US20100279587A1

US20100279587A1 - Apparatus and method for particle radiation by frozen gas particles

Info

Publication number: US20100279587A1
Application number: US12/450,827
Authority: US
Inventors: Robert Veit; Eckart Uhlmann
Original assignee: Technische Universitaet Berlin
Current assignee: Technische Universitaet Berlin
Priority date: 2007-04-13
Filing date: 2008-04-14
Publication date: 2010-11-04
Also published as: DE102007018338A1; EP2136965A1; EP2136965B1; DE102007018338B4; WO2008125648A1

Abstract

The invention relates to a device for pressure blasting by means of a mixed jet made of particles of a frozen gas and a carrier gas, comprising a nozzle housing, which encloses an outer cavity and an inner cavity, wherein the inner cavity forms an expansion space, which comprises an inlet at its upstream longitudinal end for inducting a liquefied gas into the expansion space, and comprises an outlet opening at its downstream longitudinal end, wherein the outlet opening comprises a much larger cross section than the inlet, and wherein the outer cavity envelops the inner cavity at least partially in the portion of the outlet opening, at least one liquid gas supply, which is connected to the inlet of the expansion space, a support gas supply, which is connected with the outer cavity, and an acceleration nozzle having a nozzle outlet downstream of the outlet opening of the expansion space, the acceleration nozzle initially contracts in flow direction, wherein a carrier gas defined laterally from the outlet opening and has cross section which is variably adjustable.

Description

The invention relates to a device and a method for pressure blasting by means of a mixed jet made of frozen gas particles and a carrier gas. The invention relates in particular to a device and a method for CO₂snow blasting by means of a mixed jet made of frozen CO₂gas particles and a carrier gas.
Frozen gas particles are particles made of a substance, which is gaseous at normal ambient temperature and normal ambient pressure.
Blasting with solid carbon dioxide became popular in recent years in different fields of application. When sensitive surfaces need to be uncoated or cleaned, or a secondary contamination through blasting media is undesirable, this technology can play out its advantages.
The low hardness of solid carbon dioxide facilitates treating a large spectrum of materials without damaging them. Due to the sublimation of the blasting media only the removed pure coating or contamination has to be disposed of.
When blasting by means of frozen gas particles, the blasting media is pneumatically accelerated and applied to the surface to be treated. Contrary to the purely mechanical effect of other blasting media, blasting with frozen gas particles is based on three different effects. Through the low temperature of the blasting media, a thermal tension between the coating and contamination of the substrate is created. Furthermore, the kinetic energy of the frozen gas particles leads to a mechanical separation which is supported by the third effect, the pressure shock due to the instantaneous sublimation of the frozen gas particles.
Such devices and methods are known in principle and there is a multitude of different configurations, which give the mixed jet made of frozen gas particles and the carrier gas different properties with respect to e.g. velocity, volume flow, size, number and characteristics of the frozen gas particles, so that a desired effect can be created on the work piece or the surface during operation.
Thus, in particular a differentiation is made between two basic configuration principles. The first configuration, which is also designated as dry ice blaster differs from the second configuration which is also designated as snow blaster, in that the first type generates the mixed jet from the solid phase and the second type generates the mixed jet from the liquid phase. For dry ice blasting, the blasting media is provided in a separate process in the form of pellets or blocks, and subsequently added to the compressed airflow in a blasting apparatus.
Since it is an object of the present invention, to provide a device for pressure blasting with frozen particles, which device is small in size and can thus be integrated in machinery and equipment easily, the present invention relates to a device for pressure blasting by means of a mixed jet comprised of frozen gas particles and a carrier gas according to the second configuration. Accordingly, the blasting medium, in particular CO₂, is stored under pressure in liquid phase in the devices described herein.
Also for this configuration, which is also designated as a snow blaster, two configurations are differentiated in turn: the two-material ring nozzle and the blasting nozzle with an agglomeration chamber.
In the two-material ring nozzle, the liquid gas is expanded to ambient pressure at the exit of the nozzle. The snow particles created are focused and accelerated by an enveloping jet made of supersonic compressed air.
The frozen gas particles formed in the two-material ring nozzle have a lower diameter compared to the ones formed in the blasting nozzle with agglomeration chamber, and thus have lower kinetic energy at the same velocity. Therefore, the particles which are generated according to said configuration have little abrasive effect, and such devices are therefore used primarily for cleaning highly sensitive components with a fine structure. Such a device is described in DE 199 26 119 C2.
In a device of the type with the second configuration, liquefied gas is inducted into an agglomeration chamber together with the carrier gas flow and expanded. Thus, larger snow particles are created compared to the two-material ring nozzle, which are then accelerated through the compressed air in a subsequent nozzle and cause a significantly stronger abrasive effect. Such a method and such a device are described in DE 102 43 693 B3.
While the first configuration with two-material ring nozzle has the disadvantage of a low abrasive effect, the second variant of the pressure blasting device with agglomeration chamber has the disadvantage that a high pressure drop occurs during operation. Furthermore, frozen gas particles accumulate in the interior of the agglomeration chamber at the outer walls and disengage from the outer walls in uneven time intervals and with undefined size. Thus, the materials removal rate increases in pulses, which creates an inhomogeneous blasting pattern.
A CO₂cold gas jet for pressure blasting by means of a mixed jet comprised of CO₂particles and compressed air is known from DE 202 14 063 U1.
Thus, it is the object of the present invention to provide a mixed jet technology which is not provided by the known configurations and variants.
The deficiency of the prior art solutions is that they cannot provide a strong abrasiveness of the mixed jet while consuming a small amount of compressed air. Such a device can also be connected to typical compressed air lines in shops, while providing the required strong abrasiveness.
Furthermore, it is the object of the present invention to configure the abrasiveness, this means in particular the size of the frozen gas particles and their volume adjustable and thus to make their abrasiveness variable.
This object is accomplished by a device for particle blasting with frozen particles, which stores the blasting medium in liquid form. Thus, the device is among the group of snow blasting devices.
The device according to the invention comprises a nozzle housing which encloses an outer and an inner cavity.
Thus, the inner cavity forms an expansion—or agglomeration space, which comprises an inlet connected to the supply for liquefied gas for inducting a liquefied gas at a longitudinal end disposed upstream, and an outlet opening at its longitudinal end disposed downstream. The outlet opening thus comprises a much larger cross section than the inlet.
This inner cavity is enveloped by an outer cavity at least in the portion of the outlet of the inner cavity, which outer cavity is connected to at least one carrier gas supply.
The inner cavity and the outer cavity preferably comprise circular cross sections.
An initially converging acceleration nozzle connects in flow direction to the outlet opening of the expansion space and to the outer cavity, which acceleration nozzle comprises a lateral carrier gas inlet as an outlet for the outer cavity, which carrier gas inlet is in particular disposed on all sides of the outlet opening.
Since it is an object of the present invention as described supra to configure the abrasiveness, this means in particular the size of the frozen gas particles and their volume, adjustable, and thus to configure their abrasiveness variable, the cross section of the carrier gas inlet is variably adjustable according to the invention.
In a transition portion between the supply for the liquefied gas and the expansion space, there is a dosage device which forms the inlet for the expansion space and which is configured preferably as an expansion—or needle valve nozzle preferably with a variably adjustable interior diameter. In the flow direction behind the dosage device, the flow diameter instantaneously expands from the inner diameter of the dosage device to the inner diameter of the expansion space. This expands the liquefied gas in the expansion space which forms a mixture made from frozen gas particles and gas.
During the flow of the mixture of frozen gas particles and gas through the expansion space, particular particles agglomerate with one another, so that an increase of the particle size occurs downstream in the expansion space, or also in the agglomeration space.
Preferably, the diameter of the expansion space is configured, so that the cross section of the expansion space continuously increases downstream.
Said cross section expansion of the expansion space towards the nozzle exhaust provides a continuous flow and thus a safe removal of the snow particles created. With a constant cross section, accretion and accumulation of solid gas particles occurs in the so-called “dead spaces” directly after jetting in the liquefied gas. These accretions come off in uneven time intervals, so that an inhomogeneous and pulsating blasting pattern of the nozzle is created, which is also designated “coughing” in the art. The comparatively large particle agglomerations have a higher kinetic energy and thus impact the blasted surface more strongly. This effect is negative for reproducible application of the snow blasting technique. Furthermore, the accumulation of frozen gas particles can create a plugging of the blasting nozzle.
In order to configure the abrasiveness, this means in particular the size of the frozen gas particles and their volume adjustable, the volume flow of the liquefied gas flowing into the expansion space and also the carrier gas flow flowing into the outer cavity is variably adjustable.
Since the abrasiveness substantially is a function of the particle size, which is also a function of the length or the volume of the expansion space or the agglomeration space according to a preferred embodiment of the invention, the volume of the agglomeration space is also variably adjustable according to a preferred embodiment of the invention.
Preferably, the volume of the agglomeration space can be varied in that the dosage device which is disposed in the transition portion between the supply for the liquefied gas and the expansion space can be moved in the transition portion and parallel to the flow direction, so that the length or the volume of the agglomeration space changes.
According to another embodiment of the invention, also the agglomeration space can be configured movable in the direction of the longitudinal axis, so that also here the relative position of the dosage device is movable in the transition portion so that the volume of the agglomeration space is variable.
The volume of the expansion space can also be configured variable through a variably adjustable inner diameter of the expansion space according to another embodiment of the invention.
Through the variably adjustable volume flows of liquid gas supply and carrier gas supply and furthermore also through the variable length of the agglomeration space, the flow properties and the size of the frozen gas particles can also be quite different in the portion of the inlet of the acceleration nozzle. When these flow properties are disadvantageous, it can occur, that the frozen gas particles sublimate while being mixed with the carrier gas and before they can impart the desired effect upon the work piece. In order to prevent this, an essential feature of the invention is comprised in that the outlet cross section of the carrier gas inlet which is formed between the outer contour of the expansion space and the inner contour of the inlet of the acceleration nozzle is configured variably adjustable.
Preferably, the device for pressure blasting by means of a mixed jet made of frozen gas particles and a carrier gas is configured, so that the outlet cross section can be varied in that the expansion space can be moved in axial direction relative to the acceleration nozzle with reference to the longitudinal axis of the acceleration nozzle. According to other embodiments of the invention, said outlet cross section is configured variably adjustable in that the expansion space is movable in orthogonal direction relative to the longitudinal axis of the acceleration nozzle. According to another embodiment of the invention, the outlet cross section at said location can be varied in that the inner contour of the acceleration nozzle and/or the outer contour of the outlet of the expansion space can be configured variably at least on a partial portion of its circumference.

Additional features and advantages of the present invention are described infra with reference to the appended drawing figure.

The singular FIG. 1 shows a preferred embodiment of the invention in a cross sectional view.

The illustrated device for pressure blasting comprises a nozzle housing 4 which encloses an outer cavity 6 and an inner cavity 2.
The inner cavity 2 is connected to a supply 7 for inducting liquefied gas into the inner cavity 2. The outer cavity 6 in turn is connected with a supply 3 for inducting pressurized carrier gas into the outer cavity 6.
The inner cavity 2 is defined by an inlet 8 at one longitudinal end, which inlet 8 is defined by the inner diameter of a dosage device 1 according to the illustrated embodiment. The dosage device 1 is disposed in a transition portion between the supply 7 and the inner cavity 2. The dosage device 1 is configured as a needle valve nozzle in the illustrated preferred embodiment and preferably has a diameter between 0.1 mm and 2 mm. After the dosage device 1 operating as an inlet 8 for the inner cavity 2, the inner cavity 2 itself is connected, which comprises a much larger diameter of 3 mm to 50 mm. Due to the instantaneous diameter increase directly behind the inlet 8 to the diameter of the inner cavity 2, the liquefied gas instantaneously evaporates when entering the inner cavity 2 while generating coldness, and a portion of the liquefied gas freezes into small particles. Therefore, the inner cavity 2 is also designated as expansion space.
The inner cavity 2 is defined by an outlet opening 9 at its other longitudinal end, which outlet opening is disposed downstream. From the inlet 8 of the inner cavity 2 to the outlet opening 9, the diameter of the expansion space 2 continuously expands in flow direction, and preferably has a dimension between 5 mm and 70 mm at the outlet opening 9. While flowing through the inner cavity 2, particular particles agglomerate with other particles. Therefore, the inner cavity 2 which forms the expansion space is also designated as agglomeration space.
Directly after the outlet opening 9 of the expansion space 2 and after the outer cavity 6, an acceleration nozzle 5 is connected, which initially contracts in flow direction and which protrudes into the outlet opening 9 of the expansion space 2. The acceleration nozzle 5 has a diameter of preferably between 2 mm and 20 mm at its tightest location. Since the outer contour of the expansion space 2 comprises a smaller diameter in the portion of its outlet opening 9, than the diameter of the inner contour in the transition portion between the inner contour of the outer cavity 6 and the inlet of the acceleration nozzle 5, an annular carrier gas inlet 10 into the acceleration nozzle 5 is created, which annular carrier gas inlet simultaneously forms the outlet of the outer cavity 6.
The inner cavity 2 is configured movable in axial direction with respect to the longitudinal axis of the acceleration nozzle 5 and leads into the acceleration nozzle 5 which contracts at this location. Thereby, the cross section of the carrier gas inlet 10 into the acceleration nozzle 5 can be varied through longitudinal movement of the inner cavity 2. The carrier gas inlet 10 preferably comprises a variably adjustable offset between 0 mm and 2 mm, transversal to the longitudinal axis of the device as a function of the position of the nozzle opening 9 within the device between the outer edge of the nozzle opening 9 of the inner cavity 2 and the inner wall of the outer cavity 6 or of the acceleration nozzle 5.

Claims

1. A device for pressure blasting by means of a mixed jet made of particles of a frozen gas and a carrier gas, comprising:

a nozzle housing, which encloses an outer cavity and an inner cavity,

wherein the inner cavity forms an expansion space, which comprises an inlet at its upstream longitudinal end for inducting a liquefied gas into the expansion space, and comprises an outlet opening at its downstream longitudinal end, wherein the outlet opening comprises a much larger cross section than the inlet, and

wherein the outer cavity envelops the inner cavity at least partially in the portion of the outlet opening,

at least one liquid gas supply connected to the inlet of the expansion space,

a carrier gas supply connected with the outer cavity; and

an acceleration nozzle having a nozzle outlet downstream of the outlet opening of the expansion space, the acceleration nozzle initially contracts in flow direction, wherein a carrier gas inlet is defined laterally from the outlet opening and has a cross section which is variably adjustable.

2. A device according to claim 1, wherein the cross-sectional area of the carrier gas inlet can be varied through relative movement of the inner cavity with respect to the acceleration nozzle.

3. A device according to claim 2, wherein moving the expansion space is performed in axial direction with reference to the longitudinal axis of the acceleration nozzle.

4. A device according to claim 1, wherein the inner diameter of the expansion space continuously expands in flow direction from its upstream longitudinal end to its downstream longitudinal end.

5. A device according to claim 1, wherein the outer cavity and the inner cavity have substantially circular cross sections.

6. A device according to claim 1, further comprising a dosing device having an inner diameter defining the inlet into the expansion space, the dosing device is an expansion nozzle with a substantially smaller diameter than the inner diameter of the expansion space.

7. A device according to claim 1, wherein the carrier gas inlet comprises a variably adjustable distance between 0 mm and 2 mm, transversal to the longitudinal axis of the device between the outer edge of the outlet opening of the inner cavity and the inner wall of the outer cavity or of the acceleration nozzle.

8. A device according to claim 1, wherein the expansion space comprises an inner diameter of the outlet opening between 5 mm and 70 mm.

9. A device according to claim 1, wherein the volume flow of the liquefied gas and also of the carrier gas can be varied.

10. A device according to claim 1, comprising a variably adjustable volume of the expansion space.

11. A device according to claim 10, wherein the volume of the expansion space is configured variably adjustable by moving the dosage device, which is disposed in the transition portion between the supply of the liquefied gas and the expansion space, in the transition portion and parallel to the flow direction, so that the length or the volume of the expansion space changes.

12. A method for cleaning or treating surfaces comprising the steps of:

(a) providing a device for pressure blasting by means of a mixed jet made of particles of a frozen gas and a carrier gas, comprising:

a nozzle housing, which encloses an outer cavity and an inner cavity,

at least one liquid gas supply connected to the inlet of the expansion space,

a carrier gas supply connected with the outer cavity; and

an acceleration nozzle having a nozzle outlet downstream of the outlet opening of the expansion space, the acceleration nozzle initially contracts in flow direction, wherein a carrier gas inlet is defined laterally from the outlet opening and has a cross section which is variably adjustable;

(b) generating a mixed jet made of frozen CO₂particles and a carrier gas using the device; and

(c) cleaning or treating at least one surface with the mixed jet.