The filter on the roof of a cement silo needs, as a rough guide, in the region of 100 to 200 l/min of air while a bulk tanker fills the silo, at a steady pressure of about 6 bar at the filter. On a concrete batching plant, only one silo is filled at a time: with the mixer filter and the end of the previous silo's cycle, pulse cleaning rarely accounts for more than 0.5 m³/min (about 18 CFM). The critical point is not the air flow but dry air: a trace of water on a filter bag and the cement forms a crust.
The general method for pulse-jet cleaning is explained in our article on the compressor for bag filter pulse cleaning. Here, we deal with the case specific to cement, and the page compressor for concrete batching plants covers the other uses of air on site.
Why does the silo filter work mainly during filling?
The bulk tanker blows the cement into the silo with air from its own compressor. That conveying air has to get out: it passes through the filter on the silo roof, which holds back the dust. During filling, the bags or cartridges load up quickly. The sequencer then fires pulses of compressed air, row by row, to knock the cement back down into the silo.
Outside filling, the filter sees almost no air. Most sequencers run a cleaning cycle of a few minutes after the tanker leaves, then stop. The plant compressor therefore only supplies the air for the pulses, never the tanker's conveying air.
This is why the filter's demand comes in waves. On a plant receiving four or five tankers a day, the filters may pulse for only two or three hours in total. The compressor still has to deliver the full requirement during those hours, but the rest of the day the filters cost almost nothing.
How much air is needed to clean silo filters?
The calculation is based on the filter manual: the volume of air per pulse and the interval between two pulses. Indicative values for a four-silo plant:
- Silo being filled: 25 L per pulse, one pulse every 10 seconds, so 6 per minute: 6 × 25 = 150 l/min.
- Previous silo at the end of its cycle: when tankers arrive back to back, the filter on the silo filled just before carries on pulsing for a few minutes. Same filter, same rate: 150 l/min.
- Mixer or cement weigh hopper filter: 15 L every 10 seconds, so 90 l/min, during production.
Total: 150 + 150 + 90 = 390 l/min. With a 25 % margin for diaphragm wear and leaks: 390 × 1.25 ≈ 490 l/min. If you plan to add a silo, add 20 %: 490 × 1.2 ≈ 590 l/min, or 0.6 m³/min.
The four filters are not counted at the same time: that would be another 600 l/min for filters that are idle. On the other hand, if two tankers discharge into two silos at once, count two silos being filled.
If the manual only gives the number of valves and the collector volume, ask the filter supplier for the air per pulse at the working pressure. Guessing from the size of the filter is unreliable: two filters of the same area can differ by a factor of two in air per pulse depending on the valve size.
What pressure at the filter on top of the silo?
The filter sits 15, 20 or 30 metres from the compressor, at the end of a pipe that runs up the side of the silo. The manual gives the working pressure of the header, often around 6 bar. If it is not reached, the pulses are weak, the cement stays stuck, the filter pressure drop rises and the tanker's conveying air struggles to escape.
Three rules:
- A pipe of adequate diameter, with no unnecessary reductions. Our pressure drop tool checks the loss over the actual length.
- A reserve near the filter. The filter header recharges between two pulses. A small buffer receiver at the foot of the silo, or a generously sized pipe, stops it recharging poorly at the end of the line. The air receiver size calculator gives a starting volume.
- A setting of 7 to 8 bar at the compressor, no more. Measure the pressure at the filter during filling: that is where it matters.
A simple test tells you a lot: fit a pressure gauge on the filter header and watch it during a pulse sequence. If the needle has not returned to the working pressure before the next pulse, the supply pipe or the reserve is too small.
Why does cement demand dry air?
Cement reacts with water. A pulse containing moisture wets the dust stuck to the bag: it forms a hard crust that the following pulses can no longer remove. The filter blinds for good, and the bags have to be replaced.
Water also damages the sequencer. It makes the solenoid valve diaphragms stick, so they stay open or no longer open. A leaking diaphragm empties the header continuously, and the whole row stops cleaning.
The basic treatment is a refrigerated dryer at the compressor outlet, with an automatic condensate drain on the receiver and at the low points. But the pipe that runs up to the filter is outdoors. In winter, the air in it cools below the refrigerated dryer's dew point and the remaining water condenses, then freezes. Our article dry air in winter on a concrete batching plant explains when to move to a desiccant dryer, and the guide on pressure dew point gives the reference points.
Which compressor for cement silo filters?
| Case | Requirement (margins included) | Recommended model |
|---|---|---|
| Filters only, 2 to 4 silos | 400 to 600 l/min | K 75, 1.1 m³/min at 8 bar, built-in dryer |
| Filters, cylinders and aeration for a plant | 800 to 1,100 l/min | K 75, or K 110 if output is high |
| Large plant, receiver and dryer already in place | 1,200 to 1,600 l/min | S 110, 1.7 m³/min at 8 bar |
In practice, the same compressor also supplies the gates, the valves and the aeration. The full calculation for the plant is in concrete batching plant compressor, the sizing. The K and S series are variable speed: they follow a demand that rises with each tanker delivery and drops back afterwards.
Pitfalls specific to cement silo filters
- A filter that no longer cleans. If pulse cleaning weakens, the tanker's conveying air escapes poorly and the silo builds up pressure. The silo is fitted with a pressure relief valve: a blinded filter makes it work. Watch the filter's differential pressure gauge and have the whole assembly checked according to the silo manufacturer's instructions.
- A sequencer that never stops. Some filters pulse day and night, even with the silo full. That is wasted air and needless wear. Have the sequencer set to stop after the end-of-filling cycle.
- Leaks at height. A leaking solenoid valve on a silo roof cannot be heard from the ground. The leak cost calculator shows what it is costing you.
- Oil. Our compressors are lubricated: the outlet filtration retains carried-over oil. A fine filter at the foot of the silo gives the diaphragms extra protection.
Enter your silos and filters in the configurator, or send their manuals with a quote request: we will check the air flow and the pressure at the filter with you.
For your trade: Compressor for concrete batching plants, quarries and demolition.