US3087534A - Process for spraying liquiform materials - Google Patents

Description

April 30, 1963 Tlal.,

/4 TTRA/EYS United States Patent O Mice 3,037,534 PRCESS FOR SPRAYING LIQUIFORM MATERIALS Leander H. Walker, 23 The Crescent, Berkeley, Calif. Filed Aug. 17, 1959, Ser. No, 834,126 5 Claims. (Cl. 159-48) The present invention relates generally to the spraying of liquiform materials such as concentrated food liquids, purees, and the like, and more particularly to a method of spraying such materials within a relatively high vacuum system.

The spraying of liquiform materials at Vatmospheric pressure is commonly accomplished by means of air spray guns or the like wherein the liquid is forced under pressure through a central opening, jet, or nozzle and one or more air streams are directed upon the liquid issuing therefrom to break up the liquid stream into minute particles. The air streams, moreover, direct the liquid particles in a desired pattern of spray upon a surface to be coated and serve the important function, particularly Where a at spray pattern is produced, of insuring an equal depth of coverage over the entire pattern to enable the application of a uniform thickness of spray over a given width of continuously moving surface. Although compressed air satisfactorily accomplishes the foregoing functions for substantially all common applications where materials are sprayed in the open air, viz., at atmospheric pressure, I have found such common and accepted practice involving the use of air in spray guns vastly unsuited to the spraying of liquiform materials in a vacuum system maintained at an absolute pressure of less than about l5 mm. of mercury. More explicitly, when liquiform materials, particularly concentrated food purees and juices, are introduced into an air spray gun for the purpose of spraying a flat pattern on a moving belt in such a relatively high vacuum system, the amount of heat absorbed by the expanding air in issuing from the gun is sufficient to cause growth of whiskers of frozen liquiform material at the nozzle tip and eventual freezing of the entire tip area. Even when the air is preheated a uniform fiat pattern can not be maintained for more than a few minutes at absolute pressures of less than about 12 mm. of mercury absolute because the air tends to dry out the feed material at the tip of the nozzle which therefore becomes coated with essentially dry material, grows whiskers and becomes plugged up in short order. Therefore, the conventional practice of utilizing air in a spray gun is unsatisfactory when it is desired to continuously produce a uniform coating of a liquiform material on a moving surface within a vacuum system maintained at absolute pressures less than about l2 mm. Hg. Both the cooling and drying effects utilizing air produce undesirable effects.

The present invention overcomes the above-noted difficulties encountered in spraying liquiform materials within relatively high vacuum by providing a method for spray ing liquiform materials within a vacuum system at ab solute pressures less than about three times the absolute pressure at which the solvent or suspending liquid of the liquiform material freezes. More particularly, provision is made for the operation of a conventional spray gun in accordance with an improved method whereby the disadvantages and limiting factors encountered in the conventional operation of a spray gun in a relatively high vacuum are overcome and the range of reduced pressures in which successful operation is readily achieved is materially extended. Solvents or suspending agents in addition to water are included within the generic scope of this invention and the liquiform materials are generally liquiform suspensions including nely divided solids dispersed in a lluid medium.

It is therefore an object of the present invention to 3,087,534 Patented Apr. 30, 1963 provide a method of continuously applying a uniform coating of a water suspended liquiform material on a moving belt within a relatively high vacuum system even at absolute pressures as low as l mm. of mercury.

Another object of this invention is the provision of a method of breaking up a stream of liquiform material into minute particles and continuously directing the particles to form a desired pattern on a surface disposed in a relatively high vacuum at pressures below about 12 mm. of mercury.

It is still another object of my invention to provide a method of spraying liquiform material from a conventional spray gun in relatively high vacuum without drying and freezing of the liquiform material at the tip of the nozzle of the gun.

Yet another object is the provision of a method of spraying a relatively Wet coating of liquiform material upon a surface disposed in relatively high vacuum with a minimum of dusting of the sprayed material upon the surface.

It is a further object of the present invention to provide a method of spraying liquiform material in a vacuum system maintained at extremely low absolute pressure which facilitates a material reduction in the size of the vacuum equipment.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of the preferred form of the invention which is illustrated in the drawing accompanying and forming part of the specification. It is to be understood, however, that variations in the showing made by the said drawing and description may be adopted within the scope of the invention as set forth in the claims.

FIGURE 1 is a schematic flow diagram of preferred means for conducting the spraying method in accordance with the present invention, and

FIGURE 2 is a sectional view of one type of tip which may be used with the gun.

Considering now the invention in some detail and referring to the drawing, there is provided a vacuum enclosure 11 housing a relatively low pressure region 12 established by conventional vacuum equipment (not shown) associated with the enclosure. Within the enclosure there is provided an endless belt '13 which continuously moves forward in the direction depicted by the arrow with a constant rate of travel. Disposed adjacent the moving belt in spaced relation to the surface thereof Vis disposed a conventional spray gun 14 into which liquiform material, eg., concentrated food purees or juices, is introduced by means of a pump 16, or equivalent means, for spraying upon the surface of moving belt 13 to produce a uniform coating of the liquiform material thereon. The conventional spray gun 14 typically includes a central opening, jet, or nozzle 17 through which the liquiform material is forced under pressure by the pump 16. lmmediately after leaving the nozzle, the stream of liquiform material is struck by one or more streams of impelling vapor, preferably a fluid material in a superheated vaporous state, introduced to the gun through the gas inlet 18 thereof under pressure. The inlet 18 communicates with an annular passage 19 disposed concentrically about the nozzle 17. The stream or streams of vapor issuing from the passage 19 are controlled and directed radially inward towards the opening of nozzle 17 by means of a cap 21 located at the operating tip of the gun. The cap surrounds all but the tip of the nozzle and therefore the high velocity vapor stream breaks up the liquiform material stream issuing therefrom into minute particles. Such particles are directed from the tip of the cap 21 by the vapor issuing therefrom in the desired pattern of spray, i.e., flat, round, oval, etc. for impingement upon a surface to be coated.

Although the spray gun tip detailed above is of the conventional internal mix type, tips of the external mix variety may also be employed. Such spray gun tips are similar in overall operation except in the external mix type the break up function takes place after the stream of liquiform material leaves the cap. In addition to the foregoing types of spray guns other devices capable of performing a similar spray forming function may also be employed.

As indicated previously herein, air has been heretofore utilized as the impelling gas introduced to the inlet 18 of spray gun 14 or to otherwise accomplish the spray forming process when the liquiform material is sprayed at atmospheric pressure. However, when the pressure in region 12 is reduced to less than about l2 mm. of mercury, under no conditions can a uniform flat spray upon belt 13 be maintained for extended periods of time. This is due to the cooling effect produced by the rapid expansion of the air in leaving the cap ports and entering the low pressure region to thereby absorb a substantial quantity of heat from the liquiform material issuing from the nozzle, and thus rapidly `freeze the nozzle tip. To this is added the drying effect of the rapidly expending air upon the liquiform material issuing from the nozzle which causes eventual plugging of the tip.

In order to overcome the foregoing difficulties, I provide a method of spraying liquiform material wherein a stream of the liquiform material directed into low pressure region 12 is broken up into a spray for impingement upon the surface of belt 13 by the direction of at least one pressurized stream of vapor, preferably the suspending medium of the liquiform material in vapor form, upon the entering stream of liquiform material. Where the spray nozzle 14 is employed to produce the spray, the vapor is directed under pressure to the inlet 18 of the gun. The temperature of the vapor, moreover, is best regulated at a temperature well above the freezing temperature of the liquid at the pressure maintained in region 12. The differential between the pressure of the entering vapor and pressure in region 12 is preferably adjusted sufficiently great to achieve the desired pattern Vand breakup of the liquiform material stream and yet sufficiently small that the amount of heat absorbed from the liquiform material by the vapor in expanding through the pressure differential is less than the amount of heat loss required to lower the temperature of the liquiform material to its freezing point. The liquiform material at or near the nozzle tip of gun 14 is thereby prevented from freezing. In addition the moisture content of the vapor is best regulated sufficiently great to keep the nozzle tip wet but not so great as to form droplets thereon and interfere with gun operation.

The vapor for the accomplishment of the spraying method in accordance with the present invention may be prepared by any one of various methods and means therefor well known in the art. One highly flexible arrangement by which water vapor, for example, can be prepared having temperature, pressure, moisture content, and the like variable over a wide range to -facilitate spray operation of the gun 14 over a wide range of low pressures in region 12 in accordance with the considerations described hereinbefore is schematically depicted in the drawing. In the illustrative example, water vapor is superheated by high pressure steam in a heat exchanger 22 in a readily adjustable manner and the super heated vapor fed to inlet 18 of spray gun 14. More specifically, the high pressure steam is fed by conduit 29 to shell 31 the enclosed space of which constitutes the hot side of exchanger 22 and the condensate is removed from the high pressure steam at the hot side of exchanger 22 as by passing the steam and condensate therefrom through conduit 32 to a steam trap 23. The pressure of the steam is next reduced preferably to the range of l to p.s.i.g. by feeding the steamy through conduit 33 to a pressure reducing valve 24 or equivalent means. Condensate in the resulting low pressure steam is then removed by passing same through conduit 34 to a trap 26 and the low pressure steam is then led to the cold side of heat exchanger 22 by means of conduit line 36 for superheating to the desired degree. The quantity of steam or water vapor fed to the exchanger is controlled as by means of a valve 27 inserted in the line between trap 26 and the cold side of the heat exchanger. The cold side of heat exchanger 22 constitutes the header-tube bundle 37 within the shell 31 thereof wherein the low pressure vapor absorbs heat from the steam in shell 31 and is thereby provided with the desired degree of superheating. Superheated steam from the heat exchanger 22 is applied to the inlet 18 of gun 14 through conduit line 39 in which is provided a control valve 28. With the foregoing arrangement for preparing superheated water vapor, the amount, pressure, temperature and degree of undersaturation of the water vapor is readily adjustable through a combination of settings of reducing valve 24 and control valves 27 and 2S.

As an operating example of the method of the present invention, the vacuum system was set to establish a pressure of 5 mm. of mercury absolute in low pressure region 12. Tomato paste containing 30% solids by weight was pumped to spray gun 14 by means of pump 16. Water vapor at atmospheric pressure (212 F. steam) from the cold side of heat exchanger 22 was expanded through valve 28 to supply heated water vapor at about 140 F. to the spray gun. The differential between the pressure of water vapor in the vapor feed line (approximately mm. of mercury) and that in the low pressure region was suicient to provide a ow of vapor through the gas ports of spray gun 14 sufficient to break up the stream of tomato paste into minute particles and to direct these particles into a liat and uniform spray pattern on the moving belt 13. The vapor supplied to the gun was observed to contain particles of liquid water due to the adiabatic expansion of the vapor taking place in valve 28. The nozzle tip remained wet, no whiskers were formed and a wet, non-dusting spray was obtained in continuous operation over a period of many hours.

In another operating example of the present invention, the vacuum system was `set to establish an absolute pressure of 2.5 mm. in region 12. Tomato paste containing 32% solids by weight was pumped to spray gun 14 by means of pump 16. Water vapor at 14 p.s.i.g. and containing 12 F. superheat was supplied from the cold side of heat exchanger 22 and expanded through valve 28 to supply heated Water vapor at about 160 F. to the spray gun. The water vapor supplied to the gun contained fewer droplets of condensed water than in the preceding example, but suilicient heat and moisture was supplied to the nozzle tip so that the tip remained wet, no whiskers were formed and a wet, non-dusting spray was obtained in continuous operation over a period of many hours.

:In still another operating example of the present invention, the Vacuum system was set to establish an absolute pressure of 3.2 mm. in region 12. Tomato paste containing 32% solids by weight was pumped to spray gun 14 by means of pump 16. Water vapor at 7 p.s.i.g. and containing 20 F. superheat was supplied from the cold side of heat exchanger 22 and expanded through valve 28 to supply heated water vapor at about 150 F. to the spray gun. The water vapor supplied to the gun contained fewer droplets of condensed water than in the preceding example, but sufficient heat and moisture was supplied to the nozzle tip so that the tip remained wet, no whiskers were formed and a wet, non-dusting spray was obtained in continuous operation over a period of many hours.

Similarly, at any given absolute pressure in region 12, it was found that several combinations of the settings of valves 24, 27, and 28 would produce the desired type of pattern and continuous operation. The valves are set to provide the necessary difierential pressure between the vapor entering the spray gun and the vacuum region to achieve the desired pattern and stream breakup, suificient heat to prevent yfreezing of material at Vor near the nozzle tip in the 4form of heat carried in the vapor, and sutlcient moisture in the vapor to keep the nozzle tip wet rbut insuicient moisture to `form droplets and interfere with gun operation,

It will be appreciated that the spraying method of the present invention is -further advantageous in that with such a method the admission of air to the vacuum -system providing low pressure region `12 is eliminated. Vacuum systems primarily designed ifor the removal of solvent or suspending liquid are conventionally provided with internal condensing systems capable of condensing considerable volumes of solvent or supending liquid but are provided with only enough air removal capacity to handle small air leaks. "Hence considerably larger equipment would be required if air were used as the spraying gas.

What is claimed is:

1 In a process for continuously spraying a liquiform aqueous suspension in a closed system evacuated to a lou subatmosp-heric pressure, the steps comprising directing at least one jet stream of said suspension into an intermixing zone in communication with said closed system, directing a Stream of superheated steam into said intermiXing zone to impin-ge upon, intermix with and heat said suspension stream, said steam being superheated to a temperature sufficient to eliminate freezing of the suspension at least in the area of introduction into said closed system and being introduced at such a rate as to maintain a subatmospheric zonal vapor pressure in said intermiX-ing zone above the pressure in said closed system, and discharging the intermixed streams from said intermixing zone through a restrictive orifice zone into said closed system to maintain said vapor pressure, said zonal vapor pressure thereby propelling and dispersing said suspension into said closed system to -form a spray stream of suspension droplets in water vapor whererby drying of said droplets is minimized, wherein said reduced pressure of said closed system is below about l2 mm. Hg, the zonal vapor pressure in said intermixing zone is of the order of about 150 mm. Hg and said steam is superheated to a temperature of about 12 to 20 F. above the boiling point in said system.

2. In a process for continuously spraying a uniformly distributed layer of a liquiform suspension upon a surface in a `closed system evacuated to a low sub-atmospheric pressure of about 12 mm. Hg absolute, the steps cornprising directing at least one jet stream of said suspension into an intermixing zone in communication with said closed system, the pressure in the intermixing zone being on the order of about 150 Hg, directing a stream of superheated vapor comprising the superheated vaporized uid phase of said suspension into said intermixing zone to impinge upon, intermix with and heat said suspension jet stream7 said vapor comprising the vaporized fluid phase of said suspension being superheated to a temperature of about 12 to 20 F. above the boiling point in the system sufficient to minimize freezing of the suspension in the area of introduction into said system and being introduced at such a rate as to maintain a subatmospheric vapor pressure in said intermixing zone above the lpressure in said closed system, and discharging the intermixed streams through a restrictive orice zone into said closed system to maintain dierential vapor pressure therebetween thereby propelling and dispersing said suspension into said closed system to -form a spray stream of liquid droplets in vapor of said Huid phase whereby drying of said droplets in the vicinity of said orifice zone is minimized, and directing said spray stream to deposit upon said surface in the closed system to deposit thereon as a distributed fluidic layer.

3. In a process Ifor continuously spraying a uniformly distributed layer of a liquiform suspension -food-stui upon a surface in a closed system evacuated to a low subatmospheric pressure of below about 12 mm. Hg absolute, the steps comprising directing at least one jet stream of said suspension into an intermixing zone in cornmunication with said closed system, directing a stream of vapor comprising the superheated vaporized fluid phase of said suspension into said intermixing zone to impinge upon, intermix with and heat said suspension jet stream, said vapor being superheated to a temperature in the range of about 12 to 20 F. above the boiling point in said closed system sufcient to minimize freezing of the suspension in the area of introduction into said system and being introduced at such a rate as to maintain a subatmospheric vapor pressure in said intermixing zone above the pressure in said closed system, and discharging the intermixed streams through a restrictive orice zone into said closed system to maintain a dilerential vapor pressure therebetween, thereby propelling and dispersing said suspension into said closed system to form a spray stream of liquid Idroplets in vapor of said iluid phase whereby drying of said droplets is minimized in said area, and directing of said spray stream to deposit upon said surface in the closed system to deposit thereon as a distributed fluidic layer.

4. The process as defined in claim 3 wherein said pressure in said intermixing zone is of the order of about mm. Hg.

5. The process as dened in claim 3 wherein said spray stream is directed progressively along said surface whereby a continuous tluidic film is uniformly applied upon said surface.

References Cited in the tile of this patent UNITED STATES PATENTS 1,350,247 Stutzke Aug. 17, 1920 1,432,635 Stevens Oct. '17, 1922 2,578,412 Fisher Dec. 11, 1951 2,636,555 Klepetko et al Apr. 28, `1953 2,984,420* Hession May 16, 1961

Claims (1)

1. IN A PROCESS FOR CONTINUOUSLY SPRAYING A LIQUIFORM AQUEOUS SUSPENSION IN A CLOSED SYSTEM EVACUATED TO A LOW SUBATMOSPHERIC PRESSURE, THE STEPS COMPRISING DIRECTING AT LEAST ONE JET STREAM OF SAID SUSPENSION INTO AN INTERMIXING ZONE IN COMMUNICATION WITH SAID CLOSED SYSTEM, DIRECTING A STREAM OF SUPERHEATED STEAM INTO SAID INTERMIXING ZONE TO IMPINGE UPON, INTERMIX WITH AND HEAT SAID SUSPENSION STREAM, SAID STEAM BEING SUPERHEATED TO A TEMPERATURE SUFFICIENT TO ELIMINATE FREEZING OF THE SUSPENSION AT LEAST IN THE AREA OF INTRODUCTION INTO SAID CLOSED SYSTEM AND BEING INTRODUCED AT SUCH A RATE AS TO MAINTAIN A SUBATMOSPHERIC ZONAL VAPOR PRESSURE IN SAID INTERMIXING ZONE ABOVE THE PRESSURE IN SAID CLOSED SYSTEM, AND DISCHARGING THE INTERMIXED STREAMS FROM SAID INTERMIXING ZONE THROUGH A RESTRICTIVE ORIFICE ZONE INTO SAID CLOSED SYSTEM TO MAINTAIN SAID VAPOR PRESSURE, SAID ZONAL VAPOR PRESSURE THEREBY PROPELLING AND DISPERSING SAID SUSPENSION INTO SAID CLOSED SYSTEM TO FORM A SPRAY STREAM OF SUSPENSION DROPLETS IN WATER VAPOR WHEREBY DRYING OF SAID DROPLETS IS MINIMIZED, WHEREIN SAID REDUCED PRESSURE OF SAID CLOSED SYSTEM IS BELOW ABOUT 12 MM. HG, THE ZONAL VAPOR PRESSURE IN SAID INTERMIXING ZONE IS OF THE ORDER OF ABOUT 150 MM. HG AND SAID STEAM IS SUPERHEATED TO A TEMPERATURE OF ABOUT 12 TO 20*F. ABOVE THE BOILING POINT IN SAID SYSTEM.

Images