In response to Richard Matthews' Debunking the Myth column, “Enhanced clean devices,” (Dec. 1999, p. 42) he states “Some would argue … that there has been nothing new in cleanrooms lately.” If 25 years qualifies an item as an antique, there are a couple of antique theories out there that should not be allowed into the current millennium.
This discussion does not question the various theories for contamination control. That's not my area of expertise. I'll leave that one for the filtration folks.
The myths that I am addressing deal with the “efficiency of air flow or movement” and “temperature control and balancing of the tool or process within this contamination free environment.”
I have a broad thermal analysis background that extends back a number of years and covers long list of applications and configurations.
As a manager of electronic packaging in the mid 1960's I inherited a large group of 19-inch equipment racks that were packed with electronic sub-assemblies consistent with that period's technology. These “electronic pizza ovens” were over heating, and it was my responsibility to bring them into spec.
These racks utilized the typical base-mounted base blower, which essentially pressurized the rack and exhausted air near the top. (Pressurization was the accepted solution of that period.) It was clear that horse power alone wasn't going to solve the configuration issues that existed.
Utilizing my esoteric understanding of air flow behavior and my analysis guru we set out to clarify a myth. Knowing that pushed air stalls and/or buckles when it encounters resistance we relocated all shelf located blowers so they were “pulling” air through their respective units (all to the same side). Once this had been done we created an inlet chimney on one side and an exhaust chimney on the other side with large “pulling” blowers located at the top of the exhaust chimney. The base-mounted blowers were eliminated while leaving the filters and grilles in place. Most of these units were operational with that configuration refinement. Bringing them into specification, while no easy task, was now feasible.
Keep in mind that gas turbine engines (jets) and propeller aircraft pull themselves through the air. In auto racing drivers use the draft of the car in front of them to pull them along and at times for conserving fuel. If you are fortunate enough to have a fireplace in the room you sleep in, try this. Some cool winter evening enjoy a fire in the fireplace. At bedtime leave it burning with the flu open and the door closed. In the morning you will be wondering why the room is so cold. The hot chimney “pulled” all the warm air out of the room.
Response to Myth #1, air behaves like a rope, you can “pull” it but you will have great difficulty “pushing” it. I understand why low velocity air flow is utilized for laminar air flow temperature control in contamination free environments and, by itself, it makes sense.
However, low velocity air that is encountering an object/processing tool on its way to the floor isn't doing what you think it is doing, most likely. More than likely it needs help i. e., “pulling” blowers from the floor. (HVAC systems encounter many “pushed air” problems and are generally inefficient.)
“Hot spots” within the tool can be controlled with air but not low velocity air. The tool requires its own dedicated cooling so that long-referred-to “point of use cooling” can be integrated. “Point of use cooling” air ducts can be introduced while remaining vibration isolated from the tool.
Response to Myth #2, tool hot spot temperature control and balancing can not (in most cases) be accomplished with low velocity laminar air flow. Nor is it fair to assume that the control solution for a chamber is adequate for the control of the internal processing device.
Charlie Middleton, consultant
Liquids vs. gases
In your Electronics column “Particle filtration in liquids vs. gases,” (Sept. 1999) Robert Donovan states, “The van der Waals forces that hold a particle to a surface are also typically lower in liquids, so reentrainment should be more likely in a liquid.”
It is my understanding that van der Waals forces are higher for a particle in a fluid than a gas with similar surface interaction potentials. The intermolecular forces on a particle in a polar fluid also allow chelating, which wold explain why capture by sieving would predominate over capture by diffusion.
1. The literature I've read disagrees with the first point: For example, from p. 172 of Ranade [Aerosol Science and Technology 7: 161-176 (1987)] “Use of a liquid, rather than air or vacuum as the surrounding medium, facilitates particle removal. Two features involving liquid media may be exploited. First, the van der Waals interactions are one to two orders of magnitude smaller…” Ranade also includes a Table that shows the van der Waals forces of adhesion between various particles and surfaces in air are an order of magnitude larger than the van der Waals adhesive forces of those same particles to those same surfaces in water.
2. I'm not sure chelation is the right term. I think of chelates as ringed complexes surrounding a central metallic cation. Typically these complexes are of molecular dimensions, much smaller than the pore size of the filter depicted in the Figure reproduced from Grant et al. Particle agglomeration by chemical bond-ing could no doubt increase sieving. However, in the data cited, no metals were present for chelation. The forces investigated by Grant et al. and considered in my column are strictly physical, not chemical. RD