The blast machine is the second purchase. The first one, and the one most often got wrong, is the compressor. Not because it is hard to choose, but because it gets chosen on price and motor power, and blasting asks for neither. It asks for a given flow at a given pressure, held steady, with dry air.
An undersized compressor does not break down. It runs. The pressure at the nozzle simply drops after the first few minutes, the operator spends twice as long on every part, and the electricity bill does not go down at all. The abrasive gets blamed, then the nozzle, then the operator. Rarely the compressor.
Two numbers, not one
A compressor is described by two values that belong together: the flow, in cubic metres per minute (or cfm), and the pressure at which it delivers that flow, in bar. One without the other means nothing. A “10 bar” compressor may deliver, at 10 bar, a third of the flow printed in large type on its housing.
That is the first trap: the number on the label is often the displacement, the volume the compressor draws in. What comes out after compression and internal losses is less, sometimes by a third. The figure that matters is the delivered flow, listed in data sheets as FAD (free air delivery). If it is not in the catalogue, ask for it. If nobody can tell you, that is a sign in itself.
The nozzle decides how much air you use
The machine does not consume the air. The nozzle does. The whole air-and-abrasive stream passes through its orifice, and the flow depends on the orifice diameter and the working pressure.
The relationship is not linear. Flow grows with the square of the diameter: an 8 mm nozzle uses nearly twice the air of a 6 mm one, not a third more. And raising the pressure from 6 to 8 bar increases consumption in proportion to absolute pressure.
The table below is indicative, for new nozzles at a working pressure of around 7 bar. Exact values differ between manufacturers and rise as the nozzle wears, so before ordering a compressor, ask for the data sheet of the nozzle you will actually run.
| Nozzle bore | Indicative flow (7 bar) | Typical use |
|---|---|---|
| 6 mm (No. 4) | about 2–2.5 m³/min (70–90 cfm) | small cabinets, small parts, touch-up |
| 8 mm (No. 5) | about 3.5–4 m³/min (125–140 cfm) | workshop cabinets, short runs |
| 9.5 mm (No. 6) | about 5–6 m³/min (180–210 cfm) | blast rooms, steel structures |
| 11 mm (No. 7) | about 7–7.5 m³/min (250–265 cfm) | continuous production, large surfaces |
| 12.5 mm (No. 8) | about 9–10 m³/min (320–350 cfm) | site work, tanks, ships |
What happens in practice: people size the compressor for the 8 mm nozzle that comes with the machine, then move up to 9.5 mm to go faster, and find the pressure no longer holds. Size the compressor for the largest nozzle you will ever run, not the one in the box.
Label pressure is not nozzle pressure
The compressor delivers 8 bar at its outlet. The nozzle sees 6. The difference was lost on the way: in the hose, in couplings, in filters, in valves that are not full-bore.
And that loss is expensive. Abrasive velocity, and therefore productivity, depends directly on pressure at the nozzle. Every bar lost along the line shows up as time spent on every part. An 8 bar compressor on a badly laid-out line does the same work as a 6 bar unit installed properly; it just cost more.
The only reliable check is to measure at the nozzle, with a needle pressure gauge, while blasting, not with the hose shut. Pierce the hose just before the nozzle holder, read it, compare with what the compressor shows. If the gap is more than a bar, the problem is on the line, not in the compressor.
The hose: thicker and shorter than you think
The most common source of loss is an air hose that is too narrow. The sizing rule equipment makers use: the internal diameter of the air hose should be at least three times the nozzle bore. For an 8 mm nozzle that means a hose of at least 25 mm bore. The 13 mm hose from the hardware store throttles everything.
The second source is quick couplings with a narrow passage. Every ordinary pneumatic coupling is an extra nozzle on the line. For blasting, use full-bore couplings whose internal diameter matches the hose.
The third is length. Every metre of hose costs pressure. Put the compressor as close to the machine as possible; if that cannot be done, run large-bore fixed pipe up to the machine and leave hose only for the last few metres.
Water ruins blasting
Compressed air leaves the compressor hot and saturated with vapour. As it cools in the hose, water condenses and reaches the abrasive. The consequences are immediate:
- the abrasive clumps in the pot and the hose, flow becomes erratic, blockages appear
- the part comes out stained, and cleaned steel flash-rusts within hours, before it can be painted
- the dust filter clogs faster, because damp dust sticks to the bags
The fix depends on how much air you pass and how sensitive the application is. The minimum is a water separator with a drain, mounted near the machine, not at the compressor. For continuous work, or for parts painted straight after blasting, add an aftercooler and a refrigerant dryer. For fine or hygroscopic abrasives, the dryer is not optional.
One detail that gets forgotten: the compressor receiver must be drained daily. A tank with water on the bottom sends water into the machine no matter how good the dryer after it is.
How to check your compressor before buying the machine
Five questions, in this order:
- Which nozzle will I use most? The flow in the table, for the largest nozzle, is the starting point.
- How much does the compressor actually deliver? FAD at working pressure, not displacement and not at maximum pressure.
- How much of the day will it run? Piston compressors are not built for continuous duty. For blasting in shifts, go for a screw compressor or an oversized piston unit that gets rest periods.
- What is on the line? Hose bore, coupling type, length. They all add up.
- What happens to the water? Separator, aftercooler, dryer, depending on the application.
Then leave a margin. The nozzle wears and consumes more over time, and a compressor running permanently at its limit wears out faster too. Tight sizing saves money at purchase and pays it back every month afterwards.
When the compressor is already there
The most common situation in practice: the workshop has a compressor and wants a machine that runs on it. It can be done, but then the nozzle is chosen to suit the compressor, not the other way round. A smaller nozzle at steady pressure blasts better than a large one at pressure that keeps dropping. And if productivity still falls short, the sums have to be done honestly: a new compressor or a smaller machine, not a large machine on an old compressor.
Send us the compressor data sheet and the type of parts you blast, and we will tell you which nozzle and which machine make sense for it, or whether none does. Get in touch.
Read next
- Manual sandblasting (compressed air): types, benefits and uses
- Blasting nozzles: how long they last and how to tell when one is worn
- How to choose the right sandblasting equipment
- Compressed air blasting — equipment and solutions
- Sandblasting equipment
- What you need before you install a blasting machine
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