream pressure of 50 psi, i.e, a pressure differential of 750 psi, is about 0.06 gpm. An orifice of this size is practically as small as possible because smaller orifices are prone to clogging and disrupting the recirculation flow therethrough. Clogging is very detrimental to the operation of the system because it causes a fluctuation in the pressure, and therefore in the amount of coating material being sprayed by the gun associated therewith. Thus, the ability to reduce the size of the restrictor has a practical limit. Assuming an orifice 99 of 0.010 inches for each of the eight circulation manifolds, the total amount of flow being recirculated is 0.48 gpm (8 times 0.06 gpm) and the maximum amount of total flow being sprayed is .72 gpm (8 times 0.09 gpm). Thus, there is a need for a total capacity of 1.2 gpm from the pump in order for the system to properly operate. Since the pump of our example can deliver 1.2 gpm, the system can operate properly in accordance with the design.

However, suppose the system required the addition of another two guns. In that case, assuming that all of the system operating characteristics remains the same as just described, the total amount of flow being recirculated is 0.60 gpm (10 times 0.06 gpm) and the maximum amount of total flow being sprayed is 0.90 gpm (10 times 0.09 gpm). Thus, there is a need for a total capacity of 1.50 gpm from the pump in order for the system to properly operate. Since the pump of our example can deliver a maximum of 1.2 gpm, the system cannot operate properly in accordance with the design.

Now, taking the identical system with eight guns, but adding an adjustable back pressure regulator set at a pressure of about 600 psi, the pressure differential across the circulation manifold is about 200 psi, i.e.

800 psi - 600 psi. At 200 psi, the flow rate through an orifice 99 of 0.010 inches is .03 gpm for each of the eight circulation manifolds and the total amount of flow being recirculated is 0.24 gpm (8 times 0.03 gpm). Since the maximum amount of total flow being sprayed is 0.72 gpm (8 times 0.09 gpm), there is a need for a total capacity of 0.96 gpm from the pump in order for the system to properly operate. Using the pump of our example which can deliver 1.2 gpm, the system can easily operate within the capacity of the pump.

However, suppose the previous system with the adjustable back pressure regulator 114 required the addition of another two guns for a total of ten guns. In that case, assuming that all of the system operating characteristics remains the same as just described, the total amount of flow being recirculated through the circulation manifolds is 0.30 gpm (10 times 0.03 gpm) and the maximum amount of total flow being sprayed from the ten guns is 0.90 gpm (10 times 0.09 gpm). Thus, there is a need for a total capacity of 1.2 gpm (0.30 + 0.90) from the pump in order for the system to properly operate. Since the pump of our example can deliver a maximum of 1.2 gpm, the system, even with the addition of two guns, can still operate properly in accordance with the design. Moreover, the back pressure regulator could be adjusted so that the pressure differential is still lower to further reduce the amount of recirculated coating material and enable another gun to be added to the system, if necessary.

It is apparent that there has been provided in accordance with this invention apparatus and methods for controlling the spray pressure of each spray coating gun used in a multiple gun spray coating system to spray coat surfaces with accurately controlled coating weights that satisfy the objects, means and advantages set forth hereinbefore. According to the invention, the system and method of operation includes supplying individual spray coating guns mounted to an hydraulic circuit with high pressure coating liquid from a pumping system and recirculating a portion of the coating liquid to the pumping system. This recirculated portion of the coating liquid flows through a fixed orifice in a circulation ^' manifold associated with each gun to maintain a minimum circulation of the coating liquid through the spray guns. An adjustable pressure regulator lowers the pressure differential across each of the circulation manifolds whereby the amount of coating liquid recirculated to the pump is reduced without effecting the pressure of the coating liquid at each of the spray guns.

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.

Claims

We claim:

1. A multiple gun spray coating system adapted for spray coating the insides of cans, comprising: a pump system for supplying high pressure coating^' liquid; an hydraulic circuit connected to said pump system for providing a plurality of individual spray coating guns with high pressure coating liquid and for recirculating a portion of said coating liquid from each of said individual guns to said pump system, said hydraulic circuit including: a control device for controlling the pressure of said coating liquid delivered to each of said individual guns; a plurality of manifolds each connected to one of said spray coating guns for directing the recirculated portion of said coating liquid from each of said guns, each of said manifolds having a restrictor; and a regulator for reducing the pressure differential across said restrictors.

2. The multiple gun spray coating system of claim 1 wherein said regulator is an adjustable back pressure regulator in a return flow line connecting each of said plurality of manifolds to said pump system.

3. The multiple gun spray coating system of claim 2 wherein each of said plurality of manifolds is a circulation manifold with one of said restrictors therein, and said restrictors each have a fixed orifice through which said coating liquid being recirculated flows to said adjustable back pressure regulator.

4. The multiple gun spray coating system of claim 3 wherein said control device is a plurality of pressure^' regulators, each associated with one of said guns.

5. The multiple gun spray coating system of claim 4 wherein said pump system includes a pump, a high pressure line connecting said pump to said hydraulic circuit, and a pressure relief valve in said high pressure line to regulate the pressure of said coating liquid.

6. The multiple gun spray coating system of claim 3 including a valve connected to an outlet of said circulation manifold for directing said recirculated flow to either a means for measuring the flow rate of said liquid through said orifice or to said adjustable back pressure regulator.

7. The process of spray coating the insides of cans with a multiple gun spray coating system, comprising the steps of: supplying high pressure coating liquid from a pump to a hydraulic circuit having a plurality of individual spray coating guns and recirculating a portion of said coating liquid from each of said individual guns to said pump, said step of supplying high pressure liquid including the steps of: controlling the pressure of said coating liquid delivered to each of said individual guns to minimize pressure variations within each of said individual guns; directing the recirculated portion of said coating liquid from each of said guns through a restrictor to maintain a minimum circulation of said coating liquid through said spray guns; and reducing the pressure differential across said restrictor for reducing the amount of said coating liquid recirculated to said pump without effecting the pressure of said coating liquid at each of said spray guns.

8. The process of claim 7 wherein the step of controlling the pressure of said coating liquid delivered to each of said individual guns includes the step of setting the pressure differential across a pressure regulator associated with each of said individual spray gun so that the input pressure is between about 100 psi and about 300 psi higher than the output pressure.

9. The process of claim 8 wherein the step of controlling the pressure includes the step of setting said pressure differential across each said pressure regulator so that the input pressure is between about 150 psi and about 250 psi higher than the output pressure.

10. The process of claim 7 wherein the step of controlling the recirculation includes providing a circulation manifold having said restrictor with a fixed orifice between each of said individual guns and said pump.

11. The process of claim 9 wherein the step of controlling the recirculation includes the step of setting the pressure across said circulation manifold to between about 150 psi and about 300 psi.

12. The process of claim 11 wherein the step of controlling the recirculation includes the step of setting the pressure across said circulation manifold to between about 175 psi and about 225 psi.

13. The process of claim 10 including the step of measuring the flow rate of said recirculated portion of said coating liquid through each fixed orifice to determine the amount of clogging or wear of said orifice.