In a grain silo, a refrigerated dryer is enough as long as every air line stays inside buildings that do not freeze. As soon as a line runs outdoors, up an unheated elevator tower or along a cold gallery in winter, the moisture left in the air can condense and then freeze. That is where a desiccant dryer earns its place, on the full flow or only on the exposed branch.
The other air needs of a grain intake site are covered on our page on the compressor for grain silos. Here we only look at drying.
Why is water such a problem in a silo?
The air drawn in contains water vapour. Once compressed and cooled, it gives that water back as droplets. In a silo, those droplets cause three problems:
- dust collector bags blind, because damp dust sticks;
- solenoid valves and slide gate cylinders rust and seize;
- in winter, water freezes in exposed lines and blocks the hatches, often just when the grain dryer is running.
Pressure dew point, the one figure that matters
The pressure dew point is the temperature below which the remaining water condenses in the line.
- Refrigerated dryer: it cools the air to a few degrees above zero, condenses the water and drains it. The dew point is therefore above zero.
- Desiccant dryer: the air passes through a desiccant that captures the moisture. The dew point drops well below zero.
The rule is simple: as long as the pipe stays warmer than the dew point, there is no water. An outdoor pipe on a frosty night is colder than the dew point of a refrigerated dryer.
The method, section by section
- Map the network and classify each section: heated room, unheated building, outdoors.
- Note the lowest temperature each section sees in winter.
- Compare it with the dew point of the dryer you are considering.
- Work out the flow of the branches at risk, so the dryer is sized where it is needed.
Example. A K 110 delivers 1.7 m³/min at 8 bar in a plant room. Its built-in refrigerated dryer treats all the air.
- Plant room, workshop and weighbridge office, all heated: no risk.
- Unheated elevator tower: frost risk.
- Overhead line to the loading dust collector: outdoors, frost risk.
Flow of the two exposed branches, for guidance only: one dust collector at 200 L/min and six cylinders at about 5 L/min each on average, so 230 L/min. With a 25% margin: 290 L/min, about 0.3 m³/min.
Two options:
- Desiccant on the full flow, 1.7 m³/min. Simple, but the dryer is larger and its regeneration uses part of the air across the whole flow.
- Refrigerated for the whole site, desiccant on the exposed branch, sized for 0.3 m³/min. The dryer is smaller and only treats the air that needs it.
Either way, regeneration takes a share of the air: include it in the compressor flow. We size the desiccant dryer on quotation.
Three common situations
- The whole network is frost-free. A 4-in-1 compressor is enough, with its refrigerated dryer built in, like the K 110 above.
- A site that dries grain in autumn and winter, with outdoor lines. Refrigerated for the whole site, desiccant on the outdoor branches.
- A large site with a standalone compressor. An S 110 or an S 150 (2.2 m³/min at 8 bar) takes a refrigerated dryer sized for 15 kW, then desiccant drying where it freezes.
Pitfalls to avoid
- Protect the desiccant. Oil damages the desiccant. A desiccant dryer is fitted with a fine filter upstream and a dust filter downstream. The desiccant is replaced as the manufacturer recommends.
- A refrigerated dryer needs a frost-free room. Its condenser quickly clogs with grain dust: clean it before harvest, not during.
- Low points. Lay the lines with a fall and a drain at every low point. Our guide to connecting a compressor to the air network shows the layout.
- Condensate. It contains oil: collect it and have it treated. See our article on condensate draining.
- Air that touches the grain. If the air comes into contact with the product, define the air class with our technicians using our ISO 8573-1 guide.
To compare both technologies for other uses, read which air dryer to choose. For your silo, send the network layout with a quote request: we will tell you where each dryer should go.
For your trade: Compressor for grain silos and agricultural cooperatives.