Coastal air, impregnated with higher concentrations of salt (NaCl), is known to be corrosive to HVAC equipment and fittings. It is virtually impossible to ventilate coastal buildings without salt ingress from the incoming air stream. Three strategies help lower the NaCl intensity on HVAC equipment:
Sea salt aerosols, one of the most common aerosols in the environment, form naturally as waves and wind interact. Analysis shows sea salt aerosol sizes follow a normal distribution across 0.5 µm to 10 µm, with a mean near 4.0 µm. Roughly half the particles are smaller than 4 µm and half are larger. Because the sizes range so widely, it helps to understand filtration performance across the full particle-size spectrum.
ANSI/ASHRAE Standard 52.2 rates filters on their ability to remove particles from the air stream across a range of sizes. The standard sorts 12 particle-size ranges into three bins, E1, E2, and E3. For aerosolized sea salt, the expected distribution is:
Table 1 sorts aerosolized sea salt particle sizes by ANSI/ASHRAE bin. Aerosolized sea salt is highly common, particularly in coastal regions, and the smallest sizes sit in the E1 bin. Sub-micron particles (E1) are the hardest to capture with passive filtration. Common MERV 8 filters have a 0% removal rate for these sizes, and the ultra-fine particles can linger in air streams for over a week.
Both MERV 8 and Opti-Lok M14 filters are compared here for aerosolized sea salt removal. Opti-Lok M14 was selected because it is compatible with most equipment in the field. It has been tested against 100% outdoor air for over a year, holding operating static pressure below 1.0" wg. That pressure drop is comparable to industry-standard MERV 8 filters, which means the M14 filter fits most DOAS/RTU systems without in-field modifications.
| Range | Size (µm) | Bin | Sea Salt Particles |
|---|---|---|---|
| 1 | 0.3 – 0.4 | E1 | 10% |
| 2 | 0.4 – 0.55 | ||
| 3 | 0.55 – 0.7 | ||
| 4 | 0.7 – 1.0 | ||
| 5 | 1.0 – 1.3 | E2 | 35% |
| 6 | 1.3 – 1.6 | ||
| 7 | 1.6 – 2.2 | ||
| 8 | 2.2 – 3.0 | ||
| 9 | 3.4 – 4.0 | E3 | 55% |
| 10 | 4.0 – 5.5 | ||
| 11 | 5.5 – 7.0 | ||
| 12 | 7.0 – 10.0 |
The industry norm is to apply MERV 8 filtration for outdoor air in DOAS/RTU systems. As Table 2 shows, MERV 8 filters remove 0% of the smallest particles (E1), 20% or less of the medium-sized particles (E2), and 70% or less of the larger particles (E3). Tested against aerosolized sea salt, a standard MERV 8 filter system reached an overall 45% removal rate, shown in Table 3.
| MERV Rating | Range 1 (0.3 – 1.0 µm) |
Range 2 (1.0 – 3.0 µm) |
Range 3 (3.0 – 10.0 µm) |
|---|---|---|---|
| 8 | n/a | 20 ≤ E2 | 70 ≤ E3 |
| 9 | n/a | 50 ≤ E2 < 60 | 75 ≤ E3 |
| 10 | n/a | 50 ≤ E2 < 65 | 80 ≤ E3 |
| 11 | 20 ≤ E1 | 65 ≤ E2 < 80 | 85 ≤ E3 |
| 12 | 35 ≤ E1 | 80 ≤ E2 | 90 ≤ E3 |
| 13 | 50 ≤ E1 | 85 ≤ E2 | 90 ≤ E3 |
| Bin | Sea Salt Particles (a) | Filter Removal (b) | Results (a × b) |
|---|---|---|---|
| E1 | 10% | 0% | 0% |
| E2 | 35% | 19% | 7% |
| E3 | 55% | 69% | 38% |
| MERV 8 Removal Rate for Aerosolized Sea Salt | 45% | ||
Run the same aerosolized sea salt test with a system using Opti-Lok M14 filters and the removal rate climbs to 92% (see Table 5), with static pressure not exceeding the MERV 8 sample. That is 100% better than the MERV 8 filter, allowing only 8% of the particles to penetrate the HVAC system versus 55% with MERV 8.
| MERV Rating | Range 1 (0.3 – 1.0 µm) |
Range 2 (1.0 – 3.0 µm) |
Range 3 (3.0 – 10.0 µm) |
|---|---|---|---|
| 8 | n/a | 20 ≤ E2 | 70 ≤ E3 |
| 11 | 20 ≤ E1 | 65 ≤ E2 < 80 | 85 ≤ E3 |
| 13 | 50 ≤ E1 | 85 ≤ E2 | 90 ≤ E3 |
| M14 | 95 < E1 | 95 ≤ E2 | 90 ≤ E3 |
| Bin | Sea Salt Particles (a) | Filter Removal (b) | Results (a × b) |
|---|---|---|---|
| E1 | 10% | 95% | 9.5% |
| E2 | 35% | 95% | 33% |
| E3 | 55% | 90% | 49.5% |
| Opti-Lok M14 Removal Rate for Aerosolized Sea Salt | 92% | ||
References
[1] Katherine L. Ackerman, Alison D. Nugent, and Chung Taing, "Mechanisms controlling giant sea salt aerosol size distributions along a tropical orographic coastline," Atmospheric Chemistry and Physics, Volume 23, 2023.
[2] Lynn M. Russell, Richard H. Moore, Susannah M. Burrows, Patricia K. Quinn, "Ocean flux of salt, sulfate, and organic components to atmospheric aerosol," Earth-Science Reviews, Volume 239, 2023.
[3] ANSI/ASHRAE Standard 52.2-2017, "Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size."
