12 Jul The media test results on my HEGA filters show a ‘Negative Remaining Life at 20% breakthrough’. How do I interpret this, and manage change-outs more effectively?
When HEGA filters are first installed, they capture a very high percentage of acid compound — say 98%. As the carbon media begins to adsorb the target compounds, more of the compound breaks through to the other side (this is why it’s called “breakthrough”). This breakthrough increases over time until the percentage coming through goes past the point where you set as your change out limit (i.e. – Filtration Group might say 20% – 100 ppb in 20 ppb out). The negative number is simply an increasing breakthrough until ultimately the filter is completely spent and everything in passes through your HEGAs.
Now, how to equate this to life time. How much time, under your conditions, will it take to reach a 20% break through? Your cleanroom activity is obviously key. Is it consistent, or do you have spikes in activity? Remember that coastal areas that experience smog will create larger volumes of Hydrogen Sulfide (H2S). For Fabs in these areas, your filters will need to work overtime. Do you have an impinger set up yet to monitor cleanroom conditions? It’s a good idea to set this up during the known bad times of year (inversion, smog) and monitor those conditions. If the bad time comes around February, for example, start monitoring in early January and into April/May. Take a filter and put it into service in January and pull it in out after April/May and see how much life is left and compare it to the impinger’s readings. Put another in and leave it until the end of December and compare again. That would give you a good idea what’s going on both during a known high H2S period vs. the rest of the year. If you get hit with a big spike and the filter catches it all, but is spent, then it did its job and protected the process.