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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Wed, 6 Jul 2005 11:28:47 +0300
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OK, lesson on US coming up, sorry for the simplistic didactic matter! :-)

How does US cleaning work? You apply a mechanical vibration to the 
liquid, which is virtually incompressible. When the pressure rises, the 
fluid is pushed and the level rises (imperceptibly, of course). When the 
pressure decreases, minute cavities form where there are nuclei. These 
are essentially vacuums, but they do contain vapour, as well. At the 
next positive pressure, the mechanical inertia of the mass of fluid is 
too great for the cavities to be refilled. At the next negative 
pressure, the cavities which are still present form the nuclei for the 
cavitation and they thus grow bigger. After n cycles, the cavity will 
have grown to a critical size and, being essentially a vacuum, they 
collapse instantaneously (in picoseconds). This causes the vapour to be 
adiabatically compressed, resulting in an astronomical temperature rise, 
often of the order of thousands of degrees. The heat is dissipated in 
the solvent and the energy contained therein becomes a mechanical shock 
wave, a micro-tsunami if you like, and this shock wave is what scrubs 
your parts clean. The effective radius of this scrubbing action is of 
the order of µm, so you want the cavitation to occur where the dirt is.

Now, the favourite place for cavitation to occur is on gas molecules 
which can be anywhere in the fluid, nowhere near the place to be 
cleaned, so the energy is wasted there. This is why the solvent must be 
degassed before use. In the case of some organic solvents, such as the 
late-lamented CFC-113, applying the US for, say, 15 minutes before 
starting to clean would degas the solvent sufficiently. Water (or 
aqueous solutions) are a horse of another colour and degassing became a 
complex procedure requiring heating and low pressures, then cooling, 
before use.

What is the difference between liquid and vapour phases of a substance? 
Only the size of the molecule. In an apparently homogeneous liquid, you 
actually have some molecules in vapour phase (which is why the liquid 
evaporates, as the larger molecules happen to randomly reach the 
surface). This is also the motive power for Brownian movement. This is a 
question of the equilibrium conditions of the vapour pressure at a given 
temperature. The more molecules in vapour phase within the body of the 
liquid, the more cavitation nuclei are formed remote from where you want 
them, so the cleaning becomes less efficient.

However, it doesn't stop there. The higher the temperature, the more 
vapour there is within a cavity and the lower the adiabatic compression 
during collapse, resulting in a shockwave of lesser amplitude.

For the most efficient US cleaning, choose the lowest temperature 
DEGASSED solvent you can. This is why, in the traditional vapour phase 
degreaser, the US transducers are always placed in the cold sump, 
receiving the condensate from the refrigerated freeboard condensers and 
never in a heated sump.

I hope this makes it clear.

Brian

Dehoyos, Ramon wrote:
>         I plead ignorance on this matter. I was taught that the fluid *had* to be heated.  Could you enlighten us please?
>         Thanks Brian,
>         Ramon 
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
> Sent: Tuesday, July 05, 2005 10:24 AM
> To: [log in to unmask]
> Subject: Re: [TN] Ultrasonic Cleaner
> 
> You do realise that US cavitation becomes less effective if you heat the solvent, do you?
> 
> Brian
> 
> David Tremmel wrote:
> 
>>Happy 4th Everyone,
>>
>>I am in the market for a benchtop ultrasonic cleaner and am interested 
>>in purchasing a Branson 1510DTH (1/2 gallon capacity with a heater).
>>
>>It will be removing post-reflow flux residue from stencils and other 
>>equipment and will be operating 5-10 hours per day.
>>
>>If anyone has any experience with this model or has a suggestion based 
>>on their experience with another brand and model, please let me know.
>>
>>As always, I appreciate whatever feedback I receive.
>>
>>David
>>Technical Manager
>>BT Mexico
>>
>>_________________________________________________________________
>>Consigue aquí las mejores y mas recientes ofertas de trabajo EE.UU.
>>http://latino.msn.com/empleos
>>
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