Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MEl~IOD FOR BLASTING ICE PARTICLES
IN A SURFACE TREATMENT PROCESS -
FIELD OF THE INVENTION
This invention relates to material removal from a substrate surface by
air blast techniques and more particularly by the air blasting of freshly
formed ice particles to perform work on the substrate surface.
BACKGROUND OF THE INVENTION
Particle blasting has been employed for some time to remove material
from surface structures. Sand blasting and other types of grit blasting have
been used to remove surface finishes from building ex~eriors, vehicle
surfaces, mechanical parts and the like. Sand or grit blasting, however,
requires expensive recovery systems to reduce pollution and other
environme~tal hazards. Water can be used in conjunction with the grit
blasting procedure to reduce particle losses and consequent harm to the
environment.
Al~ough grit blasting is very effective in treating building and vehicle
surfaces, great care has to be exercised in treating more sensitive surfaces
such as airplane skin which cannot be abraded during the surface treatment
process.
The blasting of ice particles resolves a number of the above problems
so that several attempts have been made in providing cornmercially viable
ice blasting equipment. It is appreciated that the blasting of ice particles
provides significantly less environmental harm because subsequent to irnpact
the ice particles melt hence assisting in the removal and disposal of abraded
material. As a result there is a considerable reduction in fL~es contributed to
the environment. Due to the nature of ice particles, there are several
problems associated with blasting the ice particles to achieve sufficient work
on the surface to be treated. By their nature, ice particles are not free-
flowing. Normally, to provide an accumulation of ice particles during
,
machine shut-down and the like, an inventory of ice particles is provided by
various mechanical devices interposed between the ice making system and
the blast nozzle. However, this results in the ice particles packing and
causing plugging problems in the system at various points in the intervening
S mechanical devices. The variety of mechanical devices normally employed
in developing and transporting the ice particles are rotors, augers,
classifiers, cyclone separators, metering devices, overflow receivers, surge
tanks and the like. All of these components are provided in an attempt to
manage the problems associated with ice packing in the system due to the
10 development of ice accumulation. But, by virtue of the provision of these
various components, their own interaction can inadvertently result in packing
of ice particles at various points in the processing system. Furthermore, the
operation of these mechanical components has to be precisely controlled with
special precautions to attempt to avoid ice packing in their components and
15 also avoid system clogging. As is appreciated, these problems can be
further magniffed when ice blast sys~ems are required to operate at distances
some 20 to 50 metres from the ice-forming equipment.
U.S. Patent 4,703,590 di~closes a particle moulding apparatus suitable
for moulding ice particles for blasting purposes. As the ice particles are
20 formed, they are collected in a reservoir at the base of the moulding
rnachine. As the blast system is operated, particles are sucked from the
reservoir in the moulding apparatus and transported to the nozzle for
purposes of doing work. However, it has been found that the inventory of
ice particles within the reservoir of the ice particle making device still causes
25 ice packing and subsequent system clogging, particularly during intermittent
blasting operations.
Another system which provides for the delivery of ice particles to the
blast nozzle is disclosed in U.K. Patent Application 2,171,624. The ice
particles are delivered in segments to a Venturi restriction for pick up by the
30 high speed air. However, when blasting ceases, the ice particles tend to
,:
clog up and pack in their segment portions, resulting in further down-time of
the system until the clogging is dislocated.
An attempt to overcome these problems is disclosed in PCT
International Publication Nos. WO90/14927 and WO91/04449. The systems
5 which are disclosed in these applications, provide for the development of
ice, ice crushing, particle sizing, cyclone separation, fluidization and
delivery of ice particles to the blast nozzle. As already mentioned, such
systems require very precise control and, by virtue of the number of -~
interactive components, defeated ~he objective in attempting to deal with the
accumulation of ice particles and their resultant, by their nature, packing in
the system and causing clogging with consequent significant down-time.
In accordance with this invention, a method and apparatus is now
provided which overcomes the above problems in a direct procedure where
intermittent delivery of the ice particles is readily accommodated. This has
been achieved by creating, on demand, the ice particles required at the blast
nozzle. The freshly formed ice particles are immediately delivered to the
blast nozzle at the mass flow-rate at which they are created. Accumulation
of the ice particles in the system is thereby avoided.
SUMMARY OF TH~ INVENTION
According to an aspect of the invention, a method for air blasting
freshly formed ice particles toward a substrate to perform work on a
substrate surface is provided. The method comprises:
fonning fresh ice particles of a desired particle sizing and at a
desired mass flow rate;
immediately drawing downwardly the freshly folmed ice
particles at the mass flow rate from the ice particle forming step directly intoa proximal end of an ice particle transport hose at reduced pressure;
developing the reduced pressure in the hose by introducing to a
blast nozzle at the hose distal end, air at a first volumetric flow rate which
develops a sufficient velocity at the blast nozzle to blast the freshly forrned
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ice particles at the mass flow rate towards the substrate surface to perform
work on the surface; and
providing in the proximal end of the hose a cool stream of air
at a second volumetric flow rate to cool the fresh ice particles as such
5 particles flow along the hose towards the nozzle, the second volumetric flow
rate of cool air being determined by the reduced pressure in the hose.
According to another aspect of the invention, an apparatus for air
blasting freshly formed ice particles toward a substrate to perform work on a
substrate surface is provided. The apparatus comprises:
means for forming fresh ice particles of a desired particle
sizing and at a desired mass flow rate to perfolm the work;
means for immediately transferring do~,vnwardly at the mass
flow rate, from the ice particle forming means the fresh ice particles to a
pro~imal end of an ice particle transport hose;
means for introducing blast air at a distal end of the hose at a
first volumetric flow rate to a blast nozzle located at the distal end, the blast
air introducing means developing due to blast air velocity at the nozzle, a
reduced pressure in the ice particle transport hose to draw at the rnass flow
rate the fresh ice particles from the ice particle transferring means into the
20 hose; and
means for providing cool air into the pro~imal end of the hose
at a second volumetric flow rate to cool such fresh ice particles, the second
volurnetric flow rate of cool air provided by said means being detelmined by
the reduced pressure in said hose.
25 BRIEF DESCRIPrION OF THE DRAWINGS
Figure 1 is a schematic of the apparatus in accordance with a
preferred embodiment of this invention in which the process of this invention
is carried out.
Figure 2 is a schematic of an alternative embodiment of this
30 invention.
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DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS
A schematic flow diagram for the ice blast apparatus and process
according to a first preferred embodiment is shown in Figure 1. The
apparahls comprises an ice particle forming device 10, a high pressure air
5 compressor 12 and a blast nozzle 14. High pressure air is delivered to the
blast nozzle 14 through air hose 16 where the nozzle 14 is connected thereto
at hose distal end 15. The ice particles generated in system 10 are delivered
to the blast nozzle through the ice particle transport hose 18. The hose 18 is
usually insulated to minimize heat gain by the flowing particles.
The air compressor system comprises a high pressure, high volume
air compressor 20 which may be a single- or multi-stage compressor. Due
to high volume, high pressure compression, the high air temperature, as it
emerges from the compressor in line 22, is passed through an after-cooler
and filter 24 to remove condensate and reduce the temperature to a desired
15 operating temperature which may be close to ambient. It is understood that
the air in line 16 may be at a temperature higher than ambient as long as the
blast nozzle is not heated by the air to a temperature which is uncomfortable
for the operator to hold.
The ice particle forming device 10 may comprise in accordance with
20 this embodiment an ice maker 26 a~d an ice fracturing device 28. Water is
introduced to the ice maker 26 through line 30. The water is converted by
the ice maker 26 into flakes or sheets of ice which are removed from the ice
maker by a tool 31 and then immediately and directly delivered downwardly
to the ice fracturing unit 28 via chute 32. The ice maker 26 is driven by
25 motor 34 which generates ice at a reasonably constant mass flow rate
sufficient to meet the needs of the work to be performed.
The ice fracturing unit 28 fractures the ice sheets at the same mass
flow rate as generated by the ice maker 26, so that ~here is no build up of
ice in the ice maker or on chute 32. As schematically indicated, the ice
- 30 fracturing uI~it 28 may comprise counter-rotating rollers having fracturing
,3
devices on their surfaces to fracture the ice into ice particles of a desired
sizing. The ice fracturing unit 28 is driven by motor 36. The fractured ice
particles as fres~ly made are then delivered to the ice transport hose 18
through the ice particle chute 38. Such delivery of ice particles to the ice
hose 18 is at the sarne mass flow rate as the ice is generated in ice maker
26. The immediate downward delivery avoids any accumulation of ice
sheets or ice particles above the ice fracturing unit or above the hose 18. By
virtue of gravity the ice particles fall freely towards and into the hose 18
without hang-up on any portion of the transfer device in moving ice from the
ice maker through to the hose 18.
In accordance with the illustrated embodiment, the ice maker 26 may
comprise a rotating drum which is partially immersed in a water bath. The
drum has a cooled surface to cause water picked up by the drum to freeze
into sheets of ice which are removed from the drum by docter blade 31.
The ice is then fractured by the fracturing unit 28. In accordance with this
embodiment, the refrigeration unit 40 supplies refrigerant through line 42 to
the rotating drum of the ice maker 26. The refrigeration unit 40 delivers
sufficient refrigerant through line 42 to ensure formation of ice sheets at the
desired mass flow rate.
Refrigerant is also delivered through line 44 to an air chiller 46.
According to this embodiment, a portion of the high pressure air in hose 16
is drawn off through the tee connector 48 through line 50. The portion of
air drawn off of line 16 is first dried in dryer 52 and then chilled to a ~ ;
temperature below 0C to provide high quality cool, dry air in line 54. -
That air is used to provide cooling within the housing 56 for the ice particle
forming device 10 and as well provide make up air in the manner to be
discussed for delivering the freshly formed ice particles through the ice hose
18. This cool dry air also functions to reduce and maintain at a minimum
any moisture which develops in the ice fracturing unit, ice particle transfer
unit and ice hose 18.
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The blast nozzle 14 is of the typical blast nozzle design where the air
hose 16 is connected to the throat of the venturi portion of the nozzle and
the ice hose 18 is connected to the low pressure side of the venturi. By
virtue of high speed air travelling through the throat of the venturi of blast
5 nozzle 14 at a first volumetric flow rate, a reduced pressure is developed in
ice hose 18. The blast nozzle is opened by actuating switch 58 to permit the
high pressure air to flow therethrough, ~e developed reduced pressure in ice
hose 18 draws the cool air from the housing 56 into the hose proximal end
17 at a second volumetric flow rate. The cool air enters via line 54 into
10 housing 56. The cool air provided at the second volumetric flow rate is
sufficient to satisfy the negative pressure within the hose 18 as generated at
the nozzle and to provide necessary cooling and ice particle transport
through the hose 18 to the blast nozzle 14. The flow of the cool dry air
through the hose 18 is sufficient to ensure fluidization of the particles as
15 they move along the hose 18 to avoid accumulation of the particles in the
hose. The second volumetric flow rate is determined by the reduced
pressure in ice hose 18. The secondary flow rate is directly proportional to
the first flow rate through hose 16 because the velocity of the first flow rate
through the nozzle 14 determines the extent of reduced pressure in line 18.
20 That is, the greater the first volumetric flow rate, the greater the reduced
pressure or vacuum developed in line 18.
In accordance with this invention, ice accumulation is not permitted in
the system. Once the ice particles are formed, in accordance with this
invention, there are no interveni~g mechanical devices to disrupt flow of the
25 ice particles to the blast nozzle. In order to achieve this feature, ice is only
made when ice particles are required at the blast nozzle 14. By use of
appropriate process controls, activation of blast nozzle switch 58 initiates -
one or more components of the system to effect ice blasting. It may be that
on closing switch 58 of the blast nozzle, the air compressor is ramped up to
30 develop the high volume, high pressure air in line 16. The refrigeration
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unit 40 is operating and solenoid control valve 60 is opened to allow
refrigerant to cool the ice maker 26. At the same time, refrigerant is
provided to the chiller 46 to cool the air delivered through line 50. As the
high pressure air cornmences to flow through the blast nozzle 14, a reduced
S pressure is formed in line 18 so that the air flowing through line 54 into
housing 56 flows through the housing 56 and into line 18. Sensors may be
provided to determine the temperature of the ice maker 26 and when it has
achieved a des~red ice making temperature, motor 34 is turned on to start
rotation of the cold drum surface of the ice maker whereby the blade 31
10 removes the ice flakes which are immediately conveyed along chute 32 into
the ice fracturing unit 28 which processes the ice flakes at the same mass
flow rate to form an equivalent amount of ice particles in chute 38. By
virtue of the reduced pressure in line 18, and as well the make up air
introduced through line 54, the ice particles are immediately delivered along
15 the ice hose 18 to the blast nozzle 14. The ice particles are then introducedto the high velocity air travelling through the blast nozzle 14 to accelerate
the ice particles to perform work on the sur~ace of the desired substrate.
When the switch 58 is deactuated to signal a waiting period, the air
compressor is ramped down to supply air in line 16 at a reduced flow rate
20 and continue to supply air to line S0. In addition, the refrigeration unit
continues to operate. However, t~e processor controlled solenoid operated
valve 60 is closed to recycle refrigerant and to stop continued forrnation of
ice flakes. This prevents accumulation of ice within the ice generating
system so that any ice remaining is imrnediately processed through the ice
25 fracturing unit 28 and delivered to the blast nozle 14. It is appreciated that
when blast ceases, the blast air reduction in line 16 is delayed, until the ice
particles in the process line back as far as the ice maker are cleared from the
system, to avoid any accumulation of the particles.
The reduced amount of air flowing through the blast nozzle continues
30 to develop a reduced pressure in line 18 where, again, air travelling through
-
line S4 provides the make up air to cool and satisfy the reduced pressure
within the housing 56 and hose 18.
The ice fracturing unit 28 fractures ice sheets into particles of desired
sizing. The fracturing unit may be set to deliver particles of a desired size
5 and in particular a nominal particle size selected from the range of 2 mm to
10 rnm. The preferred type of ice fracturing unit has, as already noted, the
counter-rotating rollers ice fracturing formation. Such fracturing systems
are designed to minimize ice crushing so that few, if any, fines are produced
in the fracturing operation. The ice fracturing system operates at a rate to
10 ensure that all ice produced is irnmediately fractured and delivered to chute
38. By virtue of the cool, dry air from line 54 equalizing the reduced
pressure in line 18, there is a flow of air which fluidizes the ice particles as
they enter line 18 and ensures that they remain cool in transport to the blast
nozzle. This is particularly important when the blast nozzle is connected to
15 hose which may delher the ice particles over extended distances to the work
surface. Normally this is in the range of 20 to 100 metres. It is -
understood, however, that the hose lengths may be considerably shorter in
the range of 5 to 20 metres.
In accordance wi~h a preferred embodiment of the invention, the -
20 compressor delivers air in the range of 150 to 350 SCFM at a pressure in
the range of 60 to 250 psig. In this operating configuration, approximately
100 to 300 SCFM is delivered to the blast nozzle and in the range of 30 to
80 SCFM is delivered to the chiller 54. The pressure of the air delivered
through line 50 to the chiller 46 is normally in the range of 80 psig. The
25 chiller is capable of cooling the air down to less than 0C and preferably in
the range of -lSC. As the high quality air exits the chiller 46, the pressure
is usually dropped down to approximately 10 psig. With these operating
parameters it has been found that the mass flow rate of ice should preferably
be in the range of 120 lbs/hr to 300 Ibs/hr.
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In accordance with the invention, it is understood that when blasting
ceases and solenoid valve 60 is turned off, the refrigeration fluid is diverted
back to the refrigeration unit 40 to ensure optimum operation of the
refrigeration unit. As already noted, blast air may be continuously delivered
5 to the blast nozzle 14 at a rate in the range of 50 SCFM. In addition, cool
dry air continues to circulate through the housing 56 to keep the components
cool so that upon resumption of ice making everything is at sub-~ero
temperature to ensure immediate delivery at the desired mass flow rate of
the ice particles.
In accordance with an alternative embodiment of the invention, the ice
blast system of Figure 2 includes the same components as that of Figure 1 as
identified by like numerals. The distinction over the embodiment of Figure
1 is with respect to the handling of the ice particles when blasting ceases. In
accordance with the embodiment of Figure 2, when the blast nozzle switch
15 58 is opened to indicate a pause in blasting, solenoid valves 62 and 64 are
actuated by an electro~ic controlled system to divert the ice particles to a
discard 66. In accordance with standard process-controlling techniques
solenoid valve 62 is closed and valve 64 is open. Conversely, when switch
58 is closed to indicaee resurnption of blasting, valve 62 is opened and valve
20 64 is closed to permit the ice particles to pass through ice hose 18. In thismanner, the ice particle generation unit operates at full capacity at all times.When the blasting ceases, high quality air continues to flow through housing
56 to transport the formed ice particles through line 18 and out through
discard 66. Such diverting of the ice particles ensures that the ice particle
25 making system operates at optimum temperatures at all times.
In accordance with the preferred embodiments of the invention, the
principle thereof has been demonstrated in providing, on demand, ice
particle blasting without any accumulation of ice particles in the system.
This approach ensures consistent blast quality widl no shut down time to
30 clear ice accumulation in the system.
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Although preferred embodiments of the invention are described herein
in detail, it will be understood by those slcilled in the art that variations may
be made thereto without departing from the spirit of the invention or the
scope of the appended claims.
