On a concrete batching plant, a refrigerated dryer is enough as long as the air stays in frost-free rooms. But the pipes that run up to the filters and silo cones are outdoors: at −10 °C, the water left after a refrigerated dryer condenses in them and then freezes in the solenoid valves. On those branches, you need a desiccant dryer, whose dew point goes well below zero. The compressor itself must stay in an enclosed room, above zero.
The other air requirements of the plant are covered on our page compressor for concrete batching plants. Here, we deal only with water and frost.
What happens when the air freezes on a plant?
On the first frosty morning, the symptoms appear in this order:
- A bin gate that will not open, or only opens halfway: the cylinder or its valve is held by ice.
- A cement silo valve stuck: the cement weighing does not progress and the batch waits.
- The silo filter pulse cleaning no longer cracks: the solenoid valve diaphragm is stuck by frost, and the filter blinds with the first tanker.
- The aeration no longer blows: the cement bridges in the cone.
When the thaw comes, the released water reaches the filter bags and aeration pads all at once. Combined with cement, it forms crusts that last well beyond the winter. The details are in our articles on cement silo filter pulse cleaning and cement aeration in the silo.
The worst damage often comes from the quick fixes. Pouring hot water on a frozen valve, or warming it with a torch, gets the first batch out but leaves more water in the system for the next night. The lasting cure is to stop the water reaching the cold sections in the first place.
How much water is left in the air after a refrigerated dryer?
A refrigerated dryer cools the air to a few degrees above zero, generally around +3 °C under pressure, and removes the condensed water. The rest of the water passes into the network. The step-by-step calculation for the silo branch:
- Water held at +3 °C: saturated air at +3 °C carries about 5.9 g of water per m³ of compressed air.
- Converted to free air: at 8 bar gauge, or 9 bar absolute, 1 m³ of compressed air contains 9 m³ of free air. That gives 5.9 ÷ 9 ≈ 0.66 g of water per m³ of free air.
- What the air can hold at −10 °C: about 2.1 g per m³ of compressed air, or 2.1 ÷ 9 ≈ 0.24 g per m³ of free air.
- What drops out: 0.66 − 0.24 = 0.42 g per m³ of free air passing through the cold pipe.
- Over a day: the silo branch uses 0.5 m³/min for 10 hours, or 300 m³. 300 × 0.42 ≈ 126 g of ice per day.
A hundred grams of ice is nothing in a receiver. In a pulse-cleaning solenoid valve or the valve of a gate cylinder, it is enough to block everything. Our condensate volume calculator gives the total quantity of water to remove upstream, and our guide on pressure dew point explains these concepts.
Which dryer for a concrete batching plant?
Work through the plant section by section:
| Section | Minimum winter temperature | Suitable dryer |
|---|---|---|
| Heated compressor room, control cabin, workshop | above +5 °C | refrigerated |
| Covered bins, conveyors, mixer in an enclosed unheated building | around 0 °C | refrigerated, to be monitored |
| Pipes running up to the roof filters and silo cones | −10 to −20 °C depending on the region | desiccant |
Choose a dew point well below the coldest temperature of the section. A desiccant dryer goes well below zero; its data sheet gives the exact value. Our article which air dryer to choose compares the two technologies.
Two possible arrangements:
- Desiccant on the full air flow, when most of the consumers are outdoors. It is the simplest, but the dryer has to handle the compressor's full output.
- Refrigerated for everything, desiccant on the silo branch only, when the valves and the mixer are under cover. The desiccant dryer is smaller.
Example of sizing the branch. Silo filters 300 l/min and aeration 160 l/min, so 460 l/min. With a 25 % margin: 460 × 1.25 = 575 l/min. You need a desiccant dryer of at least 0.6 m³/min. A desiccant dryer also uses part of the air to regenerate itself: add this to the compressor's air flow, based on its data sheet. The same method is applied to grain silos in our article compressed air dryer for grain silos.
Where should the compressor be installed in winter?
The dryer does not do everything. The compressor and its receiver must stay in an enclosed, frost-free room.
- Above zero. Below it, condensate freezes in the receiver, the drain and the refrigerated dryer, and the machine may refuse to start in the morning. Our guide air compressor, altitude and temperature gives the operating limits.
- No direct cold draught. An intake grille wide open on the north side cools the machine too much in winter. Adjust the ventilation louvres according to the season: see the compressor room.
- Compressor heat for the room. A screw compressor gives off most of its energy as heat. Properly ducted, it keeps the room frost-free and can heat the control cabin. See our guide on heat recovery.
- Protected from dust. Closing the room in winter also keeps cement dust out. Check the air filter at every visit.
Which compressor for a plant that runs through the winter?
The K 75 delivers 1.1 m³/min at 8 bar, with a 420 L receiver, refrigerated dryer and filtration built in. It covers most plants with three or four silos; a desiccant dryer is added on the silo branch. For a larger plant, the K 110 goes up to 1.7 m³/min. If you choose a desiccant dryer on the full air flow, the S 75, as a compressor only, avoids doubling up on dryers.
Our compressors are lubricated: the outlet filtration retains carried-over oil, and a fine filter upstream of the desiccant dryer protects its desiccant.
Jobs to do before winter
- Drain the low points and check every automatic condensate drain. A frozen drain lets water through into the network.
- Insulate the exposed pipes on the silos, especially horizontal sections where water collects.
- Give the pipes a fall towards a drain point, never towards a solenoid valve.
- Test the gates and valves with no load at the start of the day, before the first customer.
- Service before the cold weather. First service at 500 hours, then every 1,500 hours: use the quiet season to change the air filter and check the dryer.
Describe your plant and the routing of your pipes in a quote request, or start with the configurator: we will work out the right dryer with you.
For your trade: Compressor for concrete batching plants, quarries and demolition.