Wednesday, April 10, 2013

Waterjetting 8a - cleaning with heat

Water is used almost everywhere as a way of cleaning surfaces. Several times a day we typically rub our hands together with water, and usually with some soap, to clean them. Pediatricians and others suggest that children recite a short rhythm, such as a chorus of “Happy Birthday” while doing so to allow the water, soap and mechanical actions to combine and effectively remove dirt. That teaches the child that it takes some 20 seconds for the cleaning action to be effective. The cleaning action is not to sterilize germs, viruses and other obnoxious things on the hands. Rather it is to ensure that they and other dirt particles are physically removed, leaving the hands clean. (This is a different action to the chemical washes that are becoming popular.)

This is not an instantaneous process since the soap and water must reach into all the dirt-collecting parts of the hand, hence the need for the nursery rhythm. The same basic sequence occurs in the cleaning action of a high-pressure waterjet on a surface, although the pressure of the spray means that the water can penetrate faster. But it is why, in using a car wash lance in cleaning a car, it is smart to spray the body of the car with a detergent first, then allow this to work in creating micelle clusters around the dirt particles, so that the mechanical action of the subsequent jet spray will dislodge and remove them. Merely adding detergent to the cleaning water as it goes through the cleaning lance, and strikes the car surface does not give the chemicals in the water time to act before they are gone. Bear in mind that the jet is moving at several hundred feet per second, and that it hits and rebounds from the surface over a path length of perhaps an inch or two. As a result the residence time of the jet on the surface is measured in fractions of a millisecond. This is not enough time for the chemicals to work. (On the other hand it does help keep the sewers under the car wash cleaner than might be otherwise expected.)

With an increase in jet pressure, the speed of the mechanical removal of dirt and other particles from a surface can be fast and effective. The ability of the jet to penetrate into and flush out surface cracks, and joints, means that it becomes a good tool for removing debris from the joints in concrete decks, and, at a little higher pressure, it can also be used to remove deteriorated concrete from surfaces. But I am going to leave that topic until next week.

The other “treatment” that we use when we wash our hands is to heat the water. When used with soap it helps to remove the surface oils on the skin that act as a host to bacteria. Heat is becoming a less common tool than it used to be in high-pressure jet cleaning. At one time steam cleaning, which was followed by hot pressure-washing, had a larger sector of the market. It is a bit more difficult to work with (the handles of the gun get hot, and the operator needs more protection) but for some work it is still the more effective way to go.

Steam, however, loses both heat and mechanical energy very quickly after it leaves the nozzle. It will, for example, lose some 30% of its temperature within a foot of the nozzle. Hot sprays of water can thus be more effective, but when cleaning grease and oils a lower temperature spray will merely move the globs of grease around the surface. Heating the water to around 185 degrees Fahrenheit, or 85 degrees C, will stop that happening and works much more effectively in getting the surface clean.

Figure 1. The effect of water temperature on cleaning different surfaces (A, B and C) of different types of dirt.

But, as with many tools, heated water needs to be applied with a little bit of background knowledge. I mentioned that just pointing a large jet of water at, for the sake of discussion, a boulder covered with an oil spill would, at lower water temperatures, just move the oil around the surface. At higher temperatures the oil would break into smaller fragments that are removed from the surface, but they need to be captured, otherwise the treatment is just spreading the problem over a larger area. This is why it becomes more effective to use smaller, higher pressure systems that have lower contained jet energy, and which can be used with a vacuum collection system to pick up the displaced water, oil and debris.

Figure 2. Using hot, pressurized water streams in cleaning up after the Exxon Valdez oil spill (NOAA )

With the streams used in the picture shown in Figure 2, the energy in the jet will move the oil, but without containment it was being washed down to the water, where it was collected using booms. This is not particularly effective, since in the process the jets also washed the silt out of the beach, and drove some of the oil down into the underling beach structure, so that it continued to emerge in later years contributing to an ongoing problem.

What is needed is to provide enough energy to drive the oil away from the surface, and yet not enough to move it great distances or to disrupt the surrounding material. This can be achieved by using a higher-pressure, but lower flow rate jet. Because some of the water will turn to steam as it leaves the nozzle, Short (PhD U Michigan, 1963) showed that the droplet size will fall from 250 microns to 50 microns when the water is heated above 100 degC.

Obviously that also will reduce the distance that the jet is effective, and so a balance needs to be achieved between the heat put into the water, and the size of the orifice(s) if the jets are to remove the contamination, but in such a way that it can be captured. And here again there is a benefit from having a suction tool associated with the cleaning spray. Because of the problems that oil and grease can cause, it will require special care in designing the capture systems downstream. Incidentally it is generally better if the water is heated downstream of the pump, since there are higher risks of cavitation in the inlet ports if the water is too hot.

And sometimes the two can be combined in ingenious ways. For example Bury (2nd BHRA ISJCT, Cambridge, 1974) added a steam shroud around a conventional waterjet at 5,000 psi as a way of cleaning hardened plastic from the insides of a chemical plant pipe.

Figure 3. Wrapping a conventional waterjet in a steam shroud (Bury et al 2nd BHRA ISJCT, Cambridge, 1974)

Without the steam assist the plastic was not removable, even at higher jet pressures, but with the steam to soften the plastic the pipe was successfully cleaned.

Figure 4. High-pressure water fails to remove hardened plastic, (lhs) but with a steam shroud a lower-pressure jet effectively cleans the pipe (rhs). (Bury et al 2nd BHRA ISJCT, Cambridge, 1974).

1 comment:

  1. Unique method of cleaning.. Using warm water with pressure for cleaning is mostly used by reputed companies like Bond Clean Sunshine Coast