Abstract:
Apparatus and method to establish permissible ranges of the gas/dry ice ratio in a dry ice blasting stream. Logic and circuitry determine whether a proposed ratio is permissible. If it is, it enables the process to start. If it is not, it forbids operation of the process.

Description:
FIELD OF THE INVENTION  
         [0001]    In dry ice blasting, enabling the selection and dynamically maintaining a gas/dry ice ratio which is selected and is within an allowable range of ratios, and forbidding the selection of a ratio outside of the allowable range.  
         BACKGROUND OF THE INVENTION  
         [0002]    The art of dry ice blasting involves the impact on a surface to be cleaned by a blasting stream consisting of particles of dry ice in a gas carrier stream. Important parameters include the size (mass) of the particles and the ratio of the masses of gas and the dry ice (“gas/dry ice ratio” herein).  
           [0003]    Recently-improved apparatus for supplying dry ice particles have presented the opportunity more precisely to match the size of particles and their rate of supply to various flow rates of the gas in varieties of blast cleaning applications. However, those advantages can be lost if the operator is unaware of what is actually occurring in the apparatus, or if he has selected some ratio which is inappropriate to the intended application.  
           [0004]    For example, it is possible that an operator to reduce operating costs, would choose a ratio so heavily favoring the gas that there would be insufficient particles to do the job, and that too many of them might be reduced in mass by sublimation. Even more troubling would be his selection of a ratio too heavily favoring the dry ice particles such that the nozzle would plug up and the process stopped. Furthermore excessive dry ice mass could tend to harm delicate objects.  
           [0005]    An obvious solution to this problem is simply to train the operator better. This is frequently unworkable because of employee reassignments, and the sporadic use of this equipment.  
           [0006]    Another seemingly obvious solution is to limit the operating ranges narrowly and provide many configurations. This leads to confusion and increased equipment expense.  
           [0007]    Charts and diagrams are similarly useless because the operator may not understand them, or may choose incorrectly. Perhaps he would not even use them.  
           [0008]    Accordingly, it is an object of this invention to provide a system and method based on mass flow of the gas, which will automatically limit selection of gas/dry ice ratios to only an allowable range for a given application, and prevent selection or operation outside of the range. Operation of such a system requires no more basic input than the selection of a gas flow rate respective to a selected gas pressure. Experience and instruction will teach what pressure is desirable for a given task.  
           [0009]    This in itself can establish a range of acceptable gas/dry ice ratios. Depending on the nature of the cleaning work to be done, the system may select the ratio, or the operator may select an exact ratio, but only in the permitted range.  
           [0010]    This arrangement also makes available the association of specific nozzles in establishing allowable ranges and ratios.  
           [0011]    Its is therefore an object of this invention to simplify and authenticate the choice and maintenance of gas/dry ice ratios in a blasting stream.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0012]    Apparatus for dry ice blasting conventionally includes a particle source which supplies dry ice particles of selected size and mass at a regulated rate, a source of pressurized gas such as an air compressor when air is used as the carrier gas, a hose receiving the mixed gas and dry ice, and a nozzle for discharging the blasting stream toward a surface to be cleaned.  
           [0013]    A major parameter in dry ice blasting is the pressure of the stream. The operator is expected to select an appropriate pressure. The gas flow rate at this pressure is sensed either directly or indirectly. Under control of an adjustable regulator, it will pass the gas from the supply at the selected flow rate.  
           [0014]    In one embodiment of the invention, the flow rate is directly measured by a conventional flow meter. In another embodiment it is indirectly determined by measuring the pressure, and with knowledge of the properties of the nozzle orifice, deducing the flow rate. In both embodiments, the control is by a responsive regulator which will pass gas at a selected rate at a given pressure.  
           [0015]    The ratio of the combined stream is established by the rate of supply of the dry ice. The dry ice supply is from a particle source which will be operated at a rate to produce the necessary amount of particles to make the correct ratio in the established flow of gas. Accordingly, the regulator control over the particle source is slaved to the gas flow regulator, so the correct ratio will be maintained.  
           [0016]    According to this invention, the intended pressure is first selected. Then the airflow rate will be learned or known. Logic within the system establishes a range of gas/ice dry ice ranges that is acceptable. Then the control will establish a ratio, or the operator can select other ratios, but only within the operating range established by the control.  
           [0017]    Thereafter, the master/slave relationship of the two regulators will dynamically maintain the supply of dry ice to provide a stream with the correct ratio.  
           [0018]    The flow rate may also be deduced from pressure and identity of the nozzle. Nozzles can be physically configured or color coded to indicate their properties as related to measurement of flow at a given pressure.  
           [0019]    The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    FIGS.  1 - 4  are logic diagrams of control systems useful in this invention; and  
         [0021]    [0021]FIG. 5 is a semi-schematic view of an idealized system incorporating the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    FIGS.  1 - 4  show various control schemes to maintain a proper gas/dry ice ratio for various ratios of airflow. They all begin with the assumption that pressure of the stream is a major criterion, and that the gas flow (often for convenience referred to simply as “airflow”, as is “ice” used to denote dry ice and not water ice) rate at a given pressure may vary depending on the nozzle configuration. In all cases, however, the gas/ice ratio must be in a range of values where the stream will not plug the nozzle, or where it would provide too little ice to do the intended work. In fact, while the acceptable (permissible) operating range for the ratio may extend from about 1:1 to about 7:1, the best rate based on wide experience is between about 3:1 and about 3.5:1. In some embodiments of this invention the system will provide ice at a rate to establish a ratio in the preferred range, subject to override by the operator to a more individualized setting, but within the permissible range.  
         [0023]    It is evident that to establish a correct ratio, there must be the correct amount of ice per unit of gas flow. However, the gas flow measurement in cubic feet per minute (cfm) is of little or no use to an operator who is skilled only in the use of the apparatus. He does, however, have instructions or knowledge about the pressure to be used. The same high pressure, for example, would not be used on delicate articles as would be needed to clean paint from an oil well tool. Also, he may wish to put more or less ice in the blasting stream, in effect “fine-tuning” an automatic setting more exactly to meet the needs of a specific job.  
         [0024]    Accordingly this control begins with selection of pressure. Depending on downstream configuration namely of the nozzle, the airflow rate will be deducible, or without specific input of nozzle conditions, directly measured.  
         [0025]    With this knowledge an airflow rate and the rate of supply of ice can be regulated to constitute a stream with the desired air/ice ratio. This is done by slaving the ice regulator to the gas regulator.  
         [0026]    [0026]FIG. 1 constitutes a logic flow chart in the simplest system. It begins with selection of air pressure by the operator (block  10 ), who then starts the blasting operation, usually by pressing a trigger that controls a valve at the nozzle (block  11 ). At this moment the apparatus measures the airflow rate (block  12 ).  
         [0027]    At this point it is repeated that the airflow rate may be directly measured by a flow meter or may indirectly be deduced from knowledged of the pressure and nozzle configuration. In block  12  direct measurement is presumed, but not exclusive.  
         [0028]    A reference (block  13 ) in the nature of a look-up table is consulted for best ratio (best ice supply rate) for the measured airflow, and the best rate is determined (block  14 ).  
         [0029]    Responsive apparatus will regulate the selected ice rate for the measured airflow rate (block  15 ). Thus, the ice regulator is slaved to the airflow regulator.  
         [0030]    The control scheme of FIG. 2 is generally similar to that of FIG. 1, except that at the outside the operator can select a variance in the ratio from the best rate that otherwise would have been selected.  
         [0031]    Again, the operator selects the intended pressure (block  20 ), but he also enters a variance in the ratio—more or less ice per unit of gas mass (block  21 ). This variance will be limited so it does not result in an improper ratio, and blasting is started (block  22 ).  
         [0032]    Thereafter, as in FIG. 1, airflow is measured (block  23 ), reference is made to block  24 , and best ice rate is determined (block  25 ). In this embodiment, adjustment of the best rate is made (block  26 ) to conform with the variance and to be certain it is a valid ratio.  
         [0033]    Blocks  27  and  28  provide for the continuing operation and control.  
         [0034]    Blocks  29  and  30  attend to maintaining control when output conditions change, or are charged by the operator.  
         [0035]    [0035]FIG. 3 shows control logic which is similar to that of FIG. 1. In FIG. 1, the airflow is directly measured. In FIG. 3 it is deduced from the known pressure and the known properties of the selected nozzle. Block  35  shows the selection of a nozzle, whose properties will be known in the reference. Again, the operator selects the pressure (block  36 )  
         [0036]    Knowledge of the flow rate is obtained from the measurement of pressure (block  37 ), and knowledge in the reference (block  38 ) of the nozzle properties. The remainder of the logic system is evident from the discussion of FIG. 1.  
         [0037]    The logic of FIG. 4 closely resembles that of FIG. 2, to which reference is made. It differs (as FIG. 3 differs from FIG. 1) in the use of measured pressure to deduce the airflow rate (block  40 ). It further includes the opportunity for the operator to select variance from best rate (block  41 ). For this feature reference is again made to FIG. 2.  
         [0038]    An illustrative example of a system under the control of the logic of FIGS.  1 - 4  is shown in FIG. 5.  
         [0039]    This system  50  is intended to clean an undesired substance  51  from surface  52  of a work piece  53  by emission  54  from a nozzle  55 .  
         [0040]    Carrier gas under pressure is derived from a pressure source  56  such as a compressor or a tank of stored liquified gas, as desired. Although air is the most frequently used gas, gassified liquid nitrogen, carbon dioxide or argon are other examples. Much depends on the convenience of the source. Most often this system will utilize a compressor and air rather than a cryogenic tank.  
         [0041]    A flow regulator  60  (“first regulator”) acting as a valve under feedback control from a flow meter establishes the rate of flow of the gas at the desired pressure. The regulated gas is supplied to a mixer  61 , whose purpose is to combine the pressurized gas and dry ice particles and thereby provide a blasting stream to hose  62 .  
         [0042]    Dry ice particles are supplied to the mixer at a rate determined by a regulator  63  (“second regulator”) that controls the output of a particle source  64 . The mixed stream is presented to the entry port  65  of nozzle  55 . Nozzle  55  has an orifice  67 , with an initial converging section  68 , and a diverging section  69 . The divergence can be quite small, perhaps 1½ degrees half conical angle. The orifice is often circular but may be rectangular or any other suitable shape.  
         [0043]    The regulators are respectively controlled to establish the relative rates of supply of gas and dry ice. They are adapted to deliver air and dry ice in relative mass ratios which will entrain and suitably deliver particles of the sizes intended to be used.  
         [0044]    A suitable type of particle source shaves particles from a block of dry ice. A particle source able to make dry ice particles is available from CAE Alpheus, Inc. In addition to using this type of device directly to supply the dry ice, previously-prepared dry ice in a hopper can be dispensed. Accordingly, the term “particle source” means any controllable source of suitable dry ice particles.  
         [0045]    In the operation of this system, the desired air flow rate at the selected pressure is set by regulator  60 , acting as a flow meter-controlled regulator or other device which responds to conditions respective to flow rates, and as directed by the logic being used.  
         [0046]    In turn, as shown by schematic lines  67 ,  68  and  69 , regulator  63  is slaved to regulator  60  to cause the particle source to produce an appropriate amount of particles which, when added to the gas stream will produce a blasting stream with the desired gas/dry ice ratio. Line  70   a  indicates the control of the particle source by regulator  63 .  
         [0047]    The ultimate objective of this invention is to enable a correct gas/dry ice ratio along with a selected pressure and flow rate.  
         [0048]    In FIG. 5, the logic, selection and control of any of the logic controls of FIGS.  1 - 4  is schematically shown in block  70  indicating reactive and dynamic apparatus and circuitry to control the regulators and the particle source. In fact, these three elements are the controlled ones. Once an agreed pressure and logic system have been selected, the consequence is ultimately to control the supply of ice to gas stream. Of course it will be recognized that a start-stop circuit may be interposed, but persons skilled in the control art will have no difficulty understanding such an arrangement.  
         [0049]    The logical steps can be attended to by manual manipulation of controls in response to output from the logic, but when this invention is used as intended, the system is self-monitoring and automatically determine a best ratio and best ice supply rate, subject to override by the operator.  
         [0050]    The determination of the flow rate for purpose of regulating the flow of gas can be made directly or indirectly. The classical directly responsive flow meter can readily be used.  
         [0051]    However, indirect measurement of other parameters can enable one to deduce the flow rate. A potentially important arrangement involves knowledge of the flow through a given nozzle at a known pressure. It follows that, with a given nozzle, maintenance of pressure will provide a known and agreeable gas flow rate.  
         [0052]    Accordingly, in this disclosure, the term “regulator” means a valve or other gas flow control device responsive to a signal which directly or indirectly measures or deduces the flow rate. The control relationships will be identical.  
         [0053]    Because the permissible range may be extended toward a higher-number ratio when heavier pellets are used, and toward a lower-number ratio when lighter particles or shavings are used, the logic in all systems may be provided with responses to inputs relating to the nature of the dry ice heavier or lighter, and particle size.  
         [0054]    When different nozzles are to be used, they can be substituted for one another and if desired color coded or otherwise identified. Then their properties can be placed in the reference, to add further limiting instructions for proposed uses of the invention.  
         [0055]    This invention is not to be limited by the embodiments shown in the drawings and described in the description, which are given by way of example and not of limitation, but only in accordance with the scope of the appended claims.