Spray nozzle unit

Abstract

A nozzle unit having a pressure nozzle for spraying feed liquid and a cylindrical outer tube disposed around the pressure nozzle for high-speed gas blowing. The tip of the nozzle unit is of converging construction. When water is sprayed at low pressure in the nozzle unit, water is atomized to fine droplets.

Description

FIELD OF THE INVENTION

The present invention relates to a spray nozzle unit which can function satisfactorily even when low pressure is applied during a period of start-up, and a spray drying apparatus equipped with the nozzle unit.

BACKGROUND OF THE INVENTION

In general, temperatures in a spray drying chamber should be stabilized during a start-up period to avoid product overheating and to provide thermal protection for equipment downstream of the spray drying chamber. Therefore, water is usually injected through the pressure nozzle which is used for a feed liquid.

The rate of water to be sprayed must be equivalent to the water content of the feed liquid, which is usually in the range of 30-80% by weight. Thus, the rate of water to be sprayed is also 30-80% by weight of that of the feed liquid. Because of pressure nozzle characteristics, the spray nozzle pressure decreases to 10-80%, depending on the liquid viscosity when this low rate of water is fed. Water droplets thus produced are likely to be so coarse that they may adhere to the surface inside the drying chamber. Subsequent spraying of the feed liquid causes dried powder to adhere to the wet surface to form deposits.

The prior art devices as shown in FIGS. 6 and 7 are used to cope with this situation. In FIG. 6, a plurality of spray nozzles 10 are disposed at the top of and inside the spray drying chamber. When water is injected for spraying, the number of spray nozzles used is limited to avoid low pressure spraying. On the other hand, FIG. 7 illustrates an example in which a water spray nozzle 11 is disposed separately from a feed liquid spray nozzle 12.

However, the device of FIG. 6 has disadvantages of uneven liquid droplet dispersion and nonuniform temperature distribution because of a relatively large distance between the nozzles and the very existence of unused spray nozzles. In addition, plugging problems are likely to occur around the nozzles left unused when water is injected.

On the other hand, the device of FIG. 7 has a disadvantage of feed liquid clogging inside the feed liquid spray nozzle since cooling or flushing cannot be conducted through the feed nozzle.

In view of the foregoing disadvantages of the prior art, it is one object of the present invention to provide a spray nozzle and a spray drying apparatus using the spray nozzle, in which water is atomized into such fine particles to permit complete drying and is used to cool the spray nozzle as well to prevent plugging of feed liquid.

SUMMARY OF THE INVENTION

In order to attain the above object, the present invention provides a nozzle unit comprising a centrifugal pressure nozzle for spraying feed liquid, and a cylindrical outer tube disposed around the centrifugal pressure nozzle for high-speed gas blowing, characterized by the converging construction of the tip of the nozzle unit. A spray drying apparatus using the nozzle unit is further provided according to this invention.

A gas slit for providing swirling motion in a high-speed gas stream is desirably formed between the pressure nozzle and the cylindrical outer tube to obtain the larger spray angle of liquid droplets.

According to the present invention, a gas is blown at high speed through the annulus formed between the pressure nozzle and the cylindrical outer tube so as to atomize water into very fine droplets even when only low pressure, which would otherwise produce coarse droplets, is applied in the pressure nozzle. Therefore, complete drying is carried out, and no water droplets adhere to the inside wall of the spray drying apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a preferred embodiment of the nozzle unit of the present invention.

FIG. 2 shows a partial cross sectional view of the end portion of the nozzle unit illustrated in FIG. 1

FIGS. 3 (a), (b) and (c) depict an example of a slit used for the nozzle unit of the invention. FIGS. 3 (a), (b) and (c) are top plan, bottom plan and side views, respectively.

FIG. 4 depicts an example of the slit used for the nozzle unit of the present invention.

FIG. 5 is a schematic illustration of an embodiment of the spray drying apparatus equipped with the spray nozzle unit of the present invention.

FIGS. 6 and 7 show conventional nozzle units.

FIG. 8 shows an enlarged cross sectional view along line X-X' in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The following examples describe preferred embodiments to illustrate the present invention with particular reference to the drawings. However, it is to be understood that the invention is not intended to be limited to the specific embodiments.

Referring now to the drawings, there are shown a feed liquid (or water) pump 1, a Roots blower 2, a jacket pipe 3, an air nozzle 4, a feed liquid (or water) pipe 5, and a centrifugal pressure nozzle 6 for discharging feed liquid (or water) by imparting a spin thereto. Feed liquid pipe 5 and centrifugal pressure nozzle 6 form a conduit nozzle structure 14. The jacket pipe 3 is disposed around the conduit nozzle structure 14 to form a volume therebetween. The end portion 15 of the centrifugal pressure nozzle 6 and the end portion 16 air nozzle 4 are of converging construction or so shaped that their diameters diminish as they near the tip of the nozzle as shown in FIGS. 1 and 2.

For the purpose of increasing spray angles of feed liquid or water, it is preferable that a gas slit 7 is provided between the pressure nozzle 6 and the air nozzle 4 or on the outer area of the pressure nozzle 6 to give swirling motion to the discharging air stream as shown in FIGS. 3 (a), (b) and (c), and FIG. 4.

The spraying pressure of feed liquid or water required for the pressure nozzle 6 is appropriately determined using the following Equation I and Equation II. The former is the general equation expressing flow characteristics of a pressure nozzle while the latter expresses droplet diameters for a specific pressure nozzle used, which is an SX nozzle (centrifugal type) manufactured by Spraying Co. for one preferred embodiment of this invention.

W=K.sub.1 .multidot.D.sup.2 .multidot.P.sup.0.6. . . I where W is flow rate (kg/h), K.sub.1 is coefficient, D is orifice diameter (mm), and P is pressure (kg/cm.sup.2). D.sub.P =K.sub.2 .multidot.W.sup.-0.44 .multidot..mu..sup.0.16 .multidot.D.sup.1.52. . . II where W is flow rate (kg/h), D.sub.P is liquid droplet diameter (.mu.m), K.sub.2 is coefficient, and .mu. is liquid viscosity (cp).

The air nozzle 4 disposed around the pressure nozzle 6 has an air velocity of 80 m/s or higher, preferably 100 m/s or higher, and generally has an air pressure of 0.1 kg/cm.sup.2 or higher, preferably 0.2 kg/cm.sup.2 or higher, but both air velocities and air pressures are not limited to theses values. Other values beyond the above ranges may be used depending on the construction of the nozzle used.

As shown in FIG. 8, A.sub.G represents a cross-sectional area of a gas emitting portion between end portions 15 and 16, where the speed of gas flow is the largest. A.sub.L represents a cross-sectional area of a liquid emitting portion of the tip of the nozzle 6, along the cross-sectional plane X-X'. According to an important feature of the present invention, a ratio A.sub.G /A.sub.L is preferably in a range of 10-200, and more preferably within a range of 20-150. In contrast, the A.sub.G /A.sub.L ratio of conventional two-fluid nozzles is in a range of 0.5-7.

In conventional two-fluid nozzles, a gas pressure of 1-10 kg/cm.sup.2 provides high speed gas flow to disperse the water flowing through its central nozzle. Thus, the gas-emitting portion (annulus) of such nozzles must be made relatively small. Further, the liquid emitting portion is relatively large since liquid is fed at low pressure, typically about 1 kg/cm.sup.2.

The present invention permits feeding of liquid at relatively high pressures about 1.5-100 kg/cm.sup.2, and feeding of gas at relatively low pressures, about 0.1-0.9 kg/cm.sup.2. This is accomplished by providing the above A.sub.G /A.sub.L ratio within a range of 10-200, preferably 20-150, as noted above. According to this development of the present invention, the cross sections of the liquid-emitting and gas-emitting portions are relatively small and large, respectively.

Provided below is a summary of comparative data for a typical conventional two fluid nozzle and the present nozzle.

CONVENTIONAL TWO-FLUID NOZZLE

Conditions:

The ratio of gas amount G to the liquid amount L is 1 (G/L=1). A.sub.L =1.times.10.sup.-6 m.sup.2 u.sub.L (speed of liquid flow)=1 m/s, u.sub.G (speed of gas flow)=300 m/s .rho..sub.G (density of gas)=1.2 kg/m.sup.3, .rho..sub.L (density of liquid)=1000 kg/m.sup.3

Data:

L=u.sub.L .times.A.sub.L .times..rho..sub.L =10.sup.-3 kg/s G/L=1.thrfore.G=10.sup.-3 kg/s A.sub.G =G/(u.sub.G .times..rho..sub.G)=2.78.times.10.sup.-6 m.sup.2 .thrfore.A.sub.G /A.sub.L -2.78

PRESENT NOZZLE

Conditions:

G/L=0.5 A.sub.L =1.times.10.sup.-6 m.sup.2 u.sub.L =10 m/s, u.sub.G =140 m/s, .rho..sub.G =1.2 kg/m.sup.3, .rho..sub.L =1000 kg/m.sup.3

Data:

L=u.sub.L .times.A.sub.L .times..rho..sub.L =10.sup.-2 kg/s G/L=0.5.thrfore.G=5.times.10.sup.-3 kg/s A.sub.G =G/(u.sub.G .times..rho..sub.G)=2.98.times.10.sup.-5 m.sup.2

.thrfore.A.sub.G /A.sub.L =29.8

Referring now to FIG. 5 which is a schematic illustration of an embodiment of the spray drying apparatus equipped with the spray nozzle unit of the present invention, it will be described how the spray drying apparatus is operated.

For start-up, a feed liquid pump 1 discharges water via a feed liquid pipe 5 to a pressure nozzle 6 for spraying. Spraying of water is carried out at significantly low pressure. However, since air is blown off at high speed around the pressure nozzle 6 and swirling motion is formed in the air stream, preferably with the use of a slit 7, water is atomized into fine droplets of the desired particle size even at low pressure.

With fine water droplets of the desired particle size, every water droplet is dried with hot air as referred to as A which is blown into a drying chamber 8 of the spray drying apparatus. Thus, no liquid water is present in the drying chamber 8 and almost no temperature distribution is found in the chamber. In other words, the temperature in the drying chamber 8 is maintained constant.

Then, a feed liquid to meet a specific objection is blown into the drying chamber 8 of the spray drying apparatus via the pressure nozzle 5 of the above nozzle unit and is dried via hot air blown in through inlet 9 to obtain a powder product of specified grade.

When the feed liquid is actually sprayed using this nozzle unit, it is desirable to let a little air flow through the air nozzle 4 because the air can cool the nozzle unit for preventing feed liquid plugging. Now the present invention will be described in detail in connection with the following examples:

EXAMPLE 1

Atomization tests for a feed liquid and water were made under the conditions shown in Table 1. An SX nozzle having a hexagonal cross-section manufactured by Spraying Co. was used for a pressure nozzle 6. The circumference of the nozzle tip was covered with a cylindrical pipe having a circular cross-section to obtain an annular space used for an air nozzle 4. The distance between the SX nozzle and the cylindrical pipe were about 5 mm at their widest site, and about 3 mm at their closest site. The inner diameter of the cylindrical pipe was 7 mm at its tip. The nozzle of the present invention had an A.sub.L =4.83.times.10.sup.-7 m.sup.2, A.sub.G =3.73.times.10.sup.-5 m.sup.2 and A.sub.G /A.sub.L =77.2.

The results of these tests are shown in Table 1 below.

TABLE 1______________________________________ Conventional Nozzle of Conventional Nozzle This Invention Nozzle______________________________________Feed Liquid Poly vinyl- Water Water chloride (PVC)Orifice Dia./Core 0.787/425 0.787/425 0.787/425(mm)Spray Pressure 23 6 6(kg/cm.sup.2)Feed Rate (kg/h) 50 30 30Liquid Viscosity 110(cp)Solids Content 40(%)Air Pressure 0.26(kg/cm.sup.2)Air Flow Rate 20.5(kg/h)Air Blow 127.1Speed (m/s)Inlet Temp. (.degree.C.) 102 102 102Outlet Temp. 55 55 58(.degree.C.)Particle Size (.mu.m) 91 40* 120*Spray Angle about 15 about 60(deg.)Dryness Good No Good No Poor Wet Wet Material Wet Material Material ad- adhered adhered hered to dry chamber cone section______________________________________ *denotes droplet size.

The pressure nozzle used was an SX nozzle manufactured by Spraying Co.

In addition, the test was made for the case in which swirling motion was provided in the high-speed air stream in the nozzle unit of the present invention. The results are shown below.

______________________________________ Nozzle According to This Invention______________________________________Average Droplet Diameter (.mu.m) 40Spray Angle (Deg.) Approx. 30______________________________________

EXAMPLE 2

The same nozzle as described in Example 1 was used, but nozzle diameters were changed to obtain particles of larger sizes. The present nozzle had an A.sub.L =8.94.times.10.sup.-7 m.sup.2, A.sub.G =3.73.times.10.sup.-5 m.sup.2 and A.sub.G /A.sub.L =41.7. Additionally, a large drying chamber was used in this example.

The results of these tests are shown in Table 2 below.

TABLE 2______________________________________ Conventional Nozzle of Conventional Nozzle This Invention Nozzle______________________________________Feed Liquid Poly vinyl- Water Water chloride (PVC)Orifice Dia./Core 1.067/425 1.067/425 1.067/425(mm)Spray Pressure 7 2 2(kg/cm.sup.2)Feed Rate (kg/h) 50 30 30Liquid Viscosity 110(cp)Solids Content 40(%)Air Pressure 0.26(kg/cm.sup.2)Air Flow Rate 20.5(kg/h)Air Blow 127.1Speed (m/s)Inlet Temp. (.degree.C.) 102 102 102Outlet Temp. 55 55 65(.degree.C.)Particle Size 150 60* 640*(.mu.m) Abnormal sprayingSpray Angle about 15 about 60(deg.)Dryness Good Good No Poor Wet Wet Material Material ad- adhered hered to dry chamber cone section______________________________________ *denotes droplet size.

As clearly seen from the above results, water is atomized to give fine water droplets with the nozzle according to the present invention. Dryness in the spray drying apparatus is improved since spray angles increase when swirling motion is provided in air streams discharging from the nozzles.

The effects of the present invention include:

1) Even when water is sprayed at low pressure in the spray nozzle of the present invention, water is atomized to fine droplets which is then completely evaporated. 2) Feed liquid plugging can be prevented by the cooling of the spray nozzle unit. 3) In a spray drying apparatus equipped with this spray nozzle unit, the spraying of a feed liquid can be performed effectively and stably even after water is sprayed at low pressure.

Claims

1. A nozzle unit for atomizing liquid sprayed under low pressure therethrough, comprising:

a feed liquid conduit;

liquid spraying means for swirling and spraying liquid passed from the feed liquid conduit, said liquid spraying means including a centrifugal pressure nozzle connected to said feed liquid conduit, said centrifugal pressure nozzle having a frustoconical tip having an open end through which the feed liquid is sprayed, said open end having a cross-sectional area equal to A.sub.L, said feed liquid conduit and said centrifugal pressure nozzle forming a conduit nozzle structure; and

a tubular member disposed about said conduit nozzle structure to form an area through which a gas stream is passed, said tubular member including a convergent end portion surrounding said frustoconical tip of said centrifugal pressure nozzle, such that a gas-emitting annulus is formed between said convergent end portion and said frustoconical tip along a plane of said cross-sectional area A.sub.L, said gas-emitting annulus having an area A.sub.G ;

wherein a ratio A.sub.G /A.sub.L is within a range of 10-200.

2. The nozzle unit of claim 1, wherein A.sub.G /A.sub.L is within a range of 20-150.

3. The nozzle unit of claim 1, further comprising gas swirling means for swirling a gas stream passed through said gas-emitting annulus.

4. The nozzle unit of claim 3, wherein said gas swirling means comprises a frustoconical member having at least one slit through which the gas stream passes.

5. The nozzle unit of claim 1, wherein said gas-emitting annulus is formed at a position where a distance between said frustoconical tip and said convergent end portion is smallest such that a velocity of the gas stream is highest at said position.

6. The nozzle unit of claim 1, wherein said frustoconical tip has an outer surface which continuously converges to said open end.

7. The nozzle unit of claim 1, wherein a diameter between said frustoconical tip and said convergent end portion continuously decreases to the open end of the frustoconical tip.