Aerating a cement silo uses little air: as a rough guide, in the region of 50 to 200 l/min per silo during discharge, depending on the number of aeration pads and the sequence. The air is reduced to a low pressure, specified by the silo manufacturer, and must be dry: a drop of water in a pad and the cement forms lumps that block the fabric. On a batching plant, this requirement adds to the valves and filters, and a compressor such as the K 75 covers the lot.
The other uses of air on a batching plant are covered on our page compressor for concrete batching plants. This article deals only with silo aeration, also called fluidisation.
What does aeration do in a cement silo?
Cement is a very fine powder. At rest, it packs down in the silo cone. It can form a bridge (or arch) above the outlet, or a rathole down the middle with cement stuck to the walls. The screw conveyor then runs empty and the weighing stops progressing.
Aeration solves this problem. Aeration pads or nozzles, fitted on the cone, blow air through a fabric or a membrane. The air slips between the cement particles, which behave like a liquid for a moment and flow towards the outlet. This is not pneumatic conveying: only a little air is introduced, just enough to loosen the cement.
There are two ways of doing it:
- Aeration controlled by the weighing: the solenoid valves open during discharge to the cement weigh hopper, pad by pad or in groups.
- Booster pulses: a short cycle, triggered when weighing slows down or after a long stop, for example on Monday morning.
Some silos also use a pneumatic vibrator on the cone. The principle is different, but the air comes from the same network.
Fly ash, GGBS and limestone filler behave differently from cement. Some flow more easily, others pack down even harder. If a silo has changed product, check the aeration settings with the silo supplier rather than assuming the old ones still apply.
How much air does silo aeration use?
Each pad has its own consumption, given by the manufacturer at a working pressure. These values are a rough guide; take the ones for your silo from its manual.
Example: a cement silo fitted with six aeration pads.
- Consumption of one pad: in the region of 40 l/min at working pressure.
- Sequence: the valve opens two pads at a time, in turn. During discharge, there is therefore a constant 2 × 40 = 80 l/min.
- Silos discharged at the same time: a plant often draws two cements in the same batch, or one cement and one filler. Two silos: 2 × 80 = 160 l/min.
- 25 % safety margin for fabric wear and leaks: 160 × 1.25 = 200 l/min.
- 20 % expansion margin, if a silo is to be added: 200 × 1.2 = 240 l/min.
If the sequence opens all the pads at once, the calculation changes: six pads at 40 l/min make 240 l/min per silo, or 480 l/min for two silos before margins. Read the sequence specified by the manufacturer carefully before calculating.
On a typical plant, aeration therefore accounts for about a quarter of the total requirement. The full calculation, with the cylinders and the filters, is in our article concrete batching plant compressor, the sizing.
At what pressure should the aeration pads be supplied?
Aeration pads work at low pressure. They are never supplied directly at network pressure: too much pressure tears the fabric, creates ratholes in the cement and blows air into the silo for nothing. The working pressure is given in the silo manual.
The usual arrangement:
- a pressure regulator with gauge on the aeration line, set to the manufacturer's value;
- a non-return valve on each pad, so that cement does not travel back up the pipe when the air stops;
- a solenoid valve for each pad or group, controlled by the plant PLC;
- a fine filter just upstream of the regulator, to protect the fabrics.
The network itself stays set at 7 or 8 bar for the cylinders and filters. The regulator does the rest. Our guide to connecting to the air network explains these arrangements in detail.
Fit the regulator where it can be read from the ground or from a platform. A regulator hidden halfway up the cone is rarely checked, and its setting drifts without anyone noticing until the cement stops flowing.
Why must aeration air be very dry?
Aeration air goes straight into the cement. If it carries water, it delivers it to the worst possible place: against the pad fabric, where the cement sets in small patches. The fabric blocks, the pad no longer blows and the bridge comes back. Higher up in the silo, moisture forms lumps that end up in the screw conveyor or the weigh hopper.
A refrigerated dryer at the compressor outlet is the minimum. It is built into the K series. But the aeration pipe runs over the silo cone, outdoors. In winter, the air in it cools below the refrigerated dryer's dew point and the remaining water condenses, then freezes in the solenoid valves. On these exposed branches, a desiccant dryer is justified. See dry air in winter on a concrete batching plant and our guide on pressure dew point.
Our compressors are lubricated: the outlet filtration retains carried-over oil. For air that goes into the cement, the quality class is defined with our technicians using the ISO 8573-1 guide.
Which compressor for aeration and the plant?
Aeration never justifies a compressor on its own: it shares the plant's network.
- Small site, one or two silos, few cylinders: the S 55 delivers 0.6 m³/min at 8 bar. It comes without a receiver or dryer: these must be added.
- Plant with three or four silos: the K 75 delivers 1.1 m³/min at 8 bar, with a 420 L receiver, refrigerated dryer and filtration. This is the most common case.
- Large plant or precast works: the K 110 goes up to 1.7 m³/min at 8 bar.
The S and K series are variable speed: they follow a demand that changes with every batch.
Pitfalls specific to silo aeration
- Blowing continuously. A pad that blows while the silo is idle only ventilates the cement. Have the aeration controlled by the discharge.
- The forgotten non-return valve. Without it, cement travels back up the pipe at every stop and blocks it.
- The invisible leak. An aeration solenoid valve left slightly open blows into the silo day and night, with no sound audible from the ground. The leak cost calculator gives the order of magnitude.
- Condensate. An automatic condensate drain at the low point of the pipe, before the cone, stops water being sent into the cement.
To size the air for your plant, use the configurator or send your silo manual with a quote request.
For your trade: Compressor for concrete batching plants, quarries and demolition.