For a concrete batching plant with four silos and one mixer, the requirement is around 0.9 m³/min (about 32 CFM) at 8 bar, margins included. A 7.5 kW 4-in-1 screw compressor such as the K 75, which delivers 1.1 m³/min at 8 bar with a dryer and filtration, covers this case. A small mobile site plant with two silos manages with 0.5 m³/min; a plant with two mixers needs more like 1.8 m³/min, which means a K 150.
Our page compressor for concrete batching plants covers all the uses of air on site. Here, we go through the full calculation, valve by valve.
How much air does a gate or mixer door cylinder use?
With every stroke, a cylinder uses the volume of its chamber multiplied by the absolute pressure. The calculation takes three lines. Take the cylinder on an aggregate bin gate, with an 80 mm bore and a 200 mm stroke, supplied at 7 bar.
- Volume of one stroke: 3.14 × 0.4 dm × 0.4 dm × 2 dm ≈ 1.0 L.
- Out and back (double-acting cylinder): 2.0 L.
- As free air: 7 bar gauge is 8 bar absolute, so 2.0 × 8 = 16 L per cycle.
The rod is ignored here, which gives a small margin. Our article on pneumatic cylinder air consumption explains the method in detail.
What remains is to count the cycles per batch. On a typical plant:
| Cylinder | Free air per cycle | Cycles per batch | Air per batch |
|---|---|---|---|
| 4 bin gates (80 mm bore, 200 mm stroke) | 16 L | 2 each (coarse feed then fine dosing) | 128 L |
| 2 cement silo valves (63 mm bore, 100 mm stroke) | 5 L | 2 each | 20 L |
| Mixer door (100 mm bore, 250 mm stroke) | 31 L | 1 | 31 L |
| Total | ≈ 180 L |
With one batch every 90 seconds, or 40 batches an hour: 180 L ÷ 1.5 min = 120 l/min. The cylinders hardly count. It is the filters and the aeration that make up the air flow.
The same method applies to any other cylinder on the plant: a diverter flap on the aggregate conveyor, a water valve actuator or the admixture dosing valves. Each one is small, but list them all; on some plants they add a further 20 to 40 l/min.
The full calculation for a four-silo plant
These values are a rough guide; take your own figures from the manual for each filter and each silo.
- Gate, valve and door cylinders: 120 l/min, worked out above.
- Pulse cleaning of the filter on the silo being filled: 20 L per pulse, one pulse every 8 seconds, so 7.5 per minute: 7.5 × 20 = 150 l/min. Only one silo is filled at a time, one tanker after another.
- Pulse cleaning of the mixer filter: 15 L every 10 seconds, so 6 × 15 = 90 l/min.
- Aeration of two silos during discharge: in the region of 80 l/min each, so 160 l/min.
- Cleaning blow gun: 150 l/min while it blows, counted at half since it is used intermittently: 75 l/min.
Subtotal: 120 + 150 + 90 + 160 + 75 = 595 l/min.
- 25 % safety margin for leaks, diaphragm wear and faster cycles: 595 × 1.25 ≈ 744 l/min.
- 20 % expansion margin for a fifth silo or a higher output: 744 × 1.2 ≈ 893 l/min, or about 0.9 m³/min.
These are the margins our configurator applies. If a manual gives consumption in Nm³/h, the flow and pressure converter turns it into l/min.
Which model for which size of plant?
| Plant | Requirement (margins included) | Recommended model |
|---|---|---|
| Mobile site plant, 2 silos | ≈ 500 l/min | M 75, 1.0 m³/min at 10 bar, on wheels |
| Static plant, 3 to 4 silos, 1 mixer | ≈ 900 l/min | K 75, 1.1 m³/min at 8 bar, 420 L receiver |
| Plant with 2 mixers, 6 to 8 silos | ≈ 1,800 l/min | K 150, 2.2 m³/min at 8 bar |
For the mobile site plant, the calculation is done without an expansion margin: 120 l/min of cylinders at a slower output, reduced to 80, plus 150 for the filter, 80 for aeration and 75 for the blow gun. That gives 385 l/min, then 481 l/min with a 25 % margin. The M 75 moves with the plant and plugs into a 380 V three-phase site distribution board. It has no built-in dryer: if the plant runs through the winter, add a dryer or, at the very least, drain daily and fit a filter at the outlet.
For the large plant, if the site already has a receiver and a dryer, the S 150 offers the same air flow as the K 150 as a compressor only.
What pressure should be set on a batching plant?
Cylinders and pulse-cleaning solenoid valves generally work at around 6 bar. Note the minimum pressure in each manual and take the highest. Add the losses in the dryer, the filters and above all the pipes that run up to the silo roofs: a setting of 7 to 8 bar is enough. Check the pressure at the filter on the furthest silo, during filling, not while everything is at rest.
If a piece of equipment needs more, the K 75 is also available at 10 bar: it then delivers 0.95 m³/min, which still covers the 893 l/min in the example.
Avoid the reflex of turning the pressure up to cure a sluggish gate. Each extra bar makes the compressor work harder for the same air flow, and a gate that is slow at 8 bar usually has a worn seal, a clogged silencer or an undersized pipe. Find the cause before touching the setting.
The pitfalls of sizing a batching plant
- Counting all the filters at once. Silo filters only really pulse during and just after filling. If you add up four silos, you oversize. Count the silo being filled, plus the end-of-filling cycle of a second one if tankers arrive back to back.
- Forgetting the aeration. It blows throughout discharge. With cement that flows poorly, the sequence can last a long time. See cement aeration in the silo.
- Confusing it with the tanker's air. The bulk tanker blows the cement into the silo with its own compressor. That air never passes through your network: only the filter cleaning is down to you.
- Overlooking the receiver. A mixer door and four gates moving at the same moment create a short peak. The receiver absorbs it. The air receiver size calculator checks the volume.
- One compressor, no backup. When the air fails, the batch stays in the mixer and the truck mixers wait. Many plants keep a second machine on standby: see backup compressor and redundancy.
The K and S series are variable speed. On a batching plant, demand changes with every batch: the compressor follows it instead of switching between load and idle.
Describe your plant in the configurator or send us the list of your silos, filters and cylinders with a quote request: we will check the calculation with you.
For your trade: Compressor for concrete batching plants, quarries and demolition.