Method of cleaning passageways using a mixed phase flow of a gas and a liquid

Abstract

A method of cleaning surfaces using a mixed phase cleaning mixture of an aqueous solution and a flow of gas sufficient to produce droplets of the liquid which are entrained by the gas for a time sufficient to clean tubing of various lengths and geometries and porous membranes to remove biofilm, debris and contaminants.

Description

[0001] This application is a continuation-in-part of copending application Ser. No. 08 / 880,662.[0002] This invention relates to a method for removing biofilm, debris, contaminants and the like from the surfaces of passageways, including passageways which have irregular or complex shapes or whose walls are permeable, using a mixed-phase cleaning solution. BACKGROUND OF THE INVENTION [0003] Interior surfaces of passageways such as small-bore tubing, pipes, ducts and the like, which may carry fluids such as liquids, gases, slurries or aerosols, are very difficult to clean and to maintain in a clean condition. When the flow path is long and narrow, or hard to reach, it is difficult to clean the surfaces by conventional liquid phase flushing because such a long, narrow passageway limits liquid flow velocities by creating a high resistance to flow. As a result, shear stresses which could aid in the removal of contaminants from such surfaces are limited. Low flow velocities also limit the ...

AI Technical Summary

Benefits of technology

[0050] The present cleaning method is applicable to extremely long 1 / D tubing, and it is especially useful to be able to re-form droplets on a continuing basis after the initial impact of droplets with the tubing wall. The ability to clean long lengths of tubing is a significant advantage of the present invention. This requires operating in the appropriate region of parameter space so that the gas is able to form and entrain droplets repeatedly by means of the hydrodynamic instability. It can be estimated that prior art techniques which use a total of only a drop or two of liquid would not provide enough liquid for the surface area of a dental handpiece tube to achieve significant re-formation of droplets. One or two drops equals tenths of a milliliter. The present method uses a continuous flow for a period of time such that the amount of liquid used for the same purpose would be tens of milliliters, some two orders of magnitude higher.

[0051] Additionally, some geometries which are capable of being cleaned by the present invention have abrupt changes of geometry such as right-angle elbow or U-bends. It might be expected that at such changes of direction, many of the droplets will impact the elbow or bend and coalesce, which presents a danger of depleting the population of droplets which are the supposed mechanism of erosion. Nevertheless, it will be shown hereinbelow that good cleaning using the method of the present invention has been observed in passageways which have as many as seven reversals of direction. This indicates that the appropriate mixed-phase flow condition re-establishes itself quite well within a short distance after such a disturbance.

[0052] The formation and re-formation of droplets is influenced by, among other parameters, the surface tension of the liquid, the viscosity of the liquid, and the velocity of the gas relative to a liquid layer on the wall. In regard to surface tension, it is believed that, particularly for small diameter tubing, it is important that the liquid have a lower surface tension than pure water, which is about 72 dynes / cm. The addition of typical surfactants can lower that value to as low as 17 dynes / cm, depending on the specific surfactant added and its concentration. It is believed that a surface tension lower than about 72 dyne / cm, but higher than about 17 dyne / cm, encourages the formation of an appropriately small droplet size. It is believed that such a size results in high localized shear stresses when a droplet moving at or near the gas velocity in the principal direction of motion makes contact with the stationary wall and exerts a viscous shear on the wall. Such a value of surface tension may also extend the region of shear by encouraging wetting of the wall, and it is believed to help in reforming droplets after initial impact with the wall. However, if the surface tension is rather large and the diameter of the passageway is small, slugs may form, see FIG. 1, a regime which does not achieve good cleaning. It has been discussed elsewhere that surface tension higher than that of pure water (even as high as 150 dynes / cm) can be achieved with certain additives and this may be desirable for one mechanism of removal of particles. It is believed that the surface tension which is best used for comparing to the values just discussed is the dynamic surface tension of the liquid at the time scale characteristic of the droplet formation processes, rather than the static surface tension. For some substances, static and dynamic surface tension are indistinguishable, but for other substances they are different.

[0053] A cleaning solution can be used without the addition of a surfactant, providing its surface tension is within the above parameters.

[0054] It is also believed that an extremely small surface tension is not desirable because the droplets might become too small. In such case the droplets follow the motion of the gas so completely that the droplets do not often impact the surface of the passageway to be cleaned, referred to as channelizing the flow. Thus that amount of surfactant is added that provides good cleaning both by virtue of re-forming of droplets and a requirement that the droplets follow the gas motion closely, but not too closely.

[0055] The viscosity of the liquid is also important. The viscosity of water is affected by additives, most additives acting to increase the viscosity. Too large a liquid viscosity may prevent the liquid from breaking up into sufficiently small droploets, and will result in the liquid staying attached to the wall after the first impact, which is deleterious to cleaning. We have found that 500 centipose (cp) is too viscous.

Problems solved by technology

Interior surfaces of passageways such as small-bore tubing, pipes, ducts and the like, which may carry fluids such as liquids, gases, slurries or aerosols, are very difficult to clean and to maintain in a clean condition.

When the flow path is long and narrow, or hard to reach, it is difficult to clean the surfaces by conventional liquid phase flushing because such a long, narrow passageway limits liquid flow velocities by creating a high resistance to flow.

As a result, shear stresses which could aid in the removal of contaminants from such surfaces are limited.

Low flow velocities also limit the usefulness of solvents for the same reasons.

Cleaning of small diameter passageways is also difficult because of the nature of certain types of residues.

Removal of biofilm from passageways is also necessary for other applications, including medical, industrial and food service applications, because such biofilms are the main cause of high bacterial counts and high levels of endotoxins.

However, removing biofilm from fluid passageways is quite difficult, which makes disinfecting the surface more difficult as well.

However, these agents do not readily diffuse into the entire thickness of the biofilm.

Thus these treatments are generally only partially effective, and the original levels of viable bacteria return quite rapidly.

However, in addition to the fact that autoclaving does not actually remove debris from the handpiece, this autoclaving procedure can be damaging to the turbine and various seals in the handpiece.

While this may flush non-adherent biofilm and debris from passageways, it can be shown that it provides little or no removal of adherent biofilm and debris such as blood, mucous and the like.

While this is a useful method of distributing a cleaning solution, it does not ensure complete cleaning of adherent debris.

Simple spraying does not ensure that these conditions are met.

It can be estimated that prior art techniques which use a total of only a drop or two of liquid would not provide enough liquid for the surface area of a dental handpiece tube to achieve significant re-formation of droplets.

Infections arising from the use of endoscopic devices have been reported and traced to the inefficient cleaning and debris removal by conventional methods.

However, this method is time-consuming and suffers from inefficiency in removing all pathogens and other debris, as well as being subject to variations in technique from one operator to another.

Such substances decrease the thermal efficiency of heat exchangers.

Cleaning permeable membranes is more difficult than cleaning solid surfaces, because whatever is held back by the membrane can lodge either immediately at the membrane exposed surface or within the membrane pore structure, with the surfaces within the membrane pore structure being more difficult to clean.

Some medical devices contain materials or components which suffer damage from one or more of these processes, or there may be times when for other reasons it may be impractical to use them.

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