Silica-Resist Heat Recovery Media

Many industrial processes contain silica fume, along with the typical VOC. When the silica fume reaches around 1300℉ it combines with oxygen (oxidizes) and is converted to Silica Oxide (SiO2). At this point it changes state and goes from a fume to a fine solid particulate. It has the consistency of talcum powder.

RTO media is made of the high temperature bonding of alumina and silica.  When it is kiln fired, the 2 ingredients bond together to form alumina-silicate.  Unfortunately not all of the ingredients bond together, and some silica and alumina just remain as silica and alumina. Here is where the problems begin... The silica fume that has now gone to a solid because of oxidation, bonds with the un-bonded surface silica in the heat recovery ceramic.  As more silica fume enters the system and is oxidized, it also bonds to the previous silica that has bonded.

In time, there is so much silica bonding, on top of silica, on top of silica.  Eventually and inevitably, the RTO heat recovery bed becomes seriously clogged. If using an RTO push-fan system, blowing the silica out of the access door, gives a short-term temporary relief.  Some inexperienced RTO owners mistakenly wash it down, but all that does, is pack the silica deeper into the RTO bed.

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Pressure Drop and Thermal Efficiency Comparison of Heat Exchange Media

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Regenerative Thermal Efficiency

Just about any media sold on the market…random packed saddles, monolith or structured media, can only achieve the maximum allowed thermal efficiency, which is about 96%. This is dictated by the laws of physics!

In a given system, to reach the theoretical 96%, it’s all a matter of cycle interval and how much combustion air is introduced to unbalance the system. The faster you cycle the chambers, the greater the thermal efficiency will be.

The less combustion the air burner uses, the higher the actual thermal efficiency will be TER = (Tc-To/Tc-Ti)(Flow in/Flow out). The actual thermal efficiency determines your fuel usage, not the theoretical. Typically with just enough air to support combustion (10:1 fuel/air combustion ratio) will equal about 94% actual thermal efficiency.

Keep in mind, if your original heat recovery product was doing 92% TER and achieved your mandated VOC destruction with a transfer cycle of 3 minutes, there’s a good chance that 95% can be attained with a 1 minute cycle.  Now, the number of unburned VOC slugs entering the atmosphere will be increased by a factor of 3. Good chance you’re now out of compliance.

Without question, your prime directive is to achieve compliance with your RTO.  Conversely, media suppliers sell based on thermal efficiency. The reason you purchased a VOC destruction device is to keep you in VOC compliance. The reason you purchased an RTO was for thermal efficiency and low fuel usage.
Very often it may be necessary to mix different types and configurations of media to get your best outcome. The amount of VOC present in the process airstream and thermal efficiency required for the lowest overall utility costs, will dictate this.