By Jim Rosenthal
In a previous article I summarized tests conducted using Corsi-Rosenthal Boxes with MERV 13 Filters at very high Air Change Rates. It was determined that two Corsi-Rosenthal Boxes could reduce particle counts in a small office to Class 10,000/ISO 7 levels in less than 15 minutes.
Questions have been raised about the age of the filters used in the CR Boxes for these tests. Some people believe that the MERV 13 filters decrease in efficiency relatively quickly and that “used” filters would not perform at comparable levels. So we decided to repeat the tests with Corsi-Rosenthal Boxes that have been used for a considerable amount of time – one for 4 months and the other for 5 months. These units run 24/7. The 5 month old system was in an office with 2 to 3 workers. The 4 month old system was used in an employee breakroom with an entrance door opening to the outside.
Here is the 5 month old system (dirty looking filters and all).
So, let’s see what these “used” CR boxes can do. Remember, this is in a 12’X14’X8′ relatively messy office with carpeting, a drop-down ceiling, and a man (me) working at his desk without a mask. These are hardly “cleanroom” conditions! The two Corsi-Rosenthal Boxes running on the highest speed give about 40 Air Changes Per Hour (ACH).
Again, particle counts dropped dramatically and reached Class 10,000 / ISO 7 levels in less than 15 minutes. The determining numbers are: at 0.5 um less than 10,000/cubic foot and at 5 um less than 83/cubic foot. In those first 15 minutes, the 0.5 um particles had dropped from 47,800 to 6,600/cubic foot and the 5 um particles had dropped from 1,100 to 0/cubic foot. Pretty impressive!
| Particle Size | 1:00 PM | 1:15 PM | 1:30 PM | 1:45 PM | 2:00 PM |
| 0.3 um | 890,100 | 170,700 | 150,400 | 141,700 | 149,900 |
| 0.5 um | 47,800 | 6,600 | 5,800 | 4,500 | 5,000 |
| 1 um | 8,700 | 500 | 500 | 0 | 200 |
| 2.5 um | 3,400 | 100 | 0 | 0 | 0 |
| 5 um | 1,100 | 0 | 0 | 0 | 0 |
| 10 um | 700 | 0 | 0 | 0 | 0 |
These MERV 13 filters are made with electret media. Both systems with well-used filters performed at a high level – comparable to systems using new filters. They are additional proof that filters using electret media will continue to have very high efficiency – even with prolonged use.
But, why is this important? It is important because some filter tests try to show this is not possible!
There are two major filter test standards – ASHRAE 52.2 and ISO 16890. Both incorporate test methods that are intended to decrease the electret properties of filters to simulate what could happen in use. In the case of ASHRAE 52.2, the test is called Appendix J and uses KCL particles to neutralize the electret charge on a filter. This appendix is optional and results in what is known as a MERV-A value. In my experience Appendix J tests results are not consistent and generally overstate the loss in efficiency of filters in use. For example, the MERV 13 filters used on these Corsi-Rosenthal Boxes oftentimes are found to have a MERV-A of 10. It is clear that they do not perform like MERV 10 filters – either new or used.
ISO 16890 uses a different procedure to simulate the potential loss of the electret charge of a filter. There are two tests involved. The first is of a new filter. The second incorporates Isopropyl Alcohol (IPA) vapor that “neutralizes” or eliminates the charge. (It has also been shown to damage the fibers of the media.) Then the “neutralized” filter is tested. The results of the first test with a new filter and the second test are averaged resulting in the number used to determine an ePM value. I am not aware of any research showing that these values are consistent with what happens with electret filters in use. Once again, we do not see anything close to this drop in efficiency in the MERV 13 Corsi-Rosenthal Boxes used in these tests.
Another important question has to do to the “mixing” of the air cleaning in the room. Are the air cleaners working to “mix” the air and provide consistent particle levels throughout the room? To test this we took particle counts in the corners of the room after the boxes had been in operation for 30 minutes. The boxes were in two corners of the room – 3 feet away from the walls. Measurements were taken with the particle counter 18 inches from each corner. Here are the results
| Size | Corner #1 – No CR Box | Corner #2 – CR Box | Corner #3 – No CR Box | Corner #4 – CR Box |
| 0.3 um | 147,800 | 136,000 | 237,500 | 167,000 |
| 0.5 um | 5,700 | 5,300 | 8,700 | 6,800 |
| 1 um | 1,000 | 700 | 900 | 300 |
| 2.5 um | 200 | 200 | 600 | 100 |
| 5 um | 100 | 0 | 0 | 0 |
| 10 um | 100 | 0 | 0 | 0 |
The particle counts in all 4 corners are in the same range of a Class 10,000/ISO 7 cleanroom. The air in the room appears to be “well mixed.” I would attribute this to the design of the Corsi-Rosenthal Boxes. The air enters on the sides of the box and exits vertically from the fan on top. The air then hits the ceiling and disperses throughout the room. The result is that all areas of the space have about the same particle counts and low exposure to indoor contaminants.
There are several “take aways” from these tests. First, high air change rates can produce extremely low particle counts reinforcing the filter effectiveness formula of Filter + Fit + Flow. Second, even MERV 13 electret filters that have been in continuous use for 4 or 5 months produce the same types of positive results as new filters. Third, Corsi-Rosenthal Boxes effectively mix the air in a space to provide uniform coverage and safety.