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Для объектов менее подверженных износу от высокого давления таких как металл
бетон облицовочные материалы мы рекомендуем одношланговые бластеры так как на
выходе из сопла пистолета они используют больший диаметр гранул под большим
При деликатной очистке таких материалов как ткани электронные платы стекло мы
рекомендуем двухшланговые бластеры так как в них гранулы при выходе из сопла
имеют меньший диаметр и подаются более сбалансировано
При расстоянии до м мы рекомендуем двухшланговые системы
При большом удалении до м очищаемой поверхности от бластера подойдет
- С компрессором низкого и среднего давлеия идеально подойдет двухшланговая
- С компрссором высокого давления мы рекомендуем одношланговую систему.
Для очистки больших поверхностей мы рекомендуем двухшланговые бластеры так как
эти системы расходуют меньшее количество гранул сухого льда чем одношланговые
Для очистки локальных участков с более толстым слоем загрязнения мы рекомендуем
Today, CO2 blasting is being effectively used in a wide array of applications from heavy
slag removal to delicate semiconductor and circuit board cleaning. Imagine a process that
can be used on-line without damaging equipment or requiring a machine “teardown”.
Unlike conventional toxic chemicals, high pressure water blasting and abrasive grit
blasting, CO2 blasting uses dry ice particles in a high velocity air flow to remove
contaminates from surfaces without the added costs and inconvenience of secondary
waste treatment and disposal.
What Is Dry Ice?
Dry Ice is the solid form of Carbon Dioxide (CO2) which is colourless, tasteless, odourless
gas found naturally in our atmosphere.
With a low temperature of –78,5° (-190 °F), Dry Ice solid has an inherent thermal energy
ready to be tapped. At atmospheric pressure, solid CO2 sublimates directly to vapour
without a liquid phase. This unique property means that the blast media simply disappears,
leaving only the original contaminant to be disposed of. In addition, cleaning in water
sensitive areas is now practical.
Carbon dioxide is a non-poisonous, liquefied gas which is both inexpensive and easily
stored at work sites. Of equal importance, it is non-conductive and non-flammable.
Table 1. Carbon Dioxide (CO2) properties.
Molecular Weight 44,01 g/mole
Density (Solid) 1.562 kg./mc at –78°,5 C
Density (Liquid) 1.022 kg./mc at –18° C
Density (Gas) 1.977 kg./mc at 0° C
Melting Point -56,5° C at 5,2 bar (triple point)
Boiling Point -78,5° C (sublimates)
Liquid-to-Snow Conversion Rate 0,46 kg. snow/kg. liquid at –17,6° C
0,57 kg. snow/kg. liquid at –47,8° C
In Dry Ice blasting, there are several methods used to manufacture the Dry Ice blasting
media. One technique is to shave dry ice granules from solid Dry Ice block at the blasting
machine. This generally produces sugar-crystal sized Dry Ice granules, which must be
used quickly due to fast sublimation (due to their high surface area-to volume ratio).
Another technique is to manufacture hard pellets of Dry Ice in a pelletizer then
immediately blast with the pellets of store the pellets in an insulated container until the
pellets are required. These pellets are generally on the order of 2-3 mm. in diameter, and
2,5-10 mm. in length.
In this method, Dry Ice is manufactured by flashing pressurized liquid CO2 into snow,
followed by compression of the snow into solid form. The snow is either directly
nuggetized into pellets (mechanical compression) or is extruded into solid pellet form
through a die under hydraulic pressure. The latter process allows for more efficient
conversion from the liquid phase to the solid phase. Generally, it is desirable to have
pellets which are well compacted, to minimize entrapment of gaseous CO2 and/or air which
may affect product quality.
As can be seen in Table 1, the yield achieved when flashing liquid carbon dioxide into
snow increases as the temperature of the liquid CO2 decreases, so it is important to prechill
the incoming liquid CO2 via heat exchangers with the outgoing CO2 vapor. Figure 1 is a
block diagram showing a basic pelletization process.
Figure 1. Pelletization Process.
CO2 vapor CO2 Snow
Into a Solid
Several manufacturers make Dry ice pelletizers which may prove beneficial to have onsite
for customers with high pellet demand. Facilities required for such an arrangement are
generally as follows: a refrigerated liquid CO2 tank, a pelletizer, and liquid CO2 lines to
reach the equipment. Some manufacturers make combined Dry Ice pelletizer/blast
machines which manufacture Dry Ice and blast all in one operation. Facilities required for
such an arrangement are: an air compressor (5 mc/min at 7 bar), a liquid CO2 tank, a
pelletizer/blast machine, compressed air hose and liquid CO2 lines to reach the equipment,
blast hose from the machine to the blasting operation, and the appropriate nozzle(s) for the
application. This equipment is best suited to high volume, continuous blasting applications
where the cost savings of manufacturing pellets on-site justifies the capital expenditure
for the system.
How Does Dry Ice Blasting Work?
The Basic Process
Dry Ice particle blasting is similar to sand blasting, plastic bead blasting, or soda blasting
where a media is accelerated in a pressurized air stream (or other inert gas) to impact the
surface to be cleaned or prepared. With Dry Ice blasting, the media that impacts the
surface is solid carbon dioxide (CO2) particles. One unique aspect of using Dry Ice
particles as a blast media is that the particles sublimate (vaporize) upon impact with the
surface. The combined impact energy dissipation and extremely rapid heat transfer
between the pellet and the surface cause instantaneous sublimation of the solid CO2 into
gas. The gas expands to nearly eight hundred times the volume of the pellet in a few
milliseconds in what is effectively a “micro-explosion” at the point of impact. Because the
CO2 vaporizing, the Dry Ice blasting process does not generate any secondary waste. All
that remains to be collected is the contaminate being removed.
As with other blast media, the kinetic energy associated with Dry Ice blasting is a function
of the particle mass density and impact velocity. Since CO2 particles have a relatively low
hardness, the process relies on high particle velocities to achieve the needed impact
energy. The high particle velocities are the result of supersonic propellant or air stream
Unlike other blast media, the CO2 particles have a very low temperature of –78,5° C. This
inherent low temperature gives the Dry Ice blasting process unique thermodynamically
induced surface mechanism that affect the coating or contaminate in greater or lesser
degrees, depending on coating type. Because of the temperature differential between the
Dry Ice particles and the surface being treated, a phenomenon known as “fracking” or
thermal shock can occur. As a material’s temperature decreases, it becomes embrittled,
enabling the particle impact to break-up the coating. Refer to Figures 2 and 3.
Figure 2. Thermal shock induces micro-cracking in the surface coating
Figure 3. CO2 gas expansion and pellet kinetic effects break away and remove coating
Also, the thermal gradient or differential between two dissimilar materials with different
thermal expansion coefficients can serve to break the bond between the two materials.
This thermal shock is most evident when blasting a non-metallic coating or contaminate
bonded to a metallic substrate.
Quite often companies examining this process are concerned with the effect the thermal
shock will have on the parent metal. Studies have shown that the temperature decrease
occurs on the surface only, there is no chance of thermal stress occurring in the substrate
metal. To illustrate this principle, an experiment was performed where thermocouples
were imbedded into a steel substrate a varying depths (flush with the surface to 2 mm.
deep). Refer to Figure 4.
Figure 4. Thermocouple Distance From Plate Surface.
A CO2 blast jet was constantly traversed across the test specimen for 30 seconds (a
relatively long dwell time for this process) and the thermal couples recorded the changing
temperatures at the various depths. As shown in Figure 5, the surface mounted
thermocouple shows a temperature drop each time the blast jet impinged directly upon the
thermocouple (50° C in about 5 seconds). In contrast, the thermocouples imbedded at
various depths in the substrate recorded a slow gradual drop in temperature
corresponding to the overall test plate temperature drop. The thermocouple 2 mm. deep
only dropped 10° C after 30 seconds. This curve illustrates that the “Thermal Shock”
occurs only at the surface where the coating or contaminate is bonded to the substrate
(Reference 1) and has on detrimental effect to the substrate.
Figure 5. Temperature Response Of Thermal Couples Placed At Various Depths In the
Another approach to looking at thermal stress is by studying the use of Dry Ice blasting in
the molded rubber industry. Here, hot steel molds operating at 150° C are blasted with –
78,5°C Dry Ice particles. The temperature difference between the hot mold and cold Dry
Ice will not cause cracking. There are two reasons for this phenomenon. First, as seen
above, the temperature gradient occurs at the surface. Secondly, the thermal stresses
involved are much less than those encountered during normal heat treatment.
The thermal stress due to a temperature differential can be estimated using equation 1
where sy is stress, ΔT is temperature gradient, a is coefficient of expansion and Υ is
The corresponding parameter values are
and the thermal stress (N/cmq) is
where the temperature differential will be about 57° C. This temperature leads to a low
tensile stress of 20,85 N/cmq. Even if the mold temperature was brought down to the
temperature of the ice (an unrealistic extreme), the temperature gradient would be about
235° C. The corresponding tensile stress is 70,0 N/cmq. This calculated stress is below
the yield point of steel in the hardened condition. Again, these thermal stresses would be
far less than those encountered during normal heat treatment where the temperature
differentials would exceed 260° C.
Even at high impact velocities and direct “head-on” impact angles, the kinetic effect of
solid CO2 particles is minimal when compared to other media (grit, sand, PMB, etc.). This
is due to the relative lack of hardness of the particles and the almost instantaneous phase
change to a gas on impact which effectively provides an almost nonexistent coefficient of
restitution in the impact equation. Because CO2 blasting is considered non-abrasive and
relies on the thermal effects discussed above, the process may be applied to a wide range
of materials without damage. Soft metals such as brass and aluminium cladding can be CO2
blasted for the removal of coatings or contaminates without creating surface stresses
(pinging), pitting, or roughness (Reference 3).
Blast Machine Types
There are two general classes of blast machines as characterized by the method of
transporting pellets to the nozzle: two-hose and single-hose systems. In either type of
system, proper selection of blast hose is important because of the low temperatures
involved and the need to preserve particle integrity as the particles travel through the
In the two-hose system, Dry Ice particles are delivered and metered by various
mechanical means to the inlet end of a hose and are drawn through the hose to the nozzle
by means of vacuum produced by an ejector-type nozzle. Inside the nozzle, a stream of
compressed air (supplied by the second hose) is sent through a primary nozzle and
expands as a high velocity jet confined inside a mixing tube. When flow areas are properly
sized, this type of nozzle produces vacuum on the cavity around the primary jet and can
therefore draw particles up through the Ice hose and into the mixing tube where they are
accelerated as the jet mixes with the entrained air/particle mixture. The exhaust mach
number from this type of nozzle, in general, slightly supersonic. Advantages of this type of
system are relative simplicity and lower material cost, along with an overall compact
feeder system. One primary disadvantage is that the associated nozzle technology is
generally not adaptable to a wide range of conditions (i.e. tight turns in a cavity, thin-wide
blast swaths, etc.). Also, the aggression level and strip rate of the two-hose system is less
than comparable single-hose blast machines.
In a single-hose system, particles are fed into the compressed air line by one of several
types of airlock mechanisms. Reciprocating and rotary airlocks are both currently used in
the industry. The stream of pellets and compressed air is then fed directly into a single
hose followed by a nozzle where both air and pellets accelerate to high velocities. The
exhaust Mach number from this type of nozzle is generally in the 1.4 and 2.5 range,
depending on design and blast pressure. Advantages of this type of system are wide
nozzle adaptability and the highest available blast aggression levels. Disadvantages include
relatively higher material cost due to the complex airlock mechanism.
Blast machines are also differentiated into Dry Ice Block Shaver blasters and Dry Ice
Pellet blasters. The Block Shaver machines take standard 20 kg. Dry Ice blocks and use
rotating blades to shave a thin layer of ice off the block. This thin sheet of Dry Ice
shatters under its own weight into sugar grain sized particles. These particles then fall
into a funnel for collection. A two-hose delivery system is used to transfer the particles at
the bottom of the funnel to the surface to be cleaned. The low mass of these particles
combined with the inefficient two-hose system limits the block shavers to light duty
cleaning. Because the shaved ice machines deliver a particle blast with high flux density
(Number of particles striking a square area of surface per second), they are effective on
thin moderately hard coatings such as an air dried oil based paint. The disadvantage of the
ice shaver is the particle size and flux density is fixed, as well as, the particle velocity.
In contrast, Pellet Blast machines have a hopper that is filled with pre-manufactured CO2
pellets. The hopper uses mechanical agitation to move the pellets to the bottom of the
hopper and into the feeder system. As stated earlier, the pellets are extruded through a
die plate under great pressure. This creates an extremely dense pellet for maximum
impact energy. The pellets are available in several sizes, ranging from 1,5 to 3 mm. in
diameter. With a single-hose delivery system, the final pellet size and blast flux density
exiting the nozzle is governed by the type of blast hose (hose diameter and interior wall
roughness) and nozzle used. Because of its design, the single hose pellet blast units are
capable of “dialling-in” the correct blast type needed for a wide range of individual
coatings or contaminate removal.
For example, soft coatings such as rubber, silicone, foams and waxes, release agents, food
ingredients, etc. need large pellets with low flux density for maximum strip rate and
efficiency. These coatings require maximum thermal energy (i.e. pellets with large mss)
and large spacing between pellets (i.e. low flux density) for optimum cleaning
performance. In contrast, hard coatings such as paints, varnish, baked on sugars, carbon
build-up, etc. require smaller particle size with high flux density and high particle velocity.
Blast machines are further differentiated into all-pneumatic and electro-pneumatic types.
All-pneumatic machines have particle feed mechanism and controls operated
pneumatically. This may include the use of air motors. The advantage of such a machine is
the availability of compressed air at the blast locations, especially outdoors. One
disadvantage is that the operation of the machine may be susceptible to disruption due to
moisture or contamination in the compressed air supply. In addition, these machines are
more prone to freeze-ups and are better suited for light duty spot cleaning applications.
Also, if the machine is powered by an air motor, it will have a continuous exhaust of oily
air. This same air motor can be easily flooded with water if the air system is not
Electro-pneumatic machines are truly “Environmentally Friendly” because there is no oily
exhaust and these machines are more tolerant of moisture and contaminants in the air
supply. The electro-pneumatic machines rarely freeze-up which makes them ideal for
automated line applications where around–the-clock blasting is required. Also, these
machines provide pulse free blasting for uniform cleaning and efficient use of the Dry Ice.
There is, however, a slight inconvenience factor associated with supplying both electrical
power and compressed air to the machine at each blast location.
One of the most challenging technologies associated with either type of blast machine is
the achievement of smooth, continuous pellet feed. One surprising property of Dry Ice is
that it is not smooth or slippery like water ice nor smooth-flowing like sand or glass bead.
Instead it is somewhat resistant to flow. Because of this, Dry Ice blast machines tend to
have various agitators, augers and other devices in the hopper to improve pellet flow.
Generally, the poorer the quality of dry ice, containing; for example, water ice build-up or
a large percentage of CO2 “fines” or snow, the more difficult it is to flow through a
system. An additional property of Dry Ice is that it is extremely cold and will draw
moisture out of the surrounding air in the form of frost. The machine, therefore, must be
tolerant of repeated freeze-thaw cycles and the associated moisture accumulation that will
take place over time.
Generally the difference between a high quality blast machine and a mediocre one lies in
the units ability to do a cleaning job quickly, cost-effectively, and in the reliability of
smooth and continuous pellet flow under real-world conditions.
The nozzle is where the Dry Ice particles are accelerated to the highest velocity possible
in order to create an effective blast stream. Figure 6 shows schematics of the two types of
nozzles used for Dry Ice blasting. The science of two-hose ejector nozzles compared to
single-hose convergent-divergent supersonic nozzles operating under the same conditions
(i.e., air volume, pressure, temperature, CO2 particle mass etc.), shows significantly higher
efficiency capability for the described single-hose type nozzles. This difference in
capability directly relates to the two-hose ejector nozzle’s overall supplied energy being
used not only to accelerate the CO2 particles, but also to create the vacuum pulling the
secondary pellet flow through the secondary hose. Then more energy is drained to mix
this low velocity particle flow with the high velocity jet flow in order to accelerate the
particle through the two-hose nozzle. In simple terms, the net resultant energy available
for pellet acceleration is inherently lower for two-hose systems because much of the
available energy is lost simply in combining the CO2 particle flow with the air-jet flow.
Figure 6. CO2 Particle Blast Nozzle Types
Since the size of the Dry Ice particles effect the cleaning performance, a blast system
should have the flexibility to “Dial-In” the correct particle size. This can be done a couple
of different ways. First, the size of the pellet being produced by the pelletizer may be
varied. Once the pellet is in the blast machine hopper, the size of the pellet reaching the
surface to be cleaned can be varied several ways. The diameter and type of blast hose
used will either keep the pellet intact or break the pellet up into smaller particles. Also,
the nozzle may be intentionally mes-expanded to produce partially destructive shock
waves in the nozzle. Both techniques are used independently or together to optimise the
particle size, blast stream velocity, and flux density for any cleaning job.
When sand or any similar media with very small diameter is used in blasting, the size of
the nozzle throat is very large compared to the blast media. In Dry Ice blasting, however,
the nozzle throat may only be slightly larger than the dry ice particle being accelerated.
Table 2 is a chart indicating the approximate size of a round nozzle throat for four
different levels of blast pressure at a constant airflow of 200 Standard Cubic Feet per
Minute (SCFM) and typical flow rate available for blasting operations. At higher pressures,
the Dry Ice particle size needs to be smaller to correspond with the smaller throat size.
The high pressure blast stream is described as high velocity small particles with high flux
density. Again, this particle blast profile is suited best for removing hard coatings such as
paint. The chart shows a larger nozzle throat diameter corresponding to low pressure
operations. As stated above, large pellets impacting the surface with low flux density is
ideal for cleaning soft coatings.