2015年12月24日星期四

Dry Ice Blasting -- a Better Cleaning


What Is Dry Ice?

Dry ice pellets are made by taking liquid carbon dioxide (CO2) from a pressurized storage tank and expanding it at ambient pressure to produce snow. The snow is then compressed through a die to make hard pellets.

The Process

With the dry ice blasting process, dry ice (CO2) particles are propelled to supersonic speed impacting and cleaning a surface. The particles are accelerated by compressed air, just as with other blasting methods. Overall, there are three steps involved in dry ice blasting. Also, it can viewed in diagram form (see below).

Step #1 - Energy Transfer

Dry ice pellets are propelled out of the blasting gun at supersonic speed and impact the surface. The energy transfer knocks off the contaminant without abrasion. The force of this impact is the primary means of cleaning.

Step #2 - Micro-Thermal Shock

The cold temperature of the dry ice pellets hitting the contaminant creates a micro-thermal shock (caused by the dry ice temperature of -109F) between the surface contaminant and the substrate. Cracking and delamination of the contaminant occurs furthering the elimination process.

Step #3 - Gas Pressure

The final phase has the dry ice pellet explode on impact, and as the pellet warms it converts to a harmless CO2 gas, which expands rapidly underneath the contaminant surface. This forces off the contaminant from behind. The contaminant is then relocated, typically falling to the ground. Since the dry ice evaporates, only the contaminant is left for disposal.

Operating Details

To operate dry ice blasting equipment a few details need to be considered. First, though using dry ice is extremely safe, basic safety matters need to be followed including wearing hand, eye and ear protection. First, the process is noisy and ear protection should be used. Second, the temperature of dry ice is -109 degrees Fahrenheit. Gloves should always be worn when working with it. Thirdly, in dusty areas, a mask is recommended for eye and nose protection. Otherwise, wearing standard eye and ear protection would be acceptable just as with sand blasting or pressure washing. Fourthly, the gun should never be pointed at another individual or serious injury could occur.

Also, blasting in an enclosed area is safe with proper ventilation. Because CO2 is 40% heavier than air, placement of exhaust fans at or near ground level is necessary when blasting in an enclosed area. In an open shop environment, existing ventilation is sufficient to prevent undue CO2 buildup.
Window mounted copper motor round exhaust fan

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