For  a  Clean  Safe  Shop,  Start  With
Clean Shop Air,  LLC
Why  Not  Use  Room-Air  Filter  Systems?
There are some very good reasons why they are not a very good idea

Ceiling-mounted, whole-room air filter systems are quite popular. So why not use one of them instead of a fancy filter system on a big cyclone? It seems it would be a lot more efficient, wouldn't it? And wouldn't it certainly cost less in total?

Popularity is not
the Same as

Lots of people make decisions based on what other people do and what other people think. Sometimes that saves a lot of trouble and mistakes, but what if other people like what they have but do not recognize the danger in their decision?

A classic example of this "herd mentality" is the popularity of smoking 50 years ago when the tobacco industry was claiming that there were no health risks associated with tobacco use. Yet, how many have died from smoking-related illnesses.

Let's look at the ceiling-mounted (or wall-mounted or mounted anywhere else) room air filter from a viewpont of sound, scientific reasoning.

Let's Try a Pot
of Coffee,
Cleaning Up
the Urn
Suppose we have a workshop about 25 x 30 feet, probably a slightly oversized 2-car garage. With an 8-foot ceiling, the total air volume is about 6000 cubic feet. With a room-air filter that filters 1200 CFM, that means a complete air change every five minutes, so we should have clean air in that same amount of time, right? Well, let's conduct an experiment to find out:

  • Imagine a large five-gallon clear, glass coffee pot or urn full of stale, nasty coffee. Now let's use some nice, clean, clear water to clean it up.

  • First, we connect an imaginary hose to our imaginary water source, set the 5-gallon urn in the sink, turn the water on so it is flowing at the rate of one gallon per minute so that we have a "water change" in the same five minutes as with our air filter system in the workshop.

  • Next we place the hose in the coffee so the end of it is about an inch or two below the top surface, and let the water flow in as the coffee spills out over the side. Will the coffee be cleared out and the urn full of clean water in five minutes? No. It is impossible. It cannot happen.

  • So why doesn't the coffee clear out in five minutes? Simple. The water is diluting the coffee, not directly replacing it. Suppose we could replace the water hose with a perfect filter system that would pump coffee out at the rate of one gallon per minute, remove all impurities, then return clean water to the urn. How long would that take? Five minutes? Again, no. For the same reason. As coffee is removed, filtered, and the water returned, the effect is exactly the same as pumping clean water in. In fact, in theory, each time you do a complete coffee change-out with water, you cut the amount of coffee in the urn by half. But this is in theory only, and assumes you are stirring the coffee. If not, the coffee in the bottom may sit there and take a lot longer to clear out. So, with tubulent coffee, in 10 minutes, you remove up to 75% of the coffee, in 20 minutes, you remove 93.8%, and in an hour, you remove about 97.5%. But in two hours, you are only up to 99.94%.

Mathematically speaking, you cannot ever remove ALL of the dust from the air in a closed space by filtering the air over and over and over. Every five minutes, you reduce it by half, but you can never get all the way to zero. Furthermore, if you "stir up the air" for more efficient cleaning by the air cleaner, you also increase the number of particles in the air that find their way deep into your lungs where they can cause health problems for you.

Start with
    Clean Air
    Keep it Clean

To understand how to do the job right, consider municipal water systems. They start with dirty water, process it, then deliver a clean product for human consumption. Apply that same concept to your shop, start with dusty, dirty air coming from a tool or machine, capture it before it can escape into the air you are breathing, then clean it and return the clean air to the room for safe breathing.

Municipal water treatment facilities usually take incoming water from a lake or stream (preferring a lake so that silt and heavier dirt can settle out before treatment), run it through a series of filter beds followed by activated charcoal or some other agent to remove various organic contaminants, then they add a disinfectant, usually chlorine, to make it safe from most disease-causing organisms (called pathogens).

To attain clean shop air, one must start by collecting enough air around the tool or machine to capture all of the dust escaping from the tool bit, knife, or blade, then use a highly effective separator to remove all but a small amount of the very finest dust from the air stream before sending the air through a micro-fine, certified filter medium that then captures the last very few dust particles that escaped from the separator. Using a HEPA (high-efficiency particle arrestor) filter, or a reasonably equivalent non-HEPA equivalent, is practical in this scenario because the air is already mostly clean and contains very little, and only very fine dust. This is not unlike the activated charcoal filters in public water systems where most of the fine dirt particles (called "turbidity") have already been removed from the water so they do not plug the charcoal beds with clay and other solid matter.

The Issue Is

The primary purpose of dust collection is:

  1. First: Keep the air in the shop clean, safe, and dust-free.

  2. Second: Collect all dust-laden, contaminated air from around tools and/or machines and strip the dust from the captured air before returning it to the shop environment.

Keeping a clean, attractive shop and shop floor is entirely secondary to those primary considerations. But as in money management where if you keep track of pennies and nickels, the dollar bills tend to take care of themselves, so is it with having a clean, safe shop. If you pay proper attention to the capturing and processing of dust and dirt from tools and machines, including the really fine dust that is often missed due to inadequate air flow, the rest of the shop tends to take care of itself to a surprising extent.

Room-Air Filters
   Don't work

For the reasons explained, whole-room air filters, regardless of where or how they are mounted and configured, simply cannot maintain dust levels and air conditions in a working shop at levels that are compatible with good health. Yes they can filter out micro-fine dust particles, but they cannot ensure a shop with dust levels at acceptable concentrations while tools and machines are in use. That requires capture and processing of dust, starting at the source of the dust. It is far more practical to use a good quality, well-designed cyclone with adequate dust-collection hoods and adaptations, ample ducting of appropriate size (no smaller than 6-inch), and effective, high-performance air filters in a balanced and properly installed configuration. Such an arrangement is more pleasant to work with, ensures reasonable freedom from health risks, gets rid of the sneezing, and delivers a lot of other advantages that the room-air filter cannot provide.

Don't Forget
Filter Costs

Not only does a whole-room air filter not get all of the dust out of the air, but if you do not use a cyclone dust collector that is exceptionally effective in removing micro-fine dust from the air stream, you can actually end up with more dust being trapped by the whole-room filter than would be caught in the cyclone filter. Why? Because conventional collectors allow too much dust to escape through the filters, and that dust is what poses the greatest health risks. That means more money spent on filter elements, and over the long term they could use up more money than a good cyclone system would have cost in the first place.

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Updated August 3, 2004

©Copyright 2004, Clarke F. Echols and Clean Shop Air, LLC. All rights reserved.
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